Study On The Promoting Effect Of Aba On Cd Accumulation Of Sedum Alfredii Based On Omics Technology

Sedum alfredii,as a cadmium(Cd)and zinc(Zn)hyperaccumulator,plays an essential role in the remediation of heavy metal contaminated farmland.However,like many other hyperaccumulators,it also has some shortcomings such as long growth periods,low shoot biomass,which limit its application.Thus,increasing Cd accumulation in the shoot of S.alfredii is a critical way to improve the efficiency of phytoextraction.It might have a decisive significance for the application of S.alfredii in phytoremediation.Phytohormones are widely used as plant growth regulators in agricultural production.Abscisic acid(ABA)is a plant stress response hormone.Most studies have shown that ABA can inhibit the uptake and accumulation of Cd by plants and alleviate the toxic effects caused by Cd.However,our previous research found that ABA promotes the accumulation of Cd in S.alfredii.That may be a unique mechanism for hyperaccumulators to uptake and accumulate an ultra-high level of heavy metals,but similar studies have been rarely reported.In this paper,S.alfredii,a hyperaccumulator,was used to systematically analyze the influence of ABA and Cd on the accumulation of Cd.Furthermore,through transcriptome,metabolome,and ion omics analysis,the regulation of ABA on promoting the accumulation of Cd in S.alfredii was studied from a systematic level.The relevant research results not only provide new ideas for further revealing the hyperaccumulation mechanism of S.alfredii on Cd but also lay a foundation for expanding the application of S.alfredii in the phytoremediation on heavy metal contaminated soil.The research results of this paper are as follows:(1)There was a dose-effect of ABA on the accumulation of Cd in the shoot of S.alfredii.Under the conditions of 0.1 mg/L to 0.5 mg/L ABA treatment,0.2 mg/L ABA treatment promoted the accumulation of Cd in the shoot most obviously.Too low or too high ABA treatment concentration will weaken the promotion effect.Also,the effect of ABA was affected by the concentration of Cd treatment.In our test,ABA could promote Cd accumulation in S.alfredii shoot only in 100 ?mol/L Cd treatment.At too high or too low Cd treatment concentration,ABA can inhibit the accumulation of Cd in the shoot of S.alfredii.The endogenous ABA content in roots was significantly positively correlated with the content of Cd(Pearson correlations = 0.52,P <0.05)and the biomass(Pearson correlations = 0.44,P <0.05)of the shoot.With the analysis of ROS content,it was found that exogenous ABA may affect endogenous ABA by affecting the resistance enzyme activity to affect ROS content.(2)ABA affects the expression of genes known related to Cd accumulation in S.alfredii.Characteristic genes were filtered by orthogonal partial regression discriminant analysis(OPLS-DA)and correlation analysis of gene expression and endogenous ABA content.It was found that the HMA family,Hsf A4 c,CAX,and NAS play essential roles in ABA affecting Cd accumulation.Among them,HMA4 had the most significant effect.When ABA was applied under 100 ?mol/L Cd treatment,the expression above ground was up-regulated 2.39 times,and the expression in roots was up 23.72 times.By analyzing the expression rules of all characteristic genes,it can be inferred that exogenous ABA affected the expression of Cd accumulation-related genes of S.alfredii by affected the content of endogenous ABA,and then affected Cd accumulation.Endogenous ABA mainly switches the main Cd upward transporter from HMA2 to HMA4,and at the same time,more Cd was stored in the cell wall of the shoot to promote the accumulation of Cd in the shoot.(3)Transcriptomics studies found that the application of ABA under 75 ?mol/L Cd treatment produced 389 and 1588 differentially expressed genes in the shoots and roots.Furthermore,the application of ABA under 100 ?mol/L Cd treatment produced 1107 and 2625 differentially expressed genes in shoots and roots,respectively.Based on the analysis of differentially expressed gene sets and GO and KEGG enrichment results,cell wall synthesis,cell wall components,and energy metabolism-related pathways were enriched in the shoots at 100 ?mol/L Cd,and essential node genes in these pathways were up-regulated by ABA.Regarding metal transporters,ABA up-regulated expression of shoot ZIP1 promoted Cd transport from