A "defeated" rice resistance gene acts as a QTL against a virulent strain of Xanthomonas oryzae pv. oryzae

The genetic components responsible for qualitative and quantitative resista nce of rice plants to three strains (CR4, CXO8, and CR6) of Xanthomonas ory zae pv. oryzae (Xoo) were investigated using a set of 315 recombinant inbre d lines (RILs) from the cross Lemont (japonica) x Teqing (indica) and a com plete linkage map with 182 well distributed RFLP markers. We mapped a major gene (Xa4) and ten quantitative trait loci (QTLs) which were largely respo nsible for segregation of the resistance phenotype in the RILs. The Teqing allele at the Xa4 locus, Xa4(T), acted as a dominant resistance gene agains t CR4 and CXO8. The breakdown of Xa4(T)-associated resistance mediated by t he mutant allele at the avl Xa4 locus in the virulent strain CR6 results fr om significant changes in both gene action (lose of dominance) and the magn itude of gene effect (approximate to 50% reduction). Nevertheless, Xa4(T) s till acted as a recessive QTL with a significant residual effect against CR 6. The mutant alleles at the avrXa4 locus in CX08 and CR6 that lead to a re duction in effect, or "breakdown", of Xa4(T) were apparently accompanied by corresponding penalties for their fitness. The quantitative component of r esistance to Xoo in the RILs was largely due to a number of resistance QTLs . Most resistance QTLs mapped to genomic locations where major resistance g enes and/or QTLs for resistance to Xoo, blast and sheath blight were identi fied in the same cross. Most QTLs showed consistent levels of resistance ag ainst all three Xoo strains. Our results suggest that a high level of durab le resistance to Xoo may be achieved by the cumulative effects of multiple QTLs, including the residual effects of "defeated" major resistance genes.