Study On The Alleviating Effect Of Exogenous Melatonin On Cadmium Toxicity In Brassica Juncea

Cadmium(Cd)pollution of soil is one of the major global environmental problems.It not only reduces the yield and quality of crops,but also harms human health through the food chain.Cd has toxic effects on plants,especially on plant growth,photosynthesis and antioxidant activity.Plants have complex mechanisms of detoxification and tolerance to Cd.However,due to their limited antioxidant and detoxification abilities,plants often suffer from toxicity under the stress of high concentration of Cd.Increasing plant tolerance to Cd by adding exogenous substances has attracted extensive attention.Melatonin(MT)is a multifunctional signaling molecule that is widely distributed in different parts of plants and can alleviate the inhibitory effect of stress on plant growth and development.At present,the mechanisms of melatonin action are mainly focused on plant physiological responses and other aspects,but its molecular mechanism is not well understood.Brassica juncea,widely cultivated in China,is an annual herb that can easily absorb and accumulate Cd from contaminated soil.In this paper,we investigated the effect of foliar melatonin spray on Cd tolerance in potted B.juncea and analyzed its molecular mechanism using transcriptomic approach.The results were as follows:(1)The effect of Cd stress on B.juncea and the alleviating effect of exogenous MT on Cd toxicity.With the increase of Cd concentration,the growth and development of B.juncea seedlings were inhibited more and more obviously,mainly the height,dry weight,chlorophyll content,carotenoid content,and relative activity of leaf cells of B.juncea seedlings were inhibited,and the Cd content of each tissue also showed significant differences.The Cd content of B.juncea was higher at 50 mg kg-1 Cd treatment,and the translocation factor(TF)of Cd was greater than 1.The effects of different concentrations(0、25、50、100、200 and 400 μmol L-1)of MT on the growth,photosynthesis,relative activity of leaf cells,Cd content in each tissue and enrichment and transport coefficients of B.juncea under 50 mg kg-1 Cd stress were investigated.The results showed that foliar spraying of 100 μmol L-1 MT had the best effect on the mitigation of B.juncea seedlings under 50 mg kg-1 Cd stress.(2)Effect of exogenous MT on the physiology of B.juncea under Cd stress.Cd stress inhibited growth,decreased chlorophyll and carotenoid contents,reduced photosynthesis,and increased ROS and lipid peroxide contents in B.juncea.In addition,Cd stress increased the activities of antioxidant enzymes(SOD、POD、CAT、GR and APX)and the contents of non-enzymatic antioxidant substances(AsA and GSH)in plants compared to the control.Compared with Cd-stressed plants,MT treatment increased the activities of antioxidant enzymes and synthesis of non-enzymatic antioxidants in the plants,and ROS and MDA contents in seedlings decreased significantly,thus,protecting cells from injury and improving the ability of B.juncea plants to resist Cd stress.(3)Effect of exogenous MT on the transcriptome of B.juncea under Cd stress.Gene expression changes in the control,MT-treated,Cd-treated and MT+Cd-treated groups of B.juncea were analyzed by using transcriptome sequencing technique after 21 d of Cd stress.A total of 2889 differential expressed genes(DEGs)were identified after comparative analysis of the four data sets.8 differentially expressed genes were verified by qRT-PCR,and the correlation coefficient R2 between RNA-Seq data and qRT-PCR data was 0.7949.166 DEGs were detected in MT+Cd-treated plants compared to Cdtreated plants,including 98 up-regulated and 68 down-regulated genes.619 DEGs were detected in MT+Cd treated plants compared to control treated plants,of which 458 genes were up-regulated and 161 genes were down-regulated.435 DEGs were detected in Cdtreated plants compared to control-treated plants,of which 274 genes were up-regulated and 161 genes were down-regulated;872 DEGs were detected in MT+Cd-treated plants compared to MT-treated plants,of which 616 genes were up-regulated and 256 genes were down-regulated.There were 702 DEGs detected in MT-treated plants compared to+Cd-treated plants,of which 445 genes were up-regulated and 257 genes were downregulated;95 DEGs were detected in MT-treated plants compared to control-treated plants,of which 53 genes were up-regulated and 42 genes were down-regulated.These differential genes in KEGG pathway classification are mainly pathways for plantpathogen interactions,photosynthesis,and glutathione metabolism.The biological processes annotated by GO mainly regulate transcription factors,osmotic stress,photosynthesis,carbohydrate metabolism,and phytohormone signaling.