Rf5 is able to partially restore fertility to Honglian-type cytoplasmic male sterile japonica rice ( Oryza sativa ) lines

Three-line japonica hybrids have been developed mainly on Chinsurah Boro II (BT)-type cytoplasmic male sterile (CMS) lines of Oryza sativa L., but the unstable sterility of some BT-type CMS lines, and the threat of genetic vulnerability when using a single cytoplasm source, have inhibited their use in rice cultivation. Previously, the sterility of Honglian (HL)-type japonica CMS lines derived from common red-awned wild rice ( Oryza rufipogon ) has been proven to be more stable than that of BT-type japonica CMS lines. Here, we genetically characterized HL-type japonica CMS lines and the restorer-of-fertility ( Rf ) gene for breeding HL-type japonica hybrids. HL-type japonica CMS lines displayed stained abortive pollen grains, unlike HL-type indica CMS lines. The BT-type japonica restorer lines, which contain Rf , had different capabilities to restore HL-LiuqianxinA (HL-LqxA), an HL-type japonica CMS line, and the restorers for the HL-type japonica CMS lines could be selected from the preexisting BT-type japonica restorers in rice production. A genetic analysis showed that the restoration of normal fertility to HL-LqxA was controlled by a major gene and was affected by minor effector genes and/or modifiers. The major Rf in SiR2982, a BT-type japonica restorer, was mapped to a ~100-kb physical region on chromosome 10, and was demonstrated to be Rf5 ( Rf1a ) by sequencing. Furthermore, Rf5 partially restored fertility and had a dosage effect on HL-type japonica CMS lines. These results will be helpful for the development of HL-type japonica hybrids.