Author: Dr. Kiran B. Gaikwad, Scientist, Division of Genetics, IARI, New Delhi
Introduction
Mitochondria are often described as "power house of cell" because they generate most of the cell's supply of adenosine triphosphate (ATP), used as a source of chemical energy. In addition they are also involved in other important cell functions viz., signalling, cellular differentiation, cell death, control of the cell cycle and cell growth. Several characteristics make mitochondria unique. Although most of a cell's DNA is contained in the nucleus, the mitochondrion has its own independent genome and its DNA shows substantial similarity to bacterial genomes. Plant cells is well equipped with three types of genomes, viz., nuclear, mitochondrial and chloroplast. Among these genomes, nuclear genes are inherited by both the parents whereas chloroplast and mitochondrial genes are inherited maternally, therefore chloroplast and mitochondrial genomes could act as a useful candidates for transgene containment which is a crucial concern in genetically modified crops, where transgene esacpe is a major concern (Siddra Ijaz 2010). Mitochondria are thought to have originated from an ancient symbiosis mechanism and were originated from incorporated K-purple bacteria (Grey et al 1992). They are well capable of transferring many of its essential genes to the nucleus. Mitochondria use an N-formylmethionyl- tRNA (fMet-tRNA) as initiator of protein synthesis, which is a peculiar characteristic of prokaryotic organism like bacteria.
Plant mitochondrial manipulation:
Genetic manipulations of mitochondria have been attempted in many organisms but it is mostly successful to yeast and C. reinhardtii. Sequencing of mitochondrial genomes in most of the plant species has been done. Plant mitochondria offer great advantages in its manipulation because of following reasons.
1. Maternal inheritance
2. No pleiotropic effect
3. Absence of gene silencing
4. Multigene engineering
5. No position effects
6. No specific degradation of transgene RNA at post transcriptional level.
Despite of these advantages, mitochondrial transformation is less employed due to difficulties in:
1. The introduction of foreign DNA into the mitochondria.
2. Incorporation of transgene into the mitochondrial DNA.
3. Absence of selectable marker for mitochondria.
4. Relatively large number of mitochondria per cell and mitochondrial genome per mitochondria.
5. RNA editing may render foreign genes ineffective that are introduced into the mitochondrial genome.
Methods of mitochondrial transformation:
ïÆ'Ëœ Protoplast fusion
ïÆ'Ëœ Particle bombardment of cell culture
ïÆ'Ëœ Agrobacterium mediated gene transfer
ïÆ'Ëœ Microinjection method
Application of plant mitochondrial genetic manipulation in crops:
Transfer of transgene through pollens to related plant/ crop species is a big environmental concern. Mitochondrion is a unique candidate for transgene carrier as it has its own transcription and translation machinery, and maternal inheritance gives assurance of transgene containment with high expression level. These features make mitochondria amenable to engineer and therefore reduce the risk of transgene escape in nature. Genetic manipulation or engineering will add to the efficiency of crop improvement. In plants, the mitochondrial genome is the source of cytoplasmic male sterility (CMS), a trait of major interest to crop breeders for hybrid generation. Many of the economically important traits including yield, male sterility, heterosis, disease resistance, temperature or drought tolerance etc are affected by genetic interactions between the organellar and the nuclear genes (Frie et al 2004, Mackenzie SA 2005, Atienza et al 2007)). The mechanisms explaining effects of cytoplasmic inheritance on these traits are still largely unexplored and roles of mitochondrial and chloroplastic genetics remain to be established (Romain Val et al. 2011). Due to lack of molecular approaches to engineer and control the genetic system in this organelle, progress in the understanding of mitochondrial genetic processes and regulation pathways, are hindered.
Conclusion:
• Many economically important crop species are devoid of CMS system due to unavailability of cytoplasmic genetic male sterility. In such species mitochondrial manipulation could provide a novel means to develop CMS lines as non-GMO/ transgenic materials
• Once the mitochondrion is transformed with "gene of interest", their maternal inheritances will confind the gene through successive generations thus reducing the risk of transgene escape.
• Use of mitochondrial plasmid as a vector for transgene would be more compatible than the bacterial plasmid being its origin from the plant genome itself.
References:
1. Atienza SG, Martin AC, Ramirez MC, Martin A, and Ballesteros J. (2007) Effects of Hordeum chilense cytoplasm on agronomic traits in common wheat. Plant Breeding, 126, 5-8.
2. Frei U, Peiretti EG, and Wenzel G. (2004) In Janick,J. (ed.), Plant Breeding Reviews, Vol. 23. John Wiley & Sons, Hoboken, pp. 175-210.
3. Gray MW, Hanic-Joyce PJ, Covello PS (1992). Transcription, processing and editing in plant mitochondria. Annu. Rev. Plant Physiol. Plant Mol. Biol. 43: 145-175.
4. Mackenzie SA. (2005) In Janick,J. (ed.), Plant Breeding Reviews, Vol. 25. John Wiley & Sons, Hoboken, pp. 115-138.
5. Romain Val, Wyszko E, Valentin C, Szymanski M, Cosset A, Alioua M, Theo W. Dreher Jan Barciszewski and Dietrich A. (2011) Organelle trafficking of chimeric ribozymes and genetic manipulation of mitochondria. Nucleic Acids Research. 39(21): 9262-9274.
6. Siddra Ijaz (2010) Plant mitochondrial genome: "A sweet and safe home'' for transgene. African Journal of Biotechnology. 9(54): 9196-9199.
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