Exploration of Sweet Immunity to Enhance Abiotic Stress Tolerance in Plants: Lessons from CAM

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The concept of ‘sweet immunity’ or ‘sugar-enhanced defence’ is based on the accumulating evidence that sweet, endogenous saccharides might act as signalling molecules that are activated by exposure to stress and hence initiate signal amplification and lead to more rapid and robust activation of defence, immunity and stress tolerance. Sugars such as glucose, fructose and sucrose have acquired important regulatory functions in evolution and are becoming more and more recognized as signalling molecules in plants controlling gene expression related to plant metabolism, stress resistance and development. This offers opportunities for ‘sweet priming’, defined as a physiological process that prepares plants for a faster and/or stronger defence response to future stress conditions, but does not impose the costs associated with full implementation of an induced defence response. Future possibilities to substitute toxic agrochemicals with biodegradable sugar-(like) compounds in agricultural and horticultural practice requires a thorough understanding of how sugars can play a crucial role in perceiving, anticipating and counteracting abiotic stresses. In this review, the physiological responses of crassulacean acid metabolism (CAM) plants to different conditions of abiotic stress will be discussed with particular attention to sucrose dynamics. CAM plants are ideally suited to different abiotic stress conditions and carbohydrate cycling and availability are of paramount importance for plant growth, photosynthesis and homeostasis. By evaluating the plethora of effects sugars can exert on plant metabolism, growth and development the possibilities for sugars as potential priming agents to enhance abiotic stress tolerance will be explored.