Mitochondrial nucleoid organization and biogenesis of complex I require mTERF18/SHOT1 and ATAD3 in Arabidopsis thaliana.

Mitochondria play critical roles in eukaryotes in ATP generation through oxidative phosphorylation (OXPHOS) and also produce both damaging and signaling reactive oxygen species (ROS). Originating from endosymbiosis, mitochondria have their own reduced genomes that encode essential subunits of the OXPHOS machinery. MTERF (Mitochondrial Transcription tERmination Factor-related) proteins have been shown to be involved in organelle gene expression by interacting with organellar DNA or RNA in multicellular eukaryotes. We previously identified mutations in Arabidopsis thaliana MTERF18/SHOT1 that enable plants to better tolerate heat and oxidative stresses, presumably due to low ROS and reduced oxidative damage. To understand molecular mechanisms leading to shot1 phenotypes, we investigated mitochondrial defects of shot1 mutants and targets of the SHOT1 protein. shot1 mutants have problems accumulating OXPHOS complexes that contain mitochondria-encoded subunits, with complex I and complex IV most affected. SHOT1 binds specific mitochondrial DNA sequences and localizes to mitochondrial nucleoids, which are diffuse in shot1 mutants. Furthermore, three homologues of mammalian ATAD3A proteins, which are suggested to be involved in mitochondrial nucleoid organization, were identified as SHOT1-interacting proteins (designated SHOT1 BINDING ATPASES (SBA)1, 2 and 3). Importantly, disrupting SBA function also disrupts nucleoids, compromises accumulation of complex I and enhances heat tolerance. We conclude that proper nucleoid organization is critical for correct expression and accumulation of complex I, and propose that nucleoid disruption results in unique changes in mitochondrial metabolism and signaling that lead to heat tolerance. SignificanceIn all eukaryotes, mitochondria are critical organelles that supply chemical energy for life, which is produced by the oxidative phosphorylation (OXPHOS) machinery on the inner mitochondrial membrane. The OXPHOS machinery comprises multiple protein complexes with subunits encoded by both nuclear and mitochondrial genes. Nuclear-encoded mTERF proteins are important for expression of mitochondrial genes, interacting with mitochondrial DNA or RNA. Our study reveals that the Arabidopsis mTERF18/SHOT1 protein interacts with mtDNA and homologs of human ATAD3A proteins, and that both proteins are critical for mitochondrial nucleoid organization and accumulation of OXPHOS Complex I. Further, the data indicate nucleoid disruption leads to unique mitochondrial and cellular responses such that mutant plants have enhanced heat tolerance.