The Mechanism Of Arbuscular Mycorrhizal Fungi In Improving Cd Tolerance And Bioenergy Quality For Switchgrass

Switchgrass(Panicum virgatum)is one of the most important cellulose energy grass which could be converted to produce bioenergy to replace traditional fossil fuels.Arbuscular mycorrhizal fungi(AMF)could form symbiotic relationships with 80%plants on the earth,which could improve plant growth and heavy metal tolerance by enhance nutrients absorption and changing heavy metal absorption and transfer in plants under heavy metal stress.In order to obtain excellent biomass materials and improve the ecological environment in Cd contaminated areas simultaneously,our study selected the proper switchgrass varieties based on the plant biomass,Cd absorption and extraction,bioenergy related equality with AMF inoculation,and furtherly explored the selected cultivar physiological,biochemical and transcriptome mechanism to Cd with AMF presence.In addition,we explored whether the biochar,phosphor addition and intercropping could furtherly improve the inoculated switchgrass growth and Cd tolerance under high Cd addition(20 mg/kg).The main results are as follows:(1)AMF inoculation significantly enhanced the shoots and roots biomass and P concentration for all switchgrass cultivars.AMF inoculation had no significant effects on the Cd concentrations of all cultivars under 0 and 10mg/kg Cd treatments,but their effects on Cd concentrations under 1 mg/kg were varied.The Cd concentrations in the shoots and roots of Ala and roots of Kan were reduced,and the opposite trend were observed in the roots of Bw and Per,and shoots and roots of Sum.There was no significant difference between AMF presence and absence for Cd extractions of lowland cultivars,but they varied in different cultivars:Bw contained the highest total Cd extraction(mainly in the root);Ala had the second highest Cd extraction in shoots,which attained to 32%when the soil Cd addition was 1mg/kg,while the Cd extraction in Sum was the lowest.AMF symbiosis significantly affected the biological energy related-quality of switchgrass:including total carbon,total nitrogen,C/N,gross calorific value and alkali elements(Na,Ca,Mg),while it had no significant effects on the ash,hemicellulose,cellulose and K contents.With three Cd addition levels,AMF presence decreased the lignin contents of Ala and C/N in Sum,and increased TC and GCV contents in Sum.With AMF inoculation,Bw have fixed highest Cd in its roots;Ala had strong Cd extraction ability and superior energy quality.(2)Compared with non-inoculated treatments,the growth index(plant height,leaf width,length,tillers number and main stem width),Chla contents and number of starch grains,the biomass and P content increased with AMF inoculation under different Cd levels(0-40mg/kg),and the chloroplast ultrastructures kept integrity in inoculated switchgrass(Alamo).Although AMF symbiosis reduced the Cd concentrations in different tissues,the Cd extraction significantly increased in per pot.The contents of WSC,starch and soluble protein were increasing in the shoots with AMF presence,whereas the starch contents showed the opposite trend in the roots.AMF treatment did not improve the antioxidant activity in the shoots of switchgrass,but the contents of ASAFR increased and H2O2 content decreased.The activities of POD and CAT in the roots with AMF inoculated treatments were higher than the non-inoculated treatment under all Cd levels.PCA analysis found that AMF could regulate more factors,including mineral elements,osmotic substances,antioxidant enzyme activities to protect switchgrass from Cd toxicity.(3)RNA-seq technique was used to determine the inoculated root transcriptome under Cd stress(5 mg/kg),It was found that the genes with AMF inoculation were upregulated which were involved in transporters of P and other mineral ions,the enzymes of sugar synthesis and transporter,the secondary metabolites,endonuclease and heat shock proteins related AMF colonization compared with non-inoculated treatment.Cd addition induced the genes differentially upregulated in the roots with AMF symbiosis,which was mainly involved in ion transporters,antioxidant enzymes,P and sugars transport,and polyhydroxy compounds.With Cd addition,AMF inoculation up-regulated antioxidant enzymes,mineral ion transport and amino acid synthase genes compared with NM treatment.A total of 64 metabolic pathways were obtained from AM and NM treatments,including pentose phosphate pathway,tricarboxylic acid cycle,fructose mannose metabolism,and metabolism of secondary compounds.AM-Cd and AM treatment co-enriched 79 metabolic pathways,including secondary biological metabolism,phenylpropanoid metabolism,fatty acid degradation,amino acid metabolism and selenium compound metabolic pathways.AM-Cd and NM-Cd treatments upregulated 27 metabolic pathways including synthesis of secondary metabolites,nitrogen metabolism,amino acid metabolism,glutathione metabolism,sucrose starch metabolism,biosynthesis of keratin,berberine and wax,etc.(4)In our study exogenous treatments were used to furtherly improve Cd tolerance for inoculated switchgrassunder the high Cd treatments(20 mg/kg),which including control(CK),Biochar addition(B),phosphorus addition(P),B+P and intercropping(IC).The results showed that AMF+P significantly promoted the root colonization.AMF significantly increased the relative chlorophyll contents(SPAD),shoots and root biomass,growth rate irrespective of exogenous treatments;but compared with AMF+CK,only AMF+B significantly increased the SPAD,there was no significantly differences between other exogenous treatments and CK with AMF presence.AMF+B and AMF+B+P treatments significantly increased P and roots Cd concentration of switchgrass,while AMF+B and AMF+IC treatment increased the Cd concentration in the shoots.Inoculation with AMF increased soil pH and decreased soil acid phosphatase activity with B+P addition treatments.The addition of B and B+P reduced the acid-extracted Cd contents in the soil,but their residual-Cd contents increased with AMF inoculation.Compared with CK,the residual-Cd content in AMF+B were lower,there was no significantly differences between AMF+B+P and AMF+CK.Thus,inoculation with AMF+B might promote Cd uptake by converting residual-Cd into acid-extracted Cd to promote Cd absorption to the shoot and root;AMF+B+P might reduce soil Cd bioactivity by increasing soil pH.