Cytoplasmic Male Sterility of Rice with Boro II Cytoplasm Is Caused by a Cytotoxic Peptide and Is Restored by Two Related PPR Motif Genes via Distinct Modes of mRNA Silencing

TLDR

Cytoplasmic male sterility (CMS) and nuclear fertility restoration are common in crops, yet the molecular basis of cytoplasmic–nuclear interaction remains unclear. Two pentatricopeptide repeat proteins, Rf1a and Rf1b, localize to mitochondria and restore fertility by blocking ORF79 production through endonucleolytic cleavage (Rf1a) or mRNA degradation (Rf1b) of the dicistronic B‑atp6/orf79 transcript. In rice with Boro II cytoplasm, the mitochondrial ORF79 peptide is expressed in microspores and causes male sterility, but Rf1a and Rf1b restore fertility—Rf1a is epistatic over Rf1b and also promotes atp6 RNA editing—demonstrating distinct modes of mRNA silencing.

Abstract

Abstract Cytoplasmic male sterility (CMS) and nucleus-controlled fertility restoration are widespread plant reproductive features that provide useful tools to exploit heterosis in crops. However, the molecular mechanism underlying this kind of cytoplasmic–nuclear interaction remains unclear. Here, we show in rice (Oryza sativa) with Boro II cytoplasm that an abnormal mitochondrial open reading frame, orf79, is cotranscribed with a duplicated atp6 (B-atp6) gene and encodes a cytotoxic peptide. Expression of orf79 in CMS lines and transgenic rice plants caused gametophytic male sterility. Immunoblot analysis showed that the ORF79 protein accumulates specifically in microspores. Two fertility restorer genes, Rf1a and Rf1b, were identified at the classical locus Rf-1 as members of a multigene cluster that encode pentatricopeptide repeat proteins. RF1A and RF1B are both targeted to mitochondria and can restore male fertility by blocking ORF79 production via endonucleolytic cleavage (RF1A) or degradation (RF1B) of dicistronic B-atp6/orf79 mRNA. In the presence of both restorers, RF1A was epistatic over RF1B in the mRNA processing. We have also shown that RF1A plays an additional role in promoting the editing of atp6 mRNAs, independent of its cleavage function.