Enhanced Phytoextraction Of Cadmium From Soil By Alfalfa Combined With Pseudomonas Monteilii PN1:Plant Growth-promoting And Rhizosphere Microbial Community Response

Cadmium(Cd)is regarded as one of the most dangerous"soft metals"globally and constitutes a certain threat to human security and environmental quality.Cd was spread into the soil typically through some industrial and agricultural activities.As a non-essential element of the human body,low doses of Cd show severe toxicity to plants and animals owing to its high fluidity and bio-accumulation.Therefore,it is significant to promote improvement of feasible remediation technologies for cadmium-contaminated soil.Some physical and chemical approaches for remediating cadmium-contaminated soil have an adverse effect on the fertility of the soil.Promising progress have been found using bio-remediation approach which is economically valuable.In this work,we introduce an alfalfa-based(with perennial and developed root systems)plant-microbe system which shows a conceptual promise to extract cadmium from soil.This remediation is achieved by the promoting effects of Pseudomonas PN1on alfalfa growth.Based on the status of cadmium-contaminated soil in China,the concentration of cadmium in soil was set to 0,35,45,55 mg·kg~-11 in this experiment.Soil inoculated with and without Pseudomonas PN1 were studied.Plants and rhizosphere soil were harvested for determination after alfalfa was grown in cadmium-contaminated soil for60 days.(1)We observed the effects of different cadmium concentrations on stem length,fresh weight,leaf area,leaf quality,chlorophyll,and cell membrane permeability of alfalfa.Without PN1 inoculation,Cd inhibited the growth of alfalfa,and the stem length and fresh weight were significantly reduced with the increase of cadmium concentration in the soil.However,the growth of alfalfa was promoted with PN1inoculation.(2)The IAA yield and ACC deaminase activity of alfalfa were measured.Due to cadmium stress,the IAA yield and ACC deaminase activity of alfalfa were inhibited.After inoculation with PN1,the IAA yield(2.98%-14.86%)and ACC deaminase activity(5.75%-25.20%)were significantly increased.The results showed that PN1can enhance IAA yield and ACC deaminase activity of alfalfa.(3)As the concentration of cadmium in the soil increased,the activities of SOD,POD and CAT were suppressed.After inoculation with PN1,the antioxidant enzyme activity of the plant was obviously improved,which can eliminate excess free radicals in alfalfa and relieve the damage of alfalfa by cadmium stress.(4)The concentration of Cd in alfalfa was raised with the increase of concentration of Cd in soil.After inoculation with PN1,the Cd extraction of alfalfa shoots and roots parts increase by 11.63-27.14%and 18.17-39.94%respectively,while the transport coefficient decreased slightly.The result shows that Pseudomonas PN1 promote the extraction of cadmium from alfalfa,and the root was the main organ for storing cadmium.(5)According to the fluorescence observation of alfalfa and rhizosphere soil,PN1 labeled by GFP was found in plant roots,stems,leaves and soil,which may be regarded as the main reason for the increase of plant hormones and antioxidant enzyme activities.According to the results of rhizosphere soil Illumina sequencing,alpha diversity,beta diversity,Cd reduces the abundance and diversity of microorganisms,which is the main reason for the change of microbial community structure,and inoculation with PN1 increases the abundance of cadmium-resistant bacteria.