Seed mitochondria are equipped with cristae and a full proteome to kickstart germination

Mitochondria of seeds face the challenge of remaining quiescent over long periods and rapidly resuming their respiratory function once environmental conditions become favorable. Despite the fundamental importance of mitochondrial functions for germination, the composition and functionality of seed mitochondria remain poorly understood. Previous work has suggested that dry seeds contain immature promitochondria largely devoid of cristae or respiratory complexes and that key mitochondrial functions need to be re-established by rebuilding a functional proteome to support germination. Here, we examined the onset of respiration in the germination of several plant species and investigated the composition of Arabidopsis seed mitochondria using cryo-preservation-based transmission electron microscopy, affinity- and centrifugation-based mitochondrial isolations, native gel electrophoresis, and advanced proteomic and protein localization analyses. Our data reveal pre-formed cristae, the full set of respiratory complexes in the mitochondria of dry seeds, as well as the presence of proteins for all key functions that mitochondria fulfill in vegetative tissues. While the overall protein composition of Arabidopsis seed mitochondria is similar to that in other developmental stages and dark-grown cell cultures, it deviates particularly strongly in selected proteoforms and yet-unassigned candidate mitochondrial or mitochondria-associated proteins. Our findings establish that Arabidopsis seed mitochondria are preserved in a structurally and functionally competent state, accounting for the immediate activation of respiration that occurs upon seed rehydration. They further expand our understanding of mitochondrial plasticity by assignment of novel proteins to the plant mitochondrial proteome, laying the foundation for future investigations of their potential significance in desiccation tolerance and metabolic regulation.