WO2009150441A1

WO2009150441A1 - Mitochondrial transformation

Info

Publication number: WO2009150441A1
Application number: PCT/GB2009/001510
Authority: WO
Inventors: Simon Geir Moller; Xiang Ming Xu
Original assignee: WEBBER PHILIP MICHAEL; Universitetet i Stavanger
Current assignee: WEBBER PHILIP MICHAEL; Universitetet i Stavanger
Priority date: 2008-06-13
Filing date: 2009-06-12
Publication date: 2009-12-17
Anticipated expiration: 2010-12-13

Abstract

The invention relates to a method for producing a transformed plant cell. More particularly, the method involves the transformation of a plant cell with a Transformation Cassette which is targeted to plant mitochondria and which comprises a selection gene, for example isopentenyl transferase (IPT), and a transgene. After selection for transformed mitochondria, expression of a recombinase is induced in the plant cell, which leads to the excision of the selection gene from the mitochondria and the expression of the transgene in the mitochondria. The invention also provides cells and plants comprising the Transformation Cassette.

Description

MITOCHONDRIAL TRANSFORMATION

The invention relates to a method for producing a transformed plant cell. More particularly, the method involves the transformation of a plant cell with a Transformation Cassette which is targeted to plant mitochondria and which comprises a selection gene, for example isopentenyl transferase (IPT), and a transgene. After selection for transformed mitochondria, expression of a recombinase may be induced in the plant cell, which leads to the excision of the selection gene from the mitochondria and the expression of the transgene in the mitochondria. The invention also provides cells and plants comprising the Transformation Cassette.

The use of genetically modified (GM) food crops in agriculture is rapidly increasing with an approximate £14 billion world market in 2005. There is, however, a growing concern amongst scientists, politicians, regulatory agencies and the general public regarding the risk of the spread of transgenes by pollen from GM crops to other plant species generating so- called "superweeds". All commercial GM crops used in today's agriculture have engineered nuclear genomes containing transgenes conferring desirable traits such as resistance to disease, insects, harsh environmental conditions and increased vitamin content, flavour, and storage time. However, there are several serious problems with nuclear transgenes including gene silencing and low levels of transgene expression. One way of resolving the problems associated with nuclear transgenes is to insert transgenes into the mitochondrial genome of plants.

All eukaryotes contain mitochondria. The number of mitochondria per cell varies depending on organism and cell type. Often cells requiring a lot of energy have more mitochondria than others. Animal cells do generally have between 1000-2000 mitochondria and plants have a similar number. Mitochondria have generally between 2 and 10 copies of mitochondrial DNA but again this varies with some cells having between 100 000 - 1 000 000 copies. Due to the large number of mitochondria albeit with a lower genome copy number transformed plant cells with all mitochondria harbouring transgenes would have comparable total transgene numbers per cell as for plastid transformation. The ability to transform mitochondria in plants has several uses: 1) By inserting genes into mitochondria one can generate mitochondrial mutants and assay for maternal inheritance effects and phenotypes for which it has not previously been possible to assay. 2) One can express proteins to high levels which will be contained inside the organelle.

3) One can express proteins in mitochondria that then are exported into the cytosol.

4) One can assay and study male sterility in plants. 5) Based on the plant system, it is possible to design ways to transform mitochondria in mammalian cells to assay for disease states such as Parkinson's disease, stroke, cancers, etc.

Mitochondrial transformation has clear advantages over nuclear transformation in that high levels of proteins can most probably be produced and that the transformed mitochondria are maternally inherited (i.e. from the mother) providing genetic control. Mitochondrial transformation will also be useful for assaying mitochondrial functions in terms of agricultural traits. The invention is based on a selection and regeneration system for mitochondrial transformation based on the over-expression of a gene such as the isopentenyl transferase (IPT) gene (cytokinin biosynthesis) in mitochondria. This system allows, for example, for the direct selection of cells containing transformed mitochondria on media lacking cytokinin due to cytokinin production within mitochondria. The system therefore provides an antibiotics-free selection and regeneration system which will overcome concerns regarding the use of antibiotic resistance genes in GMOs.

Whilst IPT has previously been used as a selectable marker in plant nuclear transformation (e.g. EP 1 069 855 A), its use in mitochondrial transformation has not previously been suggested. The person skilled in the art will be aware of the fact that mitochondria are semi-autonomous organelles within plant cells with their own genomes and metabolism. In particular, it has not previously been shown that IPT precursors are made in mitochondria or that any IPT produced is capable of diffusing out of the mitochondria into the cell cytoplasm in order to initiate cytokinin signalling to stimulate shoot regeneration. Hence the criteria for choosing mitochondrial - selection genes are distinct from those for choosing genome-selection genes.

Due to the fact that over-expression of plant-hormone biosynthetic polypeptides in adult plants leads to impaired development, the plant- hormone biosynthetic gene is preferably removed after selection and initial regeneration has occurred. The transgene in question (expressing the polypeptide of interest) might only be activated after the removal of the plant- hormone biosynthetic gene, so that any adverse effects due to transgene expression during selection and regeneration are also eliminated.

In one embodiment, the invention provides a method for producing a transformed plant cell, the method comprising the step: (i) transforming the plant cell with a genetic construct, wherein the genetic construct comprises first and second homologous recombination elements flanking a Transformation Cassette, wherein the first and second homologous recombination elements are capable of directing the integration of the Transformation Cassette into the genome of at least one mitochondria which is present in the plant cell, wherein the Transformation Cassette comprises: (a) a first promoter which is operable in said plant cell,

(b) an Excision Cassette,

(c) one or more transgenes,

(d) a first terminator element, wherein the Excision Cassette comprises: (bl) a first site-specific recombination element,

(b2) an optional second promoter

(b3) one or more nucleotide sequences encoding one or more plant- hormone biosynthetic polypeptides and optionally a nucleotide sequence encoding a polypeptide which confers resistance to an antibiotic,

(b4) a second terminator element,

(b5) a second site-specific recombination element, wherein the first and second site-specific recombination elements are capable of being recognised by a recombinase. In some embodiments, the method additionally comprises the step:

(ii) selecting for transformed plant cells on media which lacks one or more of the plant-hormone biosynthetic polypeptides or antibiotic.

In other embodiments, the method additionally comprises the step:

(iii) expressing a recombinase in the plant cell, wherein the recombinase is one which recognises the first and second site-specific recombination elements. The method of the invention is suitable for all plants that can be transformed and regenerated. The plant may be a monocot or dicot.

Examples of suitable plants are cereals (rice, wheat, barley, oats, sorghum, corn), legumes (alfalfa, lentils, peanut, pea, soybean), oil crops (palm, sunflower, coconut, canola, olive), cash crops (cotton, sugar cane, cassava), vegetable crops (potato, tomato, carrot, sweet potato, sugar-beet, squash, cucumber, lettuce, broccoli, cauliflower, snap bean, cabbage, celery, onion, garlic), fruits/trees and nuts (banana, grape cantaloupe, muskmelon, watermelon, strawberry, orange, apple, mango, avocado, peach, grapefruit, pineapple, maple, almond), beverages (coffee, tea, cocoa), and timber trees (oak, black walnut, sycamore). Other suitable plants include mosses and duckweed. Preferably, the plant is tobacco or lettuce.

The plant cells which are being transformed may be used in any convenient form, for example, as individual cells, groups of cells, in dissociated form or undissociated form, or as part of a plant tissue or plant part. Preferably, the cells are present in leaves that are removed from intact plants. It is preferable to use actively-growing leaves.

As used herein, the term "genetic construct" refers to a nucleic acid molecule comprising the specified elements and Cassettes. The genetic construct may, for example, be in the form of a vector or a plasmid. It may also contain other elements which enable its handling and reproduction, such as an origin of replication, selection elements, and multiple cloning sites. Generally, the genetic construct will be a double-stranded nucleic acid molecule, preferably a dsDNA molecule. The first and second homologous recombination elements are ones that are capable of directing the integration of the Transformation Cassette into the genome of at least one mitochondria which is present in the plant cell.

Upon transformation of the genetic construct into the plant cell, the first and second homologous recombination elements recombine with corresponding sequences in the genome of the selected mitochondria, resulting in the insertion of the Transformation Cassette into the genome of the selected mitochondria.

The nucleotide sequences of the homologous recombination elements are selected such that the Transformation Cassette is specifically targeted to one or more selected mitochondria. In particular, the nucleotide sequences of the homologous recombination elements are selected such that no or essentially no Transformation Cassettes become integrated into the nuclear genome of the plant or into the mitochondrial genome of the plant. In other words, the nucleotide sequences of the homologous recombination elements are preferably mitochondria-specific, i.e. corresponding sequences are not present in the nuclear genome and preferably not present in the plastid genome of the plant in question. This may be done by avoiding sequences which are present in the nuclear genome of the plant and optionally in the plastid genome also. The skilled person will readily be able to detect whether a specific sequence is or is not present in the nuclear/plastid genome by standard means, for example, by Southern Blotting of the nuclear/plastid genome with a labelled sequence probe or by sequence analysis.

Apart from the above, any sequences can be used from the mitochondrial genome as long as the selected insertion site is not lethal to the cell, i.e. it does not result in the death of the cell. Preferably, the insertion sites are not in coding regions of mitochondrial genes.

The orientation of the sequences of the first and second homologous recombination elements should be the same as the orientation in the mitochondrial genome to allow for efficient homologous recombination.

In order to target the Transformation Cassette to the mitochondrial genome, the nucleotide sequences of the first and second homologous recombination elements must be identical or substantially identical to sequences in the genome of the selected plant mitochondria.

In the context of the present invention, the term "substantially identical" means that the nucleotide sequences of the first and second homologous recombination sequences are independently more than 95%, preferably more than 98% or more than 99% and particularly preferably 100% identical to sequences which are present in the mitochondria to be transformed. Percentage sequence identities may be determined using the Clustal method of alignment with default parameters, e.g. KTUPLE 1, GAP PENALTY=3, WINDOW=5 and DIAGONALS SAVED =5.

Similarly, the nucleotide sequences of the first and second homologous recombination elements should preferably not be identical or substantially identical to sequences in the nuclear genome of the selected plant. In this context, the term "substantially identical" means that the nucleotide sequences of the first and second homologous recombination sequences are independently less than 50%, more preferably less than 70% or less than 90% identical to sequences which are present in the nuclear genome of the plant to be transformed. Percentage sequence identities may be determined using the Clustal method of alignment with default parameters, e.g. KTUPLE I₃ GAP PENALTY=3, WINDOW=5 and DIAGONALS SAVED=5.

Preferably, the lengths of first and second homologous recombination sequences will independently be 50-2500, 50-2000, 50-1500 or 50-1000 nucleotides each, more preferably about 150, about 1000 or about 1200 nucleotides in length.

