Sequence tagged site marker-aided selection for bacterial blight resistance genes and against fertility restoring genes in mestizo hybrid rice (Oryza sativa L.) maintainer and cytoplasmic male sterile lines

Introgression of several bacterial blight (BB) resistance genes into three-line hybrid rice (Oryza sativa L.) components is deemed important in order to improve rice production. Initial gene pyramiding was done to incorporate BB resistance genes into Mestizo maintainer lines using donors IRBB4/7 and IRBB21 for Xa4, Xa7, and Xa21 genes, respectively. IRBB62 was also used (Borines, 2001). Selected BB lines were advanced until BC sub 3 F4 generation. Subsequently, selected BC sub 3 F4 plants were used as donors for crosses with BB susceptible parents IR58025B and IR58025A (cytoplasmic male sterile line) thereby generating BC sub 4 F1 B-line and A-line progenies, respectively. Since fertility restoring (Rf) genes present in IRBB donors were also inadvertently transferred, two sequence tagged site (STS) markers RG140 F/R and S100119 F/R linked to Rf3 and Rf4 genes, respectively were used to select against progenies detected with either one or both Rf genes. Three other STS markers MP1/MP2, MSF/R, and Xa21 F/R lined to Xa4, Xa7, and Xa21 BB resistance genes, respectively, were used to select for progenies with single genes, and two-, or three-gene combinations. In addition, phenotype analysis of individual plants for resistance to X. oryzae pv. oryzae Race 3, PX079 (dominant race in Isabela) was also conducted. Selection of progenies was carried out based on the following parameters: a) presence of Xa genes in progenies with corresponding resistance reaction to Race 3 PX079, and b) absence of dominant Rf genes. Subsequent DNA fingerprinting of selected lines using two resistance gene analog (RGA) primers indicated parental genotype recovery of at most 91% on all progenies. Progenies selected with Xa genes and without Rf genes, even those heterozygous for one or both Rf genes, are considered valuable selections for subsequent breeding applications. Selfing will allow obtaining next generation segregants without the Rf genes. Further backcrossing will increase the chance of recovering the background of recurrent parents IR58025A and IR58025B in succeeding generations. All plants selected are potential BB-resistant IR58025B and IR58025A lines that can be used to produce a BB-resistant Mestizo, the most widely used hybrid in the Philippine government's hybrid rice commercialization program. Moreover, the IR58025A selected lines with BB resistance are the female parents of most hybrids released in Asia, with the AXB and AXR seed production technology for IR58025 well worked out and found to be economically feasible. Hence, these BB-enriched lines promises to create both national and international impact.