Study On The Effect Of Sulfur On Cadmium Accumulation In Tobacco And Its Mechanism

ABST ACT: Soil cadmium pollution seriously threatens plant growth and human health.Sulfur is an important biogenic element in soil and an essential nutrient element for plants.Studies have shown that sulfur can affect the absorption and accumulation of cadmium in plants.However,there are great differences in the effects of different forms and concentrations of exogenous sulfur on different types of crops.At present,there is still a lack of systematic research on the mechanism of sulfur affecting plant uptake and accumulation of cadmium.Tobacco is a widely planted economic crop in the world and also an accumulator of cadmium.In order to systematically explore the effect of sulfur on the accumulation of cadmium in plants and its mechanism,the effects of different concentrations of sodium sulfate(0,6,12 mmol·L-1)on the accumulation of cadmium in tobacco under cadmium stress(12 μmol·L-1)were studied by using the cadmium leafaccumulating tobacco variety K326(N.tabacum)and the cadmium root-accumulating tobacco variety Maxopka(N.rustica).The mechanism of sulfur affecting cadmium accumulation in tobacco was analyzed from physiological and biochemical,gene expression,transcriptomics and metabolomics.It provides a reference for reducing the accumulation of cadmium in edible parts of plants or increasing the accumulation of cadmium in phytoremediation.The main results are as follows.Firstly,by measuring the biomass and cadmium concentration of K326 and Maxopka after different sulfur or(and)cadmium treatments(including total cadmium concentration,total cadmium accumulation,cadmium concentration in different subcellular components,and cadmium concentration in different chemical forms in roots and leaves),the effects of different concentrations of sulfur on the absorption and accumulation of cadmium in cadmium leaf(root)-accumulating tobacco were studied.Compared with Cd12 S0,Cd12 S6 biomass increased,K326 increased by 33.33%(root)and 16.00%(leaf),Maxopka increased by 100.00%(root)and 51.28%(leaf);the biomass of K326 did not change significantly at Cd12 S12,and the biomass of Maxopka increased by 60.00%(root)and 28.21%(leaf),respectively.In the Cd12 S0 treatment,the total cadmium concentration in the roots of K326 was 0.57 times that of Maxopka,the total cadmium concentration in the leaves was 1.55 times that of Maxopka,and the cadmium translocation factor(TFLeaf/Root)was 2.71 times that of Maxopka.Compared with Cd12 S0,Cd12 S6 decreased Cd concentration in the root by 62.43%,increased Cd concentration in leaf by 25.61%,and increased cadmium translocation factor by 234.34%.The cadmium concentration in the roots of Maxopka increased by 0.40%,the cadmium concentration in leaves decreased by 12.95%,and the cadmium translocation factor decreased by 17.23%.Under Cd12 S6 treatment,the cadmium accumulation in the roots of K326 decreased by 49.91%,and the cadmium accumulation in leaves increased by 45.71%.In Maxopka,Cd accumulation increased by 121.50%(root)and 38.68%(leaf),respectively.When Cd12 S6 was applied,the proportion of cadmium in the root cell wall component of K326 decreased(from 17% to 9%),and the percentage of cadmium extracted from root deionized water increased(from 27% to 30%),which increased the percentage of cadmium with higher activity in roots.However,Maxopka cell wall cadmium increased(from 24% to 27%),the percentage of sodium chloride extracted cadmium increased(from 34% to 41%),and the percentage of cadmium with higher activity decreased(from 49% to 45%)in roots.Secondly,By measuring the growth status(biomass,root activity,chlorophyll and photosynthetic characteristics),stress resistance level(malondialdehyde),nutrient elements(mineral elements),sulfur metabolites(sulfhydryl substances)related indicators and gene expression(cadmium transport gene,sulfur assimilation gene,sulfur transport gene)of K326 and Maxopka after different sulfur or(and)cadmium treatments,the causes of sulfur affecting cadmium accumulation in tobacco were studied from physiological,biochemical and gene expression.Compared with Cd12 S0,the root activity of Cd12 S6 increased by 131.42%(K326)and 197.63%(Maxopka),respectively.There was no significant difference in the root activity of Cd12 S12.The chlorophyll content,net photosynthetic rate,stomatal conductance and transpiration