The distance between the first and second homologous recombination sequences in the mitochondrial genome may be 0-4000 nucleotides or more. Preferably, the distance is about 1-100, 100-500, 500-1000 or 1000-3000 nucleotides.

The total length of the genetic elements which are present between the first and second homologous recombination (i.e. genetic elements (a)-(d)) is preferably less than 4000 nucleotides. Preferably, the first homologous recombination sequence is nucleotides

72001-72501 of the Nicotiana tabacum (GenBank accession no. BA000042) mitochondrial genome DNA, and/or preferably, the second homologous recombination sequence is nucleotides 72502-73000 of the Nicotiana tabacum (GenBank accession no. BA000042) mitochondrial genome DNA. The Transformation Cassette promoter (a) must be one that is operable in the selected plant mitochondria. The promoter is one which is capable of initiating transcription of the transgene once the Excision Cassette has been excised; and of initiating the transcription of the nucleotide sequence encoding a plant-hormone biosynthetic polypeptide, in cases where the Excision Cassette does not contain its own promoter. The promoter might, for example, be one derived from a plant or bacterial gene. Preferably, the promoter is plant specific.

Examples of suitable promoters include PsbA, RbcL, Prrn, 16S rRNA, T3, T7 and ATPase promoters. In some embodiments, the promoter is an inducible promoter. This allows inducible, controlled expression of the selection gene(s). For example, the inducible promoter may be inducible by IPTG, e.g. the PrrnL promoter. Other inducible promoters include those inducible by light, dark, ethanol, drought, metals, pathogens, growth regulators, heat, cold, galactose and other sugars.

Alternatively, the promoter is a high-expression level promoter. The Excision Cassette comprises a first site-specific recombination element; optionally, a second promoter; a nucleotide sequence encoding a plant-hormone biosynthetic polypeptide; a terminator sequence; and a second site-specific recombination element. The first and second site-specific recombination elements are sequences of nucleotides which are capable of being recognised and/or bound by the site-specific recombinase which is produced by a recombinase. The site-specific recombination elements must flank the genetic elements in the Excision Cassette, e.g. elements (b2) (if used), (b3), (b4), any other desired elements. The sequences of the first and second recombination elements will be identical or substantially identical to each other; and will be in the same orientation relative to each other (e.g. both 5'-3' or both 3'-5').

When the recombinase is Cre, the site-specific recombination sequences are preferably lox sequences. Site-specific lox recombination sites are 34 bp sequences; these act as binding sites for the Cre recombinase polypeptide. Wild- type lox sequences are preferred (Zuo J₃ Niu QW, Møller SG, Chua NH (2001) Chemical-regulated, site-specific DNA excision in transgenic plants. Nat. Biotechnol. 19, 157-161.)

The Excision Cassette promoter, when present, must be one that is operable in the selected plant mitochondria. The promoter is one which is capable of initiating transcription of the plant-hormone biosynthetic gene. The promoter might be one derived from a plant or bacterial gene. Preferably, the promoter is plant specific. Examples of suitable promoters include PsbA, RbcL and Prrn promoters. The plant-hormone biosynthetic polypeptides act as selection markers, allowing the selection of plant cells which have been transformed with the Transformation Cassette. The plant-hormone biosynthetic polypeptides may be any polypeptides which are involved in the synthesis of a plant cytokinin or auxin or other plant growth regulator, or which regulate the production or metabolism of a plant cytokinin or auxin or other plant growth regulator.

Preferably, there are nucleotide sequences encoding 1, 2, 3, or 4 plant- hormone biosynthetic polypeptides. The nucleotide sequences encoding the plant-hormone biosynthetic polypeptides may be present in an operon, with a single optional promoter and terminator element. Alternatively, the plant- hormone biosynthetic polypeptide nucleotide sequences may each have their own promoters and terminator elements. A further option is that two or more of the nucleotide sequences encoding the plant-hormone biosynthetic polypeptides are present as fusion proteins, optionally with a short linker sequence joining the proteins (e.g. encoding a 1-10 amino acid linker sequence, e.g. a poly-glycine linker). In other embodiments, some of the nucleotide sequences encoding the plant-hormone biosynthetic polypeptides may be present in an operon and/or as fusion proteins, and others have their own promoters and/or terminators.

In some embodiments, the or a plant-hormone biosynthetic polypeptide is IPT (isopentenyl transferase) which is an enzyme involved in cytokinin biosynthesis. The IPT nucleotide sequence may be from any suitable source. Due to codon usage, bacterial IPT genes are preferred, because nuclear genes may not be expressed to maximum levels in mitochondria.

Preferably, the IPT nucleotide sequence is from Agrobacteriwn tumefaciens or a plant (e.g. from the plant which is being transformed). In other embodiments, the or a plant-hormone biosynthetic polypeptide is iaaH (indoleacetamide hydrolase) and/or iaaM (tryptophan mono-oxygenase), which are enzymes involved in auxin biosynthesis. The nucleotide sequences may be from any source. Due to codon usage, bacterial iaaH and/or iaaM genes are preferred. Preferably, the iaaH and/or iaaM nucleotide sequences are from

Agrobacteriwn tumefaciens.

In other embodiments of the invention where auxin biosynthetic enzymes are involved, the transformed plants may be selected on media lacking auxins. Examples of naturally-occurring auxins include 4-chloro indoleacetic acid, phenyl acetic acid (PAA) and indole-3-butyric acid (IBA).

In some preferred embodiments of the invention, the plant hormone biosynthetic enzymes are iaaH and iaaM, preferably from Agrobacteriwn tumefaciens.

The Excision Cassette terminator prevents the premature expression of the transgene(s) prior to the excision of the Excision Cassette. Any terminator can be used for this provided that it is recognised in the plant cell being transformed. The terminator may be a plant terminator or a bacterial terminator, inter alia.

Examples of suitable terminators include those of rrn, psbA, rbcL, T3, T7 and ATPase. The preferred terminator is a T3 or T7 terminator.

In some embodiments of the invention, the promoter and terminator used in the Excision Cassette do not both originate from the same mitochondrial gene.

In the context of the present invention, the term "transgene" (i.e. element (c)) is used to refer to a nucleic acid molecule which is being introduced into the genome of the mitochondria. The transgene may, for example, be a genomic DNA, cDNA or synthetic nucleic acid molecule coding for a peptide or polypeptide; a nucleic acid molecule encoding a mRNA, tRNA or ribozyme; or any other nucleic acid molecule.

Examples of transgenes include those coding for antibodies, antibiotics, herbicides, vaccine antigens, enzymes, enzyme inhibitors and design peptides.

Single or multiple antigens may be produced from viridae, bacteria, fungi or other pathogens. The antigens may be expressed as single units or as multiple units of several antigens, e.g. for broad-spectrum vaccine development. Enzymes may be produced for use in cosmetics (e.g. superoxide dismutase, peroxidase, etc.). Enzymes may also be produced for use in detergent compositions.

The invention particularly relates to the production of proteins/enzymes with specific activities, for example, immunostimulants to boost immune responses, such as interferons; and growth factors, e.g. transforming growth factor-beta (TGF-beta), bone morphogenic protein (BMP), neurotrophins (NGF, BDNF, NT3), fibroblast growth factor (FGF), proteolytic enzymes (papain, bromelain), and food supplement enzymes (protease, lipase, amylase, cellulase). The invention also relates to the production or over-expression of proteins/enzymes in mitochondria that make the plants more resistant to biotic and abiotic stress, such as salts and metals. Examples of this include the generation of trans-mitochondrial plants that chelate iron (Fe) for mopping up excess metal in agriculturally important areas for future planting. The invention further relates to the use of transgenes encoding polypeptides which modify fatty acid biosynthesis in mitochondria.

One or more transgenes may be inserted in the Transformation Cassette. Preferably, the transgene sequences are contiguous.

The transgene sequence may additionally encode a protein purification tag fused to the polypeptide of interest. Examples of protein purification tags include the N-terminal influenza haemagglutinin-HA-epitope (HA) and a sequence of six histidine amino acids (HIS 6). Once the Excision Cassette has been excised, the first promoter is capable of driving the expression of the transgene, leading to the accumulation of the product of the transgene in the mitochondria. The product of the transgene may be purified or isolated from the plant cell by any suitable means.

In some embodiments of the invention, the transgene is constitutively expressed independently of whether or not the Excision Cassette has been removed. In such embodiments, the transgene may be expressed from its own promoter. Such embodiments are useful if the plant does not show any detrimental effects of the transgene expression during regeneration.

The Transformation Cassette terminator (genetic element (d)) terminates the expression of the transgene (s). Any terminator can be used for this provided that it is recognised in the plant cell being transformed. The terminator may be a plant terminator or a bacterial terminator, inter alia.

Examples of suitable terminators include TrbcL or TspbA polyA addition sequences and the ATPase terminator. The preferred terminator is the psbA polyA addition sequence.

In the Transformation Cassette, the elements are preferably operably linked in the order (a), (b), (c), (d).

In the Excision Cassette, the first and second site-specific recombination elements must flank the optional promoter (when present), the nucleotide sequence encoding a plant-hormone biosynthetic polypeptide and the terminator element. In some embodiments of the invention, the nucleotide sequence encoding a plant-hormone biosynthetic polypeptide and the terminator element will be downstream (i.e. 3') to the promoter of the Transformation Cassette and hence the latter promoter will be capable of driving expression of the plant-hormone biosynthetic polypeptide. Thus in some embodiments of the invention, the Transformation

Cassette comprises:

(a) a first promoter which is operable in said plant cell,

(b) an Excision Cassette comprising:

(bl) a first site-specific recombination element, (b3) one or more nucleotide sequences encoding one or more plant-hormone biosynthetic polypeptides, (b4) a second terminator element, (b5) a second site-specific recombination element, wherein the first and second site-specific recombination elements are capable of being recognised by a recombinase; (c) one or more transgenes, (d) a first terminator element, operably linked in the order specified above in a 5'-3' direction.

In this embodiment of the invention, the first promoter is capable of driving expression of the nucleotide sequence encoding the plant-hormone biosynthetic polypeptides. After the removal of the Excision Cassette, the first promoter drives expression of the transgene(s).

In other embodiments of the invention, the Excision Cassette will be in the reverse orientation compared to the first promoter, transgene(s) and first terminator element. In this embodiment, the expressed parts of the Excision Cassette will be present in the nucleotide strand which is complementary to that which codes for the first promoter, transgene(s) and first terminator element, and in the reverse direction.

