| The applications of ZnO nanoparticles?nano-ZnO? in agriculture have largely contributed to crop growth regulation,quality enhancement and induction of stress tolerance.Melatonin plays an important role in regulating plant growth and development and enhancing plant stress tolerance.Due to the global climate change,the frequency and intensity of drought events have been increasing,which reduces crop yield and threatens the global food security.The effects of nano-ZnO applications on seed germination and seedling growth in maize?Zea mays L.? and the physiological mechanisms of induction of drought tolerance by nano-ZnO in maize were studied in physiological,biochemical,metabolomics and transcriptome levels.The induction of drought tolerance via regulating melatonin synthesis and metabolism by nano-ZnO in maize was highlighted,which provids a theoretical basis for the exploration of the technical approaches to effectively induce maize drought tolerance.The main results were as follows:1.The applicaitons of nano-ZnO(20,50,100 mg·L-1) shortened the seed germination time and increased the germination rate;The germination rate with 50mg·L-1 ZnO treatment was the highest among all treaments.The total germination rate,germination energy,germination index and seed vigor index increased gradually with the nano-ZnO concentration increasing,with a peak at 50 mg·L-1 nano-ZnO treatment.Low concentrations of nano-ZnO treatments(50,100 mg·L-1)significantly increased root and shoot growth.In addition,nano-ZnO(50 mg·L-1) significantly increased the activities of hexokinase and UDP-glucose pyrophosphorylase while reduced the aldolase activity,which benifited for seed germination and seedling growth.2.Nano-ZnO(100 mg·L-1) treated plant had significantly higher chlorophyll a,b and carotenoid concentrations,and ?PS?,cytoplasmic invertase and ADP-glucose pyrophosphorylase activities,while lower ?NPQ and aldozyme activity.It indicated that100 mg·L-1 nano-ZnO treatment benefited for enhancement of the photosynthetic carbon assimilation and plant growth in maize.3.100 mg·L-1 nano-ZnO regulated multiple metabolic processes in maize.Nano-ZnO treatment modulated the concentrations of 38 metabolites in maize leaves,mainly involving in the amino acids and flavonoids metabolism.Nine of various metabolites were involved in the regulation of plant stress tolerance.Nano-ZnO upregulated the level of Tricin 7-O-?-guaiacylglycerol,Luteolin,Kaempferol,Myricitrin,2'-Hydroxygenistein,L-?+?-Arginine and Acetylcholine,accounting for 77.8%.Enrichment analysis showed that the photoconductive signaling pathway,arginine and proline metabolism,lysine synthesis,ABC transporter and glutathione metabolism and other metabolic pathways were regulated by nano-ZnO,which was helpful for improving plant stress tolerance.4.Nano-ZnO(100 mg·L-1) up-regulated the expressions of glucosidase,Rubisco activase and trehalose phosphorylase genes.Nano-ZnO also up-regulated the expression of genes related to phospholipid metabolism,including phospholipase D,thiolytic enzyme,adenosine phosphate transferase,etc.,which was conductive to maintaining the stability of cell membrane.In addition,nano-ZnO also enhanced the gene expression involving in plant biotic stress and abiotic stress responses,and the expression of carotene oxygenase,heme peroxidase,LEA proteins,defensins,Mpv17/PMP22 family genes and HB,G2-like,Orphans,b ZIP,any Zinc finger transcription factors,hence improving the drought tolerance.5.100 mg·L-1 nano-ZnO significantly increased the chlorophyll content,the activities of antioxidant enzymes?SOD,CAT,APX?,and the activities of glucose phosphate isomerase?a key enzyme in glycolysis? in maize leaves under drought stress.However,MDA content,hexokinase and aldolase activities were decreased by nano-ZnO treatment,which alleviates the damage caused by drought stress in maize.6.Drought stress decreased the plant leaf area and aboveground fresh weight,limited the plant growth,caused stomatal closure,decreased the net photosynthetic rate and water use efficiency,and increased the concentration of osmolytes and antioxidant enzyme activities.In drought stressed maize,nano-ZnO(100 mg L-1)treatment significantly increased zinc and chlorophyll concentrations,net photosynthetic rate,water use efficiency,soluble sugar and protein concentrations,hence enhancing drought tolerance in maize.In addition,the nano-ZnO increased the activities of UDP-glucose pyrophosphorylase,phosphoglucoisomerase and cytoplasmic invertase by 17.8%,391.5% and 126.0%,respectively,which enhanced the starch and sucrose biosynthesis and glycolysis metabolism in leaves under drought stress.The nano-ZnO application(100 mg L-1) alleviates the negative effect of drought stress via improving photosynthetic carbon assimilation and water use efficiency in maize.7.Drought stress resulted in the damage of chloroplasts and mitochondria structure characteristics,the accumulation of malondialdehyde and osmolytes and the increase of antioxidant enzymes activities in maize leaf.The nano-ZnO(100 mg L-1) application promoted the melatonin synthesis,and tryptophan,tryptamine,serotonin and melatonin accumulation,and activated the antioxidant enzyme systems,which alleviated drought-induced damage to mitochondria and chloroplasts and maintained the integrity of the cell membrane structure.These changes were associated with the upregulations of the relative transcript abundance of Fe/Mn SOD,Cu/Zn SOD,APX,CAT,TDC,SNAT,COMT and ASMT induced by nano-ZnO.It was suggested that nano-ZnO(100 mg L-1) increase drought tolerance in maize via regulating endogenous melatonin synthesis and starting the antioxidant defense system.In conclusion,optimal concentration of nano-ZnO regulated the carbon metabolism and promoted seed germination and seedling growth in maize.Under drought,nano-ZnO enhanced water use efficiency and photosynthetic carbon assimilation in maize.Also,it induced the biosynthesis of melatonin,activated the antioxidant system and improved the drought tolerance in maize. |
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