Are the dominant and recessive plant disease resistance genes similar?: A case study of rice R genes and Xanthomonas oryzae pv. oryzae races

The resistance of rice to its bacterial blight pathogen Xanthomonas oryzae pv. oryzae (Xoo) has both qualitative and quantitative components that were investigated using three near-isogenic line sets for four resistance (R) g enes (Xa4, xa5, xa13, and Xa21) and 12 Xoo races. Our results indicate that these two resistance components of rice plants were associated with the pr operties of the R genes. The qualitative component of the R genes was refle cted by their large effects against corresponding avirulent Xoo races. The quantitative component of the R genes was their residual effects against co rresponding virulent races and their epistatic effects, which together coul d lead to high-level resistance in a race-specific manner. Our results reve aled important differences between the different types of R genes. Two R ge nes, Xa4 and Xa21, showed complete dominance against the avirulent Xoo race s and had large residual effects against virulent ones. They acted independ ently and cumulatively, suggesting they are involved in different pathways of the rice defensive system. The third R gene, xa5, showed partial dominan ce or additivity to the avirulent Xoo races and had relatively small but si gnificant residual effects against the virulent races. In contrast, xa13 wa s completely recessive, had no residual effects against the virulent races, and showed more pronounced race specificity. There was a strong interactio n leading to increased resistance between xa13 and xa5 and between either o f them and Xa4 or Xa21, suggesting their regulatory roles in the rice defen sive pathway(s). Our results indicated that high-level and durable resistan ce to Xoo Should be more efficiently achieved by pyramiding different types of R genes.