Physiological and iTRAQ based proteomics analyses reveal the mechanism of elevated CO concentration alleviating drought stress in cucumber (Cucumis sativus L.) seedlings.

Carbon dioxide is one of the most important anthropogenic greenhouse gases. We previously confirmed that elevated [CO] alleviated the negative consequences of drought stress to cucumber seedlings, but the physiological mechanism remains unknown. We investigated the morphological and physiological characteristics as well as iTRAQ-based proteomics analyses in this study under different combinations [CO] (400 and (800 ± 20) μmol·mol) and water conditions (no, moderate and severe drought stress simulated by polyethylene glycol 6000). The results showed: (1) elevated [CO] significantly increased plant height, stem diameter, leaf area and relative water content (RWC) under drought stress; (2) drought stress significantly increased J and K peaks of the chlorophyll a fluorescence transient, indicating the damage of photosynthetic electron transport chain, while elevated [CO] decreased them especially under moderate drought condition; (3) iTRAQ-based proteomics analyses indicated that elevated [CO] increased the abundance of psbJ and the PSI reaction center subunit VI-2 in seedlings exposed to moderate drought stress; (4) the abundance of uroporphyrinogen decarboxylase 2 and tetrapyrrole-binding protein decreased in response to elevated [CO] under severe drought condition; (5) elevated [CO] regulated the expression of chloroplast proteins such as those related to stress and defense response, redox homeostasis, metabolic pathways. In conclusion, elevated [CO] enhanced the efficiency of photosynthetic electron transport, limited the absorption of excess light energy, enhanced the ability of antioxidant and osmotic adjustment, and alleviated the accumulation of toxic substances under drought stress. These findings provide new clues for understanding the molecular basis of elevated [CO] alleviated plant drought stress.