Breeding by design of CMS lines on the platform of SSSL library in rice

Greatly increased yield by utilizing of heterosis has been a great success in rice over the past 40 years. The cytoplasmic male sterility (CMS) line is a key line of hybrid rice. However, the CMS line was very difficult to develop following the traditional breeding methods. Here, we report an example to develop and improve CMS lines sustainably by using the strategy of breeding by design under the platform of single segment substitution line (SSSL) library in the Huajingxian74 (HJX74) genetic background. The CMS lines HZA and HBA with the wild abortive (WA) cytoplasm from Zhenshan97A and BobaiA, respectively, were firstly developed in the HJX74 background through marker-assisted selection. Then, four isonuclear alloplasmic CMS lines with the WA, dwarf wild abortive (DA), and Yegong abortive (YA) types of CMS cytoplasm, respectively, were developed by using the HZB maintainer line. According to the goal of design, E5-HXA and E5-HBA were developed by pyramiding five target genes, Wx g1 , ALK , fgr , gs3 , and OsMADS50 , located on the chromosomal substituted segments in four SSSLs. The two CMS lines show the phenotypes of earlier in heading date, longer and narrower in grain shape, lower in amylose content, and fragrant smell. Thus, the goal of design was achieved on the platform of SSSL library. Compared with that in the E5-HXA line carrying the rf3 and rf4 genes of XieqingzaoA, the CMS in the E5-HBA line with the rf3 and rf4 genes from BobaiA was susceptible to environmental impact and easy to be restored. Therefore, the balance between sterility and its restored ability in CMS lines should be concerned in breeding by design.