The Powdery Mildew Resistance Gene Pm8 Derived from Rye Is Suppressed By Its Wheat Ortholog Pm3

To increase productivity, genetic improvement of tolerance to biotic and abiotic stresses is essential in wheat breeding. Wild or domesticated wheat relatives provide a rich resource for disease resistance genes which can be introgressed into elite genetic material. The most widely used introgression nowadays is the 1BL.1RS translocation derived from the rye cultivar Petkus carrying three rust resistance genes and the powdery mildew resistance gene Pm8. Pm8 has been cloned and shown to be the rye ortholog of the Pm3 wheat powdery mildew resistance gene. Both NB-LRR genes mediate race-specific resistance against powdery mildew in wheat. However, not all wheat lines carrying the translocation show resistance to Pm8 avirulent isolates due to the presence of a suppressor gene in some wheat genotypes. Interestingly, this suppressor gene was mapped to a region encompassing the Pm3 locus in wheat. In a single-cell transient expression assay we found that the Pm3 gene is sufficient to suppress Pm8-mediated resistance. This result was further confirmed in transgenic lines with combined Pm8 and Pm3 transgenes. Expression analysis revealed that suppression is not the result of gene silencing, neither in wheat 1BL.1RS translocation lines carrying Pm8 nor in transgenic genotypes with both Pm8 and Pm3 alleles. Instead, suppression is due to post-translational processes most likely involving protein interactions. Thus, our results give a first molecular explanation for a frequently observed suppression phenomenon when chromosomes from wheats with lower ploidy level and wild relatives are introgressed into wheat and suggests ways to circumvent it in future plant breeding.