Alleviation Of Arsenic And Cadmium Toxicity In Barley By Calcium And Calcium Oxide Nanoparticles(CaO NPs) And Their Mechanisms

Heavy metal(loids)such as arsenic(As)and cadmium(Cd)contamination in agricultural soils is a major issue for crop production and food security.Although several ex-situ and in-situ remediation techniques have been employed for dealing with As and Cd in the contaminated soils,they are not widely used because of the obvious drawbacks i.e.,environmental health hazards and large capital investment.Consequently,there is an imperative need to develop innovative approaches for reducing As and Cd toxicity in agricultural soils.In recent years,Ca and Ca-based nano additives have gained great consideration due to their potential functions in effectively alleviating As and Cd toxicity in plants.However,the exact roles of Ca-NPs in mitigating As and Cd toxicity and the relevant mechanisms are still unclear.Hence,the current study was conducted to investigate phytotoxic effects of As and Cd stresses on barley plants and the potential application of Ca and Ca-based nano-additives in alleviating As and Cd toxicity.The study also illustrated the physiological and molecular mechanisms of As and Cd mitigation in barley by Ca and Ca O NPs.The main results are as follows:1.L-glutamate and calcium show the opposite effects on the phyto-toxicity of arsenic in barleyExcessive arsenic(As)is toxic to living organisms,including plants.It is poorly understood how L-glutamate(Glu)and calcium(Ca)regulate the As-induced toxic effects in plants.In the present study,we investigated the effects of Glu or Ca on As toxicity at 3 d and 10 d old barley seedlings.The results showed that As drastically inhibited the plant growth and biomass,levels of photosynthetic pigments,photosynthetic efficiency,mineral nutrient uptake,and induced the production of reactive oxygen species(ROS)and malondialdehyde(MDA)in both shoots and roots.In addition,the subcellular distribution of As was more pronounced in protoplasts than cell walls,and the expressions of As-transporter genes were greatly modulated under As stress.The supply of Ca minimized As-toxicity by improving antioxidant capacity and nutrient uptake.On the other hand,the addition of exogenous Glu further escalated the As-mediated phytotoxic effects by depressing the activities of antioxidative enzymes,reducing nutrient uptake and extensively promoting root-to-shoot translocation of As.The resuts confirmed the opposite effects of Glu and Ca on the growth and physiology of barley under As stress.2.Calcium oxide nanoparticles have the role of alleviating arsenic toxicity of barleyThe rapid development of nanotechnology appears to provide a novel solution to soil As contamination.This study investigated the roles of calcium oxide nanoparticles(Ca O NPs)in alleviating As toxicity in two barley genotypes(LJZ and Pu-9)differing in As tolerance.The exposure of barley seedlings to As stress showed a significant reduction in plant growth,calcium and chlorophyll content(SPAD value),fluorescence efficiency(Fv/m),and a dramatic increase in the contents of reactive oxygen species(ROS),malondialdehyde(MDA)and As,with LJZ being more affected than Pu-9.The exogenous supply of Ca O NPs notably alleviated the toxic effect caused by As in the two barley genotypes.Moreover,the expression of As transporter genes,i.e.,Hv PHT1;1,Hv PHT1;3,Hv PHT1;4 and Hv PHT1;6,was dramatically enhanced when barley seedlings were exposed to As stress and significantly reduced in the treatment of Ca O NPs addition.It may be concluded that the roles of Ca O NPs in alleviating As toxicity could be attributed to its enhancement of Ca uptake,ROS scavenging ability and reduction of As uptake and transportation from roots to shoots.3.Exogenous calcium oxide nanoparticles can alleviate cadmium toxicity of barley seedlingsIn this study,we used two barley genotypes,LJZ(Cd sensitive)and Pu-9(Cd tolerant),to study the effects of exogenous calcium oxide nanoparticles(Ca O NPs)in alleviating Cd stress.Cd exposure of barley plants led to a significant reduction in nutrient content,photosynthetic rate,and large accumulation of Cd in plant tissues.However,Ca O NPs application significantly increased plant biomass,activities of anti-oxidative enzymes(i.e.,ascorbate peroxidase,catalase,superoxide dismutase,and glutathione reductase)and the content of non-enzymatic antioxidants(ascorbate and glutathione)accompanied by great reduction of malondialdehyde(MDA)and hydrogen peroxide contents under Cd stress.Furthermore,Ca O NPs increased the expression of the genes associated with anti-oxidative enzymes.The alleviation of Cd stress by Ca O NPs is more obvious in Pu-9 than in LJZ.It may be suggested that Ca O NPs can be used as a potential chemical to alleviate Cd uptake and toxicity of the crops planted in the Cd-contaminated soil.4.Transcriptomic analysis reveals the difference in response to arsenic and cadmium in barley roots exposed to Ca O NPsIn this study,transcriptomic analysis were performed on the response to As and Cd stresses in barley roots treated with Ca O NPs.A total of 39954 differentially expressed genes(DEGs)were inclusively expressed in the two metal stresses and Ca O NPs addition.These DEGs included stress responsive,transport related and transcription factors.Ca O NPs addition in the solution medium repressed the KEGG pathways related to ABC transporters,glutathione metabolism,and RNA transport.A novel transcript ethylene-responsive transcription factor(ERF110;novel 1758)was exclusively expressed in the As-or Cd-stressed barley roots treated with Ca O NPs,indicating the unique roles of Ca O NPs in alleviating As and Cd toxicity.Nutrient transporters were more upregulated in the Cd-than in As-treated roots,while Ca O NPs addition caused the up-regulation of nutrient transporters in As and Cd exposed roots.It may be concluded that Ca O NPs have the function of restricting As and Cd uptake and suppressing the DEGs induced by both Cd and As.