Regulation of Senescence by Sugar Metabolism

Research objectives a. Analyze transgenic plants that undergo rapid senescence due to increased expression of hexokinase. b. Determine if hexokinase-induced senescence accelerates natural senescence using senescence specific promoters that drive expression of a reporter gene (GUS) and a cytokinin producing gene (IPT - isopentyl transferase). c. Isolate and analyze plant genes that suppress sugar-induced cell death (SICD) in yeast, genes that potentially are involved in programmed cell death and senescence in plants. Background to the topic Leaf senescence is a regulated process of programmed cell death (PCD) in which metabolites are recycled to other active parts of the plant. Senescence associated genes (SAGs) are expressed throughout leaf senescence. Sugar flux and metabolism is thought to playa fundamental regulatory role in senescence. We found that transgenic tomato plants with high hexokinase activity, the initial enzymatic step of sugar (hexose) metabolism, undergo rapid leaf senescence, directly correlated with hexokinase activity. These plants provide a unique opportunity to analyze the regulatory role of sugar metabolism in senescence, and its relation to cytokinin, a senescence-inhibiting hormone. In addition, we found that sugar induces programmed cells death of yeast cells in direct correlation to hexokinase activity. We proposed to use the sugar induced cell death (SICD) to isolate Arabidopsis genes that suppress SICD. Such genes could potentially be involved in senescence induced PCD in plants. Major conclusions The promoters of Arabidopsis senescence-associated genes, SAG12 and SAGI3, are expressed in senescing tomato leaves similar to their expression in Arabidopsis leaves, indicating that these promoters are good senescence markers for tomato plants. Increased hexokinase activity accelerated senescence and induced expression of pSAG12 and pSAG13 promoters in tomato plants, suggesting that sugar regulate natural senescence via hexokinase. Expression of IPT, a cytokinin producing gene, under pSAG12 and pSAG13 promoters, delayed senescence of tomato leaves. Yet, senescence accelerated by hexokinase was epistatic over cytokinin, indicating that sugar regulation of senescence is dominant over the senescence-inhibiting hormone. A gene designated SFP1, which is similar to the major super family monosaccharide transporters, is induced during leaf senescence in Arabidopsis and may be involved in sugar transport during senescence. Accordingly, adult leaves accumulate sugars that may accelerate hexokinase activity. Light status of the entire plant affects the senescence of individual leaves. When individual leaves are darkened, senescence is induced in the covered leaves. However, whole adult plant placed in darkness show delayed senescence. In a search for Arabidopsis genes that suppress SICD we isolated 8 cDNA clones which confer partial resistance to SICD. One of the clones encodes a vesicle associated membrane protein - VAMP. This is the first evidence that vesicle trafficking might be involved in cell death. Implications Increased hexokinase activity accelerates senescence. We hypothesized that, reduced hexokinase activity may delay senescence. Preliminary experiments using a hexokinase inhibitor support this possible implication. Currently we are analyzing various practical approaches to delay leaf senescence via hexokinase inhibition. .