Analysis of Changes in the Potato Leaf Proteome Triggered by Phosphite Reveals Functions Associated with Induced Resistance Against Phytophthora infestans
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Exogenous chemicals can be used to stimulate induced resistance (IR), a process related to increased disease resistance of susceptible plants against a broad range of pathogens. The identification of molecular components related to IR provides an understanding of the mechanisms related to host resistance to pathogens. Environmentally friendly phosphite (Phi)-based fungicides are increasingly used in controlling oomycete pathogens such as Phytophthora infestans, causing late blight of potatoes and other solanaceous crops. Their efficacy was clearly proven by field trials carried out by a group of researchers led by Wang-Pruski. Nevertheless, the molecular mechanisms responsible for stimulation of IR by Phi-based fungicides have not been fully documented. In this study, 93 differentially regulated proteins (62 up-regulated and 31 down-regulated proteins) in Phi-treated potato leaves were identified by iTRAQ-based quantitative proteomics. The majority of these differentially regulated proteins have not been previously reported. Identification of the differentially regulated proteins revealed two major molecular mechanisms related to defense and metabolism for energy generation. Defense mechanisms include the hypersensitive response (HR), reactive oxygen species pathway, salicylic acid-dependent pathway, and antimicrobial activities. Energy generating metabolisms include glycolysis, photosynthesis, and starch degradation. Four days post inoculation with P. infestans, the abundance of 16 of the 93 differentially regulated proteins increased significantly in the Phi-treated plants compared with that of the control plants. On the other hand, the abundance of 9 of the 93 differentially regulated proteins was decreased in the Phi-treated plants compared to that of the control plants. The abundance of the remaining 68 differentially regulated proteins was unchanged for both challenged Phi-treated and control plants. This suggests that pre-activation of proteins in Phi-treated sample before infection is essential to increase the levels of host resistance to the pathogen. Many of the 68 differentially regulated proteins that did not change after infection play roles in HR. These proteins include cathepsin B, cysteine protease inhibitors, and proteins involved in reactive oxygen species and salicylic acid pathways. Callose deposition and subcellular changes related to HR were observed in the Phi-treated plants, indicating that Phi-responsive proteins facilitate the activation of HR against the pathogen.