The Mechanisms Of Selenium Alleviating On Chromium Toxicity In Root Tips And Selenium Combined With Functional Strains Remediating Chromium Contaminated Soil

Chromium（Cr）contaminated soil poses a potential risk to human health and ecological environment,and excessive Cr accumulation exerts deleterious effects on plants.Appropriate doses of selenium（Se）could alleviate the oxidative damage induced by Cr,and directly or indirectly remediate Cr contaminated soil by altering soil properties and modulating soil microorganisms.Based on the phytobiological functions of Se,the biological mechanisms of Se alleviating Cr toxicity in root tips was firstly revealed from the perspective of physiology and cytology.Meanwhile,two functional strains associated with Cr transformation/resistance were isolated,and the remediation effect of Se combined with functional strains on Cr contaminated soil was investigated.Further,the biochemical mechanism of Se and functional strains reducing the bioavailability of Cr in soil were elucidated via proteomics technology,and the rhizosphere process of Se and functional strains enhancing the phytoremediation of Cr-Cu-Cd contaminated soil were explored via microbiome techniques.The main results are as follows:（1）Se scavenged the overproduction of H₂O₂ and O₂^-·caused by Cr stress,and alleviated the level of lipid peroxidation in root tips.Moreover,Se effectively inhibited DNA degradation and reduced the number of apoptotic cells in root tips.Compared with Cr treatment alone,Se supplementation reduced the content of reactive oxygen species（ROS）and malondialdehyde（MDA）in mitochondria by 38.23%and 17.52%,respectively.Se application decreased the opening degree of mitochondrial permeability transition pores（MPTP）by 32.30%,increased mitochondrial membrane potential（Δψ_m）by 40.91%,alleviated the release of Cyt c from mitochondria into cytosol by 18.42%and caused 57.40%decrease of Caspase 3-like protease activity.The alteration of Se on mitochondrial physiological properties improved mitochondrial dysfunction caused by Cr stress,which further contributed to reducing the appearance of programmed cell death（PCD）in the root tip cells of Chinese cabbage.（2）Se combined with Bacillus cereus SES reduced Cr bioavailability and improved Se bioavailability in soil.The biochemical mechanism was further revealed in the liquid culture system.Se combined with Bacillus cereus SES reduced Cr（VI）to Cr（III）,and the Selenium nano particles（Se NPs）produced in the process adsorbed Cr（III）.Based on the results of proteomics,Se（IV）supplementation mediated the synthesis of Cr（VI）reductants（thiocysteine,L-Homocysteine and L-methionine）,donate electrons（pyruvate）and stress-resistant substances（cationic antimicrobial peptide and antibiotics）,thus enhancing Cr（VI）resistance and promoting Cr（VI）reduction.Meanwhile,high Se（IV）reduction rate was associated with Cr（VI）-induced electron transport processes,and Cr（VI）mediated the up-regulation of flagellar assembly,protein export and ABC transporters pathways to synthesis and export more Se NPs to adsorb Cr（III）.Results suggested that the reduction and adsorption effects mediated by Se combined with Bacillus cereus SES reduced the bioavailability of Cr in soil.（3）Se alone or in combination with Bacillus proteolyticus SES treatment enhanced the phytoextraction of Cr,Cu and Cd in the rhizosphere soil by ryegrass.Se+B.proteolyticus SES treatment played a greater role in improving the heavy metals tolerance of ryegrass,increasing the activity of soil acid phosphatase and improving the specificity of rhizosphere bacterial community.Moreover,Se application and Bacillus proteolyticus SES inoculation favored the preferential recruitment of specific taxa with the capacity of plant growth promotion and heavy metals resistance to the rhizosphere.The rhizosphere soil of Se treatment alone was specifically enriched with Lysobacter,Rhodanobacter,Micrococcales,Paenarthrobacter,and Adhaeribacter,while from class Bacilli to genus Bacillus enriched extensively and specifically in the rhizosphere of Bacillus proteolyticus SES+Se treatment.Furthermore,five functional beneficial rhizosphere microbes including:Microbacterium sp.,Pseudomonas extremaustralis,Bacillus amyloliquefaciens,Priestia megaterium,and Bacillus subtilis were isolated from the two treatments with the best remediation effect and synthetic communities（Syn Coms）were constructed.Syn Coms inoculation experiment further demonstrated the role of specific beneficial microbes in regulating the bioavailability of heavy metals.Results suggested that Se application and Bacillus proteolyticus SES inoculation showed the synergistical effect in enhancing the remediation of Cr-Cu-Cd contaminated soil by ryegrass.