The amino acid permease (AAP) genes CsAAP2A and SlAAP5A/B are required for oomycete susceptibility in cucumber and tomato

Cucurbit downy mildew (DM), caused by the obligate biotroph Pseudoperonospora cubensis, is a destructive disease in cucumber. A valuable source of DM resistance is the Indian cucumber accession PI 197088, which harbours several quantitative trait loci (QTLs) contributing to quantitatively inherited DM resistance. With a combination of fine-mapping and transcriptomics, we identified Amino Acid Permease 2A (CsAAP2A) as a candidate gene for QTL DM4.1.3. Whole-genome and Sanger sequencing revealed the insertion of a Cucumis Mu-like element (CUMULE) transposon in the allele of the resistant near-isogenic line DM4.1.3. To confirm whether loss of CsAAP2A contributes to partial DM resistance, we performed targeting induced local lesions in genomes on a DM-susceptible cucumber genotype to identify an additional csaap2a mutant, which indeed was partially DM resistant. In view of the loss of the putative function as amino acid transporter, we measured amino acids in leaves. We found that DM-inoculated leaves of line DM4.1.3 (with the csaap2a mutation) contained significantly fewer amino acids than wild-type cucumber. The decreased flow of amino acids towards infected leaves in csaap2a plants compared to the wild type might explain the resistant phenotype of the mutant, as this would limit the available nutrients for the pathogen and thereby its fitness. To examine whether AAP genes play a conserved role as susceptibility factors in plant–oomycete interactions, we made targeted mutations in two AAP genes from tomato and studied the effect on susceptibility to Phytophthora infestans. We conclude that not only CsAAP2A but also SlAAP5A/SlAAP5B are susceptibility genes for oomycete pathogens.