The role of sugars in integrating environmental signals during the regulation of leaf senescence

Although leaf senescence results in a loss of photosynthetic carbon fixation, the senescence-dependent release of nutrients, especially of nitrogen, is important for the growth of young leaves and for reproduction. Environmental regulation of senescence is therefore a vital factor in the carbon and nitrogen economy of plants. Leaf senescence is a highly plastic trait that is affected by a range of different environmental factors including light, nutrient supply, CO2 concentration, and abiotic and biotic stress. In this review, the focus is on the impact of environmental conditions on sugar accumulation and sugar signalling during senescence. By signalling a high availability of carbon relative to nitrogen in the old leaves, sugar accumulation can trigger leaf senescence. Sugar-induced senescence is therefore particularly important under low nitrogen availability and may also play a role in light signalling. Whether or not sugars are involved in regulating the senescence response of plants to elevated CO2 remains unresolved. Senescence can be delayed or accelerated in elevated CO2 and no clear relationship between sugar accumulation and senescence has been found. Plasticity in the response to environmental factors, such as daylength and sugar accumulation, varies between different Arabidopsis accessions. This natural variation can be exploited to analyse the genetic basis of the regulation of senescence and the consequences for growth and fecundity. Different evolutionary strategies, i.e. early senescence combined with a high reproductive effort or late senescence combined with a low reproductive effort, may be an important adaptation of Arabidopsis accessions to their natural habitat.