| With the increasing global warming trend,the degree and incidence of drought have increased.Drought seriously impacts plant growth,yield and quality.Actinidia deliciosa is thermophilic but not tolerant to drought,and is sensitive to water stress during growth.Kiwifruit seedlings are vulnerable to drought stress,which leads to changes in their external morphology,physiological and biochemical metabolism.In this experiment,One-year-old kiwifruit seedlings were used as the material,and watered with 0,50,100and 200μmol·L-1 melatonin solution,then supposed to drought stress.The growth and photosynthetic characteristics of the kiwifruit seedlings were measured.100μmol·L-1melatonin was screened out owing to its best effectiveness.Kiwifruit seedlings were re-selected and supposed to drought stress before treated with 100μmol·L-1 melatonin.The changes of leaf water condition and antioxidant activity of kiwifruit seedlings during drought were determined.The function of differentially expressed genes and metabolic pathway enrichment analysis were performed thought transcriptome sequencing,which is to investigate the effects of melatonin on the molecular response mechanism of kiwifruit seedlings under drought stress.The results are as follows:（1）Melatonin pretreatment could mitigate wilting and leaf curling symptoms of kiwifruit seedlings caused by drought stress,enhance the leaf area and stem diameter,increase lateral root number and root length,reduced the inhibition of biomass accumulation by drought stress.Pretreatment with 100 mol·L-1 melatonin increased leaf area,root dry mass and leaf dry mass by 87.93%,40.91%and 78.63%,respectively,compared with the drought treatment alone.（2）Melatonin pretreatment significantly enhanced the photosynthetic capability of kiwifruit seedlings under drought stress.Melatonin increased chlorophyll and carotenoid content,decreased initial fluorescence（Fo）and non-photochemical quenching（qN）,and increased the maximum PSII photochemical efficiency（Fv/Fm）,the PSII actual photochemical efficiency（ФPSII）,the PSII photosynthesis electron transport rate（ETR）and the photochemical quenching（qP）,protected photosynthetic organs,at the same time enhanced the stomatal conductance（Gs）and net photosynthesis rate（Pn）.The most significant melatonin concentration was 100μmol·L-1,which increased the content of Total chlorophyll and carotenoids by 21.64%and 22.33%respectively,compared with the drought treatment alone,and the Pn enhanced more than twice.（3）Melatonin pretreatment significantly increased osmotic adjustment ability,cell membrane integrity,and maintained higher relative water content of kiwifruit seedlings under drought stress.The content of soluble sugar increased with the improvement of melatonin concentration.100μmol·L-1 melatonin pretreatment increased the content of proline and soluble protein by 88.35%and 49.96%,respectively,compared with drought treatment alone.The（MDA）content,relative conductivity decrease,and hydrogen peroxide（H2O2）content decreased by 30.95%,42.72%,and 25.93%,respectively,and the relative water content increased by 36.01%.（4）Melatonin pretreatment enhanced the antioxidant capacity of kiwifruit seedlings under drought stress.Melatonin treatment enhanced the activity of superoxide dismutase（SOD）,peroxidase（POD）,and catalase（CAT）during drought,and also elevated the activity of ascorbate peroxidase（APX）and monodehydrogenation（MDAR）and dehydroascorbate reductase（DHAR）,promote the ascorbate-glutathione（AsA-GSH）cycle,and increased ascorbic acid（AsA）and glutathione（GSH）content by 16.35%and 91.49%compared with separate drought treatment.（5）The transcriptome sequencing analysis was performed on leaves of kiwifruit seedlings from CK（normal watering）,DR（drought）and MTDR（first melatonin-irrigation and then drought）.After comparison of DRvsCK,MTDRvsCK,and MTDRvsDR,9276（4810 up-regulated,4466 down-regulated）,2224（1096 up-regulated,1128 down-regulated）and 4833（2135 up-regulated,2698 down-regulated）differentially expressed genes（DEGs）were obtained,respectively.GO enrichment analysis indicated that a large number of DEGs were annotated to metabolic processes,biological process,catalytic activity and oxidoreductase activity.Significantly enriched KEGG pathways were carbon fixation in photosynthetic organisms,carbon metabolism,glutathione metabolism,plant circadian rhythms,flavonoid biosynthesis,and secondary metabolite biosynthesis.（6）Melatonin treatment significantly increased the expression of genes related to photosynthetic and antioxidant system.Melatonin increased photosynthetic capacity by inducing the expression of related genes involved in photosynthesis,porphyrin and chlorophyll metabolism as well as carbon fixation in photosynthetic organisms;promoted the expression of related genes envolved in glutathione metabolism and ascorbic acid metabolism to improve AsA and GSH content.Melatonin also promoted the expression of CAT6,SOD[Cu-Zn]and POD42,so as to enhance the antioxidant capacity of kiwifruit seedlings. |
