Physiological And Biochemical Mechanisms Underlying The Regulation Of ABA On Mung Bean Tolerance To Cadmium Stress

The problem of pollution caused by heavy metals to the environment is gradually emerging in the world.Nowadays,heavy metal pollution is one of the most extensive and harmful environmental problems.Cadmium（Cd）naturally exists in the environment with a small amount of concentration.Its water solubility is significantly higher than other heavy metal pollutants,resulting in high toxicity of Cd,which damages the growth and development of plants and the antioxidant system.Abscisic acid（ABA）is a rapidly synthesized plant growth regulator.It is not only a growth inhibitor to promote senescence and abscission,but also can comprehensively regulate the physiological functions and affect the growth and development of plants.ABA can reduce the oxidative damage of plants by regulating the antioxidant defense system and increasing the level of metabolites related to stress resistance,improve the tolerance of plants to Cd and participate in different abiotic and biological stress responses such as drought,salinity,nutrients,temperature and heavy metals.In this study,roots,stems and leaves of mung bean seedlings were used as materials,By means of hydroponics,the morphological and physiological indexes of mungbean seedlings were measured to study the regulatory mechanism of ABA on the growth and development and stress resistance of mungbean seedlings under Cd stress.To further understand the response difference of three organs tissues to Cd stress.The results show that:1.After the 10 days of different concentrations of CdCl₂ stress,the plant height,main root length,number of lateral roots,biomass of stem,leaf and root of mungbean seedlings decreased significantly,the toxic effect of 100 μM Cd on mung bean seedlings was more obvious.Heavy metal exposure seriously affected the growth and development of plants,making plant dwarfing、leaves yellowing、root browning.However,exogenous application ABA of 5 μM,10 μM,15 μM could alleviate the growth inhibition of mung bean seedlings caused by Cd,the above morphological indexes were all improved compared with Cd treatment,but still lower than the control,among which 10 μM ABA has the most significant effect on the growth inhibition of mung bean seedlings,especially promoting the growth of taproot lateral roots and increasing the biomass of roots.ABA plays an important role in regulating root system.It suggested that exogenous application of low concentration ABA can promote the growth of mungbean seedlings under heavy metal stress.2.Under 100 μM Cd stress,the contents of APX,POD,CAT,MDA,Pro,GSH,polyphenol in the roots of mung bean seedlings were increased,while the levels of SOD and ASA were decreased.Cd stress caused membrane lipid peroxidation in roots and oxidative stress in cells,which basically led to the increase of antioxidant system capacity in roots of mung bean seedlings,and enhanced the ability of scavenging reactive oxygen species in mungbean Seedlings under Cd stress.The reason for the decrease of SOD activity is that it plays a leading role in the process of root detoxification,which is faced with a large concentration of heavy metals.Excess H2O2 leads to the deactivation of root tissue enzymes and the decrease of SOD activity.The decrease of ASA content may be related to the scavenging of active oxygen.The decreased ASA content is mainly involved in the ASA-GSH cycle in mung bean seedling cells.It indicated that Cd treatment inhibited part of the antioxidant system and reduced the scavenging function of active oxygen.After exogenous application of ABA,the increase of POD,CAT,GSH,polyphenol,Pro,MDA and the decrease of SOD and ASA levels caused by Cd stress were reversed,and APX activity was further improved.ABA is most likely to control the dynamic balance of reactive oxygen species（ROS）and reduce the excessive accumulation of H2O2 in roots to directly alleviate the oxidative stress of plants,thus reducing the level of antioxidant and the degree of membrane lipid peroxidation in roots.Meanwhile,ABA also participates in the alleviation of oxidative stress induced by heavy metals by enhancing the antioxidant defense system.This may be one of the reasons why ABA can alleviate Cd stress and inhibit the growth and development of mung bean seedlings.3.After Cd treatment,the levels of APX,POD,CAT,SOD,MDA and Pro in the stems of mung bean seedlings were increased,while the content of polyphenol were decreased,ASA and GSH levels did not significantly change.The low level of Cd in the stem can cause oxidative burst,which can stimulate the antioxidant defense system of mung bean seedlings,and the activity of antioxidant enzymes gradually increases within 9 days,because the adaptability of plants in the stress environment gradually increases,and the ability of scavenging reactive oxygen radicals also increases.The decrease of polyphenol level is due to the transformation of polyphenols into quinones by polyphenoloxidase.After exogenous application of ABA,the activities of CAT and SOD increased induced by Cd were counteracted,and the changes of APX,MDA and GSH caused by Cd stress were partially restored.ABA still increased the level of POD and Pro,and further reduced the content of polyphenol.ABA largely neutralized the negative effects of Cd stress on seedling stem growth.The results showed that the application of ABA changed the activity of antioxidant system in the stem under Cd stress,and low concentration of ABA could alleviate the membrane lipid peroxidation of plant under Cd stress.4.When the leaves of mung bean seedlings were under the stress of Cd,the contents of POD,SOD,Pro,MDA,Chl and Car were up-regulated,and the levels of APX,CAT,ASA and polyphenol were down-regulated.GSH content decreased first and then increased under Cd stress.These systems are responsible for inducing plant growth and development.The stress of heavy metals in leaves can no longer as high concentrations of heavy metals in roots to mobilize most antioxidant systems.Cd inhibits some of the antioxidant systems in plants.After ABA treatment,the Cd-induced increase of SOD,MDA,Chl,Car and the change of GSH were down-regulated,ABA also up-regulate reduced APX,CAT,ASA and polyphenol levels,further enhance the activity of POD and the content of Pro.The results showed that exogenous application of ABA alleviated the heavy metal induced oxidative burst,protected membrane lipid peroxidation and reduced the harmful effects of heavy metals by regulating the antioxidant defense system in mung bean seedling leaves and reducing the level of H2O2 in plants.5.The treatment of Cd can significantly increase the CAT activity of roots and stems,but decrease the CAT activity of leaves.The activities of POD in the three tissues all increased strongly,but the activities of SOD and APX changed little except APX in the stem.These results indicate that CAT and POD play a leading role in maintaining the redox homeostasis of roots,stems and leaves under Cd stress.Interestingly,the content of polyphenol increased strongly in roots and decreased in stems and leaves under Cd stress,indicating that polyphenol plays an important role in reducing oxidative stress caused by Cd in roots.The content of MDA in roots,stems and leaves of mung bean seedlings increased significantly under Cd stress,but the change of MDA content in roots caused by Cd was significantly higher than that in stems and leaves.Compared with the control,the content of Pro in roots,stems and leaves increased significantly under Cd stress.The application of ABA reversed the change of CAT activity in roots,stems and leaves.In addition,Cd+ABA treatment had a strong inducing effect on POD activity of roots,stems and leaves.ABA could reverse the decrease of ASA level and the increase of MDA level caused by Cd in roots and leaves,also reverse the increase of GSH,polyphenol and Pro level caused by Cd in roots.ABA can reduce the toxicity of Cd by restoring the activities of antioxidant enzymes and the levels of antioxidants,MDA,Pro,photosynthetic pigments.Root plays an important role in detoxification of Cd,and stem and leaf also participate in this process.The application of ABA on the leaves transferred to the root rapidly,which regulated the response of leaves,stems and roots to Cd stress.