In such embodiments, the Excision Cassette will comprise a second promoter, capable of driving the expression of the nucleotide sequences encoding the plant-hormone biosynthetic polypeptides. In this embodiment of the invention, the Transformation Cassette comprises:

(a) a first promoter which is operable in said plant cell,

(b) an Excision Cassette,

(c) one or more transgenes, (d) a first terminator element, wherein (a), (b), (c) and (d) are operably linked in the order specified above in a 5'-3' direction, wherein the Excision Cassette comprises:

(bl) a first site-specific recombination element, (b2) a second promoter,

(b3) one or more nucleotide sequences encoding one or more plant- hormone biosynthetic polypeptides, (b4) a second terminator element, (b5) a second site-specific recombination element, wherein the first and second site-specific recombination elements are capable of being recognised by a recombinase, and wherein the parts of the Excision Cassette are operably linked and wherein the Excision Cassette is in reverse orientation compared to (a), (c) and (d).

In this embodiment, the second promoter drives expression of the nucleotide sequences encoding the plant-hormone biosynthetic polypeptides. After the removal of the Excision Cassette, the first promoter drives expression of the transgene(s).

The Transformation Cassette is not restricted to the parts (a)-(d) specified herein. It may, for example, additionally comprise a 5'-UTR to increase the expression level of the transgene(s) and 3'-additional amino acids to increase protein stability. In some embodiments of the invention, the Transformation Cassette additionally comprises a second selectable marker gene, e.g. an antibiotic resistance gene, preferably a nucleotide sequence encoding spectinomycin adenyltransferase (e.g. the aadA gene). This polypeptide confers resistance to the antibiotic spectinomycin. The nucleotide sequence enoding spectinomycin adenyltransferase may be placed downstream of one of the promoters in the Transformation Cassette. It may, for example, be placed downstream and operably linked to the first promoter; or downstream and operably linked to the nucleotide sequences encoding the plant-hormone biosynthetic polypeptides (e.g. IPT); or upstream and operably linked to the nucleotide sequences encoding the plant-hormone biosynthetic polypeptides (e.g. IPT).

In yet other embodiments, the Transformation Cassette excludes a second selectable marker gene and/or excludes a nucleotide sequence which confers resistance to an antibiotic. In some embodiments, the Transformation Cassette additionally comprises the Lad gene, preferably under control of an appropriate promoter (e.g. T7), in order to allow inducible expression.

In some embodiments, the promoter(s) and/or terminator(s) used in the Transformation Cassette are not from mitochondrial genes. This avoids/minimises vector rearrangement.

After successful delivery of the Transformation Cassette to the mitochondria, transformed cells are selected on media lacking the plant- hormone biosynthetic polypeptide. In general, plants are regenerated by adding cytokinin and auxin. Because the transformed mitochondria will preferably produce IPT and therefore cytokinin, plants can be selected and regenerated in the presence of auxin only. The cells for selection will preferably be leaf cells. In some embodiments of the invention, plants are selected in the presence of cytokinin inhibitors (e.g. cytokinin oxidase or chemical inhibitors) in order to minimise leakage of the cytokinin from the site of biosynthesis to other parts of the plant. This reduced mosaicism in the plant. In some embodiments of the invention, the method additionally comprises the step:

(iii) expressing a recombinase in the plant cell, wherein the recombinase is one which recognises the first and second site-specific recombination elements. The recombinase is a site-specific recombinase. A site-specific recombinase is a polypeptide which is capable of binding to site-specific recombination elements and inducing a cross-over event in the nucleic acid molecule in the vicinity of the site-specific recombination elements. In the present invention, the expression of the recombinase leads to the excision of the Excision Cassette from the mitochondrial genome.

In the context of the present invention, the recombinase is one which is capable of binding to the first and second site-specific recombination elements which are present in the Excision Cassette, leading to the excision of the Excision Cassette in a standard manner. Examples of site-specific recombination elements/site-specific recombinases include Cre-lox, the FLP-FRP system from Saccharomyces cerevisae (O'Gorman S, Fox DT, Wahl GM. (1991) Recombinase-mediated gene activation and site-specific integration in mammalian cells. Science. 25, 1351-1555.), the GIN/gix system from bacteriophage Mu (Maeser S and Kahmann R. (1991) The Gin recombinase of phage Mu can catalyse site- specific recombination in plant protoplasts. MoI Gen Genet. 230, 170-176.) or the R/RS system from Zygosaccharomyces rouxii (Onouchi H, Yokoi K, Machida C, Matsuzaki H, Oshima Y, Matsuoka K, Nakamura K, Machida Y. (1991) Operation of an efficient site-specific recombination system of Zygosaccharomyces rouxii in tobacco cells. Nucleic Acids Res. 19, 6373-

6378.).

The nucleotide sequence which codes for the recombinase may comprise an intron, preferably a plant-specific intron. The presence of such an intron will suppress the expression of the recombinase polypeptide in prokaryotes, for example bacteria.

The preferred recombination site is lox in combination with the recombinase Cre. Preferably, the recombinase sequence used is a cDNA sequence encoding a Cre polypeptide.

Preferably, all or substantially all of the Excision Cassettes are excised from the mitochondrial genome by the recombinase. The skilled person will understand, however, that some or all of the sequences of one or more recombination elements might remain in the mitochondrial genome.

The recombinase may be expressed in the cell by any suitable means. In some embodiments of the invention, the plant cell is one which already comprises an expressible construct which is integrated into the nuclear genome, wherein the expressible construct comprises a nucleotide sequence encoding a mitochondria-targeting transit peptide and a recombinase (operably linked, i.e. in frame). The expressible construct might, for example, have been introduced into the nuclear genome by homologous recombination. In such cases, the recombinase must be under the control of an inducible promoter. Such an inducible promoter may have been introduced with the construct or the construct may have been integrated adjacent to an endogenous inducible promoter.

Plants containing nuclear-located sequences encoding recombinases may be removed from a desired population by crossing, wherein the sequences may be lost due to segregation of this trait. In other embodiments of the invention, the plant cell is one which already comprises an expressible construct which is integrated into the genome of the desired mitochondria, wherein the expressible construct comprises a nucleotide sequence encoding a recombinase. The expressible construct might, for example, have been introduced into the mitochondrial genome by homologous recombination. In such cases, the recombinase must be under the control of an inducible promoter. Such an inducible promoter may have been introduced with the construct or the construct may have been integrated adjacent to an endogenous inducible promoter.

Thus in some embodiments of the invention, step (iii) comprises: (iii) inducing the expression of a recombinase in the plant cell from an inducible promoter operably linked to a nucleotide sequence encoding a recombinase which is present in the plant cell, wherein the recombinase recognises the first and second site-specific recombination elements.

Preferably, the inducible promoter operably linked to a nucleotide sequence encoding a recombinase is present in the nuclear genome of the plant cell. In other preferred embodiments, the inducible promoter operably linked to a nucleotide sequence encoding a recombinase is. present in a mitochondrial genome of the plant cell.

In yet other embodiments of the invention, the plant cell is transformed with a Recombinase Vector which comprises a promoter operably linked to a nucleotide sequence encoding a recombinase, either before step (i), simultaneously with step (i) or after step (i); or before step (ii), simultaneously with step (ii) or after step (ii).

The Recombinase Vector is a nucleic acid vector that comprises a promoter element that is capable of driving the expression of a downstream recombinase. The vector is preferably designed such that the recombinase is either expressed only or substantially only in mitochondria or is targeted specifically or substantially specifically to mitochondria.

The promoter in the Recombinase Vector must be one that is operable in the plant cell which is to be transformed. The promoter might, for example, be one derived from a plant or bacterial gene. Preferably, the promoter is plant-specific or mitochondria-specific. Most preferably, the promoter is an inducible promoter such as XVE (Zuo J, Niu QW, Chua NH. (2000) Technical advance: An estrogen receptor-based transactivator XVE mediates highly inducible gene expression in transgenic plants. Plant J. 24,

265-273.).

In the context of the present invention, the term "plant-specific" means plant-specific or substantially plant-specific. Similarly, the term "mitochondria-specific" means specific or substantially specific to mitochondria.

The promoter may or may not be an inducible promoter. If the Recombinase Vector is introduced to the plant cell before or during selection (step (ii)), it is preferable that the promoter is inducible. Examples of inducible promoters which are capable of operating in plants include light inducible promoters, metal inducible promoters, heat-shock promoters and other environmentally-inducible promoters.

Preferably, the promoter is an inducible promoter, for example an XVE promoter (Zuo J, Niu QW, Chua NH. (2000) Technical advance: An estrogen receptor-based transactivator XVE mediates highly inducible gene expression in transgenic plants. Plant J. 24, 265-273.) or a lac promoter.

In one embodiment of the invention, the Recombinase Vector comprises a promoter, operably linked to a nucleotide sequence encoding a mitochondria— targeting transit peptide and a recombinase.

Upon expression, a polypeptide product will be produced comprising a mitochondria-targeting transit peptide operably linked to a recombinase polypeptide. In this embodiment, the promoter may or may not be mitochondria-specific.

In the context of the present invention, the term "mitochondria- targeting transit peptide" means a peptide sequence which is capable of targeting the recombinase polypeptide to a mitochondria in a specific or substantially specific manner. Upon expression, the recombinase polypeptide will be produced and specifically imported into mitochondria by means of the mitochondria-targeting peptide.

Examples of mitochondria-targeting transit peptides include mitochondria-targeting transit peptides from mitochondria-targeted proteins.

Specific mitochondrial targeting transit peptides include: 1) 28 amino acid mt targeting peptide of mtSSB

Edmondson AC, Song D, Alvarez LA, Wall MK, Almond D, McClellan DA₃ Maxwell A, Nielsen BL. Characterization of a mitochondrial^ targeted single- stranded DNA-binding protein in Arabidopsis thaliana. MoI Genet Genomics. 2005 Apr; 273(2): 115-22. 2) 23 amino acids or more of the Fl-ATPase beta subunit

Chaumont F, Silva Filho Mde C, Thomas D, Leterme S, Boutry M. Truncated presequences of mitochondrial Fl-ATPase beta subunit from Nicotiana plumbaginifolia transport CAT and GUS proteins into mitochondria of transgenic tobacco. Plant MoI Biol. 1994 Feb; 24(4):631-41. Most preferably, the Recombinase Vector comprises an XVE promoter, operably linked to a nucleotide sequence encoding a mitochondria- targeting transit peptide and CRE recombinase.

The Recombinase Vector may also comprise other elements, for example, the nptll gene (kanamycin resistance) to allow for selection of transformed cells.

Thus in some embodiments of the invention, step (iii) comprises: (iii) transforming the plant cell with a Recombinase Vector comprising a promoter, a nucleotide sequence encoding a mitochondria -targeting transit peptide and a recombinase, wherein the recombinase is one which recognises the first and second site-specific recombination elements.

Particularly preferably, step (iii) of the invention comprises: (iii) transforming the plant cell with a Recombinase Vector comprising a promoter, a nucleotide sequence encoding a mitochondria-targeting transit peptide and a recombinase, wherein the promoter is capable of driving the expression of the nucleotide sequence encoding the mitochondria-targeting transit peptide and the recombinase in the plant cell, and wherein, upon expression in the plant cell, the recombinase polypeptide is targeted by the transit peptide to the mitochondria. More preferably, the promoter is an inducible promoter.

