Lessons Learnt from Transgenic Gene Pyramiding for Classical Resistance Breeding in Wheat

Major plant resistance (R) genes are highly effective to protect plants against diseases, but once they are present in cultivars grown at large scale, pathogens frequently overcome such genes. We have used transgenic lines of wheat to test possible strategies to make the use of R genes more sustainable. As a model system, we use alleles of the resistance locus Pm3 in wheat, conferring resistance to wheat powdery mildew. We wanted to combine R genes or alleles in F1 hybrids or in genetically stable lines by crossing. Surprisingly we found that a suppression activity among the Pm3 alleles interfered with resistance. Using a transient-expression system in the heterologous species Nicotiana benthamiana we found that the suppression occurs at the posttranslational level. We also studied the suppression of disease resistance in wheat observed for the powdery mildew resistance gene Pm8 introgressed from rye. Some wheat lines with this translocation do not show resistance to isolates of the wheat powdery mildew pathogen avirulent to Pm8. We found that the wheat gene Pm3, an ortholog of rye Pm8, suppressed Pm8-mediated mildew resistance. Expression analysis revealed again that suppression is caused by a post-translational mechanism. These results suggest that the expression of closely related NB-LRR resistance genes in the same genotype can lead to dominant-negative interactions, which is a molecular explanation for and a first step to overcome the frequently observed ineffectiveness of resistance genes introduced from the secondary gene pool. We propose that resistance suppression could be predicted in classical breeding approaches.