Organ-Dependent Effectiveness of Potato R Genes Against Phytophthora infestans: Resistance Phenotypes and Expression Patterns

Major resistance (R) genes are widely used to protect potato against Phytophthora infestans, but their effectiveness may differ between foliage and tubers. Based on our previous findings, we used two contrasting models of organ-dependent resistance: R2-like, which is effective mainly in foliage, and Rpi-phu1, which is effective in both foliage and tubers. We investigated whether these contrasting resistance phenotypes could be explained by differences in R-gene expression. Resistance and R-gene expression were evaluated across potato genotypes following inoculation with three P. infestans isolates. Both genes produced the expected isolate-specific resistance patterns in detached leaflets, whereas their effectiveness in tubers differed markedly. R2-like provided little tuber resistance even against isolates avirulent towards this gene in foliage, while Rpi-phu1 conferred high tuber resistance in some, but not all, genotype-isolate combinations. Analysis of basal and post-inoculation expression revealed no simple relationship between R-gene transcript abundance and resistance phenotype. Strikingly, strong R-gene induction was often associated with susceptible interactions, while effective resistance could occur without pronounced induction at the sampled time points. Therefore, differences in R-gene expression did not explain the contrasting resistance phenotypes observed between foliage and tubers. These results highlight organ-, isolate- and genotype-dependent differences in R-gene effectiveness. Progeny carrying both R2-like and Rpi-phu1 showed higher tuber resistance than either single-gene group in interactions where single-gene resistance was incomplete, suggesting that R-gene pyramiding may enhance not only the spectrum but also the level of tuber resistance.