Melatonin Stimulates Lipid Remodeling And Contributes To The Maintanence Of Ion Homeostasis In Sweetpotato(Ipomoea Batatas [L.] Lam) Cuttings Under Salinity Stress

Salinity stress is one of the most important abioic stresses affecting crop yield worldwide.Sweetpotato(Ipomoea batatas [L.] Lam)is an important food and industrial crop that ranks seventh in terms of worldwide staple food production(ranks fourth in China).The investigations of the defensive responses in sweetpotato under salinity condition and of the physiological/molecular mechanisms involved in the salt resistance of sweetpotato are important for breeding salt-tolerant cultivars and are also useful to find alternative ways for improving sweetpotato salt tolerance.Plant salt tolerance are usually correlates with the alterations of metabolic process and many primary or secondary metabolites are involved in the regulation of plant salt toelrnace.Our non-target metabonomic data showed that melatonin may plays an important role in the regulation of salt resistance in sweetpotato.In this dissertation,we compared the NaCl stress-triggered alterations of melatonin synthesis-related metabolites concentrations in two sweetpotato cultivars with contrasting salt tolerance(relative salttolerant Xushu 22 and salt-sensitive Xushu 32).In addition,we investigated the effects of exogenous melatonin on the ion homeostasis and lipid remodelling in salt-sensitive Xushu 32.The interrelation between melatonin-altered lipid metabolism and ion homeostasis was discussed.The main results of this dissertation are listed as follows:(1)An obvious difference of melatonin-related metabolites concentrations was observed between the two cultivars under salinity stress.The concentrations of tryptophan,tryptamine,serotonin,N-acetylserotonin and melatonin were significantly enhanced by salinity in root and leaf tissues of Xushu 22.However,the levels of these metabolites in Xushu 32 were not obviously altered by salinity.The concentration of melatonin was significantly lower in Xushu 32 than that of Xushu 22 during the period of salinity stress.(2)Exogenous melatonin markedly decreased the MDA content and electrolyte leakage rate,while obviously enhanced the chlorophyll content and relative water content in salinized Xushu 32 leaves,indicating melatonin-improvement of salinity resistance in salt-sensitive Xushu 32.(3)Exogenous melatonin improved mineral nutritions(K,Ca,Mg,Mn,Fe,Cu,Se)in salinized Xushu 32 and inhibited the massive accumulation of Na~+ in the leaves.The melatonin-imporved K~+ homeostasis was mainly achieved by the enhanced plasma membrane H~+-ATPase activity,which contributes to the maintenance of less depolarized membrane potential and K~+ efflux in salinized Xushu 32.(4)Melatonin treatment altered leaf lipidome in Xushu 32,including the enhanced abundance of phosphatidic acid(PA),phosphatidylcholine(PC),phosphatidylinositol(PI),diacylglycerol(DAG),monogalactosyldiacylglycerol(MGDG)and decreased abundance of triacylglycerol(TAG).Salinity stress markedly lowered membrane lipids abundahcne in Xushu 32 leaves,such as phosphatidylserine(PS),phosphatidyl ethanolamine(PE),PC,phosphatidylglycerol(PG);while significantly enhanced the accumulation of storage lipids,e.g.TAG.Exogenous melatonin almost completely abolished the salt-triggered accumulation of TAG in Xushu 32 leaves.In addition,the abundance of PS and PG was enhanced and the abundance of MGDG was decreased by melatonin in salinized Xushu 32 leaves.(5)Real time PCR data revealed that exogenous melatonin enhanced the expression levels of lipase(SDP1),fatty acid ?-oxidation-related genes(LACS6/LACS6/ACX1/ACX2/ACX4)and energy turnover-related genes(isocitrate lyase: ICL;citroyl synthetase,CSY2)in salinized Xushu 32 leaves.These results indicate that melatonin stimulates TAGs breakdown,FA ?-oxidation and energy turnover in salinized Xu 32 leaves.(6)Inhibition of fatty acid ?-oxidation pathway by DMP caused massive accumulation of lipid droplets(LDs)in NaCl-or NaCl plus melatonin-treated Xushu 32 root and leaf cells.The K~+ content,PM H~+-ATPase activity and the cellular ATP level in NaCl-or NaCl plus melatonin-treated Xushu 32 were all significantly reduced in the presence of DMP.(7)Inhibition of melatonin synthesis by p-CPA caused an obously enhanced accumulation of LDs in salinized Xushu 22 root and leaf cells;while the salt shocktriggered K~+ efflux was significantly enlarged and the salt shock-induced H~+ efflux was markedly reduced in root and leaf cells of Xushu 22.Taken together,this dissertation revealed that endogenous or exogenous melatonin plays an important role in the stimulation of triacylglycerols(TAGs)breakdown,FAs ?-oxidation and energy turnover under salinity condition,and thus contributes to the better energy state,which is required for the maintenance of plasma membrane H~+-ATPase activity and K~+/Na~+ homeostasis in sweetpotato.The results herein are important to understanding the salt tolerance mechanisms in sweetpotato and this dissertation provides a novel insight into the mechanism of melatonin on the regulation of plant salt tolerance.