Cytoplasmic male sterility: Sterility induction, fertility restoration and gene-for-gene interaction of CMS/Rf genes

Cytoplasmic male sterility (CMS) is a maternally inherited agronomic trait. The CMS lines cannot produce viable pollens (incomplete or nonfunctional), properly dehisce pollen, germinate on the stigma or be accessible to the stigma, thus unable to fertilize the ovule/ egg. Although CMS is not beneficial to the plants itself, it is a valuable resource for hybrid breeding. CMS is caused by the mitochondrial genome carrying chimeric genes (orfs), which frequently originate in the re-arrangements of the mitochondrial genome. The nuclear genome's restorer-of-fertility (Rf) gene suppresses the CMS conditions to restore fertility. The Rf genes interact with CMS-inducing genes at various levels to regulate their activity and restore fertility. In various crop species, different cytotypes carry specific CMS-inducing genes, which require a specific Rf gene for fertility restoration. Although some Rf genes can restore the fertility of more than one cytotype, most cytotypes require specific Rf genes. In this review, we briefly discussed the specificity of Rf genes and cytotypes based on B. napus, Oryza spp, and H. annuus. The findings from these crop species suggest that the relationship of CMS/Rf genes occurs in a gene-for-gene fashion. However, the molecular mechanism behind the Rf-cytotype specificity is not much understood. Studying the in-vitro expression system can help to elucidate the relationship between the CMS and Rf genes. The evolution of PPR in response to the emergence of CMS-inducing mitochondrial orfs and the induction of male sterility via genome editing technologies is also briefly discussed.