The Physiological Mechanism Of Exogenous 5-Aminolevulinic Acid Regulating Cadmium Stress In Chinese Cabbage(Brassica Pekinensis L.)

Cadmium poisoning is a complex phenomenon,which can not only destroy the structure of chloroplast,affect the absorption of energy and plant growth,but also increase reactive oxygen species（ROS）activity,lead to lipid per-oxidation,destroy protein structure and function,and affect the expression of genes encoding metal transcription factors.Chinese cabbage belongs to small Chinese cabbage,which is one of the important ball vegetables.When under the Cd stress,the yield and quality were decreased significantly,and the Cd enrichment phenomenon was easy to occur in the above ground leaf bulb.Therefore,to effectively reduce the Cd content in cabbage leaves is an important measure to ensure the safety of vegetables.5-aminolevulinic acid（ALA）,as a new plant hormone,has attracted much attention because of its alleviating effects of salt,drought,temperature and other stresses.In this study,the effects of exogenous ALA spraying on the growth morphology,photosynthesis,antioxidant system and ion absorption of Chinese cabbage seedlings under Cd stress were studied by using hydroponic method.The main results are as follows:1、50μM CdCl₂ significantly inhibited the seedling growth of cabbage.The fresh weight of roots and shoots decreased by 52.9%、46.7%,The dry weight of roots and shoots decreased by 39.6%and 5.4%,respectively.Spraying 25 mg L^-1ALA significantly increased leaf area,promoted root development,repaired root tip cell damage,improved root activity and biomass accumulation of seedlings under Cd stress.ALA significantly alleviated the inhibition on the growth of cabbage under Cd stress.2、ALA can repair the damaged chloroplast structure,form orderly thylakoid and grana lamella,and restore the function of swollen chloroplasts.ALA promoted the synthesis of chlorophyll and accumulation.The maximum photochemical efficiency（Fv/Fm）and light chemical quenching coefficient increase significantly（q P）,the light capture efficiency and light energy conversion efficiency were improved.The activities of Rubisco、FBPase、FBA and TK in calvin cycle were enhanced.Pn、Gs and Tr were increased by 74%、59%and 44%,respectively,and photosynthetic capacity was significantly improved.3、ALA can also increase the activities of key enzymes（APX、DHAR、MDHAR）in the As A-GSH cycle,and increase the contents of As A and GSH.It improved the scavenging efficiency of MDA and H₂O₂in Chinese cabbage.4、The contents of cadmium in root and leaf of cabbage under Cd stress was 200and 70 times higher than that of control.ALA inhibited the absorption of Cd ions in roots by down-regulating the expression of IRT1、IRT2,Nramp1 and Nramp3,and inhibited the transport of cadmium ions in leaves by down-regulating the expression of HMA2 and HMA4 genes.5、ALA could promote the absorption of N,P and K in roots and leaves,and the content of K in the root increased by 25%.ALA promoted the absorption of Ca and Mg,and Ca content in roots increased by 18%.ALA enhanced the absorption of Fe、Zn and Mn,Fe content increased 30.4%and Mn content increased 63.5%.ALA improved the absorption and utilization of essential mineral elements and balanced the ion concentration in cabbage.6、Transcriptome analysis were performed on leaves and roots of cabbage in different treatment groups.There were 5621 DEGs in leaves and 3367 DEGs in roots.In the Cd-Cd+ALA treatment group,there were 83 DEGs in leaves and 85 DEGs in roots.Through GO and KEGG analysis,ALA in leaves regulates the growth of Chinese cabbage under Cd stress through carbon metabolism and photosynthetic-antenna protein pathway in photosynthesis and glutathione metabolism.ALA up-regulated GST gene expression,increased GSH content and antioxidant activity in cabbage.ALA up-regulated the expression of chlorophyll a/b binding protein gene,up-regulated the expression of GAPDH gene,increased the activity of carbon assimilation enzyme,and promoted the biomass accumulation of cabbage.ALA regulates the resistance of cabbage to Cd stress by affecting the metabolism of amino acids in cabbage root.