rate of Cd12 S6 increased significantly,and Cd12 S12 inhibited photosynthesis.The MDA content in roots and leaves of K326 and Maxopka decreased sharply under Cd12 S6,while the MDA content of Cd12 S12 increased significantly compared with Cd12 S6.The correlation between cadmium accumulation and sulfhydryl content in roots and leaves of K326 and Maxopka was analyzed.The accumulation of cadmium in roots of K326 was significantly positively correlated with glutathione,non-protein sulfhydryl and phytochelatin,and the accumulation of cadmium in leaves was significantly positively correlated with glutathione.The accumulation of cadmium in the roots of Maxopka was significantly positively correlated with glutathione,non-protein thiol and metallothionein,and the accumulation of cadmium in leaves was significantly positively correlated with non-protein thiol and metallothionein.Compared with Cd12 S0,Cd12 S6 significantly increased Nt ZIP1,Nt ZIP3,Nt ZIP4,Nt HMA2α,Nt HMA2β,Nt YSL7,Nt NRAMP3,Nt GSH1,Nt PCS1,Nt SAT1,Nt SAMS1,Nt SULTR1 in K326 roots.The expression levels of Nt YCF1 and Nt CAX2 were significantly reduced.The expression levels of Nr ZIP3,Nr HMA2β,Nr YSL7 and Nr SAMS1 in Maxopka roots were significantly decreased,and the expression levels of Nr ZIP1,Nr ZIP4,Nr NRAMP3,Nr YCF1,Nr CAX2,Nr GSH1,Nr PCS1,Nr SAT1,Nr SULTR1 and Nr SULTR2 were significantly increased.Thirdly,the causes of exogenous sulfur affecting cadmium accumulation in tobacco were analyzed by transcriptomics and metabolomics.Gene-metabolite joint analysis showed that the response of Cd12 S6 to cadmium stress had the same metabolic pathwaylinoleic acid metabolism in K326 and Maxopka.In K326,glutathione metabolism plays a role in Cd12 S6 and Cd12 S12.In Maxopka,cyanoamino acid metabolism and alanine,aspartate and glutamate metabolism play a role in Cd12 S6 and Cd12 S12.Three conclusions are drawn from the above results.First,Cd12 S6 enhanced the ability of K326 to accumulate cadmium in leaves.Cd12 S12 aggravated the cadmium toxicity of K326 and reduced its ability to absorb and accumulate cadmium.Both Cd12 S6 and Cd12 S12 enhanced the ability of Maxopka to accumulate cadmium in roots.Second,in K326,compared with Cd12 S0,Cd12 S6 enhanced the ability to absorb cadmium.The reasons are as follows.For one thing,the gene expression of proteins related to cadmium absorption increased.For another thing,regulating a variety of key genes and metabolites in the linoleic acid metabolic pathway changed the content of unsaturated fatty acids,and reduced the degree of membrane lipid peroxidation.And thirdly,regulating the glutathione metabolic pathway increased the content of antioxidants such as ascorbic acid,and alleviated cadmium-induced oxidative stress.And Cd12 S6 increased the mobility and transport capacity of cadmium in roots and enhanced the accumulation of cadmium in leaves.The reasons are as follows.For one thing,the gene expression of proteins related to cadmium transport from roots to shoots increased.For another thing,the gene expression of proteins related to the transport of cadmium to root vacuoles decreased.And thirdly,glutathione may accelerate the transport of cadmium from roots to shoots,rather than storing cadmium in vacuoles.Third,in Maxopka,compared with Cd12 S0,Cd12 S6 enhanced the ability to absorb cadmium.The reasons are as follows.For one thing,the gene expression of proteins related to cadmium absorption increased.For another thing,regulating a variety of key genes and metabolites in the linoleic acid metabolic pathway changed the content of unsaturated fatty acids,and reduced the degree of membrane lipid peroxidation.And thirdly,cyanoamino acid metabolism and alanine,aspartate and glutamate metabolism increased the content of plant chelating agents that could bind to cadmium.Cd12 S6 reduced the mobility and transport capacity of cadmium in roots and enhanced the accumulation of cadmium in roots.The reasons are as follows.For one thing,it enhanced the ability of the root cell wall to retain cadmium.For another thing,the gene expression of proteins related to the transport of cadmium to vacuoles in roots increased,which enhanced the ability of vacuoles to separate cadmium.And thirdly,the increased phytochelatins in cyanoamino acid metabolism and alanine,aspartate and glutamate metabolism combined with cadmium reduced the activity and fluidity of cadmium.