In other embodiments of the invention, the genetic construct additionally comprises the Recombinase Vector as defined herein. The person skilled in the art will be aware of numerous methods for transforming plant cells with nucleic acid vectors. These include direct DNA uptake into protoplasts, PEG-mediated uptake to protoplasts, microparticle bombardment, electroporation, heat shock, micro-injection of DNA, micro- particle bombardment of tissue explants or cells, vacuum-infiltration of plant tissues, and T-DNA mediated transformation of plant tissues by Agrobacterium, and plant (preferably tobacco) liquid cultures.

For transformation of the plant cell containing the selected plastid, any such suitable method may be used.

In some embodiments of the invention, the plant cells to be transformed are guard cells, i.e. stomatal guard cells. Such cells have been shown to be totipotent and therefore regeneration will be more efficient. Guard cells may be used as epidermal strips or as isolated guard cell protoplasts. (Hall et al. 1996. 112 889-892, Plant Physiology; Hall et al. 1996, 14. 1133-1138, Nature Biotechnology). For targeting the genetic construct to mitochondria, biolistic transformation is preferred. This involves shooting nucleic acid vector-coated gold particles (micro-projectiles) into mitochondria of plant tissues, followed by selection of the transformed mitochondria and plant regeneration. Preferably, the plant tissue is a plant leaf, although callus, as for rice transformation, may also be used.

The method of the invention preferably also comprises the additional step of inducing the expression of the recombinase in the plant cell.

This step will take place after the Recombinase Vector/expressible construct and Transformation Cassette are both present in the plant cell. Preferably, this step will take place after selection of the plant cells on media lacking the plant-hormone cytokinin. The expression of the recombinase may be induced by applying an inducing agent which results in the activation of the promoter which is present in the Recombinase Vector or endogenous promoter or expressible construct. The recombinase polypeptide is expressed and it then binds to the first and second site-specific recombination elements in the Excision Cassette, leading to the excision of that Cassette. (As will be understood by the person skilled in the art, one of the site-specific recombination elements and some adjacent sequence may be left in the mitochondria genome).

Once the nucleotide sequences encoding the plant-hormone biosynthetic polypeptides have been removed from the mitochondria genome, the promoter which is present in the Transformation Cassette will then be able to direct expression of the downstream transgene(s), thus producing the polypeptides (s) of interest.

In general, plants are regenerated in the presence of cytokinin (shoot formation) and auxin (root formation). In the case of regenerating plants containing the IPT gene (producing cytokinin), appropriate cells/tissues (e.g. leaf segments) will be placed on media containing auxin only (for root regeneration) whilst shoot regeneration will occur due to the presence of the IPT gene. The invention also provides a method for making a transgene product, comprising the method for producing a transformed plant cell, as described hereinbefore, and additionally comprising purifying the transgene product from the mitochondria.

A particularly preferred embodiment of the invention includes a method for producing a transformed plant cell, the method comprising the step:

(i) transforming the plant cell with a genetic construct, wherein the genetic construct comprises first and second homologous recombination elements flanking a Transformation Cassette, wherein the first and second homologous recombination elements are capable of directing the integration of the Transformation Cassette into the genome of at least one mitochondria which is present in the plant cell, wherein the Transformation Cassette comprises:

(a) a first promoter which is operable in said plant cell, (b) an Excision Cassette,

(c) one or more transgenes,

(d) a first terminator element, wherein the Excision Cassette comprises:

(bl) a first site-specific recombination element,

(b2) an optional second promoter

(b3) one or more nucleotide sequences encoding one or more plant- hormone biosynthetic polypeptides,

(b4) a second terminator element,

(b5) a second site-specific recombination element, wherein the first and second site-specific recombination elements are capable of being recognised by a recombinase, and wherein the first and second site- specific recombination elements are lox elements and the recombinase is Cre.

A further particularly preferred embodiment of the invention includes a method for producing a transformed plant cell, the method comprising the step:

(a) a first promoter which is operable in said plant cell,

(b) an Excision Cassette,

(c) one or more transgenes,

(d) a first terminator element, wherein the Excision Cassette comprises:

(bl) a first site-specific recombination element, (b2) an optional second promoter

(b3) one or more nucleotide sequences encoding one or more plant- hormone biosynthetic polypeptides, wherein one or more of the polypeptides is IPT,

(b4) a second terminator element,

(b5) a second site-specific recombination element, wherein the first and second site-specific recombination elements are capable of being recognised by a recombinase. A yet further particularly preferred embodiment of the invention includes a method for producing a transformed plant cell, the method comprising the step: (i) transforming the plant cell with a genetic construct, wherein the genetic construct comprises first and second homologous recombination elements flanking a Transformation Cassette, wherein the first and second homologous recombination elements are capable of directing the integration of the Transformation Cassette into the genome of at least one mitochondria which is present in the plant cell, wherein the Transformation Cassette comprises:

(a) a first promoter which is operable in said plant cell,

(b) an Excision Cassette, (c) one or more transgenes,

(d) a first terminator element, wherein the Excision Cassette comprises:

(bl) a first site-specific recombination element, (b2) an optional second promoter (b3) one or more nucleotide sequence encoding one or more plant- hormone biosynthetic polypeptides, wherein one or more of the polypeptides is IPT, (b4) a second terminator element, (b5) a second site-specific recombination element, wherein the first and second site-specific recombination elements are capable of being recognised by a recombinase and wherein the first and second site- specific recombination elements are lox elements and the recombinase is Cre. Yet a further particularly preferred embodiment provides a method for producing a transformed plant cell, the method comprising the steps: (i) transforming the plant cell with a genetic construct, wherein the genetic construct comprises first and second homologous recombination elements flanking a Transformation Cassette, wherein the first and second homologous recombination elements are capable of directing the integration of the Transformation Cassette into the genome of at least one mitochondria which is present in the plant cell, wherein the Transformation Cassette comprises:

(a) a first promoter which is operable in said plant cell,

(b) an Excision Cassette,

(c) one or more transgenes, (d) a first terminator element, wherein the Excision Cassette comprises:

(b3) one or more nucleotide sequences encoding one or more plant- hormone biosynthetic polypeptides, wherein one or more of the polypeptides is IPT, (b4) a second terminator element,

(b5) a second site-specific recombination element, wherein the first and second site-specific recombination elements are capable of being recognised by a recombinase and wherein the first and second site- specific recombination elements are lox elements and the recombinase is Cre, (ii) selecting for transformed plant cells on media which is lacking IPT; and (iii) expressing a Cre recombinase in the plant cell at a level which results in excision of the Excision Cassette from the mitochondrial genome.

The invention also provides a Transformation Cassette as herein defined, and a Recombinase Vector as herein defined. The invention further provides a plant cell comprising a

Transformation Cassette of the invention, a plant cell comprising a Recombinase Vector of the invention, and a plant cell comprising a Transformation Cassette and a Recombinase Vector of the invention. The invention further provides a transgenic plant comprising a Transformation Cassette of the invention, a transgenic plant comprising a Recombinase Vector of the invention, and a transgenic plant comprising a Transformation Cassette and a Recombinase Vector of the invention.

The invention further provides a plant seed comprising a Transformation Cassette of the invention, a plant seed comprising a Recombinase Vector of the invention, and a plant seed comprising a

Transformation Cassette and a Recombinase Vector of the invention.

The invention further provides a plant mitochondria comprising a Transformation Cassette of the invention, a plant mitochondria comprising a Recombinase Vector of the invention, and a plant mitochondria comprising a Transformation Cassette and a Recombinase Vector of the invention.

Additionally, the invention provides a plant cell obtainable or obtained using a method of the invention. BRIEF DESCRIPTION OF THE FIGURES

Figure 1: Schematic diagram showing the overall principle of the

IPT gene excision and transgene activation. pPTI001-YFP containing the IPT gene sandwiched between two lox sites which allows for CRE-mediated IPT excision after regeneration. The removal of the IPT gene results in simultaneous transgene (YFP) activation.

Figure 2: Mitochondrial Transformation Vector pMTIOOl

HOMl: Left homologous region, 72001-72501, Nicotiana tobacum mitochondrial genome DNA, GenBank BA000042; HOM2: right homologous region, 72502-73000; Nicotiana tobacum mitochondrial genome DNA, GenBank BA000042. Pτ7: T7 promoter; aadA: aminoglycoside adenine transferase CDS; IPT: Tn₅ T3 terminator; Tτ7÷ T7 terminator. IPT: isopentenyltranferase gene from Agrobacterium tumefaciens. aadA and IPT are expressed as separate transgenes.

Figure 3: Mitochondrial Transformation Vector pMTI002

HOMl: Left homologous region, 72001-72501, Nicotiana tobacum mitochondrial genome DNA, GenBank BA000042; HOM2: right homologous region, 72502-73000; Nicotiana tobacum mitochondrial genome

DNA, GenBank BA000042. P_T7: T7 promoter; aadA: aminoglycoside adenine transferase CDS; IPT: Tτ3₅ T3 terminator; Tτ7: T7 terminator. IPT: isopentenyltranferase gene from Agrobacterium tumefaciens. IPT and aadA are expressed as an operon or as a fusion protein.

Figure 4: Mitochondrial Transformation Vector pMTAOOl HOMl: Left homologous region, 72001-72501, Nicotiana tobacum mitochondrial genome DNA, GenBank BA000042; HOM2: right homologous region, 72502-73000; Nicotiana tobacum mitochondrial genome DNA, GenBank BA000042. P_τ?: T7 promoter; iaaH: indoleacetamide hydrolase from Agrobacterium tumefaciens; iaaM: tryptophan monooxygenase from Agrobacterium tumefaciens; Tτ3_} T3 terminator; Tτ7: T7 terminator. iaaH and iaaM are expressed as an operon or as a fusion protein.

Figure 5: Mitochondrial Transformation Vector pMTA002

HOMl: Left homologous region, 72001-72501, Nicotiana tobacum mitochondrial genome DNA, GenBank BA000042; HOM2: right homologous region, 72502-73000; Nicotiana tobacum mitochondrial genome DNA₃ GenBank BA000042. Pτ7: T7 promoter; aadA: aminoglycoside adenine transferase CDS; iaaH: indoleacetamide hydrolase from Agrobacteήum tumefaciens; iaaM: tryptophan monooxygenase from Agrobacteήum tumefaciens; Tτ3, T3 terminator; Tn: T7 terminator. iaaH and iaaM are expressed as as an operon or as a fusion protein.

