Mitochondrial Mutations in Plants

Mitochondrial mutations are widespread in the plant kingdom. They are easily detected when they result in maternal-defective or male-sterile plants. Neutral mutations that do not result in visible phenotypes also occur and are likely to be reservoirs for mitochondrial genome evolution. Because plant mitochondrial genes usually exhibit a slow rate of nucleotide Substitution, most of the reported mitochondrial mutations are rearrangements and/or deletions. Nuclear genes influence the generation and recovery of mitochondrial mutations because they control the Organization of mitochondrial genomes, as well as the expression of mitochondrial genes. The most extensively studied plant mitochondrial mutations are rearrangements resulting in chimeric genes that confer cytoplasmic male sterility (CMS), and deletions that either restore fertility to CMS plants or that cause abnormal growth. Chimeric genes, and novel arrangements of coding and regulatory sequences, can result from recombination across repeats. A model explaining the generation of abnormal growth mutants, as well as reversions of CMS-associated genomes to male fertility, is discussed. Analysis of mutants also reveals the roles of mitochondria in stress responses and mitochondrial-nuclear signaling. Plant Systems offer the advantage that mitochondrial-nuclear combinations are readily manipulated, the experimental materials are easily accessible, and generation times are usually short. Thus, they represent useful modeis for the generation and analysis of mitochondrial mutations and for the understanding of nuclear-cytoplasmic interactions.