Silencing susceptibility genes in potato hinders primary infection with Phytophthora infestans at different stages

Most potato cultivars are susceptible to late blight disease caused by the oomycete pathogen Phytophthora infestans. Here we report that the genetic loss of host susceptibility is a new source of resistance to prevent or diminish pathogen infection. Previously, we showed that RNAi-mediated silencing of the potato susceptibility (S) genes StDND1, StDMR1, and StDMR6 leads to increased late blight resistance. The mechanisms underlying this S-gene-mediated resistance have thus far not been identified. In this study, we examined the infection process of P. infestans in StDND1-, StDMR1-, and StDMR6-silenced potato lines. Microscopic analysis showed that penetration of P. infestans spores was hampered in StDND1-silenced plants. In StDMR1- and StDMR6-silenced plants, P. infestans infection was arrested at a primary infection stage by enhanced cell death responses. Histochemical staining revealed that StDMR1- and StDMR6-silenced plants display elevated ROS levels in cells at the infection sites. Resistance in StDND1-silenced plants, however, seems not to rely on a cell death response as ROS accumulation was found to be absent at most inoculated sites. Quantitative analysis of marker gene expression suggests that the increased resistance observed in StDND1- and StDMR6-silenced plants relies on an early onset of salicylic acid- and ethylene-mediated signaling pathways. Resistance mediated by silencing StDMR1 was found to be correlated with the early induction of salicylic acid-mediated signaling. These data provide evidence that different defense mechanisms are involved in late blight resistance mediated by functional impairment of different potato S-genes.