New generation maintainer lines with multiple BLB resistance genes and good grain quality

Marker-assisted breeding of bacterial blight resistant maintainer lines started at PhilRice in 2003. Although carried out successfully, there were difficulties in using them for cytoplasmic male sterile (CMS) conversion due to residual fertility restoration capacity. Thus, the first generation of improved maintainer lines were not effectively used in developing bacterial blight resistant hybrids. In this study, the authors intended to develop a new generation of maintainer lines with two or multiple resistance genes and no residual fertility in converted CMS lines. Maintainer lines assembled at PhilRice were screened by inoculating them with Xanthomonas oryzae pv. Oryzae (Xoo) isolates and confirmed what possible genes were present using gene markers. The authors used IRBB62, a pyramided line having Xa4, Xa7 and Xa21 genes, and maintainer lines that showed broad resistance for bacterial leaf blight, as donor parental. Forward breeding and marker aided selection (MAS) were used in development and improvement of maintainer lines. After six generation, ten advanced lines were already uniform. Initial pollen sterility evaluation of F1 progenies, using three cytoplasmic sources, showed different reactions to iodine straining. Four pollen classifications were observed: unstained withered sterile (UWS), unstained spherical sterile (ULL), stained round (light) sterile (SRS), and stained round (dark) fertile (SRF). This implies the occurrence of sterility inducing factor present in the cytoplasm or in the nucleus of both parents. Grain qualities of these advanced lines exhibited acceptable value for percent chalkiness and amylose content. Therefore, with appropriate CMS source and proper selection, development of new CMS lines in the background of improved maintainer line having resistance gene is possible.