| Leaf senescence is a genetically controlled developmental process that is believed to be driven by expression of senescence-associated genes ( SAGs). Thus far, only a limited number of SAGs have been isolated, and their functions are unproved. In order to fully understand the biochemical components and molecular mechanisms underlying leaf senescence, it is necessary to identify and characterize additional SAGs. In this study, a large-scale enhancer trap approach was employed to identify SAGs in Arabidopsis. By screening 1300 Arabidopsis enhancer trap line, 147 senescence enhancer lines (Sels) were found to show expression of a reporter gene (GUS) in senescing leaves but not in non-senescing ones. The SAGs, represented by these Sels, were differentially regulated by jasmonic acid, ethylene, abscisic acid, brassinosteroids, darkness and dehydration, which revealed that there are multiple senescence-promoting pathways that are interconnected to form a regulatory network.;A role of JA in leaf senescence is uncertain. This study provided three lines of evidence to support JA's involvement in leaf senescence. First, exogenous JA application induced leaf senescence and senescence-specific marker genes in Arabidopsis. Second, genes involved in JA biosynthesis were found up-regulated during leaf senescence. Last, JA level in senescing leaves was found to be higher than in non-senescing leaves.;A JA-regulated SAG was isolated from one of the Sels (Sel69). This gene encodes a 12-oxophytodienoic acid-10,11-reductase (OPR1). Two regulatory cis elements, namely JASE1 (5' -CGTCAATGAA-3'; a new motif) and JASE2 (5 '-CATACGTCGTCAA-3'; a mixed A/C box motif), in the promoter region of this gene were identified. Both elements are involved in the regulation of this gene by JA and leaf senescence.;JA biosynthesis is thought to be limited by the availability of its precursor alpha-linolenic acid (LA). LA is released from membrane glycerolipids by lipolytic acyl hydrolases. A SAG encoding an acyl hydrolase (SAG 101) was cloned from Sel139. Recombinant SAG101 fusion protein overexpressed in E. coli possesses acyl hydrolase activity. Antisense RNA interference of SAG101 delayed the onset of leaf senescence whereas inducible overexpression of this gene caused precocious leaf senescence in transgenic Arabidopsis. |
