Advances in understanding the molecular mechanisms of cytoplasmic male sterility and restoration in rice.

Cytoplasmic male sterility (CMS) in plants is a male reproductive defect determined by mitochondrial genes and inherited maternally. CMS can be suppressed by nuclear restorer of fertility (Rf) genes. Therefore, CMS/Rf systems provide a classic model for the study of mitochondrial-nuclear interactions in plants. Moreover, CMS/Rf systems are economical, effective tools for the production of hybrid seeds. For example, CMS/Rf systems have been applied in over forty countries to breed hybrid rice (Oryza sativa L.) with improved yields due to hybrid vigor. The production of hybrid rice mainly depends on three types of CMS systems, namely Wild-Abortive type CMS (CMS-WA), Hong-Lian type CMS (CMS-HL) and Boro II type CMS (CMS-BT). Understanding the molecular mechanisms underlying these CMS/Rf systems will help us to understand mitochondrial-nuclear interactions, and accelerate the utilization of heterosis for improvement in yield. In the past decades, research benefitting from the availability of the high-quality, annotated mitochondrial and nuclear genome sequences of rice has isolated many CMS genes, identified the cognate nuclear Rf genes and studied the molecular mechanisms underlying CMS and restoration in rice. Here, we focus on recent advances in studies of the three major CMS/Rf systems in rice and discuss the key issues facing basic research and application of CMS/Rf systems in the future.