Heteroplasmy and stoichiometric complexity of plant mitochondrial genomes--though this be madness, yet there's method in't.

Mitochondrial heteroplasmy is defined as the coexistence of divergent mitochondrial genotypes in a cell. The ratio of the alternative genomes may be variable, but in plants, the usually prevalent main genome is accompanied by sublimons--substoichiometric mitochondrial DNA (mtDNA) molecules. Plant mitochondrial heteroplasmy was originally viewed as being associated with pathological mutations or was found in non-natural plant populations. Currently, it is considered to be a common situation in plants. Recent years have changed the previous view on the role of homologous recombination, small-scale mutations, and paternal leakage of mtDNA in the generation of heteroplasmy. Newly developed sensitive techniques have allowed the precise estimation of mtDNA stoichiometry. Mechanisms of maintenance and transmission of heteroplasmic genomes, including DNA recombination and replication, as well as mitochondrial fusion and fission, have been studied. This review describes the high level of plant mitochondrial genome complication--the 'madness' resulting from the heteroplasmic state and explains the method hidden in this madness. Heteroplasmy is described as the evolutionary strategy of uniparentally inherited plant mitochondrial genomes which do not undergo sexual recombination. In order to compensate for this deficiency, alternative types of mtDNA are substoichiometrically accumulated as a reservoir of genetic variability and may undergo accelerated evolution. Occasionally, sublimons are selected and amplified in the process called substoichiometric shifting, to take over the role of the main genome. Alternative mitochondrial genomes may recombine, yielding new mtDNA variants, or segregate during plant growth resulting in plants with mosaic phenotypes. Two opposite roles of mitochondrial heteroplasmy with respect to acceleration or counteracting of mutation accumulation are also discussed. Finally, nuclear control of heteroplasmy and substoichiometric shifting is described.