Root proteomics reveals cucumber 24-epibrassinolide responses under Ca(NO3)2 stress

NO3- and Ca2+ are the main anion and cation of soil secondary salinization during greenhouse cultivation. Brassinosteroids (BRs), steroidal phytohormones, regulate various important physiological and developmental processes and are used against abiotic stress. A two-dimensional electrophoresis gel coupled with MALDI-TOF/TOF MS was performed to investigate the effects of exogenous 24-epibrassinolide (EBL) on proteomic changes in cucumber seedling roots under Ca(NO3)(2) stress. A total of 80 differentially accumulated protein spots in response to stress and/or exogenous EBL were identified and grouped into different categories of biological processes according to Gene Ontology. Under Ca(NO3)(2) stress, proteins related to nitrogen metabolism and lignin biosynthesis were induced, while those related to cytoskeleton organization and cell-wall neutral sugar metabolism were inhibited. However, the accumulation of abundant proteins involved in protein modification and degradation, defence mechanisms against antioxidation and detoxification and lignin biosynthesis by exogenous EBL might play important roles in salt tolerance. Real-time quantitative PCR was performed to investigate BR signalling. BR signalling was induced intracellularly under Ca(NO3)(2) stress. Exogenous EBL can alleviate the root indices, effectively reduce the Ca2+ content and increase the K+ content in cucumber roots under Ca(NO3)(2) stress. This study revealed the differentially expressed proteins and BR signalling-associated mRNAs induced by EBL in cucumber seedling roots under Ca(NO3)(2) stress, providing a better understanding of EBL-induced salt resistance in cucumber seedlings. The mechanism for alleviation provides valuable insight into improving Ca(NO3)(2) stress tolerance of other horticultural plants.