The pentatricopeptide repeat protein MTSF3 is required for nad2 mRNA stability and embryogenesis in Arabidopsis

Gene expression in plant mitochondria is predominantly governed at the post-transcriptional level and relies mostly on nuclear-encoded proteins. However, the involved protein factors and the underlying molecular mechanisms are still not well understood. In this study, we report the function of the mitochondrial stability factor 3 (MTSF3) protein and we show that it is essential for accumulation of the mitochondrial nad2 transcript in Arabidopsis and not for the splicing of nad2 intron 2, as recently proposed (Marchetti et al., 2020). The MTSF3 gene encodes a pentatricopeptide repeat protein that localizes in the mitochondrion. An MTSF3 null mutation induces embryonic lethality but viable mtsf3 mutant plants could be generated by partial complementation with the developmentally-regulated ABSCISIC ACID INSENSITIVE3 promoter. Genetic analyses reveal that mtsf3 rescued plants display growth retardation due to the specific destabilization of a nad2 precursor transcript bearing exons 3 to 5. Biochemical data demonstrate that MTSF3 protein specifically binds to the 3'-terminus of nad2. The destabilization of nad2 mRNA induces a significant decrease in complex I assembly and activity, and an overexpression of the alternative respiratory pathway. Our results support that the MTSF3 protein protects nad2 transcript from degradation by mitochondrial exoribonucleases by binding to its 3' extremity.