Influence of rice development on the function of bacterial blight resistance genes

Disease resistance genes most commonly used in breeding programs are single, dominant genes with relative effectiveness that is sometimes influenced by plant developmental stage. Knowing the developmental stages at which a resistance gene is functional is important for disease management. In rice, resistance at the seedling stage is crucial, because wounding during transplanting increases the potential for bacterial blight disease, and not all bacterial blight resistance genes are effective at the seedling stage. Effectiveness of the bacterial blight resistance genes Xa4, xa5, and Xa7, all in a common genetic background, was evaluated at different developmental stages by measuring lesion length and bacterial numbers after inoculation with the bacterial pathogen, Xanthomonas oryzae pv. oryzae. The Xa4 and xa5 genes controlled disease at all growth stages. In contrast, Xa7 was not fully functional in very young seedlings, but was completely effective by 21 days after sowing (das). The effects of plant developmental stage on interactions of the Xa7 gene with X. oryzae pv. oryzae strains carrying different mutant avrXa7 alleles were also tested. If a partial or fully functional avrXa7 allele was present, Xa7 resistance was effective at all growth stages tested after the transplant stage (>21 das).