| Heavy metal(HM)pollution poses a serious threat to the quality and safety of the ecological environment.Phytoremediation is an important approach to eliminate environmental risk of HMs.At present,hyperaccumulators and transgenic technology are the two most important methods for phytoremediation of HMs pollution.However,these two technologies have some shortcomings such as difficulty in variety screening,long-term research and development,high potential risk and uncertainty.Hormesis is described as a biphasic dose-response with low-dose stimulation and high-dose inhibition.The experimental models include plants,animals,microorganisms,etc.The endpoints are biomass,root length,growth rate,metabolic enzyme activity,and stress resistance of plants.A large number of studies have shown that the stimulatory range is generally 30–60% higher than that of the control group,and the ratio of the highest to the lowest dose is frequently located in a zone usually 10–20 fold below the estimated threshold.Therefore,Hormesis is considered to be an evolutionary attribute formed by organisms in the process of coping with stressors.The hormesis mechanisms mainly include dual-receptor theory,overcorrection theory,cascade effect,bystander effect,and overcompensation theory.However,these theories are still controversial,and exploring the mechanism at the molecular level will become a new research hotspot.We supposed that hormesis in plants is regulated by some specific genes,and the key genes regulating the effects are excavated and their biological functions are verified,so that they can be overexpressed in plants,significantly promoting plant growth and enhancing plant resistance,which is expected to provide new inspiration for phytoremediation of HM-contaminated environments.In this paper,the model plant Arabidopsis thaliana(A.thaliana)was used as the research object to study the response of plant to cadmium(Cd)stress.(1)The biomass(fresh weight and dry weight)and root length of A.thaliana were measured to explore the dose-response relationship between a series of Cd doses(0,0.003,0.03,0.3,0.6,1.2,2.4,3.0,6.0,and 12.0mg/L),analyzing the potential hormetic effects of biomass and root length of A.thaliana induced by Cd;(2)Determination of photosynthetic pigments(chlorophyll a,chlorophyll b,and total chlorophyll),antioxidant enzymes(superoxide dismutase,SOD;catalase,CAT)and decomposition products of membrane lipid peroxidation(malonaldehydes,MDA)to study the effects of different doses of Cd(0 and 0.003–12.0 mg/L)on chlorophyll a,chlorophyll b,total chlorophyll,SOD,CAT,and MDA,analyzing the physiological and biochemical mechanisms of Cd-induced hormesis in A.thaliana;(3)Transcriptome sequencing analysis of A.thaliana under0 mg/L,stimulation and inhibition doses of Cd,mining the key genes regulating Cd-induced hormesis in A.thaliana based on the trend chart of differentially expressed genes(DEGs),the KEGG metabolic pathway,and the weighted gene co-expression network analysis(WGCNA);(4)Three groups of wild type(WT),overexpression type(OE),and mutant type(MT)of key genes were generated to analyze the phenotypic figures of A.thaliana under 0 mg/L Cd,stimulation and inhibition doses of Cd stress,and verifying the biological functions of key genes.The main results and conclusions of this paper are as follows:(1)The dose-response relationships between different doses of Cd(0 and 0.003–12.0 mg/L)and biomass and root length of A.thaliana exhibited the inverted U-shaped curve.The stimulatory dose range corresponding to fresh weight,dry weight,and root length were 0.003–2.4 mg/L,0.03–2.4 mg/L and 0.003–1.2 mg/L,respectively;and the stimulatory magnitude were46.9–52.8%,49.0–53.3%,and 23.2-57.1%;the corresponding inhibitory dose range of Cd was3.0–12.0 mg/L,and the inhibition rates were 22.9–84.1%,23.0–92.9%,and 18.5–87.0%,respectively.Obviously,A.thaliana induced by Cd showed a significant hormesis phenomenon with “low-dose stimulation and high-dose inhibition”.(2)Under different doses of exogenous Cd(0 and 0.003–12.0 mg/L),chlorophyll a content,total chlorophyll content,SOD activity,and CAT activity in A.thaliana exhibited a significant hormesis effect of “low-dose stimulation and high-dose inhibition”.Correlation analysis indicated that chlorophyll a,chlorophyll b,total chlorophyll,SOD,and CAT were significantly positively correlated with biomass and root length(P < 0.01).Therefore,physiological and biochemical indexes such as chlorophyll,SOD,and CAT are closely related to Cd-induced hormesis in A.thaliana.(3)Transcriptome sequencing of A.thaliana under five doses of Cd(0,0.03,0.3,3.0 and6.0 mg/L)was analyzed.The results showed that 27,091 of expression genes was detected,including 26,913 known genes and 178 new genes.Analysis of the relative expression levels of DEGs under different Cd doses revealed a total of 137 DEGs with “a low-dose stimulation and a high-dose inhibition”.Based on the WGCNA,four groups of gene module significantly related to growth and physiological and biochemical indexes were obtained,namely turquoise module,greenyellow module,black module,and blue module.Base on the trend chart of DEGs,KEGG metabolic pathway analysis,and WGCNA,the key gene of ABF1 was finally identified.(4)Compared with the WT,the relative expression level of ABF1 in OE was significantly increased(P < 0.05),while the relative expression level of ABF1 in MT type was evidently decreased.Under the exposure of 0,0.03,0.3,3.0,and 6.0 mg/L Cd,the biomass and root length of WT and OE showed a significant “inverted U-shaped” characteristics.Under the same Cd stress,the order of biomass and root length was OE > WT > MT.Compared with the WT,the maximum stimulation dosage of OE increased from 0.3 mg/L to 6.0 mg/L,and the maximum stimulation range promoted from 13.3% to 25.0% in response to Cd stress.It is thus clear that OE has significantly enhanced stress resistance to exogenous Cd stress but decreased in MT,which indicated that ABF1 is the key gene involved in regulating Cd-induced hormesis in A.thaliana.Generally,Cd induced Arabidopsis to exhibit significant hormesis.Overexpression of ABF1 can significantly improve the growth(biomass and root length)of A.thaliana and enhance the resistance of A.thaliana to exogenous Cd stress(stimulation range and maximum stimulation magnitude),which provide a new method for phytoremediation of HM-contaminated environments. |
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