Evaluation of a mitochondrial mutator system in higher plants

Plant growth and development require coordinated expression of genes located in the nucleus and in the organelles. The plant mitochondrial genome is unusual in its propensity to incorporate foreign DNA sequences and to undergo both homologous and illegitimate recombination. The maintenance of mitochondrial genome stability relies on contributions from the nuclear genome. In plants, three nuclear genes, Msh1, RecA3 and Osb1 have been shown to participate in recombination surveillance and the suppression of illegitimate recombination in mitochondria. Disruption of these loci in Arabidopsis results in reproducible rearrangement of the mitochondrial genome. Transgenic suppression of Msh1 by RNA interference in crop plants likewise produces mitochondrial genome alteration and the associated phenotype of cytoplasmic male sterility. We have demonstrated in four species (tobacco, tomato, soybean and Arabidopsis) the induction of CMS by suppressing Msh1. Interestingly, leaf variegation, stunted plant growth phenotypes and altered leaf morphology were also observed consistently in the four plant species. These phenotypes showed maternal inheritance, and a comprehensive analysis of the derived mitochondrial genomes using 23 mitochondrial gene probes revealed distinct mitochondrial genome alterations associated with each phenotype in tomato. Our observations suggest that mitochondrial genome rearrangement in response to depletion of Msh1 is followed by a process of cytoplasmic sorting to produce distinct cytotypes. Mitochondrial genome alterations were also accompanied by dramatic changes in nuclear and mitochondrial gene expression. To date, opportunities to investigate mitochondrial genetics in higher plants, and to understand the role of mitochondrial gene expression in plant growth and development, have been limited and artificial. Here we describe a natural consequence of nuclear gene modulation to produce a host of mitochondrial variants providing insight into the mitochondrial intersection of several important cellular pathways.