Understanding the expression and interaction of Rph genes conferring seedling and adult plant resistance to Puccinia hordei in barley

Leaf rust caused by Puccinia hordei is a major disease of barley and is best managed by deploying effective resistance genes. The effect of post-inoculation temperature [18 +/- 2 degrees C (low), 22 +/- 2 degrees C (mid), or 26 +/- 2 degrees C (high)] on the expression of a selection of known all stage resistance (ASR) Rph genes and their interaction in combination with the adult plant resistance (APR) gene Rph20 were investigated in barley. Rph1.a and Rph2.b expressed best at higher temperatures whereas Rph4.d lost its effectiveness at higher temperatures when tested with pathotype (pt) 200 P- (avirulent on these genes). The expression of the genes Rph3.c, Rph5.e, Rph6.f, Rph12 (9.z), Rph13.x and Rph15.ad was not affected by post-inoculation temperature. The interaction between Rph14.ab and pt 5457 P+ gave the lowest infection type at low post-inoculation temperature. Among a selection of lines carrying Rph20 ('Flagship'), Rph23 ('Yerong') and Rph24 ('ND24260MODIFIER LETTER PRIME), higher levels of resistance were observed in seedlings of 'Flagship' (Rph20) at lower temperatures, while seedlings of 'Yerong' (Rph23) and 'ND24260MODIFIER LETTER PRIME (Rph24) showed high infection types at all three temperatures with pt 5457 P+. When RphASR genes were combined with the APR gene Rph20 an enhanced level of seedling resistance was observed in the six combinations: Rph1.a+Rph20, Rph2.b+Rph20, Rph4.d+Rph20, Rph9.i+Rph20, Rph5.e+Rph20 and Rph15.ad+Rph20. Seedlings carrying the Rph5.e+Rph20 combination remained resistant when challenged with the Rph5.e-virulent pt 220 P+ + Rph13, suggesting a residual effect of Rph5.e when present in combination with Rph20. This is the first study to demonstrate that several ASR Rph genes interact with the APR gene Rph20 in an additive manner, emphasizing the importance of deploying genes in combination rather than individually to achieve durable resistance.