The chloroplast pentatricopeptide repeat protein RCN22 regulates tiller number in rice by affecting sugar levels via the TB1-RCN22-RbcL module

As an important yield component, the rice tiller number controls panicle number and determines grain yield. The regulation of rice tiller number by chloroplast pentatricopeptide repeat (PPR) proteins has not been reported. Here, we report a rice reduced culm number22 (rcn22) mutant which produces few tillers due to suppressed tiller bud elongation. Map-based cloning revealed that RCN22 encodes a chloroplast-localized P-type PPR protein. We found that RCN22 specifically binds to the 5'-UTR of RbcL mRNA (encoding the large subunit of Rubisco) and enhances its stability. The reduced RbcL mRNA abundance in rcn22 led to a lower photosynthetic rate and decreased sugar levels. Consequently, transcript levels of DWARF3 (D3) and TEOSINTE BRANCHED1 (TB1) (encoding negative regulators of tiller bud elongation) increased, whereas protein levels of a positive regulator DWARF53 (D53) decreased. Furthermore, high concentrations of sucrose could rescue the tiller bud growth defect of the rcn22 mutant. On the other hand, TB1 directly binds to the RCN22 promoter and downregulates its expression. The tb1/rcn22 double mutant showed a tillering phenotype similar to rcn22. Our results suggest that the TB1-RCN22-RbcL module plays a vital role in rice tiller bud elongation by affecting sugar levels.