Interactive effects of melatonin and cytokinin on alleviating drought-induced leaf senescence in creeping bentgrass (Agrostis stolonifera)

Drought stress induces leaf senescence while cytokinins (CK) and melatonin (ME) play roles in regulating leaf senescence. The objectives of this study were to determine whether ME alone or interacting with CK could suppress drought-induced leaf senescence through mediating CK pathways and chlorophyll catabolism in creeping bentgrass (Agrostis stolonifera). Plants of wild type ('WT') (cv. 'Penncross') or a transgenic line ('S41') harbouring isopentenyltransferase (ipt) gene were foliar sprayed with either 20 mu M ME or water and exposed to drought stress for 14 d in growth chambers. Exogenous applied ME increased visual quality, photochemical efficiency, chlorophyll content and relative water content, while it suppressed leaf electrolyte leakage, lipid peroxidation and hydrogen peroxide production in both WT and ipt-transgenic plants. Cytokinin-signaling and synthesis genes were also significantly up-regulated in ME-treated ipt-transgenic plants. Dehydration responsive genes (JUBI and DREB2A) were up-regulated and chlorophyll-degradation genes (Chlase, PPH and Chl-PRX) and enzyme activities were significantly down-regulated by ME application. Endogenous melatonin content and ME biosynthesis genes (TDC1, SNAT1, and COMT) expression increased in ipt-transgenic plants with ME treatment. ME-alleviation of drought-induced leaf senescence in creeping bentgrass was associated with the down-regulation of chlorophyll catabolism and the synergistically interaction with CK-synthesis gene and signaling pathways.