iTRAQ-based comparative proteomic analysis reveals tissue-specific and novel early-stage molecular mechanisms of salt stress response in Carex rigescens

Proteomic changes for early-stage responses to salt stress are largely unknown in Carex rigescens, a stress-tolerant turfgrass that originates in northern China. In this study, we investigated early-stage molecular mechanisms of C. rigescens under salt stress using isobaric tags for relative and absolute quantitation (iTRAQ) and mass spectrometry to characterize proteome-level changes. The relative water content, membrane stability (shown as relative electrolyte leakage), and contents of Ca2+, K+, and Na+ were significantly altered after 12 h of 300 mM NaCl treatment, indicating that C. rigescens adjusted its physiological performance in order to cope with salt stress. The comparative proteomic analysis was conducted after 12 h of salt stress treatment, and a total of 9258 proteins were identified using the UniProt database. Among them, 1893 proteins were differentially expressed after salt stress treatment, including 726 up-regulated and 1209 down-regulated proteins. Furthermore, we found 897 (47.4%) differentially expressed proteins (DEPs) uniquely expressed in leaf, and 900 (47.5%) uniquely expressed in root. The proteomic results were in accordance with the verification analysis by qRT-PCR and parallel reaction monitoring (PRM) assays. The functional analysis showed that these DEPs were involved in carbohydrate and energy metabolism, photosynthesis and the electron transport chain, signal sensing and transduction, protein synthesis, secondary metabolism, and stress defense response. Specifically, the glycolysis process, oxidative phosphorylation, and Ca2+ related signaling pathways in roots strengthened for energy generation and signal transduction; photosynthesis and hormone signal transduction were more active in leaves for light harvesting and hormone regulation; and protein synthesis and secondary metabolism-related processes changed with different patterns in tissues. The expression profiles of the DEPs demonstrated that the early-stage salt response was regulated diversely between leaf and root in C. rigescens. Moreover, ketosphingosine reductase (KRSD) and sodium-coupled neutral amino acid transporter-2 (SNAT2) were also responsive to salt stress, highlighting their roles in future research about salt tolerance in C. rigescens and other plants. Our study highlighted physiological changes in C. rigescens under salt stress, and used comparative proteomic analysis to investigate responses to salt stress in leaf and root tissues. Our results contribute to existing knowledge on the complexity of the salt stress response in plant tissues, and also provide a basis for further study of the mechanism underlying salt response and tolerance in C. rigescens and other plant species.