PEP-ASSOCIATED PROTEIN 3 regulates rice tiller formation and grain yield by controlling chloroplast biogenesis.

Plastid-encoded RNA polymerase (PEP) plays a pivotal role in chloroplast development by governing the transcription of chloroplast genes, and PEP-associated proteins (PAPs) modulate PEP transcriptional activity. Therefore, PAPs provide an intriguing target for efforts to improve yield by enhancing chloroplast development. In this study, we identified the rice (Oryza sativa) OsPAP3 gene and characterized its function in chloroplast development. OsPAP3 expression was light-dependent and leaf-specific, similar to the PEP-dependent chloroplast gene RUBISCO LARGE SUBUNIT (OsRbcL), and OsPAP3 protein localized to chloroplast nucleoids where PEP functions. Analysis of loss-of-function and gain-of-function mutants showed that expression of OsPAP3 is tightly linked to chloroplast gene expression and chloroplast biogenesis in rice. Homozygous knock-out mutants of OsPAP3 had fewer chloroplasts than wild type, whereas plants overexpressing OsPAP3 had more chloroplasts. Also, OsPAP3 knock-out suppressed PEP-dependent expression of chloroplast genes, but OsPAP3 overexpression increased their expression. These findings indicate that OsPAP3 regulates chloroplast biogenesis in rice by controlling PEP-dependent expression of chloroplast genes. More importantly, data from three seasons of field cultivation revealed that overexpression of OsPAP3 improves rice grain yield by approximately 25%, largely due to increased tiller formation. Collectively, these observations suggest that OsPAP3 regulates rice growth and productivity by promoting chloroplast development.