Influence Of Tetracycline And Cadmium On The Growth And Rhizosphere Microecology Of Tillering Stage Rice

With the large-scale development of livestock and poultry breeding and feed industry,that,a large amount of fertilizer application of breeding waste,has led to the compound pollution of heavy metals and antibiotics in farmland soil.Tetracycline（TC）is one of the most antibiotics widely detected in soil environment.At the same time,Cadmium（Cd）has attracted much attention because of its high toxicity and harmfulness,and it is easy to be absorbed and accumulated by food crops.When TC and Cd coexist together,their interaction may affect plant rhizosphere microecology,thus affecting plant growth physiology and contaminant enrichment,resulting in food safety risks.Microorganisms and root exudates play an important role in rhizosphere microecology,which can benefit plants directly or indirectly.It is necessary to figure out the influence mechanism of the coexistence of antibiotics and heavy metals on plants,so as to provide reference data for establishing effective rhizosphere regulation measures.In this study,TC and Cd were taken as the target pollutants,and rice,the main food crop,was taken as the research object.The hydroponic culture experiments were conducted to study the growth physiology and pollutant accumulation of rice under single and combined pollution of TC and Cd.Meanwhile,the diversity of rice rhizosphere microbial community was further revealed by high-throughput sequencing,and the species and content of rice root exudates were determined by high-resolution mass spectrometry.Following is the main findings.Therefore,the effects of TC and Cd combined pollution on rice rhizosphere microecology were investigated.Following is the main findings.1.The root elongation and transpiration rate of rice plants showed promotion in low concentration exposure of single pollution while high concentration single treatment limited these physiological indexes and plant height,biomass and chlorophyll content also decreased.However,the combined pollution at low concentrations of TC and Cd could alleviate the toxic effect of TC/Cd to a certain extent.but the combined pollution at high concentrations could produce joint toxicity,which synergistically inhibit the growth of rice plants.2.The uptake and accumulation of TC or Cd by each part of rice showed a obvious with the increase of the concentration of TC and Cd.In addition,the content of pollutants in aerial part was significantly lower than in root.There is a surface bridging mechanism between TC and Cd,and low concentrations of TC could promote the rice growth by acting as a carbon source,which results in synergistically promotion of pollutants accumulation in rice root.However,with the increase of toxicant concentration,the interaction between them resulted in the increase of TC accumulation and the slightly decrease of Cd accumulation.The Cd,Zn and Fe were mainly concentrated in the root tip region of rice,while the Mn had no significant difference in the root parts.Furthermore,there may be a similar absorption mechanism for Cd and Zn in rice,and the addition of TC could promote the accumulation of Cd and Zn in the root tip region,and then a certain dose of TC could promote their migration from the root tip upward.Under the combined treatment of TC(5.0μmol·L^-1)and Cd(10.0μmol·L^-1),Cd and Zn mainly accumulated in the epidermis and midcolumn of the 10-15 cm root region,Fe mainly accumulated in the epidermis of the root tip（>15 cm）,and Mn mainly accumulated in the epidermis of each root region.The difference in the distribution of Zn,Fe,Mn,as nutrient elements,and Cd,as non-essential elements,may be due to their similarity and difference in physical and chemical properties,which play different roles in plant defense.3.TC and Cd pollution changed the rhizosphere microecology of rice.The rhizosphere bacterial and fungal species decreased,and the community composition changed significantly under the single and compound treatment of TC and Cd.The relative abundance of Bacteroidetes increased under the combination pollution of low concentration Tc and Cd,but decreased slightly due to the toxic effect exerted by the combination pollution of high concentration.And the relative abundance of Proteobacteria increased with the addition of TC.In other words,the increase of relative abundance of Bacteroidetes and Proteobacteria may activate the metal availability of Cd and thus promote the accumulation of Cd in rice.In addition,Cryptomycota had certain specific resistance to Cd.4.Both TC and Cd could inhibit the secretion of rice root exudates.The combined pollution at low concentrations of TC and Cd could alleviate the physiological toxicity of rice,and then promote the compounds secreted by rice root.However,root cell integrity is impaired at high concentrations in combined pollution with toxic effects,which inhibited the secretion of root exudates.Further analysis found that,in the root exudates,the components involved in plant defense function changed differently.Under the single pollution condition,the relative contents of amino acids and derivatives,organic acids and sugar alcohols in rice root exudates were increased by Cd pollution,while the relative contents of organic acids and sugar alcohols were increased by TC pollution.The relative contents of organic acids and sugar alcohols in root exudates showed antagonistic and promoting effects under high concentrations of TC(25.0μmol·L^-1)and Cd(45.0μmol·L^-1)combined pollution,and that were significantly increased compared with that under single Cd(45.0μmol·L^-1)pollution stress.Furthermore,compared with the low concentration of TC（5.0μmol·L-1）/Cd（1.0μmol·L-1）,the combined contamination with Cd/TC could stimulate the secretion of various amino acids in rice.The results of this study showed that,the combined pollution of TC and Cd synergistically promoted the accumulation of pollutants in the rice root,and affected the growth and rhizosphere microecology of rice.The results of this study can provide reference data for ensuring the safe production of rice and establishing effective rhizosphere regulation measures for the control of such combined pollution.