SummaryLeaf senescence is a form of programmed cell death, and is believed to involve preferential expression of a speci®c set of`senescence-associated genes' (SAGs). To decipher the molecular mechanisms and the predicted complex network of regulatory pathways involved in the senescence program, we have carried out a large-scale gene identi®cation study in a reference plant, Arabidopsis thaliana. Using suppression subtractive hybridization, we isolated approximately 800 cDNA clones representing SAGs expressed in senescing leaves. Differential expression was con®rmed by Northern blot analysis for 130 non-redundant genes. Over 70 of the identi®ed genes have not previously been shown to participate in the senescence process. SAGencoded proteins are likely to participate in macromolecule degradation, detoxi®cation of oxidative metabolites, induction of defense mechanisms, and signaling and regulatory events. Temporal expression pro®les of selected genes displayed several distinct patterns, from expression at a very early stage, to the terminal phase of the senescence syndrome. Expression of some of the novel SAGs, in response to age, leaf detachment, darkness, and ethylene and cytokinin treatment was compared. The large repertoire of SAGs identi®ed here provides global insights about regulatory, biochemical and cellular events occurring during leaf senescence.