iTRAQ-based quantitative proteomics analysis of cold stress-induced mechanisms in grafted watermelon seedlings

Rootstock grafting can improve the cold tolerance of watermelon. However, the molecular mechanisms underlying this process remain unknown. Herein, we used an isobaric tag for relative and absolute quantification (iTRAQ)-based quantitative proteomics approach for the comparative analysis of protein abundances in self-grafted (SG) and pumpkin rootstock-grafted (RG) watermelon seedlings in response to cold stress. A total of 4796 distinct proteins were identified, and 752 proteins were significantly differentially accumulated in grafted watermelon seedling leaves after 48 h cold stress. Based on bioinformatics analysis, the cold tolerance of RG watermelon seedlings might be related to more energy produced through photosynthesis, carbon metabolism, and oxidative phosphorylation, compared with that of SG watermelon seedlings. RG watermelon seedlings could cope with cold stress by improving the scavenging capacity of ROS and arginine biosynthesis. Posttranscriptional regulation and protein homeostasis also play important roles for grafted watermelon seedlings to adapt to cold stress. Several protein kinases involved in signal transduction may act as positive regulators in RG watermelon seedling leaves suffering from cold stress. In addition, iTRAQ data were confirmed to be reliable by the assays of physiological indicators and relative transcript levels of eight genes. Biological significance: Rootstock grafting is regarded as an effective method to enhance the cold tolerance of watermelon seedlings. To elucidate the cold tolerance mechanism of grafted watermelon, an iTRAQ-based quantitative proteomics approach combined with bioinformatics analysis was employed to identify differentially accumulated proteins in SG and RG watermelon seedlings between cold stress and control conditions. This study provided additional insight into the molecular basis of the grafted watermelon seedlings in response to cold stress.