Mechanisms Of Melatonin Alleviating Low Temperature Stress And Water Stress In Tomato Plants

Low temperature and drought are two major environmental stress factors,affecting species distribution in nature,limiting plant growth and development,causing huge losses in agriculture and threatening food security throughout the world.Tomato(Solanum lycopersicum)is an important horticultural crop worldwide;however,tomato growth and development is often hampered by abiotic stresses,such as low temperature and drought.Under cold and drought conditions,tomato yields are greatly decreased and the quality is severely deteriorated.Thus,it is of great significance to explore effective methods to improve cold and drought tolerance in tomato plants.It has been recently demonstrated that melatonin plays various roles in the regulation of growth,development and responses to biotic and abiotic stresses in plants.However,little information is available on the roles of melatonin in responses to cold and drought in tomato plants.In the present work,through analyses of photosynthetic capacity,non-photochemical quenching,antioxidant potential,cold-regulated genes,osmolytes,cuticular waxes,osmotic potential,etc.,we investigated the impacts of exogenous melatonin on cold and drought tolerance and explored the underlying mechanisms in tomato plants.The major findings of this work include:1.Melatonin ameliorates cold-induced damages to tomato plants.In the present work,we found that application of exogenous melatonin significantly improved photosynthesis in cold-stressed tomato plants and alleviated cold-induced oxidative stress,as evidenced by low levels of MDA and electrolyte leakage.Further results revealed that exogenous melatonin enhanced activities of antioxidant enzymes and contents of non-enzymatic antioxidants,which contributed to reduced oxidative stress in tomato plants.Moreover,exogenous melatonin induced the expression of Sl SBP,a gene encoding an important Calvin cycle enzyme SBPase,and improved SBPase activity in tomato plants under cold stress.The increased SBPase activity is partially accountable for the improved photosynthetic capacity observed in this study.Finally,exogenous melatonin induced the expressions of cold-related genes and promoted the accumulation of osmolytes,which contributed to the improvement of cold tolerance in tomato plants.2.Melatonin alleviates photoinhibition in tomato plants exposed to light during chilling.In the present work,we found that exogenous melatonin accelerated non-photochemical quenching in tomato plants.Further investigations revealed that exogenous melatonin enhanced the de-expoxidation state of xanthophyll cycle,which was due to increased VDE activity.Additionally,we found that exogenous melatonin increased the expressions of VDE and DHAR as well as the level of VDE cofactor ascorbic acid,consequently improving VDE activity.Thus,the possible mechanism is that melatonin-mediated increases in VDE transcript level and ascorbic acid level contribute to higher VDE activity in tomato seedlings exposed to light during chilling,resulting in an increase in the de-epoxidation state of xanthophyll cycle and the induction of NPQ.Relieved photoinhibition is,at least in part,attributed to higher NPQ in melatonin-pretreated tomato seedlings exposed to moderate light during chilling.3.Melatonin improves drought tolerance in tomato plants.In this study,the results showed that exogenous melatonin significantly enhanced the photosynthetic capacity and relieved drought-induced oxidative stress in tomato plants.Moreover,exogenous melatonin promoted the accumulation leaf cuticular waxes,resulting in retarded non-stomatal water loss in tomato plants.Finally,application of exogenous melatonin increased the osmotic potential gradient between old leaves and young leaves.The increased drought tolerance is ascribed to increased antioxidant capacity,improved cuticular wax accumulation and increased osmotic potential gradient between leaves of different ages.4.The melatonin-mediated increase in antioxidant capacity may involve ABA-dependent pathway.In this work,using an ABA biosynthesis inhibitor fluridone,we investigated the possible link between melatonin and ABA in improving antioxidant capacity in drought-stressed tomato plants.The results showed that both exogenous melatonin and ABA were capable of reducing drought-induced oxidative stress by enhancing activities of antioxidant enzymes and accumulation of non-enzymatic antioxidants.However,feeding tomato plants with an ABA biosynthesis inhibitor,fluridone,significantly reversed the effects of exogenous melatonin on the antioxidant capacity in tomato plants,suggesting that ABA may be involved in the melatonin-mediated antioxidant capacity in tomato plants under drought stress.