other hand, catabolism, such as nucleic acid breakdown and proteolysis, becomes active through induction of a number of hydrolytic enzymes (Matile, 1992; Noodé n, 1988; Summary Smart, 1994;Thiman, 1980;Thomas and Stoddart, 1980). Leaf senescence, although a deteriorative cellular process, Four mutants that show the delayed leaf senescence phenotype were isolated from Arabidopsis thaliana.is assumed to be an evolutionarily acquired, active genetic trait that makes an important contribution to fitness of Genetic analyses revealed that they are all monogenic recessive mutations and fall into three complementation plants, for example by remobilizing nutrients from vegetative tissues to reproductive organs (Matile, 1992; Noodé n, groups, identifying three genetic loci controlling leaf senescence in Arabidopsis. Mutations in these loci cause delay in Thiman, 1980;Thomas and Stoddart, 1980). Elucidating the genetic mechanism of leaf senescence is essential all senescence parameters examined, including chlorophyll content, photochemical efficiency of photosystem II, to understanding the senescence phenomenon itself and also for practical purposes such as improvement of plant relative amount of the large subunit of Rubisco, and RNase and peroxidase activity. Delay of the senescence productivity, pre-or post-harvest storage, and stress tolerance. However, despite the biological and practical import-symptoms was observed during both age-dependent in planta senescence and dark-induced artificial senescence ance of leaf senescence, the genetic mechanism controlling the leaf senescence process remains poorly understood. in all of the mutant plants. The results indicate that the three genes defined by the mutations are key geneticWe therefore undertook a systematic genetic screening to identify the genes that control leaf senescence, using elements controlling functional leaf senescence and provide decisive genetic evidence that leaf senescence is aArabidopsis thaliana as a model system. genetically programmed phenomenon controlled by several monogenic loci in Arabidopsis. The results further Results suggest that the three genes function at a common step of age-dependent and dark-induced senescence processes.Isolation of Arabidopsis mutants with delayed leaf

a small crucifer Arabidopsis thaliana has emerged as the Vitaly Citovsky* plant of choice for most genetic and developmental studies.