Exogenous potassium phosphite application improved PR-protein expression and associated physio-biochemical events in cucumber challenged by Pseudoperonospora cubensis

In the present study, the effect of potassium phosphite on Pseudoperonospora cubensis-inoculated cucumber plants was investigated. Different defense-related enzymes including laccase, polyphenoloxidase and glucanase as well as total protein and lignin contents were analyzed. Anatomical alterations in plant tissues were analyzed using a light microscope. Expression changes in major pathogenesis-related genes were studied at different time courses. The highest expression of glucanase was observed in pre-inoculated plants (97% higher than control) at 24 h with 97% increase compared to the control plants, while Chitinase transcripts were accumulated at a maximum level in potassium phosphite-treated plants 96 h after inoculation with 93% increase over control plants. Analysis of polygalacturonase inhibitor proteins gene expression revealed a transcription peak (96% increase over control plants) 48 h after inoculation. The potassium phosphite-treated plants exhibited an increase in β-1,3-glucanase (82%) enzymatic activity as well as total protein (53%), polyphenoloxidase (21%), laccase (11%) and lignin contents (15%) in comparison to the control. The results of the anatomical assay showed an increase in the vascular bundle diameter in potassium phosphite-treated plants (174 μm) and a decrease in pathogen-treated leaves (66 μm) compared with the control (100 μm). It can be suggested that potassium phosphite treatment induced higher expression of plant defense genes and increased laccase and polyphenoloxidase activities which in turn enhanced lignin deposition in plant tissues. The findings of the present study would be implemented for designing a controlling program to decrease the adverse effect of Pseudoperonospora cubensis on cucumber plants.