The splicing factor SPR2 interacts with the RNA-binding protein PPR8 to facilitate mitochondrial intron splicing and is essential for maize seed development

Intron splicing plays a crucial role in organellar gene expression in plants, yet its mechanism remains poorly understood. We previously reported a small PPR protein, SPR2, which is a general splicing factor in the splicing of over half of the mitochondrial introns via interaction with PPR-SMR1, a small MutS-related domain protein. To gain insight into the mechanism, we identified SPR2-interacting proteins using immunoprecipitation and mass spectrometry (IP-MS). One of the proteins is a P-type PPR protein with unknown function, named PPR8. PPR8 is targeted to mitochondria, and protein interaction assays confirmed its interaction with SPR2. The ppr8 mutants are severely arrested in embryogenesis and endosperm development. Loss of PPR8 impairs the splicing of mitochondrial nad7 intron 2 and nad4 intron 3, consequently disrupting the assembly and activity of mitochondrial complex I. These two introns match the targeted introns of SPR2 and PPR-SMR1, suggesting a mechanism in which the core SPR2/PPR-SMR1 forms combinatorial complexes with specific PPR proteins to facilitate individual intron splicing. The function of PPR8 is vital for mitochondrial complex I assembly and seed development in maize.