Expression of a modified <i>Avr3a</i> gene under the control of a synthetic pathogen‐inducible promoter leads to <i>Phytophthora infestans</i> resistance in potato

Late blight resistance of potato was improved by the co-expression of the potato resistance gene R3a and the pathogen-inducible avirulence gene Avr3a of Phytopthora infestans. The synthetic pathogen-inducible promoter 2xS-4xD-NpCABE_core, which is composed of the cis-acting elements S and D and the core promoter of the NpCABE gene, was developed for potato. By analysis of 20 core promoters from Solanacea species synthetic promoters of the 2xS-2xD-type were generated which differ in their background activity, strength and promoter inducibility. These data showed that the core promoter plays an important role for the architecture of a synthetic promoter and influences the specificity and strength beside the cis-acting element. The 2xS-2xD-NpCABE_core promoter was further improved by increasing the number of the cis-acting elements resulting in the 2xS-4xD-NpCABE_core promoter. Modified Avr3a alleles, which triggered less cell death than the Avr3a^KI allele, were expressed with the optimized synthetic promoter in transgenic potatoes with an R3a gene. The transgenic lines showed less late blight symptoms and up to 60% reduction of sporangia in detached leaf assays. The absence of a negative plant phenotype in the greenhouse demonstrated that the balanced co-expression of a modified Avr3a gene under the control of an optimized synthetic promoter is a promising strategy to increase late blight resistance of potatoes. This concept might be as well applied to other crops since the co-expression of the R3a and Avr3a^KI gene induced cell death in leaves of corn, wheat and soybean in a transient assay.