Physiological Mechanisms Of Exogenous Melatonin In Alleviating Abiotic Stress In Tea Plants

Tea plant(Camellia sinensis(L.)O.Kuntze)is a perennial evergreen woody plant that is susceptible to low temperature,high temperature,drought,salinity and other abiotic stresses,which seriously affect the yield and quality of tea.Therefore,finding effective ways to improve the stress tolerance of tea trees not only contributes to the promotion and cultivation of tea trees,but also increases economic benefits.It is also of great significance for the selection of new varieties of tea plant.Exogenous substances have been widely used to improve plant stress resistance and crop yield.One of the most studied exogenous substances is melatonin(MT),which is widely used to help plants cope with stress resistance.In this study,Two-year-old vegetatively propagated cuttings of tea plant(C.sinensis cv.’Longjing 43’)were used as materials.Using the method of nutrient solution hydroponics experiment,tea plants with the same growth were selected for stress treatment.Physiological,molecular and metabolic methods were used to explore the possible mechanism of MT in response to abiotic stress in tea plants.The main results of the study are as follows:(1)The melatonin treatment groups were sprayed with melatonin(Sigma-Aldrich,St.Louis,MO,USA)concentrations(5,50,100,500 and 1000 μM).Decreasing chlorophyll contents and increasing malondialdehyde levels in leaves were observed in tea plants treated with chilling at 4℃.Melatonin-treated plants led to high levels of soluble protein and soluble sugar compared with the control.The addition of MT also enhanced the scavenging ability of reactive oxygen species(ROS)by increasing the activities of antioxidant enzymes including superoxide dismutase(SOD),catalase(CAT),and peroxidase(POD).(2)Moreover,melatonin also stimulates antioxidant enzymes and augmenting antioxidants involved in the ASA-GSH cycle under cold stress.It was observed that there is increased the contents of reduced glutathione(GSH),decreased the contents of oxidized glutathione(GSSG),increased the ratio of GSH/GSSG,and increased the activity of ascorbate peroxidase(APX)and glutathione reductase(GR)in tea leaves after melatonin treatments.Moreover,melatonin regulated the expression of genes involved in GSH metabolism.CsGSHS,CsAPX and CsGR were substantially up-regulated in melatonin-pretreated leaves,which possibly contributed to preserve the contents of GSH and maintain the higher ratio of GSH/GSSG.Exogenous MT improved the cold resistance of tea plant,and the best treatment effect was 100 μM.(3)Eight groups were set up as control(CK),CK+MT,low temperature(4 C),low temperature+MT,drought(Drought),drought+MT,high salt(NaCl),high salt+MT.Tea seedlings under low temperature,drought and high salt stress were treated with 100 μM MT,and Leaf samples(mature leaves)were harvested at 0,12,24 and 48 hours after stress treatment.The results showed that under the abiotic stress,the maximum photochemical efficiency(Fv/Fm)of tea leaves decreased significantly,and hydrogen peroxide(H2O2),superoxide anion(O2-)and MDA accumulated excessively,which caused cell membrane damage and increased ROS in tea plants.However,MT treatment effectively alleviated the damage of photosystem Ⅱ under stress,inhibited the production of H2O2 and O2-and reduced the content of MDA in tea plants.In addition,exogenous MT could significantly enhance the activities of antioxidant enzymes(SOD,CAT,POD and APX)in tea plants and increase the content of non-enzymatic antioxidants GSH and ascorbic acid(ASA).At the transcriptional level,MT significantly up-regulated the expression of CsSOD,CsCAT,CsPOD,and CsAPX genes.The results showed that the acclimation of abiotic stress improved the stress resistance of tea plants.Exogenous MT played an active role in maintaining cell membrane stability,maintaining photosynthesis efficiency,removing excessive ROS,improving antioxidant enzyme activity and regulating the expression of related genes,which could protect tea plants from abiotic stress.In conclusion,exogenous application of MT enhanced the tolerance of tea seedlings to abiotic stress in a dose-dependent manner.It is expected to provide a new idea for tea production and planting,and provide a theoretical basis for the application of melatonin,a new stress-resistant regulator,in stress production.