Production of advanced lines of rice possessing genes for resistance against Philippine bacterial blight PXO races

The government launched a comprehensive Hybrid Rice Program (HRP) in 1998 due to the yield advantage offered by this technology. A common observation, however, is the susceptibility of the hybrids and component lines to local pests and diseases, especially bacterial blight (BB) caused by Xanthomonas oryzae pv. oryzae (Xoo). Evaluation of the reaction of 'Mestizo' hybrid as well as its maintainer (B) and restorer (R) lines, to the Philippine Xoo (PXO) races and improvement of their resistance to BB are necessary before the hybrids could achieve the expected yield benefits. To evaluate the reaction of mestizo rice hybrids to PXO races, Borines et al. (LSU, IRRI, and PhilRice) incorporated bacterial blight resistance gene into Mestizo hybrid by marker-aided backcrossing, and confirmed presence of introgressed gene in advanced lines. The research was conducted for a year starting 2005. Specifically, mestizo hybrids, their B and R lines, IRBB61 donor, and checks were phenotyped using ten PXO races (12 isolates). Marker-aided backcrossing from IRBB61 donor to each B and R line was done to incorporate single and combination of target genes. The presence of the genes in the advanced lines was confirmed using diagnostic PXO races and analyses of linked DNA markers. Mestizo hybrids and most of their B and R lines showed resistance to almost half of the total number of PXO races. This was due to the presence of Xa4 in these lines. Resistance genes singly or in combination were incorporated to BC5F1 progeny of IR34686-179-1-2-IR, IR60819-34-2R, IR62161-184-3-1-3-2R, and IR6888B. Resistance genes were also incorporated to BC3F1 progeny of IR58025B (maintainer of Mestizo 1) and IR68897B. The presence of Xa genes in the advanced backross progeny was confirmed through the backcross progeny reaction to the diagnostic PXO races and linked DNA markers. The lines containing gene pyramids had increased resistance to bacterial blight and wider resistance spectrum to Xoo races. The advanced lines (BC5F1) can be used as resistance gene sources for the Mestizo hybrids' improvement. Seeds of the advanced lines had been forwarded to PhilRice. However, further backcrossing is necessary for IR58025B and IR68897B, which are still in the Backcross3 (BC3) stage, to produce genetically stable progeny possessing the characteristics of the parent and the resistance genes. This progeny will be used as parent to transfer the desired genes to the cytoplasmic male sterile (A or CMS )lines and Mestizo hybrids.