Rice bacterial blight resistance genes Xa7, xa5, and Xa4 confer resistance during all developmental stages

Resistance to bacterial blight, caused by Xanthomonas oryzae pv. oryzae follows a gene-for-gene model where a host resistance (R) gene confers recognition of a corresponding pathogen effector (avr) gene. Previous work indicated that resistance to bacterial blight conferred by the Xa21 rice gene was developmentally regulated, i.e., resistance was not observed in seedling plants (less than 21 days post-emergence). We investigated the impact of plant development stage on the interaction of the rice R genes Xa7, xa5, and Xa4 with X. oryzae pv. oryzae containing the corresponding avr genes. X. oryzae pv. oryzae isolates, PXO145 (containing avrXa7, avrxa5, and avrXa4) and PXO99 (with no relevant avr genes), were inoculated to rice cultivars with Xu7, xa5, Xa4, and the near-isogenic recurrent parent, IR24, at five developmental plant stages in the greenhouse. Lesion length and bacterial enumeration studies demonstrated that resistance controlled by Xa7, xa5, or Xa4 is effective at all tested stages of development.