Impacts of environment and plant development on rice bacterial blight resistance gene durability and potential impacts to Xanthomonas oryzae pv. oryzae populations

Understanding factors that influence durability of plant disease resistance (R) genes is critical to predicting how long those genes will remain effective after deployment. One factor previously demonstrated to contribute to the durability of the rice bacterial blight R gene, Xa7, is the cost in pathogenic fitness associated with adaptation of the pathogen, Xanthomonas oryzae pv. oryzae (Xoo) to virulence. Adaptation to virulence on Xa7 results from loss of pathogen avirulence gene function. However, we found that after planting rice with Xa7 in field sites for 10 years (20 crops), the structure of the pathogen population shifted to one with increasing virulence to rice with Xa7 and the overall aggressiveness of the population to rice with no R gene also increased. Despite these changes, the Xa7 gene remains highly effective. Because disease severity is very low in the hot, dry cropping season relative to the cool, wet season, we asked if temperature is a factor in predicting Xa7 durability. Xa7 functions to restrict disease more effectively at high than low temperature regimes. These data suggest that high temperature effects on the resistance gene Xa7 may impose oscillating selection on the Xoo population, and thereby positively impact the durability of Xa7 in the field. Another factor important to plant disease epidemics is plant developmental age at time of infection. Many sources of resistance are not effective at seedling stages but become more effective as the plant matures. Determination of the effectiveness of R genes prior to deployment is important for disease management strategies and prolonging the durability of these genes. We measured the function of three different bacterial blight R genes, Xa4, xa5, and Xa7, at six stages of plant growth, and found that all of the genes were effective at all plant growth stages. Thus, these genes are good candidates for incorporation into germplasm, particularly in areas where seedling blight is important. Overall, our data are consistent with the prediction that Xa7 is a durable resistance gene, and we have identified additional factors (temperature and seedling function) that may contribute to that durability.