Effects Of Exogenous Citric Acid On Antioxidant System And Metabolic Pathway Of Elymus Dahuricus Under Cd Stress

Citric acid is an important intermediate product of tricarboxylic acid(TCA)cycle,which can improve cadmium tolerance in plants.In order to investigate the effects of exogenous citric acid on cadmium tolerance of Elymus dahuricus,a pot experiment was conducted.In this experiment,we studied the changes of growth,cadmium accumulation,antioxidant system and metabolic pathway of E.dahuricus leaf and root under Cd(50mg/kg)stress and CA(citric acid,4mmol/L)combination treatment.To investigate the effects of exogenous citric acid on the antioxidant and metabolic pathways of E.dahuricus under Cd stress.The main results are as follows:1)There was no significant change on the growth of E.dahuricus,but Cd stress seriously affected the growth of E.dahuricus(The biomass and chlorophyll content decreased significantly,electrolyte leakage,malondialdehyde(MDA)content and H2O2 content increased significantly)(p<0.05).Compared with Cd treatment,the leaves and roots biomass of E.dahuricus increased significantly;chlorophyll content increased significantly;electrolyte leakage decreased significantly in leaves and roots;and MDA and H2O2 contents decreased significantly in roots;the Cd content of E.dahuricus leaves decreased and the Cd content of root increased significantly,under the treatment of Cd+CA(p<0.05).The change trend of Cd accumulation in leaves and roots of E.dahuricus was the same as that of E.dahuricus.2)Compared with CK treatment,there was no significant difference in the contents of antioxidant enzymes and antioxidants in leaves and roots of E.dahuricus under CA treatment,while the activity of SOD,CAT,POD,APX,MR and GR increased significantly and the activity of DR decreased significantly under Cd treatment(p<0.05).Compared with Cd treatment,the activity of POD,SOD,APX,MR in leaves and roots of E.dahuricus increased significantly,the activity of CAT and DR in root of E.dahuricus increased significantly,and the activity of GR in leaves increased significantly,under Cd+CA treatment(p<0.05).3)In this study,GC/MS technology was used to analyze the characteristics of metabolite changes in the leaves and roots of E.dahuricus by CA treatment under Cd stress.A total of 225 kinds of differential metabolites were detected.KEGG classification combined with KEGG enrichment analysis showed that there were 18 and 21 metabolic pathways in leaves and roots,including energy metabolism,amino acid anabolism and purine pyrimidine metabolism,respectively.Amino acid metabolism is the main metabolic pathway.4)12 and 18 metabolites were identified in the leaves and roots of E.dahuricus,respectively.Cadmium stress resulted in abnormal TCA cycle,increased the content of Fumaric Acid and Malate in leaves,and the conent of citric acid,Malate and 2-Ketobutyric Acid in roots decreased significantly.The relative content of sugar and sugar alcohol was significantly higher,and the relative contents of TCA-related substances in leaves and roots were obviously changed,but the trend was diferent,under Cd+CA treatment.5)There were 15 species and 24 species of differential metabolites related to amino acid metabolism in leaves and roots of E.dahuricus,respectively.With the treatment of CA,the content of resistant substances increased significantly in leaves(Putrescine)and roots(glutathione,cysteine);and it also alleviated the abnormal metabolism of several amino acids(serine,glycine,aspartic acid,glutamic acid,etc.)induced under Cd stress.6)Four differential metabolites related to pyrimidine metabolism were identified in E.dahuricus leaves and 10 differential metabolites related to purine pyrimidine metabolism were identified in roots.The content of uridine in leaves and roots of E.dahuricus was up-regulated by CA treatment under Cd stress,while the content of thymidine was decreased in leaves and roots of E.dahuricus.In conclusion,Cd stress seriously affected the growth of E.dahuricus,which not only induced the production of reactive oxygen species(ROS)in the leaves and roots of E.dahuricus,but also led to membrane peroxide,and the plant self-defense system played a role(antioxidant defense system was activated).Energy metabolism,amino acid metabolism and purine-pyrimidine metabolism were abnormal in plant metabolic pathway.The application of exogenous CA can not only effectively alleviate the inhibition of Cd stress on the growth of E.dahuricus,reduce the content of ROS,make the antioxidant defense system play a greater role,but also promote the increase of Cd content in roots.Metabonomics studies showed that exogenous CA increased the content of sugar and sugar alcohols,promoted the synthesis and metabolism of amino acids,and increased the content of resistant substances.The results showed that exogenous application of CA significantly enhanced the cadmium resistance of E.dahuricus,which was mainly reflected in shikimic acid pathway.