Phytophthora nicotianae infection of citrus leaves and host defense activation compared to root infection.

Currently, little is known about the host pathogen interaction between Phytophthora and citrus roots versus leaves. Therefore, we compared the molecular events occurring in citrus roots and leaves after zoospore inoculation with Phytophthora nicotianae (P.n.). We analyzed the physical characters and genetic responses to P.n. infection of leaves and roots for susceptible and tolerant citrus rootstocks to examine the potential for leaves to model root responses to P.n. infection. Leaves responded faster and stronger to P.n. infection than roots, and leaves showed greater differential response than roots. In addition to differences in hormonal responses, sugar, phospholipases D (PLD) and phospholipases A (PLA) involvement in the interaction between citrus and P.n. was identified. This work for the first time creates a solid P.n. zoospore infection protocol, reports P.n. infection on citrus leaves through stomata, and provides evidence that different host organs respond to the pathogen differentially in timing and magnitude. This work identifies the hormones, sugars, pathogenesis-related (PR) genes, PLDs, and PLAs that are involved in the molecular events occurring in citrus under infection of P.n. zoospore, and advances our understanding of the mechanisms underlying the interaction.