Figure 6: Mitochondrial Transformation Vector pMTA003 HOMl: Left homologous region, 72001-72501, Nicotiana tobacum mitochondrial genome DNA, GenBankBA000042; HOM2: right homologous region, 72502-73000; Nicotiana tobacum mitochondrial genome DNA, GenBank BA000042. PT₇: T7 promoter; aadA: aminoglycoside adenine transferase CDS; iaaH: indoleacetamide hydrolase from Agrobacteήum tumefaciens; iaaM: tryptophan monooxygenase from Agrobacteήum tumefaciens; Tn, T3 terminator; Tn: T7 terminator. aadA, iaaH and iaaM are expressed as an operon or as a fusion protein.

The present invention is further defined in the following Examples, in which parts and percentages are by weight and degrees are Celsius, unless otherwise stated. It should be understood that these Examples, while indicating preferred embodiments of the invention, are given by way of illustration only. From the above discussion and these Examples, one skilled in the art can ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Thus, various modifications of the invention in addition to those shown and described herein will be apparent to those skilled in the art from the foregoing description. Such modifications are also intended to fall within the scope of the appended claims. The disclosure of each reference set forth herein is incorporated herein by reference in its entirety.

EXAMPLES Example 1 Use of cytokinin or auxin biosynthetic genes as selectable markers for mitochondrial transformation The mitochondria transformation vectors pPMTIOO 1 , pMTI002 and pMTA001-pMTA003 are constructed as detailed in Figures 2-6 using a 500 bp homologous recombination sequence (72001-72501 nt) and a 498 bp homologous recombination sequence (72502-73000 nt) from the mitochondrial genome from Nicotiana tabacum on either side of the gene cassette. Shorter or longer homologous recombination sequences are also used to ensure appropriate homologous recombination events.

These vectors are bombarded into tobacco leaves as detailed in the Appendices. For transformation experiments, young actively growing leaves from plants growing in Magenta Box are used.

Example 2

Expression of proteins with toxic effects on plant development

The expression of high levels of foreign protein in plants can lead to detrimental effects on plant development because of toxic effects. The described system overcomes this by combining insertion of the transgene(s) into the mitochondrial genome where it remains dormant until the cytokinin or the auxin selectable marker genes are removed by CRE/lox mediated recombination. The transgene encoding the "plant-toxic" protein is inserted into one of the pPMTIOOl, pMTI002 and pMTA001-pMTA003 vectors (Figures 2- 6) and the construct transformed into mitochondria using the protocol shown in the Appendices followed by cytokinin-, auxin- and/or antibioltics-mediated selection and regeneration. Once regenerated, the cytokinin and/or auxin biosynthetic genes are removed by CRE-mediated recombination. Once expressed, the transmitomic plants are used conferring a desired trait or the recombinant protein is purified.

Example 3 Expression of proteins that have a role in mitochondria

Mitochondria play an integral role during plant development and it may therefore be of interest to express proteins (plant or non-plant) inside mitochondria that would have a positive effect on plant growth, development and/or confer modified characteristics to the plant as whole. The transgene encoding the mitochondrial protein is inserted into one of the pPMTIOOl, pMTI002 and pMTA001-pMTA003 vectors (Figures 2- 6) and the constructs are transformed into mitochondria using the protocol shown in the Appendices followed by cytokinin-, auxin- and/or antibioltics- mediated selection and regeneration. Once regenerated, the cytokinin and/or auxin biosynthetic genes are removed by CRE-mediated recombination. Once expressed, the transmitomic plants are used conferring a desired trait or the recombinant protein is purified.

Example 4

High level expression of eukaryotic proteins

There are often problems associated with expression of eukaryotic proteins in bacterial systems due to insolubility and/or lack of post- translational modifications. Similarly, the expression of eukaryotic proteins in mammalian cells is expensive and labour intensive. The present system can be used for the expression of eukaryotic proteins in mitochondria using IPT, iaaH and/or iaaM marker gene selection. As described in Examples 2 and 3, any gene encoding a eukaryotic protein may be inserted into one or all of the pPMTIOOl, pMTI002 and pMTA001-pMTA003 vectors followed by transformation, selection and regeneration as described previously. Following regeneration, the expressed protein may be purified and used for downstream applications. A non-exclusive list of possible eukaryotic protein families that will be expressed includes antibodies, enzymes, enzyme inhibitors and design peptides.

Example 5 High level expression of prokaryotic proteins

As described in Examples 2 and 3, any gene encoding a prokaryotic protein may be inserted into one or all of the pPMTIOOl, pMTI002 and pMTA001-pMTA003 vectors followed by transformation, selection and regeneration as described previously. Following regeneration, the expressed protein may be purified and used for downstream applications.

Example 6

Additional and new mitochondria transformation vectors

To optimize the selection and regeneration system further a new series of mitochondrial transformation vectors are constructed. The following modifications and additions are being made: 1) Inclusion of new homologous recombination sites with varying lengths and composition that may increase the efficiency of homologous recombination and transgene insertion.

2) Inclusion of a variety of inducible promoters to drive transgene expression. 3) Inclusion of different promoters to maximize transgene expression levels.

Appendix 1

PROTOCOL FOR MITOCHONDRIAL TRANSFORMATION USING CYTOKININ SELECTION

TIME COURSE

The standard procedures produce transformed plants in 3-5 months.

• Bombardment to first shoot on: 3-8 weeks • Subculture production of homoplastic plants: 3-4 weeks

• Growtfi in soil prior to analysis: 1-2 weeks

EQUIPMENT SET UP Helium Gun Biorad PDS 1000 • Rupture disk PSI: 1100

Gap between rupture disk retaining cap and macrocarrier over cover lid: ¹A"

• Spacer rings below stopping screen support: 2

• Level of macrocarrier launch assembly: 1 (from top) • Level of Petri dish holder: 4 (from top)

• Vacuum inflow rate: Maximum

Vacuum release rate: Attenuate the release so it approximates the speed of vacuum inflow.

STOCK SOLUTIONS

• 2.5 M CaCb autoclave or filter sterilize

1 M Spermidine Free Base in sterilize H2O dH₂O

DNA at 1 μg/μl in dH₂O or IX TE • lOOVo Ethanol

70% Ethanol CONSUMABLES

1100 psi rupture disks

• Stopping screens Macrocarriers • Gold particles

PREPARATION OF THE DNA-GOLD PARTICLE MIX

• 50 mg of Gold particles are suspended in 1 ml of absolute ethanol as stock. • Take 0.25 ml of Gold stock suspension and microfuge for 5 seconds.

Remove Ethanol and wash 3 times with sterile distilled H2O, microfuging 3 minutes between washings. Resuspend Gold in 0.25 ml dH₂O.

• Aliquot 50 μl Of GoId-H₂O suspension into Eppendorf tubes. • Into each Eppendorf add the following in succession:

10 μl DNA at 1 μg/μl 50 μl of 2.5 M CaCl₂ 20 μl of 0.1 M Spermidine free base

• Vortex for 5 minutes at highest speed. • Add 200 μl of absolute ethanol to each tube.

Spin in microfuge at 3000 rpm for 2 seconds.

Remove supernatant as much as possible and rinse pellet in absolute ethanol once, microfuging at 3000 rpm for 2 seconds.

• Resuspend pellet in 30 μl absolute ethanol (makes 4-5 shots). Store mixture on ice.

PREPARING THE BIOLISTIC GUN AND CONSUMABLES

• Sterilize the gun vacuum chamber and surfaces with 70% ethanol or 70% isopropanol. • Sterilize the rupture disks and macrocarrier holders in 70% ethanol for

10 minutes. Air dry in hood.

• Soak the macrocarriers in absolute ethanol to remove all traces of H₂O. Air dry in hood.

• Open the helium tank. Set the helium tank regulator to 1300 psi (or 200 psi over the rating of the rupture disks). BOMBARDMENT

• Snap the macrocarriers into their holders.

Pipet 5 μl of the gold-DNA mixture onto the centre of the macrocarrier. The mixture should spread out evenly with few chunks. • Place a rupture disk in the holder ring and tighten ring to the helium barrel.

Place a stopping screen and macrocarrier with gold-DNA (in holder) into the retaining assembly and screw down.

Place assembly into the vacuum chamber at level 1 (first from top). • Place sample at level 4. Remove Petri dish lid.

• Turn on vacuum pump.

Pull a vacuum to 27-29 in.Hg.

Press and hold fire button till the rupture disk breaks. Release vacuum and remove sample. • Remove rupture disk, macrocarrier and stopping screen.

• Repeat if desired.

At the end of the experiment, turn off the helium tank. Pull a vacuum in the gun and release the remaining helium through the gun, then turn off the helium regulator.

The key to successful bombardment is usually in the spread of particles on the macrocarrier. The ethanol/gold/DNA mixture should quickly spread out over the centre of the macrocarrier. The resulting spread should be a very fine dusting of particles, evenly spread and containing few chunks. Chunk causes increased cell death.

Each 30 μl Gold-DNA mix gives four or five bombardments. The remaining mixture is usually too dense for good results.

TOBACCO PREPARATION AND REGENERATION MEDIA

MS: MS salts and vitamins (IX)

30 g/1 sucrose

6 g/1 phytagar pH 5.8 Autoclave

RMOP: MS salts (IX)

1 mg/1 BAP 0.1 mg/1 NAA 1 mg/1 Thiamine 100 mg/1 inositol 30 g/1 sucrose 6 g/1 phytagar pH 5.8 Autoclave

RMOP- BAP pH 5.8 Autoclave

MS + MS media

1 mg/1 IBA (indole-3-butyric acid)

2 μM 17-β-estradiol pH 5.8 Autoclave

TISSUE CULTURE

• Tobacco plants are micropropagated using sterile technique in magenta boxes containing MS media. • Expanded leaves are excised and placed abaxial surface up on a

Whatman filter paper laying on top of RMOP media. The leaves are allowed to desiccate slightly (1-2 hours) prior to bombardment on the filter paper. Each bombardment can treat 1-3 leaves covering approx. 1/3 of the 9 cm Petri dish. • Post bombardment, the plates are sealed and the leaves are left at

12:12 photoperiod at 24°C for 2 days.

• After two days the leaves are cut into sections approx. 5 mm square and placed abaxial side down on RMOP media lacking BAP. Green shoots can be collected from the bleached explants in 3-8 weeks.

Leaves from these shoots are cut up (2mm square) and subcultured in the same selective media for approx. 4 weeks.

• Typically 4 shoots are collected per initial subcultured shoots. These are rooted in tubes containing MS media + lmg/1 IBA and 2 μM 17-β- estradiol.

• Rooted shoots are transferred to soil approx. 3-5 weeks after isolation. Plants are allowed to grow in standard tobacco conditions (16:8 photoperiod at 25°C).

NOTES

1. Tobacco leaves do not have to lay completely flat for bombardment. 2. Tobacco leaves will lose their turgor after 2 days on filter paper. This is OK.

