PPR9 mediates mitochondrial nad transcript maturation required for complex I biogenesis and early plant development in Arabidopsis

The biogenesis and function of plant mitochondria relies on the expression of the organellar genome, which involves extensive post-transcriptional processing of organellar transcripts in order to generate mature functional RNAs. This includes the removal of introns from many essential organellar transcripts through splicing, a process mediated by diverse nuclear-encoded RNA-processing factors, including numerous members of the pentatricopeptide repeat (PPR) protein family. Here, we report the analysis of PPR9 (At1g03560), a mitochondrial P-class PPR protein originally identified in genetic screens for essential protein cofactors. Loss of PPR9 function results in embryonic arrest at early stages of seed development. In vitro rescued homozygous ppr9 mutant plants could be recovered and maintained using a modified in vitro embryo-rescue method. The mutants display delayed growth and development which are associated with altered mitochondrial functions. Molecular analyses reveal that PPR9 functions in the splicing of several mitochondrial transcripts encoding respiratory complex I (CI) subunits, including nad2 intron 3 and nad7 introns 1 and 2. Accordingly, CI assembly is disrupted in ppr9 plants, resulting in altered OXPHOS activity and broad physiological defects. Together, these findings identify PPR9 as a key regulator of mt-RNA maturation in Arabidopsis, enabling CI biogenesis and optimal respiratory functions, and further highlight the roles of nuclear-encoded factors in coordinating mitochondria functions and (early) plant growth and development.