Arabidopsis Seed Mitochondria Are Bioenergetically Active Immediately upon Imbibition and Specialize via Biogenesis in Preparation for Autotrophic Growth

Seed germination is a vital developmental transition for production of progeny by sexual reproduction in spermatophytes. Quiescent cells in nondormant dry embryos are reawakened first by imbibition and then by perception of germination triggers. Reanimated tissues enter into a germination program requiring energy for expansion growth. However, germination requires that embryonic tissues develop to support the more energy-demanding processes of cell division and organogenesis of the new seedling. Reactivation of mitochondria to supply the required energy is thus a key process underpinning germination and seedling survival. Using live imaging, we investigated reactivation of mitochondrial bioenergetics and dynamics using Arabidopsis thaliana as a model. Bioenergetic reactivation, visualized by presence of a membrane potential, is immediate upon rehydration. However, reactivation of mitochondrial dynamics only occurs after transfer to germination conditions. Reactivation of mitochondrial bioenergetics is followed by dramatic reorganization of the chondriome (all mitochondrial in a cell, collectively) involving massive fusion and membrane biogenesis to form a perinuclear tubuloreticular structure enabling mixing of previously discrete mitochondrial DNA nucleoids. The end of germination coincides with fragmentation of the chondriome, doubling of mitochondrial number, and heterogeneous redistribution of nucleoids among the mitochondria, generating a population of mitochondria tailored to seedling growth.