Regulation Effect And Mechanism Of Melatonin On Salt Tolerance Of Rice Seedlings

The world population growth has brought great pressure on food production.It is estimated that 50%of the world’s main cultivated land will be salinized by 2050,and the rice production must double in the same period to meet the growing population demand.However,20%of the rice growing areas have serious salt related problems,which poses a great threat to rice production.On the other hand,planting rice in saline alkali soil can significantly reduce soil salinity and increase the content of organic matter;it also has the advantages of simple method,no impact on agricultural production and significant improvement effect,which makes rice become the pioneer crop in saline alkali soil improvement.Rice is a salt sensitive crop,and its seedling stage is very sensitive to salt;at the same time,the plant height of rice seedling stage is low,which requires strict water management.After transplanting,shallow water irrigation is needed,and it is difficult to implement water conservancy measures such as use water to restrain salt in the field,which will lead to the occurrence of salt stress.Salt stress can affect physiological and biochemical activities of rice at seedling stage,leading to growth retardation,tiller reduction,and then affect the number of panicles at maturity stage and reduce yield,so it is very important to improve salt tolerance of rice at seedling stage.Melatonin（MT）is involved in the regulation of plant growth and development,and plays an important role in plant stress response.Our previous study found that using MT to treat rice at seedling stage can improve rice yield in saline alkali soil by increasing effective panicles at maturity stage,but the specific regulation mechanism is not clear.In this study,the conventional japonica rice Ningjing 7 was used as the experimental material.Different concentrations of MT treatment and MT synthesis inhibitor treatment were set,and 150 m M Na Cl stress was applied to rice.The physiological mechanism of MT regulating salt tolerance in rice seedling stage was comprehensively expounded by measuring the related indexes of rice growth,photosynthesis,antioxidant system and ion homeostasis under salt stress.The main results are as follows1.The regulation and mechanism of MT on photosynthesis of rice under salt stressMT spraying increased the growth of rice under salt stress,mainly manifested as higher dry weight,fresh weight and relative water content.MT spraying also reduced the mortality of rice seedlings under salt stress,and increased the salt tolerance index of rice seedlings.MT treatment increased dry matter accumulation,relative water content,sucrose and starch content of rice under salt stress.This was mainly owing to the fact that melatonin enhanced the net photosynthetic rate and enhanced the absorption and transmission of light energy.The effect of melatonin on photosynthesis of rice under salt stress conditions was mainly due to the regulation of three processes:maintaining low ROS status by improving the total antioxidant capacity,promoting the xanthophyll cycle and increasing the xanthophyll pool size to dissipate excess light energy,increasing the activities of key photosynthetic enzymes.Taken together,these results provide a mechanism for melatonin to improve the photosynthetic capacity of rice under salt stress.3.MT regulates the antioxidant mechanism of rice in response to continuous salt stressMT can reduce the inhibition of salt stress on seedling growth and membrane damage.During the whole salt stress period,rice after melatonin priming showed lower ROS accumulation.In the early stage（1-3 d）of stress,the rice after melatonin priming showed a strong increase in antioxidant enzyme activity,while in the later stage（5,7 d）,it showed a strong increase in antioxidant content.During the whole period of salt stress,melatonin priming had a weak regulatory effect on As A-GSH cycle.Through the above regulation process,the decreasing effect of melatonin priming on ROS content of rice under salt stress did not decrease with prolonged stress time in a short time（1-7d）.In conclusion,melatonin priming improved the antioxidant capacity of rice under continuous salt stress,and rice showed variable antioxidant strategies after melatonin priming.4.MT regulates Na⁺/K⁺homeostasis of rice under salt stress by mediating NOThe results showed that MT inhibited the increase of Na⁺content and the decrease of K⁺content caused by salt stress,and increased the ratio of K⁺/Na⁺.MT increased the K⁺selective absorption index of rice under salt stress,indicating that MT pretreatment improved the K⁺absorption ability and Na⁺exclusion ability of rice.The results of root Na⁺/K⁺fluxes also confirmed this.MT treated rice showed strong root Na⁺efflux and K⁺influx after salt stress.As signal molecules,the content of NO increased and the content of H₂O₂ decreased in MT treated rice roots;and the response of ion current to NO and H₂O₂ was more sensitive in MT treated rice roots,which showed that Na⁺efflux and K⁺influx were stronger after being impacted by NO and H₂O₂.In addition,we also explored the role of NO in MT regulation of salt tolerance in rice.MT can increase the activity of NOS.The content of polyamine was also increased,which increased the NO content of rice seedlings under salt stress.Pharmacological studies have shown that NO,as an essential downstream signal molecule,is involved in the regulation of K⁺/Na⁺homeostasis by MT in rice.Under salt stress,MT increased the proton pump activity of plasma membrane and vacuolar membrane of rice roots,and increased the ATP content of rice roots by increasing the content of NO.Therefore,the outflow of Na⁺and the influx of K⁺are promoted.When endogenous NO was removed,the regulation of K⁺/Na⁺homeostasis by MT was blocked.Therefore,MT mediates K⁺/Na⁺homeostasis in rice under salt stress by mediating NO.In conclusion,this study improved the growth of rice under salt stress by applying MT,clarified the physiological mechanism of MT improving the photosynthetic capacity and antioxidant capacity of rice under salt stress,and clarified that MT regulating the ion homeostasis of rice under salt stress through mediating NO.These studies elucidated the mechanism of MT regulating rice salt tolerance,and provided technical guidance and theoretical basis for rice cultivation under salt stress.