Regulatory Effects Of Exogenous Melatonin On Carbon And Nitrogen Metabolism, Yield And Quality Of Soybean During Grain Filling Under Drought Stress

In the context of global climate change,the frequency and severity of drought stress will increase annually.The grain-filling stage is the key period for soybean yield and quality development,and also the most complex stage of carbon and nitrogen metabolism.Drought stress inevitably inhibits the assimilation,distribution and transition of carbon and nitrogen in soybean during grain-filling stage,and affects soybean yield and quality.Melatonin as a plant hormone and growth stimulant can eliminate reactive oxygen species directly or indirectly and regulate other hormone levels by participating in a series of crop metabolic processes in response to adverse stress,thus improving crops’ ability to withstand stress and increasing their yield.However,the mechanism of melatonin in promoting soybean growth and yield increase under drought stress is still lacking from the perspective of carbon and nitrogen metabolism.Therefore,this research was conducted in 2018–2019 at the National Engineering Research Centre for studying grains,potted field and type selection of drought sensitive soybean for 26 varieties using the method of potted plant water control set drought stress treatment.In addition,we also studied the exogenous melatonin on grain-filling stage of carbon and nitrogen metabolism of soybean under drought stress and the control effect of the production quality,and used the transcription proteomics and metabolomics analysis for establishing a possible regulatory mechanism.The main results and conclusions are as follows:1.The water content of the leaves significantly decreased,the content of reactive oxygen species increased and the integrity of the cell membrane decreased under drought stress.Exogenous melatonin can significantly improve soybean leaves under drought stress;osmotic regulatory substances,soluble sugar and soluble protein content increased the activity of antioxidant enzymes(SOD,POD and CAT)and glutathione system key enzymes(APX,GPX,GR and DHAR),reduced the active oxygen content(H2O2 and O2-),reduced the generation of MDA and significantly lowered the electrolyte permeability,while promoting growth hormones(IAA,GA and ZR)content,lowering ABA content,improving the leaf water content,maintaining cell membrane integrity and promoting soybean growth and development.Under normal water supply conditions,exogenous melatonin can reduce the content of reactive oxygenspecies to a certain extent,increase the content of the abovementioned growth hormones and reduce the content of ABA.2.Drought stress reduced the photosynthetic capacity of leaves and inhibited carbohydrate transport.Exogenous melatonin can significantly increase the soybean photosynthetic pigment under drought stress(Chlorophyll a and Chlorophyll b),carotenoids content and chlorophyll fluorescence parameters(Fv/Fm,Fv/F,ΦPSⅡ,ETR),improve photosynthetic gas exchange parameters(Tr,Gs and Ci),improve the photosynthetic rate,improve the carbon metabolism of key enzymes(AI,NI,SPS and SS)activity,improve the carbohydrate(starch,sucrose,glucose and fructose)content and improve the carbon assimilation and transfer level.Under normal water supply conditions,exogenous melatonin had no significant impact on the physiological indexes of carbon metabolism in soybean leaves.3.Drought stress resulted in decreased activity of key enzymes in nitrogen metabolism,increased inorganic nitrogen content,decreased amino acids content,and impeded nitrogen accumulation and transport.Under drought stress,exogenous melatonin can significantly improve the activity of key enzymes in nitrogen metabolism(NR,GS,GOGAT and GDH),decrease the inorganic nitrogen content(NH4+–N,NO3-–N),increase the acyl urea content,increase the amino acid content,promote the accumulation of nitrogen in the plant and facilitate nitrogen assimilation and transport.4.Drought stress resulted in a decrease in the number of pods and grains per plant,a decrease in the grain weight per plant and a decrease in the yield by 27.70%.Under drought stress,exogenous melatonin can increase the number of grains per plant to a certain extent,and significantly increase the weight of 100 varieties of grains,which is up to 10.86% higher than that under drought stress.Exogenous melatonin can marginally increase soybean yield under normal water supply conditions,with an increase rate of 1.94%.5.The content of protein,soluble sugar and dietary fibre increased,and the content of fat,total isoflavones,total fatty acids,Cu and Fe decreased significantly under drought stress.Under drought stress,exogenous melatonin can increase the protein content,soluble sugar and dietary fibre in soybean and effectively alleviate the decrease in fat,isoflavone,fatty acid and mineral elements,thus improving the quality of soybean.Under normal water supply conditions,exogenous melatonin can generally promote the improvement of the abovementioned indexes.6.Transcriptome analysis showed that,compared with drought stress treatment,37 and 493 genes were jointly up-regulated and down-regulated in soybean leaves treated with normal water supply and treated with exogenous melatonin under drought stress.The up-regulated genes have functional genes directly and indirectly involved in carbon and nitrogen metabolism,including the key genes involved in the cysteine synthesis pathway,photosynthesis,carbohydrate metabolism and glucose metabolism.Metabolomic analysis revealed that,compared with drought stress treatment,17 and 43 metabolites were jointly up-regulated and down-regulated in soybean leaves treated with normal water supply and treated with exogenous melatonin under drought stress.respectively,to increase the metabolites of most(14/17)belonging to amino acids,lipids,organic acids,and carbohydrates,further revealing that exogenous melatonin can improve soybean carbon and nitrogen metabolism and drought resistance.Combined with transcriptome and metabolomic analysis,melatonin,by controlling the “amino acid metabolism” and “starch and sucrose metabolism” pathways,promotes the growth of β-D-Glucosidase gene expression under drought stress,improves the content of L-Asparagine and D-Glucose-6P metabolites and ultimately improves the soybean drought resistance.