Mechanisms Of Sphingomonas Sp.-enhanced The Migration And Transformation Of Cd-phenanthrene Co-contaminants In Rhizosphere Microenvironment Of Rice(Oryza Sativa L.)

Heavy metals（HMs）and polycyclic aromatic hydrocarbons（PAHs）in paddy soils can be absorbed into rice via the root system,leading to"cadmium rice"and rice enriched with PAHs,which has seriously been threaten the safe production of rice and thus endangered human health.Exploring the interactions among rice roots,rhizobacteria and pollutants in the rhizosphere microdomain of rice is important for deep understanding of the mechanisms that how rhizobacteria affect the uptake of pollutants by rice and thus to develop blocking and control technologies of co-contaminants in paddy soils.This study explored the adaptive changes of Sphingomonas sp.PAH02 in response to the stresses of Cd and/or phenanthrene and its effect mechanisms on the translocation of Cd and phenanthrene in the rhizosphere microenvironment of rice（Huanghuazhan）.The main results were listed as follows:（1）Surface hydrophobicity affected the interaction among strain PAH02,and Cd²⁺and/or phenanthrene in the rhizosphere microenvironment.In the absence of contaminants,the biomass amount of strain PAH02 adhering to the hydrophobic surface was 1.18 times higher than that of the hydrophilic surface.However,after the addition of phenanthrene,the biomass amount of strain PAH02 adhering to the hydrophobic surface was 2.07 times higher than that of the hydrophilic interface.Notably,under the coexistence of cadmium and phenanthrene,the biomass amount of strain PAH02adhering to the hydrophobic surface was 1.26 times higher than that of the hydrophilic surface.Strain PAH02 reduced the amount of Cd²⁺absorbed by rice and the amount of by 13.6%and 49.5%while the amount of phenanthrene adsorbed on the root surface decreased by 66.1%and 81.1%at the hydroponic tillering stage,respectively,regardless of the single and combined systems.（2）Strain PAH02 coped with the stress of phenanthrene,cadmium and phenanthrene-cadmium co-contaminants by regulating energy metabolism and changing cell membrane components.Strain PAH02 can adsorb cadmium and degrade phenanthrene through the salicylic acid pathway.Compared with the single pollution system,the time of phenanthrene（1 mg/L）degraded completely by strain PAH02 was prolonged from 6 h to 8 h in the phenanthrene-cadmium system,and the adsorption rate of cadmium（0.5 mg/L）reduced from 50%to 25%.The main mechanism of strain PAH02 adapting to the environmental stress of cadmium,phenanthrene and cadmium-phenanthrene co-contaminants at the proteomic level indicated by isotope relative labeling and absolute quantification（i TRAQ）were included as follows:the expression of iron-sulfur cluster proteins involved in electron transport were regulated to ensure aerobic respiration.Also,the expression of proteins involved in the generation or consumption of ATP were regulated to improve the utilization of energy substances.Expression of proteins encoding cell membrane components was altered to protect the cellular integrity.（3）Under soil culture conditions,strain PAH02 inhibited the uptake of cadmium and/or phenanthrene into rice grains via the root system in the rice rhizosphere microenvironment,thereby reducing the content of phenanthrene and cadmium in brown rice.Strain PAH02 decreased bioavailable cadmium by increasing the p H value of rice rhizosphere soil,up-regulated expression of Os HMA3 gene to increase root cell vacuolar fixation of cadmium,and down-regulated expression of Os Cd1 gene to reduce cadmium transport to shoots and even to grains.As a result,cadmium content in brown rice reduced by 17%.Strain PAH02 reduced the phenanthrene content in brown rice by reducing the phenanthrene content in the rhizosphere soil.The phenanthrene content in brown rice reduced from 0.53-0.55 mg/kg to 0.20-0.17 mg/kg,with a decreasing range of 62.2%-69.1%.