Ability of Rf5 and Rf6 to Restore Fertility of Chinsurah Boro II-type Cytoplasmic Male Sterile Oryza Sativa (ssp. Japonica) Lines

BACKGROUND: Three-line Oryza sativa (ssp. japonica) hybrids have been developed mainly using Chinsurah Boro II (BT)-type cytoplasmic male sterility (CMS). The Rf1 gene restores the fertility of BT-type CMS lines, and is the only fertility restorer gene (Rf) that has been used to produce three-line japonica hybrids. Using more Rf genes to breed BT-type restorer lines may broaden the genetic diversity of the restorer lines, and represents a viable approach to improve the heterosis level of BT-type japonica hybrids. RESULTS: We identified two major Rf genes from ‘93-11’ that are involved in restoring the fertility of BT-type CMS plants. These genes were identified from resequenced chromosome segment substitution lines derived from a cross between the japonica variety ‘Nipponbare’ and the indica variety ‘93-11’. Molecular mapping results revealed that these genes were Rf5 and Rf6, which are the Rf genes that restore fertility to Honglian-type CMS lines. The BT-type F(1) hybrids with either Rf5 or Rf6 exhibited normal seed setting rates, but F(1) plants carrying Rf6 showed more stable seed setting rates than those of plants carrying Rf5 under heat-stress conditions. Furthermore, the seed setting rates of F(1) hybrids carrying both Rf5 and Rf6 were more stable than that of F(1) plants carrying only one Rf gene. CONCLUSION: Rf6 is an important genetic resource for the breeding of BT-type japonica restorer lines. Our findings may be useful for breeders interested in developing BT-type japonica hybrids. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s12284-017-0142-9) contains supplementary material, which is available to authorized users.