Analysis of Protein Synthesis in Cucumber Leaves after Inoculation with Corynespora cassiicola: A Proteomic Approach

Cucumber target leaf spot (TLS) disease caused by Corynespora cassiicola has become one of the most important fungal foliar diseases of cultivated cucumbers. However, the defense mechanisms of cucumber plants (Cucumis sativus) against C. cassiicola are still poorly understood. Here, proteins from resistant cucumber plants were analyzed using iTRAQ (isobaric tags for relative and absolute quantification) method. A total of 286 differentially expressed proteins were identified (p < 0.05, ratio > 1.2 or < 0.83) 6 and 24 h after pathogen inoculation in the resistant cultivar Jinyou 38 (the data are available via ProteomeXchange; identifier, PXD012903). Some of the early responses to C. cassiicola infection were revealed, and four factors related to the resistance of cucumber plants to TLS were discovered. First, the proteomic approach revealed modulation of signaling pathways in resistant cucumber plants in response to C. cassiicola infection. Second, the plant immune system recognizes the pathogen and initiates expression of immune response proteins, including those related to plant defense, stress response, signal transduction, cell metabolism, and redox regulation. Third, C. cassiicola activates common stress response pathways; in particular, mildew resistance locus O (MLO) proteins were found to play a crucial role in the TLS prevention. Fourth, rapid activation of the carbohydrate and secondary metabolic pathways, modification and reinforcement of cell walls, and adjustment of the apoplastic environment to the highly stressful conditions were crucial in the cucumber resistance to TLS. Overall, our data contribute to the understanding of interactions between plants and their pathogens and provide new insight into molecular processes involved in the resistance of cucumber plants to disease.