Identification Of Ectomycorrhizal Fungi To Cd Tolerance And Investigation Of The Mechanism In Enhancing Cadmium-Tolerant Of Quercus Acutissima Seedlings

Soil heavy metal pollution,especially cadmium pollution,has been focused on and paid much more attention in the pollution remediation due to its long history of pollution,a wide contaminated area and high toxicity.The ectomycorrhizal fungi-plant symbionts is widespread in forest ecosystems.The ectomycorrhiza is very important for the growth of woody plants and resistance to adverse environments.Therefore,it is important for phytoremediation to exploit strain resources and study the resistance mechanism of ectomycorrhizal fungi and mycorrhizal plants to heavy metals.In this study,four ectomycorrhizal fungi,Suillus luteus,Suillus bovinus,Pisolithus tinctorius,and Cenococcum geophilum,were selected.The resistance and enrichment ability of the strains to different Cd concentrations were studied under pure culture conditions,and the mechanisms of tolerance of P.tinctorius and S.luteus to be further studied by cell morphology,extracellular secretions,and antioxidant system.On that basis,the effects of S.luteus associated with oak seedlings?Quercus acutissima Carruth.?on the growth of oak seedlings,photosynthesis,antioxidant system,and element absorption and transport under Cd stress were investigated.The results of this study provide a scientific basis for clarifying the tolerance mechanism of ectomycorrhizal fungi and its associated host plant's to Cd.And the findings also provide the scientific support for phytoremediation and ecological reconstruction of heavy metal contaminated soil using ectomycorrhizal fungi.The main results are as follows:1.The lower Cd concentration(0.01,0.1,1 mg L^-1)has no significant effect on the colony diameter of C.geophilum and S.bovinus,while the colony diameter of P.tinctorius and S.luteus showed a dose-response to Cd stress.However,the dry mycelium weight of P.tinctorius and S.luteus were higher than C.geophilum and S.bovinus.C.geophilum and S.bovinus.However,the tolerances of C.geophilum and S.bovinus.C to Cd were higher than P.tinctorius and S.luteus.But their Cd enrichment capacities are opposite.2.TEM images show that heavy metal Cd stress mainly damages the membrane system of ectomycorrhizal fungi cells,causing severe deformation of P.tinctorius cells.S.luteus cells can thicken the cell wall to resist heavy metal stress,indicationg S.luteus is more resistant to Cd than P.tinctorius.3.With the increasing Cd concentration,the total amount of organic acids secreted by P.tinctorius showed a significant upward trend and S.luteus was reduced first and then increased.Succinic acid was the main composition of organic acid secreted by the two strains.Both P.tinctorius and S.luteus can secrete hydroxamic acid siderophores,and they are significantly reduced under Cd stress.4.For P.tinctorius,low concentration of Cd(0.1,1 mg L^-1)enhances SOD activity and inhibites CAT activity reduced.The high concentration of Cd(10 mg L^-1)significantly enhanced both enzyme activities.For S.luteus,the SOD activity was enhanced under a low concentration of Cd,and its activity was inhibited by a high concentration of Cd,while the CAT activity was not affected by Cd stress.With the increasing Cd concentrations,MDA content in boht strains increased significantly.The changes in APX activity and ASA content were consistent with MDA,showing a significant increase trend.However,GR activity and GSH content showed a significant opposite trend.5.Inoculation with S.luteus has significantly promoted the growth of leaves and roots of Q.acutissima seedlings.The net photosynthetic rate,transpiration rate and stomatal conductance of non-mycorrhizal seedlings gradually increased with the increasing Cd concentration.The inoculation treatment significantly decreased the photosynthetic capacity,indicating it is the possible way to enhance the tolerance of Q.acutissima seedlings under Cd concentrations.6.Compared with non-mycorrhizal seedlings,the inoculation has different effects on the antioxidant defenses of Q.acutissima seedling leaves and roots under different concentrations of Cd stress.Among them,GSH plays an important role in the antioxidant process of Q.acutissima seedlings.7.Ectomycorrhizal mainly enhances the absorption of Ca and Mg by Q.acutissima seedlings,and promotes the transport of K,Mg,Zn,Cu and P elements to the upper part of plant.The absorption and transport of Ca,Mg and Cu elements play a key role in the response of ectomycorrhizal Q.acutissima seedlings to Cd stress.The inoculation treatment can promote the absorption of low-level Cd by Q.acutissima seedlings,and alleviate the damage of high-level Cd stress on the shoots.