Distribution And Response Mechanism Of Cadmium In Arbuscular Mycorrhizal Fungus-phragmites Australis Symbiosis

With the development of economy,the demand for heavy metal gradually increases and have caused serious heavy metal pollution.Cadmium(Cd),one of the key object of "twelfth five-year" special plan and soil pollution control plan,caused a serious threat to the environment,therefore how to deal with Cd pollution in a safe,economic,and effective way has become an urgent need for the sustainable development of soil.At present,phytoremediation has been widely applied for several advantages such as low cost and environmental friendliness.However,using single plant or hyperaccumulator to remeidate Cd pollution have following drawbacks: low remediation efficiency,slow growth,long time remediation,or plant stunt.This study uses arbuscular mycorrhizal fungi(arbuscular mycorrhizal fungi,AMF)and Phragmites australis(Cav.)Trin.ex Steud.to build a symbiotic system for illustrating the response of on the growth changes under Cd stress,analyzing Cd chemical forms and distribution,and unravelling the response mechanism in AMF-P.australis symbiosis.This study builds Rhizophragus irregularis-P.australis symbiosis and illustrates the effects of AMF on plant growth,photosynthesis,and uptake of nutritive elements under different Cd stress in the substrate.The results showed that Cd could inhibit the growth of P.australis seedlings.The inoculation of AMF alleviated the inhibition of growth induced by Cd.Compared to control treatment,the stem diameter in mycorrhizal treatment increased 5.8%~36.8%.The maximal increases of root biomass and root-to-shoot ratio were 93.5% and 208% under 2 mg Cd L-1 stress,respectively.Under the same Cd stress,there were significant differences of eight nutritive elements concentrations in P.australis between mycorrhizal and mycorrhizal treatments.Zn,Mn,Cu,Fe,and Mg were mainly distributed with the proporation of 40%~98% in roots of P.australis.Most of Ca was in leaves of P.australis.The distributions of K and P in each organ of P.australis were equally.The competition between Cd and Mn in the roots and stems was alleviated via inoculation with R.irregularis,as well as the P concentration in the leaves.To unravel the effects of AMF on Cd chemical forms and distribution in P.australis,the distribution and translocation of Cd in roots,stems,and leaves of plants as well as subcellular distribution and chemical forms were illustrated under different Cd stress.The results showed that the highest Cd concnetrations were observed in the roots regardless of AMF inoculation,but the effects of AMF on Cd concentrations in stems and leaves were affeted by Cd concentration in the substrate.The AMF inoculated P.australis was regarded as “indicator”.Under low Cd stress(0-1 mg L-1),the inoculation of AMF could improve the Cd translocation to shoots of P.australis,while AMF could immobilize Cd in the roots and inhibit Cd translocation t o shoots under low Cd stress(1-20 mg L-1).Under 20 mg Cd L-1 stress,the results of nonlinear fitting equation showed that the saturated concentration of Cd in roots increased from 1252.82 mg kg-1 DW to 1546.22 mg kg-1 DW.At subcellular levels,Cd was mainly distributed in the cell wall,followed by the cell fluid,finally in organelles.The inoculation of AMF could alleviate Cd toxicity in P.australis by increasing Cd retention in cell wall and Cd sequestration in vacuoles as well as reducing the Cd distribution in organells.In addition,the six chemical forms of Cd in organs of P.australis was followed by roots,stems,and leaves.The inoculation of AMF could significantly increase the Cd concentration and proporation of inactive forms(undissolved Cd,oxalate Cd,and residue)in all organs of P.australis.To unravel the improvement of AMF on Cd response machanism in roots of P.australis,the uptake and distribution of Cd under different treatment time,the effects of AMF on antioxidative system and the uptake of nutritive elements,and effects of AMF on pathways of Cd phytotoxicity in roots of P.australis by principal component analysis(PCA)were illustrated under different Cd stress in the substrate.The results showed that Cd could be taken by roots of P.australis in 24 h,and Cd concentration in roots increased with the Cd concentration added in the substrate.The inoculation of AMF could improve and inhibit Cd uptake in roots under low and high Cd stress,respectively.increasing the treatment time and Cd concentration added in the substrate induced a marked intensity of fluorescence in roots of P.australis.AMF could alleviate oxidative stress induced by Cd via significantly decreasing malonaldehyde(MDA)and proline(Pro)concentrations.However,the effects of AMF on antioxidative enzymatic activtives and levels of antioxidants was depend on Cd concentration added in the substrate.Meanwhile,the effects of AMF on six nutritive elements concentrations and relative concentrations in roots of P.australis were varied with different Cd stress.Furthermore,the PCA analysis showed that the inoculation of AMF could be involved in the change of Cd-induced antioxidative defense,the interfered physicological process by combining Cd with thiols groups,and the competition between Cd and essential elements for the transport pathways or sites,which could alleviate the Cd phytotoxicity in the roots of P.australis and enhance the Cd tolerance to host plants.The decrease in phytotoxicity was mainly accomplished by mentinoned three pathways with the changes in superoxide dismutase(SOD),catalase(CAT),non-protein thiols(NPT),Ca,Mn,and Cu.Therefore,the R.irregularis-P.australis symbiosis could not only alleviate the inhibitation of plant growth,but also enhance Cd tolerance via regulating the uptake and translocation of Cd.This study provides new theoretical support and insight into practice for remediating heavy metals pollution by AMF-plant combined remediation technology and fill the gaps of practical application in traditional phytoremediation technology.