Leaf senescence: Physiology and molecular biology

Leaf senescence although deteriorative in nature, has been recognized as th e last phase of the organ's development,a highly ordered process regulated by genes-known as senescence associated genes (SAGs), Till now, more than 3 0 SAGs have been isolated, cloned and characterized. The leaf when young an d mature,accumulates nutrients and exports them to growing,parts of the pla nt during senescence.:The macromolecular degradation and their transport du ring senescence are reported to be strictly controlled by genes, The genes are also reported to actively participate in energy metabolism and supply m etabolic energy for the transport of nutrients. Through genetic regulation, the senescing leaves maintain cellular integrity and potential not only fo r nutrient:transport, but also for effective transcription and-translation of proteins. Although the genes specific for induction of senescence have n ot yet been precisely identified, the down-regulation of photosynthetic gen es has been proposed to be the possible signal for:up-regulation of SAGs an d induction of senescence, Leaf senescence is recognized as a process follo wing a programmed cell death (PCD), The regulatory elements of some of the SAGs are characterized and their response to senescence inducing factors in dicates scope for further studies on the molecular mechanism of signal resp onse coupling during foliar senescence.