Transgenic Resistance Genes in Wheat Breeding: Results of a Large Field Trial in Switzerland

In wheat, the highest yield losses due to pathogens are caused by fungal diseases. Among them is powdery mildew which attacks leaf epidermal cells. We are studying the molecular diversity and function of the wheat powdery mildew resistance (R) gene Pm3. It has 17 functional alleles that confer race-specific resistance and encode members of the CC-NB-LRR proteins. For the Pm3a-d, Pm3f and Pm3g alleles, we have developed transgenic lines expressing a single, epitope-tagged PM3 derivative driven by the constitutive, strong ubiquitin promoter. Powdery mildew resistance was significantly improved in all lines in the greenhouse and the field, both with naturally occurring infection or after artificial inoculation. Under controlled environmental conditions, the line with the strongest overexpression of the Pm3b gene showed a dramatic increase in resistance to several independent isolates that are virulent on lines carrying an endogenous copy of Pm3b. In one or more field environments, but never in the greenhouse, five of the 12 tested transgenic lines showed pleiotropic effects on leaves and/or spikes. The strongest overexpressing line had the most dramatic negative effects, suggesting a correlation between expression level and phenotypic changes. These results demonstrate that the successful transgenic use of R genes will critically depend on achieving an optimal level of their expression, possibly in a tissue specific way. Since single R genes may be rapidly overcome in lines with widespread cultivation, we tested if mixtures of transgenic lines carrying each a different Pm3 allele represent a long term agronomical strategy to increase R gene durability.