Die and let live: leaf senescence contributes to plant survival under drought stress.

Leaf senescence is a highly regulated physiological process that leads to leaf death and is, as such, the last developmental stage of the leaf. Plant aging and environmental stresses may induce the process of senescence. Here we will focus on the role of leaf senescence in field-grown plants as a response to adverse climatic conditions and, more specifically, on how it contributes to plant survival under drought stress. Drought induces several responses in plants including leaf senescence, which plays a major role in the survival of several species. Drought-induced leaf senescence contributes to nutrient remobilisation during stress, thus allowing the rest of the plant (i.e. the youngest leaves, fruits or flowers) to benefit from the nutrients accumulated during the life span of the leaf. In addition, drought-induced leaf senescence, especially when accompanied by leaf abscission, avoids large losses through transpiration, thus contributing to the maintenance of a favourable water balance of the whole plant. Drought-induced leaf senescence occurs gradually and is characterised by specific macroscopic, cellular, biochemical and molecular changes. Leaf yellowing (i.e. chlorophyll degradation) and specific changes in cell ultrastructure (e.g. chromatin condensation, thylakoid swelling, plastoglobuli accumulation), metabolism (e.g.protein degradation, lipid peroxidation) and gene expression occur during leaf senescence in drought-stressed plants. Cytokinins and ABA have been shown to be involved in the regulation of drought-induced leaf senescence, although the possible role of other plant hormones should not be excluded. Reactive oxygen species, whose concentrations increase during drought-induced leaf senescence, are also known to be regulators of this process. The complex mechanisms of regulation of leaf senescence in drought-stressed plants are discussed, and attention is drawn to those aspects that still require investigation.