3. Do minimum amount of transferring. The maximum amount of transferring that needs to be done: a. Excise leaves from plants grown in MS. Place leaves on filter paper on RMOP. b. 2 days post bombardment place leaf explants on RMOP - BAP c. 3-8 weeks later, remove leaves from green shoots, cut, then transfer to RMOP - BAP. d. Collect 4 shoots, transfer to MS+ lmg/1 IBA+ 2 μM 17-β-estradiol in individual tubes.

4. Transformation frequency for an average experiment is anywhere from 1.5 stably transformed plants to 0.3 stably transformed plants per bombardment.

MEDIA FOR TOBACCO MITOCHONDRIAL TRANSFORMATION Name Compositions Container Time⁴

MSS: MS Salts and Vitamins (IX) Magenta Box

30 g/1 Sucrose 6 g/l Phytagar pH 5.8 Autoclave

MFB¹ MS Salts (IX) Petri Dish (9 cm) 2 days l mg/1 BAP

O. l mg/I NAA

1 mg/1 Thiamine

100 mg/1 Inositol

30 g/1 Sucrose 6 g/1 Phytagar pH 5.8

Autoclave MTS² MFB minus BAP Deep Petri Dish 3-10 weeks⁵

MFR³ MSS Magenta Box 3-5 weeks

1 mg/1 IBA (indole-3-butyric acid)

2 μM 17-β-estradiol

Notes: 1. MFB: media for bombardement 2. MTS: media for transgenic selection

3. MFR: media for rooting

4. Time: time that tobacco leaves stay in the media

5. This time includes second selection time

Appendix 2

PROTOCOL FOR MITOCHONDRIAL TRANSFORMATION USING CYTOKININ AND ANTIBIOTICS SELECTION IN COMBINATION

EQUIPMENT SET UP Helium Gun Biorad PDS 1000

Rupture disk PSI: 1100

Gap between rupture disk retaining cap and macrocarrier over cover lid: 'Λ"

• Spacer rings below stopping screen support: 2 • Level of macrocarrier lauch assembly: 1 (from top)

Level of Petri dish holder: 4 (from top)

• Vacuum inflow rate: Maximum

• Vacuum release rate: Attenuate the release so it approximates the speed of vaccum inflow.

STOCK SOLUTIONS

• 2.5 M CaCb autocalve or filter sterilize

• I M Spermidine Free Base in sterilize H2O dH₂O • DNA at 1 μg/ul in dH₂O or IX TE

100% Ethanol 70% Ethanol CONSUMABLES

1100 psi rupture disks

• Stopping screens • Macrocarriers

• Gold particles

PREPARATION OF THE DNA-GOLD PARTICLE MIX

50 mg of Gold particles are suspended in 1 ml of absolute ethanol as stock.

Take 0.25 ml of Gold stock suspension and microfuge for 5 seconds.

Remove Ethanol and wash 3 times with sterile distilled H₂O, microfuging 3 minutes between washings.

Resuspend Gold in 0.25 ml dH₂O. • Aligquot 50 μl of GoId-H₂O suspendsion into Eppendorf tubes.

• Into each Eppendorf add the following in succession:

10 μl DNA at 1 μg/μl 50 μl of 2.5 M CaCl₂ 20 μl of 0.1 M Spermidine free base • Vortex for 5 minutes at highest speed.

• Add 200 μl of absolute ethanol to each tube. Spin in microfuge at 3000 rpm for 2 seconds.

Remove supernatant as much as possible and rinse pellet in absolute ethanol once, microfuging at 3000 rpm for 2 seconds. • Resuspend pellet in 30 μl absolute ethanol (makes 4-5 shots). Store mixture on ice.

PREPARING THE BIOLISTIC GUN AND CONSUMABLES

• Sterilize the gun vaccum chamber and surfaces with 70% ethanol or 70% isopropanol.

Sterilize the rupture disks and macrocarrier holders in 70% ethanol for 10 minutes. Air dry in hood.

• Soak the macrocarriers in absolute ethanol to remove all traces of H2O. Air dry in hood. • Open the helium tank. Set the helium tank regulator to 1300 psi (or

200 psi over the rating of the rupture disks) . BOMBARDMENT

Snap the macrocarriers into their holders.

• Pipet 5 μl of the gold-DNA mixture onto the centre of the macrocarrier. The mixture should spread out evenly with few chunks. • Place a rupture disk in the holder ring and tighten ring to the helium barrel.

• Place a stopping screen and macrocarrier with gold-DNA (in holder) into the retaining assembly and screw down.

• Place assembly into the vacuum chamber at level 1 (first from top). • Place sample at level 4. Remove Petri dish lid.

• Turn on vacuum pump.

Pull a vacuum to 27-29 in.Hg.

• Press and hold fire button till the rupture disk breaks.

• Release vacuum and remove sample. • Remove rupture disk, macrocarrier and stopping screen.

Repeat if desired.

NOTE

1. The key to successful bombardment is usually in the spread of particles on the macrocarrier. The ethanol/gold/DNA mixture should quickly spread out over the centre of the macrocarrier. The resulting spread should be a very fine dusting of particles, evenly spread and containing few chunks. And chunk causes cell death.

2. Each 30 μl Gold-DNA mix gives four or five bombardments. The remaining mixture is usually too dense for good results.

TOBACCO PREPARATION AND REGENERATION MEDIA

MS: MS salts and vitamins (IX)

30 g/1 sucrose 6 g/1 phytagar pH 5.8 Autoclave

RMOP: MS salts (IX) 1 mg/1 BAP 0.1 mg/1 NAA 1 mg/1 Thiamine 100 mg/1 inositol 30 g/1 sucrose

6 g/1 phytagar pH 5.8 Autoclave

RMOP- BAP + Spectinomycin 500 μg/ml Spectinomycin pH 5.8 Autoclave

MS + MS media

1 mg/1 IBA (indole-3-butyric acid) 2 μM 17-β-estradiol pH 5.8 Autoclave

TISSUE CULTURE

Tobacco plants are micropropagated using sterile technique in magenta boxes containing MS media

• Expanded leaves are excised and placed abaxial surface up on a Whatman filter paper laying on top of RMOP media. The leaves are allowed to desiccate slightly (1-2 hours) prior to bombardment on the filter paper. Each bombardment can treat 1-3 leaves covering approx. 1/3 of the 9 cm Petri dish.

Post bombardment, the plates are sealed and die leaves are left at 12: 12 photoperiod at 24⁰C for 2 days.

• After two days the leaves are cut into sections approx. 5 mm square and placed abaxial side down on RMOP media lacking BAP and containing spectinomycin.

• Green shoots can be collected from the bleached explants in 3-8 weeks.

Leaves from these shoots are cut up (2mm square) and subcultured in the same selective media for approx. 4 weeks. • Typically 4 shoots are collected per initial subcultured shoots. These are rooted in tubes containing MS media + lmg/1 IBA and 2 μM 17-β- estradiol. • Rooted shoots are transferred to soil appox. 3-5 weeks after isolation.

Plants are allowed to grow in standard tobacco conditions (16:8 photoperiod at 25°C).

NOTE

1. Tobacco leaves do not have to lay completely flat for bombardment.

2. Tobacco leaves will lose their turgor after 2 days on filter paper. This is OK.

3. Do minimum amount of transferring. The maximum amount of transferring that needs to be done: a. Excise leaves from plants grown in MS. Place leaves on filter paper on RMOP. b. 2 days post bombardment place leaf explants on RMOP - BAP + Spectinomycin c. 3-8 weeks later, remove leaves from green shoots, cut, then transfer to RMOP - BAP + Spectinomycin. d. Collect 4 shoots, transfer to MS+ lmg/1 IBA+ 2 μM 17-β- estradiol in individual tubes.

MEDIA FOR TOBACCO MITOCHONDRIAL TRANSFORMATION Name Compostions Container Time⁴ MSS: MS Salts and Vitamins (IX) Magenta Box

30 g/1 Sucrose 6 g/1 Phytagar pH 5.8 Autoclave

MFB¹ MS Salts (IX) Petri Dish (9 cm) 2 days

1 mg/1 BAP 0.1 mg/1 NAA 1 mg/1 Thiamine 100 mg/1 Inositol

30 g/1 Sucrose 6 g/1 Phytagar pH 5.8 Autoclave

MTS² MFB minus BAP Deep Petri Dish 3-10 weeks⁵

+ Spectinomycin

MFR³ MSS Magenta Box 3-5 weeks

1 mg/1 IBA (indole-3-butyric acid)

2 μM 17-β-estradiol

Note: 1. MFB: media for bombardment

2. MTS: media for transgenic selection

3. MFR: media for rooting

4. Time: time that tobacco leaves stay in the media

5. This time includes second selection time

Appendix 3

PROTOCOL FOR MITOCHONDRIAL TRANSFORMATION USING

AUXIN SELECTION

EQUIPMENT SET UP

Helium Gun Biorad PDS 1000 Rupture disk PSI: 1100

• Gap between rupture disk retaining cap and macrocarrier over cover lid: Vi"

Spacer rings below stopping screen support: 2 Level of macrocarrier lauch assembly: 1 (from top) Level of Petri dish holder: 4 (from top) Vacuum inflow rate: Maximum

STOCK SOLUTIONS

• 2.5 M CaCb autocalve or filter sterilize

1 M Spermidine Free Base in sterilize H₂O dH₂O

DNA at 1 μg/ul in dH₂O or IX TE 100% Ethanol 70% Ethanol

CONSUMABLES • 1 100 psi rupture disks

• Stopping screens

• Macrocarriers

• Gold particles

PREPARATION OF THE DNA-GOLD PARTICLE MIX

• 50 mg of Gold particles are suspended in 1 ml of absolute ethanol as stock.

• Take 0.25 ml of Gold stock suspension and microfuge for 5 seconds. Remove Ethanol and wash 3 times with sterile distilled H₂O₃ microfuging 3 minutes between washings.

Resuspend Gold in 0.25 ml dH₂O.

• Aligquot 50 μl of GoId-H₂O suspendsion into Eppendorf tubes.

• Into each Eppendorf add the following in succession:

10 μl DNA at 1 μg/μl 50 μl of 2.5 M CaCl₂

20 μl of 0.1 M Spermidine free base Vortex for 5 minutes at highest speed. Add 200 μl of absolute ethanol to each tube. Spin in microfuge at 3000 rpm for 2 seconds. • Remove supernatant as much as possible and rinse pellet in absolute ethanol once, microfuging at 3000 rpm for 2 seconds.

Resuspend pellet in 30 μl absolute ethanol (makes 4-5 shots). Store mixture on ice.

PREPARING THE BIOLISTIC GUN AND CONSUMABLES

• Sterilize the rupture disks and macrocarrier holders in 70% ethanol for 10 minutes. Air dry in hood. • Soak the macrocarriers in absolute ethanol to remove all traces of

H₂O. Air dry in hood.

