Mitochondria as a connected population: ensuring continuity of the mitochondrial genome during plant cell dedifferentiation through massive mitochondrial fusion.

Mitochondrial fusion in plants and its role in development are poorly understood. Cultured tobacco mesophyll protoplasts provide an excellent experimental system for visualizing mitochondrial dynamics. Before protoplasts first divide, mitochondria undergo a phase of extensive elongation before fission causes an increase in number, followed by actin filament (AF)-dependent dispersion that distributes mitochondria uniformly throughout the cytoplasm. Here, by fusing protoplasts containing either green fluorescent protein- or MitoTracker-labelled mitochondria, we show that elongation results from fusion during early (4-8 h) protoplast culture. This massive mitochondrial fusion (MMF) leads to near-complete mixing of the mitochondrial population within 24 h. Staining isolated mitochondria with 4',6-diamidino-2-phenylindole (DAPI) revealed that in freshly prepared protoplasts mitochondrial nucleoids were unequally distributed, with many mitochondria failing to stain with DAPI, suggesting the presence of an incomplete mitochondrial genome. Following MMF, nucleoids were distributed evenly throughout the population, thereby ensuring continuity of the mitochondrial genome in daughter cells. Massive mitochondrial fusion appears to be specific to dedifferentiation, since it also occurs in mesophyll protoplasts of Arabidopsis and Medicago but not in protoplasts from already dedifferentiated cells such as BY-2 or callus cultures. Efficient MMF requires an inner membrane electrical gradient, cytoplasmic protein synthesis, microtubules and functional kinesin but not ATP or AFs, indicating fundamental differences from mitochondrial fusion in non-plant systems. Our studies reveal that individual mitochondria are connected over time by fusion events, a finding that allows a clearer interpretation of how novel mitochondrial genotypes develop following cell fusion, and indicates that developmentally regulated fusion ensures continuity of the mitochondrial genome.