RNA-Dependent RNA Polymerase 6 Contributes to Double-Strand Break Formation in Meiosis.

RNA-dependent RNA polymerase 6 (RDR6) is a core component of the small RNA biogenesis pathway, but its function in meiosis is unclear. Here, we report a new allele of ( []), which causes meiosis-specific phenotypes in rice (). In , meiotic double-strand break (DSB) formation is partially blocked. We created a biallelic mutant with more severe phenotypes, , by crossing with a knockout mutant, In meiocytes, 24 univalents were observed, and no histone H2AX phosphorylation foci were detected. Compared with the wild type, the number of 21-nucleotide small RNAs in was dramatically lower, while the number of 24-nucleotide small RNAs was significantly higher. Thousands of differentially methylated regions (DMRs) were discovered in , implying that OsRDR6 plays an important role in DNA methylation. There were 457 genes downregulated in , including three genes, , , and , related to DSB formation. Interestingly, the downregulated genes were associated with a high level of 24-nucleotide small RNAs but less strongly associated with DMRs. Therefore, we speculate that the alteration in expression of small RNAs in mutants leads to the defects in DSB formation during meiosis, which might not be directly dependent on RNA-directed DNA methylation.