Effect Of Melatonin On The Antioxidant Ability Of Naked Oats Seedlings Under Cadmium And Lead Stress

Melatonin（MT）is also called N-acetyl-methoxytryptamine.There are many studies on plant functions of MT,such as responding to abiotic stresses,resisting to oxidative damage under adversity environments,and promoting the activities of antioxidant enzymes.However,the research on these functions is still unclear in higher plant naked oats.Naked oats,as a favored food crop of mankind,has been severely affected by heavy metal pollution in farmland soil in recent years,resulting in a serious decline in the production of naked oats.During the growth of naked oats,the accumulation of a large amount of metal elements in the soil causes its growth to be suppressed,which indirectly harms human dietary health and increases the risk of human diseases.Based on this,present paper mainly focuses on the effect of exogenous melatonin on the antioxidant effect of the heavy metal cadmium or lead on the target plant,with the purpose of reducing or avoiding the adverse effects of heavy metal stresses in the soil on the naked oats.In this study,naked oats were used as experimental materials,and the morphological indexes,antioxidant enzyme activities and the expression of related stress resistance genes of naked oats seedlings under the stress of metal cadmium or lead after exogenous application of melatonin were discussed.Meanwhile,naked oats were also used as experimental materials to explore whether melatonin has the function of alleviating heavy metal poisoning and related mechanisms.It provides relevant theoretical basis for melatonin to alleviate the harm caused by similar heavy metal stress.The main results are as follows:（1）The protective effect of exogenous melatonin on naked oat seedlings under cadmiumstress（1）Cadmium stress at a concentration of 25 mg/L reduced the increase of plant height,fresh weight and dry weight of naked oats seedlings.However,the exogenous application of melatonin alleviated this situation.We can see that the three indicators of plant height,fresh weight and dry weight have all begun to increase,and the chlorophyll content has also been significantly increased.The results of the study showed that melatonin can promote the growth of naked oats seedlings under cadmium stresses.（2）Cadmium stress at a concentration of 25 mg/L caused serious accumulation of reactive oxygen and cadmium content in the leaves of naked oats seedlings,and the content of H₂O₂,O₂^·–and MDA also increased significantly.After exogenous application of melatonin,the cadmium content,H₂O₂,O₂^·–and MDA content all began to drop significantly.On the contrary,the activities of SOD,POD,and CAT have all been improved to varying degrees.This shows that exogenous melatonin can improve the antioxidant capacity of naked oats seedlings,reduce the level of ROS,and increase the activity of antioxidant enzymes.（3）Cadmium stress at a concentration of 25 mg/L reduced the soluble protein content of naked oats seedlings.After the application of exogenous melatonin,the soluble protein content was increased to a certain extent in naked oats seedlings that have been subjected to cadmium stress,the proline content increased slightly,but the increase was more significant after exogenous melatonin was applied.This experiment confirmed that proline,as an osmotic regulator,can be affected under cadmium stress,and this change will increase the tolerance of naked oats seedlings to cadmium.（4）Under cadmium treatment,the expression of LOX and POX genes in the leaves of naked oats seedlings increased to varying degrees.After addition of different concentrations of exogenous melatonin,the gene expression of POX was more significantly up-regulated,while on the contrary,the expression of LOX was down-regulated.At the same time,the Asmap1 gene in the MAPK（mitogen-activated protein kinase）cascade was also significantly up-regulated.NAC and WRKY1 showed very obvious improvement after 100μM melatonin was added externally.This shows that the MAPK of naked oats seedlings under cadmium stress can mediate the H₂O₂ signal pathway,improve antioxidant enzyme activity,remove excessive ROS in plants,and regulate the expression of genes related to cadmium stress.The above multiple synergistic effects improved tolerance of naked oat seedlings to cadmium.（2）The protective effect of exogenous melatoninon naked oat seedlings under lead stress（1）The lead stress of 75 mg/L will severely inhibit the plant height,fresh weight and dry weight of naked oat seedlings,while the addition of exogenous melatonin under the lead stress will inhibit the plant height,fresh weight and dry weight of naked oats seedlings will be improved.The chlorophyll content of naked oat seedlings also began to decrease under lead stress,but it was improved after adding melatonin.The above results indicate that the addition of melatonin under lead stress can alleviate the toxicity of naked oats seedlings under lead stress to a certain extent.（2）Under lead stress,the H₂O₂ content and O₂^·–production rate were significantly higher than those of the experimental group without stress treatment,and the addition of melatonin significantly reduced the ROS in the leaves of naked oats.Under lead stress,the soluble protein content of naked oats seedlings decreased.After melatonin was added,the soluble protein content began to increase.In addition,the activities of antioxidant enzymes SOD,POD,and CAT are all significantly increased.This phenomenon reduces the oxidative stress damage of naked oat seedlings.Lead stress also increased the MDA content of naked oats leaves,but after treatment with melatonin,MDA began to decrease,so the membrane lipid peroxidation of seedlings was alleviated.At the same time,lead stress will increase the content of proline.The reason is that the addition of melatonin will increase the content of proline in the naked oats cells and maintain the osmotic balance inside and outside the cells.（3）Under lead stress,the expression of LOX and POX in the leaves of naked oats seedlings did not change significantly,but after application of exogenous melatonin,their expression levels were significantly up-regulated,and the Asmap1 gene expression level was also significantly up-regulated,and the magnitude of the up-regulation increased with melatonin.Under lead stress,after exogenous melatonin treatment,the expression level of NAC was up-regulated compared with the lead-treated group,but WRKY1 did not change significantly when treated with 50μM melatonin,and the increase was more obvious when 100μM melatonin was added.In summary,whether naked oats seedlings are under cadmium stress or lead stress,melatonin treatment can enhance the antioxidant protection function of naked oats under stresses,and enhance the defense ability of naked oats from heavy metal poisoning.The research can be used as a reference for the theory and practice of the planting of naked oats and the prevention and control of heavy metal pollution in the soil.