Rf4 has minor effects on the fertility restoration of wild abortive-type cytoplasmic male sterile japonica (Oryza sativa) lines

Wild abortive (WA)-type cytoplasmic male sterility (CMS) has been exclusively used for breeding three-line hybrid indica rice, but it has not been applied for generating japonica hybrids because of the difficulties related to breeding japonica restorer lines. Determining whether the major restorer-of-fertility (Rf) gene used for indica hybrids can efficiently restore the fertility of WA-type japonica CMS lines may be useful for breeding WA-type japonica restorer lines. In this study, japonica restorer lines for Chinsurah Boro II (BT)-type CMS exhibited varying abilities to restore the fertility of ‘WA-LiuqianxinA’, which is a WA-type japonica CMS line. Additionally, Rf genes for WA-type CMS were identified in the BT-type japonica restorers. Meanwhile, ‘C9083’, which is a BT-type japonica restorer, exhibited a limited ability to restore the fertility of WA-type japonica CMS lines, and a genetic analysis revealed that the fertility restoration was controlled by one locus. The Rf gene was mapped to an approximately 370-kb physical region and was identified as Rf4. Furthermore, Rf gene dosage effects and the temperature influenced the fertility restoration of WA-type japonica CMS lines. This study is the first to confirm that Rf4 has only minor effects on the fertility restoration of WA-type japonica CMS lines. These results may be relevant for the development of WA-type japonica hybrids.