Identifying the genes around Rf5 and Rf6 loci for the fertility restoration of WA-type cytoplasmic male sterile japonica rice (Oryza sativa) lines

Abstract

Wild abortive (WA)-type, Honglian (HL)-type, and Chinsurah Boro II-type cytoplasm are three typical sterile cytoplasms used to generate three-line hybrid rice, and the fertility restorer (Rf) genes are considered to have specificity for fertility restoration of cytoplasmic male sterility (CMS) lines. ‘93-11’, an HL-type indica restorer line used widely in China, shows a weak ability to restore the fertility of WA-type CMS lines. Rf5 and Rf6, the fertility restorer genes for HL-type CMS, are members of a multigene cluster that encodes pentatricopeptide repeat proteins in ‘93-11’. In the present study, we studied the function of Rf genes around Rf5 and Rf6 loci on fertility restoration to WA-type CMS lines. We generated plants carrying WA-type cytoplasm and different genotypes at the Rf5 and Rf6 loci. All plants exhibited no seed setting on bagged panicles but had different anther and pollen grain morphologies. Plants with the genotypes of Rf5rf5rf6rf6, Rf5Rf5rf6rf6, rf5rf5Rf6rf6, rf5rf5Rf6Rf6, and Rf5rf5Rf6rf6 exhibited degraded anthers and typical abortive pollen grains, which were same as those of WA-NipA plants (rf5rf5rf6rf6); however, plants with the genotypes Rf5rf5Rf6Rf6, Rf5Rf5Rf6rf6, and Rf5Rf5Rf6Rf6 displayed restored anthers and pollen grains. These results indicated that Rf genes around the Rf5 and Rf6 loci had minor effects on the fertility restoration of WA-type CMS lines, which were mediated by dosage effects. Furthermore, these Rf genes functioned to decrease the WA352 (the mitochondrial gene conferring CMS-WA) transcript levels. Our findings will promote the development of three-line hybrids.