• Open the helium tank. Set the helium tank regulator to 1300 psi (or 200 psi over the rating of the rupture disks).

BOMBARDMENT

• Snap the macrocarriers into their holders. • Pipet 5 μl of the gold-DNA mixture onto the centre of the macrocarrier. The mixture should spread out evenly with few chunks.

• Place a rupture disk in the holder ring and tighten ring to the helium barrel.

• Place assembly into the vacuum chamber at level 1 (first from top).

• Place sample at level 4. Remove Petri dish lid.

• Turn on vacuum pump.

Pull a vacuum to 27-29 in. Hg. • Press and hold fire button till the rupture disk breaks.

Release vacuum and remove sample.

• Remove rupture disk, macrocarrier and stopping screen. Repeat if desired.

NOTE

TOBACCO PREPARATION AND REGENERATION MEDIA

MS: MS salts and vitamins (IX) 30 g/1 sucrose

6 g/1 phytagar pH 5.8 Autoclave RMOP: MS salts (IX)

1 mg/1 BAP

0.1 mg/1 NAA 1 mg/1 Thiamine

100 mg/1 inositol 30 g/1 sucrose 6 g/1 phytagar pH 5.8 Autoclave

RMOP- NAA pH 5.8 Autoclave

MS + MS media 1 mg/1 IBA (indole-3-butyric acid)

2 μM 17-β-estradiol pH 5.8 Autoclave

TISSUE CULTURE • Tobacco plants are micropropagated using sterile technique in magenta boxes containing MS media

Expanded leaves are excised and placed abaxial surface up on a Whatman filter paper laying on top of RMOP media. The leaves are allowed to desiccate slightly (1-2 hours) prior to bombardment on the filter paper. Each bombardment can treat 1-3 leaves covering approx.

1/3 of the 9 cm Petri dish.

Post bombardment, the plates are sealed and the leaves are left at

12:12 photoperiod at 24°C for 2 days.

• After two days the leaves are cut into sections approx. 5 mm square and placed abaxial side down on RMOP media lacking NAA.

Green shoots can be collected from the bleached explants in 3-8 weeks.

• Leaves from these shoots are cut up (2mm square) and subcultured in the same selective media for approx. 4 weeks. • Typically 4 shoots are collected per initial subcultured shoots. These are rooted in tubes containing MS media + lmg/1 IBA and 2 μM 17-β- estradiol. Rooted shoots are transferred to soil appox. 3-5 weeks after isolation. Plants are allowed to grow in standard tobacco conditions (16:8 photoperiod at 25⁰C).

NOTE

1. Tobacco leaves do not have to lay completely flat for bombardment.

3. Do minimum amount of transferring. The maximum amount of transferring that needs to be done: a. Excise leaves from plants grown in MS. Place leaves on filter paper on RMOP. b. 2 days post bombardment place leaf explants on RMOP - NAA c. 3-8 weeks later, remove leaves from green shoots, cut, then transfer to RMOP - NAA. d. Collect 4 shoots, transfer to MS+ lmg/1 IBA+ 2 μM 17-β- estradiol in individual tubes.

MEDIA FOR TOBACCO MITOCHONDRIAL TRANSFORMATION

Name Compostions Container Time⁴

MSS: MS Salts and Vitamins (IX) Magenta Box

30 g/1 Sucrose

6 g/1 Phytagar pH 5.8

Autoclave

MFB¹ MS Salts (IX) Petri Dish (9 cm) 2 days

1 mg/1 BAP

0.1 mg/1 NAA

1 mg/1 Thiamine

100 mg/1 Inositol

30 g/1 Sucrose

6 g/1 Phytagar pH 5.8 Autoclave

MTS² MFB minus NAA Deep Petri Dish 3-10 weeks⁵

MFR3 MSS Magenta Box 3-5 weeks

1 mg/1 IBA (indole-3-butyric acid)

2 μM 17-β-estradiol

Note: 1. MFB: media for bombardment

2. MTS: media for transgenic selection

3. MFR: media for rooting

4. Time: time that tobacco leaves stay in the media

5. This time includes second selection time

Appendix 4

PROTOCOL FOR MITOCHONDRIAL TRANSFORMATION USING

AUXIN AND ANTIBIOTIC SELECTION IN COMBINATION

EQUIPMENT SET UP

Helium Gun Biorad PDS 1000 Rupture disk PSI: 1100 • Gap between rupture disk retaining cap and macrocarrier over cover

Hd: Vi"

• Spacer rings below stopping screen support: 2 Level of macrocarrier lauch assembly: 1 (from top) Level of Petri dish holder: 4 (from top) • Vacuum inflow rate: Maximum

STOCK SOLUTIONS • 2.5 M CaCl₂ autocalve or filter sterilize

• I M Spermidine Free Base in sterilize H₂O dH₂O DNA at 1 μg/ul in dH₂O or IX TE 100% Ethanol 70% Ethanol

CONSUMABLES

1 100 psi rupture disks Stopping screens

• Macrocarriers Gold particles

PREPARATION OF THE DNA-GOLD PARTICLE MIX

• 50 mg of Gold particles are suspended in 1 ml of absolute ethanol as stock.

Resuspend Gold in 0.25 ml dH₂O.

Aligquot 50 μul of GoId-H₂O suspendsion into Eppendorf tubes.

• Into each Eppendorf add the following in succession: 10 μl DNA at 1 μg/μl

50 μl of 2.5 M CaCl₂ 20 μl of 0.1 M Spermidine free base Vortex for 5 minutes at highest speed. Add 200 μl of absolute ethanol to each tube. • Spin in microfuge at 3000 rpm for 2 seconds.

PREPARING THE BIOLISTIC GUN AND CONSUMABLES

Sterilize the gun vaccum chamber and surfaces with 70% ethanol or

70% isopropanol.

• Soak the macrocarriers in absolute ethanol to remove all traces of H₂O. Air dry in hood. • Open the helium tank. Set the helium tank regulator to 1300 psi (or 200 psi over the rating of the rupture disks).

BOMBARDMENT • Snap the macrocarriers into their holders.

Pipet 5 μl of the gold-DNA mixture onto the centre of the macrocarrier. The mixture should spread out evenly with few chunks.

• Place a rupture disk in the holder ring and tighten ring to the helium barrel. • Place a stopping screen and macrocarrier with gold-DNA (in holder) into the retaining assembly and screw down.

• Place assembly into the vacuum chamber at level 1 (first from top). Place sample at level 4. Remove Petri dish lid.

• Turn on vacuum pump. • Pull a vacuum to 27-29 in.Hg.

• Press and hold fire button till the rupture disk breaks.

• Release vacuum and remove sample.

• Remove rupture disk, macrocarrier and stopping screen.

• Repeat if desired. • At the end of the experiment, TURN OFF THE HELIUM TANK.

Pull a vacuum in the gun and release the remaining helium through the gun, then turn off the helium regulator.

NOTE 1. The key to successful bombardment is usually in the spread of particles on the macrocarrier. The ethanol/gold/DNA mixture should quickly spread out over the centre of the macrocarrier. The resulting spread should be a very fine dusting of particles, evenly spread and containing few chunks. And chunk causes cell death. 2. Each 30 μl Gold-DNA mix gives four or five bombardments. The remaining mixture is usually too dense for good results.

TOBACCO PREPARATION AND REGENERATION MEDIA MS: MS salts and vitamins (IX)

30 g/1 sucrose 6 g/1 phytagar pH 5.8 Autoclave

RMOP: MS salts (IX)

1 mg/1 BAP 0.1 mg/1 NAA

1 mg/1 Thiamine 100 mg/1 inositol 30 g/1 sucrose 6 g/1 phytagar pH 5.8 Autoclave

RMOP- NAA + Spectinomycin

500 μg/ml Spectinomycin pH 5.8 Autoclave

MS + MS media

1 mg/1 IBA (indole-3-butyric acid)

2 μM 17-β-estradiol pH 5.8 Autoclave

TISSUE CULTURE

1/3 of the 9 cm Petri dish.

Post bombardment, the plates are sealed and the leaves are left at 12: 12 photoperiod at 24⁰C for 2 days.

• After two days the leaves are cut into sections approx. 5 mm square and placed abaxial side down on RMOP media lacking NAA and containing spectinomycin.

Green shoots can be collected from the bleached explants in 3-8 weeks.

• Leaves from these shoots are cut up (2mm square) and subcultured in the same selective media for approx. 4 weeks. Typically 4 shoots are collected per initial subcultured shoots. These are rooted in tubes containing MS media + lmg/1 IBA and 2 μM 17-β- estradiol.

• Rooted shoots are transferred to soil appox. 3-5 weeks after isolation. Plants are allowed to grow in standard tobacco conditions (16:8 photoperiod at 25⁰C).

NOTE

3. Do minimum amount of transferring. The maximum amount of transferring that needs to be done: a. Excise leaves from plants grown in MS. Place leaves on filter paper on RMOP. b. 2 days post bombardment place leaf explants on RMOP - NAA + Spectinomycin c. 3-8 weeks later, remove leaves from green shoots, cut, then transfer to RMOP - NAA + Spectinomycin. d. Collect 4 shoots, transfer to MS+ lmg/1 IBA+ 2 μM 17-β- estradiol in individual tubes.

MEDIA FOR TOBACCO MITOCHONDRIA TRANSFORMATION

Name Compositions Container Time⁴

MSS: MS Salts and Vitamins (IX) Magenta Box

30 g/1 Sucrose

6 g/1 Phytagar pH 5.8

Autoclave

MFBl MS Salts (IX) Petri Dish (9 cm) 2 days

1 mg/1 BAP

O. l mg/I NAA

MTS2 MFB minus NAA + Spectinomycin Deep Petri Dish 3-10 weeks⁵

MFR3 MSS Magenta Box 3-5 weeks

1 mg/1 IBA (indole-3-butyric acid)

2 μM 17-β-estradiol

Note: 1. MFB: media for bombardment

2. MTS: media for transgenic selection

3. MFR: media for rooting

4. Time: time that tobacco leaves stay in the media

5. This time includes second selection time

Claims

1. A method for producing a transformed plant cell, the method comprising the step: (i) transforming the plant cell with a genetic construct, wherein the genetic construct comprises first and second homologous recombination elements flanking a Transformation Cassette, wherein the first and second homologous recombination elements are capable of directing the integration of the Transformation Cassette into the genome of at least one mitochondria which is present in the plant cell, wherein the Transformation Cassette comprises:

(a) a first promoter which is operable in said plant cell,

(b) an Excision Cassette,

(b4) a second terminator element, (b5) a second site-specific recombination element, wherein the first and second site-specific recombination elements are capable of being recognised by a recombinase.

