Sugar homeostasis in leaves during senescence and abiotic stress: role of cell wall polysaccharides

Historically leaf senescence, natural or stress-induced, was not taken seriously by plant physiologists. However, it is directly associated with plant productivity and nutrient economy. The complete execution of senescence and stress response is energy dependent. On the other hand, photosynthesis declines during the senescence process that limits supply of sugar to the leaf cells. Under sugar starvation environment, the cells search for alternative sources of sugar. Of course, the breakdown of cell deposits like starch, lipids and proteins provide the respiratory sugar at initial phase. However, these sources are likely to exhaust during the last phase of leaf senescence and the leaves experiencing stress. The possible role of cell wall polysaccharides in the maintenance of the sugar homeostasis during the late phase of leaf senescence and abiotic stresses is not much explored. The review will focus on the possible role of cell wall hydrolases that participate in the breakdown of cell wall polysaccharides and provide respiratory sugar, maintain cellular sugar homeostasis to execute and complete both senescence programme and stress response. The review will also emphasize on the loss of photosynthesis and, consequently loss of cellular sugar that possibly worked as the signal for induction and enhancement of the wall hydrolases to provide sugar to energy starved cells. Different groups of cell wall hydrolases such as exopolysaccharides, endopolysaccharides, and other hydrolases are involved in the coordinated loosing and the degradation of cell wall polysaccharides.