A comparative cell wall proteomic analysis of cucumber leaves under Sphaerotheca fuliginea stress

Powdery mildew, caused by Sphaerotheca fuliginea (S. fuliginea), is the most devastating disease that hampers cucumber plants cultivation and productivity. Cell wall proteins (CWPs) play a crucial role in response to biotic stress as a frontline defense of plants. In this work, we present a comparative cell wall proteomic approach to explore differentially expressed proteins in both highly resistant and highly susceptible cucumber leaves after 24 h of exposure to S. fuliginea. After extraction conducted by a destructive procedure with salts, glucose-6-phosphate dehydrogenase (G6PDH) activity and SDS-PAGE assessments were performed to determine the cytosolic contamination. Label-free quantitative proteomics approach was used to gain a comprehensive understanding of differentially regulated CWPs between the two lines after S. fuliginea inoculation. Among more than 200 proteins identified, 71 were significantly altered between the two lines. Most of these identified proteins were predicted to be CWPs except some classical cytosolic proteins. These differentially expressed CWPs belonged to different functional categories including defense, metabolism, redox regulation and cell wall arrangement. The expression levels of seven proteins selected were determined using RT-PCR. We found that resistant cucumber line is believed to start a series of disease-resistant mechanisms against pathogen. This study provides useful information on cell wall proteomic changes between a resistant and a susceptible genotype under infected conditions.