Physiological And Molecular Bases Of Exogenous Melatonin Improving Drought Resistance In Maize Seedings

Maize is the principal grain crop in Shaanxi Province,while drought has severely restricted the growth and yield of maize in this area.Studies hold that exogenous melatonin can effectively alleviate the growth inhibition induced by drought and improve the resistance of maize to drought stress,but the molecular and physiological mechanisms by which exogenous melatonin mediates drought tolerance in maize is still largely unknown.In the present study,the maize cultivar?shaandan 609‘was used as the experimental materials for pot experiments,and the maize seedlings at the seven-leaf stage were subjected to drought stress and foliar spraying of 100μmol L^-1 melatonin.The experiment contained four treatments,well water（CK）,melatonin treatment（MT）,drought stress（DS）and drought stress plus melatonin treatment（DS＿MT）.The changes of growth characteristics,photosynthetic capacity,leaf stomatal behavior,contents of metabolites related to carbon and nitrogen metabolism,activities of key enzymes in carbon and nitrogen metabolism and leaf transcriptome of different treatments were measured.The current study aimed at elucidating the physiological responses and its molecular mechanisms of melatonin in the drought tolerance of maize plants by a combination of physiology and transcriptome analyses.We hope to explain the reason why melatonin improves drought tolerance from a comprehensive point of view,and provide a theoretical basis and technical support for the application of melatonin in crop production.The main results obtained are as follows:1.Exogenous melatonin significantly promoted photosynthesis and plant growth of maize seedlings under drought stress.Under drought stress,the stomata of maize leaves were almost completely closed,and the shoots dry weight and leaf photosynthetic capacity were significantly reduced.However,foliar spraying of 100μmol L^-1 melatonin markedly increased the stomatal aperture,dry matter accumulation,chlorophyll content,net photosynthetic rate（Pn）,and the effective quantum yields and electron transport rate of PSII and PSI in maize.These results indicated that exogenous melatonin enhanced maize photosynthetic capacity by keeping stomata partially open and strengthening light energy conversion efficiency under drought stress,thus promoting the growth of maize seedlings.2.Exogenous melatonin remarkably improved the carbon metabolism of maize seedlings under drought stress.Under drought stress,the synthesis of soluble sugar in maize leaves was increased,while the starch synthesis and the tricarboxylic acid cycle（TCA）were inhibited.However,foliar spraying 100μmol L^-1 melatonin dramatically increased the contents of starch and sucrose,the activities of ADP glucose pyrophosphorylase（AGPase）,sucrose phosphate synthase（SPS）,phosphoenolpyruvate carboxylase（PEPC）and citrate synthase（CS）were also enhanced substantially,while the glucose and fructose content,invertase（INV）activity and sucrose synthase（Su Sy）activity decreased significantly.These results showed that exogenous melatonin increased the production of photosynthate,soluble osmotic matter and energy by promoting the synthesis of sucrose and starch,glucose oxidation metabolism and TCA,thus enhancing the tolerance of maize to drought stress.3.Exogenous melatonin vastly enhanced the nitrogen metabolism of maize seedlings under drought stress.Under drought stress,the nitrate（NO₃^-）reduction and ammonium（NH₄⁺）assimilation in maize leaves were significantly inhibited.However,foliar spraying100μmol L^-1 melatonin vastly increased the contents of NO₃^-,soluble protein and proline,the activities of nitrate reductase（NR）,nitrite reductase（Ni R）,glutamine synthetase（GS）and glutamate synthase（GOGAT）were also increased,while a considerable decline was observed in free amino acid content,NH₄⁺content and glutamate dehydrogenase（GDH）activity.These results suggested that exogenous melatonin promoted the nitrogen metabolism by enhancing the activities of nitrogen metabolism enzymes and the synthesis of nitrogenous compounds,which not only detoxified ammonium,but also increased the synthesis of osmotic substances in drought-affected maize,and finally improved the drought resistance of maize seedlings.4.Transcriptome analysis showed that a total of 2174 differentially expressed genes（DEGs）were identified in maize leaves after treatment,of which 957（733 up-regulated and224 down-regulated）DEGs come from DS/DS＿MT treatment group.The results of GO annotation analysis showed that most DEGs were classified into stress response process,metabolic process,membrane,cell,binding,transporter activity,enzyme activity,photosynthesis and antioxidation.The results of KEGG enrichment analysis showed that DEGs were mainly enriched in plant-pathogen interaction,MAPK signaling pathway-plant,plant hormone signal transduction,sucrose and starch metabolism,glutathione metabolism,α-linolenic acid metabolism and nitrogen metabolism.Integrated analysis of GO and KEGG results showed that exogenous melatonin activated downstream stress regulatory factors,hormone-mediated defense response,antioxidant defense response and osmotic regulation to alleviate drought stress by mediating signal transduction pathways of drought stress.In summary,exogenous melatonin enhanced the drought tolerance of maize seedlings under drought stress.At the physiological level,exogenously applied melatonin improved the photosynthetic capacity of maize leaves and promoted the growth of maize seedlings by keeping stomata partially open and coordinating the distribution of photosynthetic energy;and exogenous melatonin also increased the synthesis of osmotic substances and energy by enhancing and coordinating the carbon and nitrogen metabolism in maize leaves,thus improving the drought resistance of maize seedlings.At the transcriptional level,exogenous melatonin activated transcription factors and metabolic processes related to drought resistance by regulating signal transduction pathways of drought stress,and finally enhanceed the drought tolerance of maize seedlings.