| When the soil salinity and alkalinity in a forested area exceeds the tolerance limit of plants,it will have negative impacts on their growth.Effective measures must be taken to alleviate salt and alkali stress.Melatonin,a small hydrophilic indole compound with strong antioxidant properties,has been found to promote plant growth,enhance related stress resistance,and increase their survival rate under adverse conditions when applied to plants.However,recent studies have found that the effect of melatonin application may vary under different types of stress.In this study,Populus cathayana × canadansis ‘Xin lin 1’ was used as the experimental subject and various physiological indices of seedlings were detected under different concentrations of exogenous melatonin treatment conditions.Transcriptional metabolism analysis was also performed to explore the molecular mechanism of the effect of melatonin application on the salt and alkali resistance of forest trees.The results are as follows:(1)By using melatonin at different concentrations(0μM,50 μM,100 μM,200 μM,400μM,800 μM)on Populus cathayana × canadansis ‘Xin lin 1’,the impact on common physiological indices was explored.The results are as follows: Melatonin treatment at 50 μM and 100 μM effectively reduced the MDA and osmoprotectant content in the leaf cells,and the results of 800 μM melatonin treatment were slightly higher than the control group.Melatonin at 50 μM and 100 μM significantly increased SOD and CAT activities in the leaf cells,while800 μM melatonin tended to induce POD activity and had a tendency to inhibit CAT activity.(2)By using different concentrations(0μM,50μM,100μM,200μM,400μM,800μM)of melatonin to treat Populus cathayana × canadansis ‘Xin lin 1’,the effects on various hormone indicators in the cytoplasm were explored,and the results are as follows: Overall,melatonin can stimulate the secretion of ABA in plant bodies under stress conditions,and has a tendency to inhibit the secretion of SA;100μM melatonin treatment is more helpful for the secretion of IAA in the plant.By combining the conclusion(1),it can be initially determined that melatonin has a dose-effect in helping seedlings relieve salt-alkali stress.(3)Through transcriptome analysis,the effects of melatonin on seedling photosynthesis mechanism were explored,and the results are as follows: Compared with the control group,100μM melatonin will stimulate the activity of magnesium-protoporphyrin IX monomethyl ester(oxidative)cyclase,which will speed up the synthesis of Protochlorophyllide.It also inhibits the activity of chlorophyllase to maintain the stability of chlorophyll content,and once chlorophyll hydrolyzes,100μM melatonin will activate the expression of magnesium dechelatase encoding for rapid hydrolysis to reduce energy consumption.800μM melatonin is more inclined to inhibit the expression of genes in the entire chlorophyll synthesis.(4)The following is the result of exploring the effect of melatonin on the signaling mechanism of seedlings through transcriptome analysis: For gibberellic acid,100 μM melatonin can induce the expression of ARF in the secretion pathway,while 800 μM melatonin tends to inhibit the expression of AUX.For abscisic acid,100 μM melatonin can significantly increase the expression of GID2.For cytokinin,800 μM melatonin tends to inhibit the expression of type A-ARRs in the secretion pathway.For jasmonic acid,100 μM melatonin treatment can inhibit its transport efficiency at the source.For ethylene,there is a positive correlation between the dose of melatonin applied and ethylene expression.For brassinosteroids,the expression of the negative regulator of BR receptors,BKI1,increases with the increase of melatonin concentration,while the reduction of membrane-bound BRI1/BAK1 complex reduces the seedlings’ sensitivity to brassinosteroids.For salicylic acid,800 μM melatonin can reduce the expression of JAR1,the salicylic acid resistance protein,leading to a reduction in the expression of downstream R genes.For hydrogen peroxide,melatonin also promotes the expression of downstream signaling pathways.In terms of MAPK cascade signaling,when the sensors on the cell membrane are subjected to extracellular stress,at the MAPKKK level,melatonin tends to induce the expression of MAP3K17/18 and MEKK1,at the MAPKK level,melatonin tends to induce the expression of MKK1/2/3/4/5,and at the MAPK level,melatonin tends to induce the expression of MPK1/2/3/4/6/8.By comparison,100 μM melatonin tends to enhance MAPKK activity,while 800 μM melatonin tends to promote MAPK expression.(5)Through transcriptome-metabolome joint analysis,the impact of melatonin on the mechanism of seedling resistance was explored,and the results are as follows: Melatonin mainly plays a role in influencing the ability of cell osmotic stress tolerance,ion toxicity tolerance,and oxidative stress tolerance.800 μM melatonin stimulates the expression of glucan1,3-beta-glucosidase,beta-glucosidase enzyme to increase the content of soluble sugar in the cell,and activation of endoglucanase enzyme also promotes cellulose hydrolysis into soluble sugar.800 μM melatonin also promotes the conversion of glutamine and arginine to proline,and increases the expression of polyamines.On the basis of the activation of the SOS signaling pathway,melatonin can promote the expression of transmembrane proteins to transfer excess sodium ions from the cytoplasm to the vacuole to relieve ion toxicity.Compared to 800 μM melatonin,100 μM treatment,and genes related to transmembrane proteins will receive more abundant expression.Melatonin protects plant cells from oxidative damage by influencing the expression of intracellular antioxidants.Finally,this study found that 100 μM melatonin can better induce the expression of yellow flavonoids including asperol,salicylic acid,gibberellin,kaempferol,naringenin,hesperetin,quercetin,apigenin,and shikimic acid in the leaves.Overall,the anti-stress resistance of Populus cathayana × canadansis ‘Xin lin 1’to exogenous melatonin has a significant dose-effect relationship.Under different melatonin treatment conditions,there were significant differences in various physiological and biochemical indices and underlying gene expression patterns of Populus cathayana ×canadansis ‘Xin lin 1’.Generally speaking,the results of the indices were better under 100 μM melatonin treatment.Throughout the anti-stress process,melatonin can regulate various aspects such as photosynthetic mechanism,signal transduction system,permeability regulation system,ion toxicity tolerance,and antioxidant stress.This study provides a theoretical basis for the application of melatonin to enhance the anti-stress resistance of poplar and can also serve as a reference for research on other woody plants. |
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