Genetic Overlap in the Quantitative Resistance of Rice at the Seedling and Adult Stages to Xanthomonas oryzae pv. oryzae

The genetic components responsible for the qualitative and quantitative resistance of rice to three Chinese races (C2, C4, and C5) of Xanthomonas oryzae pv. oryzae (Xoo) were investigated at the seedling and adult stages in two successive years in set of Lemont/Teqing cross introgression lines (ILs) in a Teqing background, to create a complete linkage map using 160 well-distributed SSR markers. Teqing was resistant to C2 and C4, but moderately susceptible to C5, whereas Lemont was susceptible to all three races. Highly significant correlations were detected among the resistance to different races at different developmental stages. A major gene (Xa4), 14 main-effect QTLs (M-QTLs), and 18 epistatic QTLs were identified in the two developmental stages over 2 years, and were largely responsible for the segregation of resistance in the ILs. In 2007, the Lemont alleles at all loci in the seedling stage, except QBbr10 to C4, increased lesion length (LL) or decreased resistance. The Teqing allele at the Xa4 locus acted as a resistance gene against C2 and C4, but acted as a M-QTL when its resistance was overcome by the virulent race C5. M-QTLs showed a degree of race specificity and had a cumulative effect on resistance. Most M-QTLs (94%) consistently expressed resistance to the same race at the seedling and adult stages, indicating that a high degree of genetic overlap exists between Xoo resistance at both developmental stages in rice. Among the digenic interactions, most co-introgressed Lemont alleles at the two epistatic loci lead to significantly smaller LL with all three races, compared to other types of interacting alleles at both development stages. The results indicate that a high level of resistance may be achieved by the cumulative effect of multiple M-QTLs, including the residual effects of defeated major resistance genes and the epistatic effects of co-introgression from diverse susceptible varieties.