RFLP mapping and race specificity of bacterial blight resistance genes (QTLs) in rice

By using a set of 315 recombinant inbred lines (RILs) from the cross Lemont (japonica) x Teqing (indica) and a complete linkage map with 186 well distributed RFLP markers and 3 morphological markers, a major gene (Xa4) and 10 QTLs and 9 pairs of epistasis loci conferring horizontal resistance to three strains of Xanthomonas oryxa pv oryza (Xoo) were mapped. The Teqing allele at Xa4 on chromosome 11 acts as a dominant resistant gene against pathogen race CR4 and CX08, but as an additive QTL with a significantly (47%) reduced effect against the virulent strain, CR6. The major gene Xa4 exhibited stronger degree of race specificity. Most QTLs showed consistent levels of resistance against all three Xoo strains. The results suggest that a high level durable resistance to Xoo may be achieved by cumulative effects of multiple QTL.