The management of Phytophthora infestans, causal agent of potato late blight, involves the use of about 15 fungicide treatments per cropping season. The combination of both induced resistance and quantitative resistance could be an alternative or a complementary strategy to decrease the use of pesticides. However, this method requires a better understanding of the interactions between Solanum tuberosum physiological responses and P. infestans ecology. Indeed, pathogen recognition by plants leads to the induction of defense responses via the PAMP-triggered immunity, but pathogens can modulate it via the secretion of effectors. In this context, we assumed that in potato, defenses triggered before infection, by a pathogen-associated molecular pattern (PAMP) of P. infestans, could alter both disease symptoms and the expression of effector genes depending on the genotype. In order to test this hypothesis, we simultaneously assessed in three potato genotypes, pre-treated or not with a concentrated culture filtrate of P. infestans (CCF), the expression of potato defense genes and of P. infestans effector genes involved in counter-defense, by qRT-PCR. The effectiveness of the induction was assessed by the measurement of disease symptoms. CCF pre-treatment induced most defense genes in Désirée and Bintje but repressed most defense genes in Rosafolia. On the contrary, effector genes were more expressed in Rosafolia than in Désirée and Bintje after CCF pre-treatment. Interestingly, induced defense responses by CCF significantly reduced lesion areas at 3 dpi only in Désirée. Together, these results show that the differential interplay between host response to elicitation and infection in the expression of defense genes on one side, and of pathogen effector genes on the other side, largely condition the effectiveness of induced defense on symptoms development. It also highlighted that a strain slowed down by a genotype and/or by a pre-treatment could adapt by secreting more effectors to counter induced defenses. These results open research prospects for combining induced resistance and quantitative resistance.