root to shoot.Under 75 ?mol/L Cd,although the shoot cell wall componentrelated metabolic pathways were enriched,the related genes were down-regulated by ABA;the expression of Nramp5 in the shoot was also down-regulated;more metal chelate metabolismrelated genes were up-regulated in the root.It was shown that under 100 ?mol/L Cd treatment,ABA promotes the shoot cell wall synthesis,cell wall component modification,metal chelate synthesis,and energy metabolism,while inhibiting the transfer of Cd into vacuoles increasing Cd accumulation in the shoot.Under 75 ?mol/L Cd treatment,ABA promoted the segments of Cd in the root,reduced the expression level of Cd transporter,reduced the modification of cell wall components,and reduced the accumulation of Cd in the shoot.In summary,the effect of ABA on the gene expression pattern of S.alfredii under different Cd treatment concentrations was different.(4)Metabolomics analysis found that the effects of ABA on S.alfredii were similar to those in transcriptomics analysis.Under 100 ?mol/L Cd treatment,ABA can also promote the accumulation of Cd in the shoot of S.alfredii by promoting the same metabolic pathway.These include: In terms of cell wall synthesis,the phenylalanine synthesis pathway is enriched upstream of the lignin precursor synthesis pathway.As to metal chelate synthesis,the glutathione metabolic pathway is enriched.In terms of energy metabolism,the starch sucrose metabolism pathway is enriched;at the same time,sucrose and glucose levels were measured and increased after ABA application.It is found that there may be spatiotemporal differences in response to ABA in S.alfredii by comparing the results of transcriptome and metabolome analysis.That is,in the transcriptomics analysis,there was more KEGG pathway enriched in the shoot of the 75 ?mol/L Cd treatment group,and there was almost no KEGG pathway enriched in the root of the 100 ?mol/L Cd treatment group.While in the metabolomics analysis,there was more KEGG pathway enriched in the root of the 100 ?mol/L Cd treatment group,and there was almost no KEGG pathway enriched in the shoot of 75 ?mol/L Cd treatment group.It can be inferred that compared with 75 ?mol/L Cd treatment,only responded at the gene transcription level,and 100 ?mol/L Cd treatment had responded at the metabolite level.It is shown that the higher the Cd treatment concentration for ABA signals,the faster the response of S.alfredii to it,and this effect eventually lead to differences in the effects of ABA on the accumulation of Cd in the shoot of S.alfredii under different Cd concentrations.(5)In the analysis of ion omics,it was found that ABA has a greater effect on the ion metabolism pattern of S.alfredii,among which the effect on the absorption of divalent metal ions is the most obvious.There was a co-transport effect between Cd and Cu in all characteristic divalent ions(Pearson correlations = 0.33,P <0.05),and there is a co-absorption effect of Cd and Fe in all characteristic divalent ions(Pearson correlations = 0.64,P <0.05).It is worth noting that the Fe content in the roots of S.alfredii under the treatment of 100 ?mol/L Cd increased by 3.89 times,suggesting that Fe might participate in Cd absorption and transport in S.alfredii.At the same time,ABA could also alleviate the toxicity of Cd to plants by promoting the absorption of B,S,and other elements.K might be involved in the energy metabolism process affected by ABA.Furthermore,Ca might be involved in ABA signal transduction,which promotes the transportation of Cd to the shoot in S.alfredii.The above results preliminarily revealed the effect of exogenous ABA on the accumulation of Cd in the shoot of the S.alfredii.That is,exogenous ABA and Cd together affect the content of endogenous ABA,leading to changes in metal transporters,cell wall,metal chelate synthesis,and energy metabolism,which ultimately affects Cd accumulation in the shoot of S.alfredii.At different Cd treatment concentrations,the differences in the effects of exogenous ABA on metal transporters and energy metabolism-related pathways,and their different effects on the cell wall and metal chelate synthesis in roots and shoots,lead to differences in Cd compartmentalization sites.This is the reason for the difference in the effect of exogenous ABA on the accumulation of Cd in the aerial parts of S.alfredii under different Cd treatment concentrations.