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Acacia farnesiana is an arsenic (As)-tolerant shrub, widely distributed in the arid zones of Mexico. Endosymbiotic plant-bacteria associations can modify As-tolerance and its bioaccumulation in plants. Both responses are related to mechanisms that stimulate antioxidant defense and compartmentalization of As, which are mediated by changes in protein abundance. Proteomic approaches allow the characterization of key proteins involved in stress-tolerance and endosymbiosis. With the aim of assessing the influence of an endosymbiotic plant-bacteria association on As-tolerance and bioaccumulation by A. farnesiana, 15-day-old plants associated, or not associated, with Methylobacterium sp. were exposed to 580 mu M arsenate for 15 days; shoots were analyzed for As concentration, physiological changes, and differential protein abundance. Compared to endophyte-free shoots under As-stress, Methylobacterium sp. increased the As concentration from similar to 600 mg kg(-1) (dry weight) to similar to 1700 mg kg(-1) without reduction in biomass production, and enhanced the protein, chlorophyll and glutathione (GSH) concentration. Proteomic analysis evidenced an As-mediated metabolic disorder involving energy and carbon metabolism, which was alleviated by bacteria. Methylobacterium sp. contributes to the plant fitness in the presence of a phytotoxic arsenic concentration, through an improvement in the maintenance of both energy and redox homeostasis. The results indicate a central role of glutathione in the As-tolerance, and the participation of plant proteins, such as a translation inhibitor and proteins involved in protein folding (immunophilins), which were upregulated by the bacterium. Endosymbiosis contributes to strengthen the plant antioxidant systemunder stress conditions, and provide the energy requirement that could help cope with arsenic. This association can improve the effectiveness of phytoremediation strategies through the enhanced tolerance and accumulation of arsenic. (C) 2017 Elsevier B.V. All rights reserved.