2. A method as claimed in claim 1, wherein the method additionally comprises the step:

(ii) selecting for transformed plant cells on media which is lacking one or more of the plant-hormone biosynthetic polypeptides.

3. A method as claimed in claim 1 or claim 2, wherein the method additionally comprises the step:

(iii) expressing a recombinase in the plant cell, wherein the recombinase is one which recognises the first and second site-specific recombination elements.

4. A method as claimed in any one of the preceding claims, wherein the first and second site-specific recombination elements are lox sites.

5. A method as claimed in any one of the preceding claims, wherein the recombinase is a Cre recombinase.

6. A method as claimed in any one of the preceding claims, wherein the plant-hormone biosynthetic polypeptides are polypeptides which are involved in the synthesis of a plant cytokinin or auxin or other plant growth regulator, or which regulate the production or metabolism of a plant cytokinin or auxin or other plant growth regulator.

7. A method as claimed in claim 6 wherein the plant-hormone biosynthetic polypeptides are selected from IPT (isopentenyl transferase), iaaH (indoleacetamide hydrolase) and iaaM (tryptophan mono-oxygenase).

8. A method as claimed in any one of the preceding claims, wherein at least one of the one or more transgenes codes for an antibody, antibiotic, herbicide, vaccine antigen, enzyme, enzyme inhibitor or design peptide.

9. A method as claimed in any one of the preceding claims, wherein the nucleotide sequences of the homologous recombination elements are selected such that the Transformation Cassette is specifically targeted to one or more selected mitochondria.

10. A method as claimed in any one of the preceding claims, wherein the nucleotide sequences of the homologous recombination elements are selected such that no or essentially no Transformation Cassettes become integrated into the nuclear genome of the plant.

11. A method as claimed in any one of the claims 3 to 10, wherein step

(iii) comprises inducing the expression of a recombinase in the plant cell from an inducible promoter operably linked to a nucleotide sequence encoding a recombinase which is present in the plant cell, wherein the recombinase recognises the first and second site-specific recombination elements.

12. A method as claimed in any one of claims 3 to 10, wherein step (iii) comprises transforming the plant cell with a Recombinase Vector which comprises a promoter operably linked to a nucleotide sequence encoding a recombinase, either before step (i), simultaneously with step (i) or after step (i); or before step (ii), simultaneously with step (ii) or after step (ii).

13. A method as claimed in claim 12, wherein the Recombinase Vector comprises a promoter operably linked to a nucleotide sequence encoding a mitochondria-targeting transit peptide and a recombinase.

14. A method as claimed in claim 13, wherein the mitochondria-targeting transit peptide is one which is capable of targeting the recombinase to a mitochondria.

15. A method as claimed in any one of claims 12 to 14, wherein the promoter is an inducible promoter.

16. A method as claimed in any one of the preceding claims, wherein the nucleotide sequence encoding a plant-hormone biosynthetic polypeptide and the second terminator element are downstream of the first promoter, wherein the first promoter is capable of driving expression of the plant-hormone biosynthetic polypeptide.

17. A method as claimed in any one of the preceding claims, wherein the Transformation Cassette comprises:

(a) a first promoter which is operable in said plant cell, (b) an Excision Cassette comprising:

(bl) a first site-specific recombination element,

(b3) one or more nucleotide sequences encoding one or more plant-hormone biosynthetic polypeptides, (b4) a second terminator element, (b5) a second site-specific recombination element, wherein the first and second site-specific recombination element are capable of being recognised by a recombinase;

(c) one or more transgenes,

(d) a first terminator element, operably linked in the order specified above in a 5'-3' direction.

18. A method as claimed in any one of claims 1 to 16, wherein the Excision Cassette is in the reverse orientation compared to the first promoter, transgene(s) and first terminator element, and the Excision Cassette comprises a second promoter which is capable of driving the expression of the nucleotide sequence encoding the plant-hormone biosynthetic polypeptide.

19. A method as claimed in claim 18, wherein the Transformation Cassette comprises:

(a) a first promoter which is operable in said plant cell,

(b) an Excision Cassette, (c) one or more transgenes,

(d) a first terminator element, wherein (a), (b), (c) and (d) are operably linked in the order specified above in a 5'-3' direction, wherein the Excision Cassette comprises: (bl) a first site-specific recombination element,

(b2) a second promoter,

(b3) one or more nucleotide sequences encoding one or more plant- hormone biosynthetic polypeptides, (b4) a second terminator element, (b5) a second site-specific recombination element, wherein the first and second site-specific recombination elements are capable of being recognised by a recombinase, and wherein the parts (bl)-(b5) of the Excision Cassette are operably linked and wherein the Excision Cassette is in reverse orientation compared to (a), (c) and (d).

20. A method for producing a transformed plant cell, the method comprising the step:

(a) a first promoter which is operable in said plant cell,

(b) an Excision Cassette, (c) one or more transgenes,

(d) a first terminator element, wherein the Excision Cassette comprises:

(b3) one or more nucleotide sequences encoding one or more plant- hormone biosynthetic polypeptides, wherein one of the polypeptides is IPT,

(b4) a second terminator element, (b5) a second site-specific recombination element, wherein the first and second site-specific recombination elements are capable of being recognised by a recombinase and wherein the first and second site- specific recombination elements are lox elements and the recombinase is Cre.

21. A method as claimed in claim 20, additionally comprising the steps (ii) selecting for transformed plant cells on media which is lacking IPT; and (iii) expressing a Cre recombinase in the plant cell at a level which results in excision of the Excision Cassette from the mitochondrial genome.

22. A method as claimed in any one of the preceding claims, wherein the Transformation Cassette additionally comprises a nucleotide sequence encoding a polypeptide which confers resistance to an antibiotic.

23. A method as claimed in claim 22, wherein the antibiotic is spectinomycin.

24. A method as claimed in any one of the preceding claims, wherein the plant is a monocot or dicot.

25. A method as claimed in any one of the preceding claims, wherein the plant is selected from the group consisting of cereals (rice, wheat, barley, oats, sorghum, corn), legumes (alfalfa, lentils, peanut, pea, soybean), oil crops (palm, sunflower, coconut, canola, olive), cash crops (cotton, sugar cane, cassava), vegetable crops (potato, tomato, carrot, sweet potato, sugar-beet, squash, cucumber, lettuce, broccoli, cauliflower, snap bean, cabbage, celery, onion, garlic), fruits/trees and nuts (banana, grape cantaloupe, muskmelon, watermelon, strawberry, orange, apple, mango, avocado, peach, grapefruit, pineapple, maple, almond), beverages (coffee, tea, cocoa), timber trees (oak, black walnut, sycamore), moss and duckweed.

26. A method as claimed in any one of the preceding claims, wherein the plant is tobacco or lettuce.

27. A method as claimed in any one of the preceding claims, wherein the plant cells are individual cells, groups of cells, in dissociated form or undissociated form, as part of a plant tissue or plant part, or a protoplast or a plant liquid culture.

28. A method as claimed in any one of the preceding claims, wherein the plant cells are present in plant leaves.

29. A method as claimed in any one of the preceding claims, wherein the first and/or second promoter is a PsbA, RbcL or Prrn promoter.

30. A method as claimed in any one of the preceding claims, wherein the first and/or second terminator is a rrn, psbA, rbcL or T7 terminator.

31. A method for producing a plant, comprising producing a plant cell by a method as defined in any one of the preceding claims, and regenerating a plant from the plant cell.

32. A transformed plant cell obtained by or obtainable by a method as claimed in any one of claims 1 to 30.

33. A plant obtained by or obtainable by a method as claimed in claim 31.

34. A method of producing a transgene product, comprising a method as defined in any one of the preceding claims, and additionally comprising the step of purifying or isolating, and optionally packaging, the transgene product.

35. A purified or isolated transgene product obtained by or obtainable by a method as claimed in claim 34.

36. A method as claimed in claim 34 or a purified or isolated transgene product as claimed in claim 35 wherein the transgene product is an antibody, antibiotic, herbicide, vaccine antigen, enzyme, enzyme inhibitor or design peptide.

37. A genetic construct comprising first and second homologous recombination elements flanking a Transformation Cassette, wherein the first and second homologous recombination elements are capable of directing the integration of the Transformation Cassette into the genome of at least one mitochondria which is present in a plant cell, wherein the Transformation Cassette comprises:

(a) a first promoter which is operable in a plant cell,

(b) an Excision Cassette,

(c) one or more transgenes,

(d) a first terminator element, wherein the Excision Cassette comprises:

(b3) one or more nucleotide sequences encoding one or more plant- hormone biosynthetic polypeptides, (b4) a second terminator element,

(b5) a second site-specific recombination element, wherein the first and second site-specific recombination elements are capable of being recognised by a recombinase.

38. A genetic construct as claimed in claim 37, wherein the first and second site-specific recombination elements are lox sites.

39. A genetic construct as claimed in claim 37 or claim 38, wherein the recombinase is a Cre recombinase.

40. A genetic construct as claimed in any one of claims 37 to 39, wherein the plant-hormone biosynthetic polypeptide is a polypeptide which is involved in the synthesis of a plant cytokine or auxin or other plant growth regulator, or regulates the production or metabolism of a plant cytokine or auxin or other plant growth regulator.

41. A genetic construct as claimed in claim 40 wherein the plant-hormone biosynthetic polypeptides are selected from IPT (isopentenyl transferase), iaaH (indoleacetamide hydrolase) and iaaM (tryptophan mono-oxygenase).

42. A genetic construct as claimed in any one of claims 37 to 41, wherein the nucleotide sequences of the homologous recombination elements are selected such that the Transformation Cassette is specifically targeted to one or more selected mitochondria.

43. A genetic construct as claimed in any one of claims 37 to 42, wherein the nucleotide sequences of the homologous recombination elements are selected such that no or essentially no Transformation Cassettes become integrated into the nuclear genome of the plant.

44. A plant cell comprising a genetic construct as claimed in any one of claims 37 to 43.

45. A plant comprising a genetic construct as claimed in any one of claims 37 to 43.

46. A plant seed comprising a genetic construct as claimed in any one of claims 37 to 43.

47. A plant, plant cell or plant seed comprising a genetic construct as claimed in any one of claims 37 to 43, wherein the one or more nucleotide sequences encoding one or more plant-hormone biosynthetic peptides (b3), and optionally the optional second promoter (b2) and/or second terminator element (b4), have been excised from the Excision Cassette, preferably through the action of a recombinase on the first and second site specific recombination elements.

48. A method of producing a transgene product, comprising purifying or isolating, and optionally packaging, a transgene product from the plant cell, plant or plant seed of claims 44 to 47.

49. A method as claimed in claim 48, wherein the transgene product is an antibody, antibiotic, herbicide, vaccine antigen, enzyme, enzyme inhibitor or design peptide.

50. A transgene product obtained by or obtainable by the method of claim 48 or 49.