Genes encoding pathogenesis-related proteins PR-2, PR-3 and PR-9, are differentially regulated in potato leaves inoculated with isolates from US-1 and US-8 genotypes of Phytophthora infestans (Mont.) de Bary

Late blight (LB) caused by Phytophthora infestans (Mont.) de Bary is a major constraint to potato production worldwide. In North America, this disease has re-emerged as a major concern to potato producers due to an important shift in its population structure. To design sustainable control strategies, it is necessary to have a better understanding of differential interactions that potato has developed in response to the currently pre-dominant versus the old populations of the pathogen. In this study, isolates from two P. infestans genotypes, US-1 (old, mildly aggressive) and US-8 (currently predominant, highly aggressive), were used to inoculate two potato cultivars, Russet Burbank (susceptible) and Kennebec (moderately resistant). Induction patterns of genes encoding three PR proteins (pr-2, pr-3 and pr-9) in response to isolates from those two genotypes were studied using northern blot analysis in three leaf strata (Local: inoculated leaflets, Proximal: un-inoculated leaflets in the same leaf inoculated with P. infestans, and Distal: un-inoculated leaflets from the leaf adjacent to the inoculated leaf) and different times after inoculation. Our results indicated that these pr genes were activated earlier in both cultivars when inoculated with P. infestans US-1 as compared to US-8 genotype. In addition, the induction of these genes occurred earlier in Kennebec than in Russet Burbank in response to both genotypes. Furthermore, pr-2 and pr-3 were found to be induced first locally, then systemically while pr-9 transcripts could be detected only locally at the site of inoculation and only in Kennebec inoculated with P. infestans US-1.