PPR21 is involved in the splicing of nad2introns via interacting with pentatricopeptide repeat–small MutS-related 1 and short P-type pentatricopeptide repeat protein 2 and is essential to maize seed development

Pentatricopeptide repeat (PPR) proteins are a large group of eukaryote-specific RNA-binding proteins that play pivotal roles in plant organelle gene expression. Here, we report the function of PPR21 in mitochondrial intron splicing and its role in maize kernel development. PPR21 is a typical P-type PPR protein targeted to mitochondria. The ppr21mutants are arrested in embryogenesis and endosperm development, leading to embryo lethality. Null mutations of PPR21reduce the splicing efficiency of nad2intron 1, 2, and 4 and impair the assembly and activity of mitochondrial complex I. Previous studies show that the P-type PPR protein EMP12 is required for the splicing of identical introns. However, our protein interaction analyses reveal that PPR21 does not interact with EMP12. Instead, both PPR21 and EMP12 interact with the small MutS-related (SMR) domain–containing PPR protein 1 (PPR-SMR1) and the short P-type PPR protein 2 (SPR2). PPR-SMR1 interacts with SPR2, and both proteins are required for the splicing of many introns in mitochondria, including nad2intron 1, 2, and 4. These results suggest that a PPR21–(PPR-SMR1/SPR2)–EMP12 complex is involved in the splicing of nad2introns in maize mitochondria.