Degradation Of Chlorimuron-ethyl Residues In Cucumber Rhizosphere Soil By Hansschlegelia Zhihuaiae S113

Chlorimuron-ethyl is one of the sulfonylurea herbicides widely used in China.However,its long half-life in the soil may cause phytotoxicity to cucumber.And it also has potential harm to the ecological environment.How to effectively remove the residual toxicity of chlorimuron-ethyl has become an urgent problem in China.Hansschlegelia zhihuaiae S113 is a highly efficient sulfonylurea herbicide-degrading bacterium that has been isolated and preserved in our laboratory.It could effectively degrade chlorimuron-ethyl and remove its toxicity.Therefore,in this paper,Hansschlegelia zhihuaiae S113 was used as the tested strain,and cucumber which was susceptible to chlorimuron-ethyl was used as the research object.The effect of strain S113 on the removal of chlorimuron-ethyl and the bioremediation of bacterial community in the rhizosphere soil of cucumber were studied,which can be provide more theoretical basis for the restoration of soil contaminated by chlorimuron-ethyl residues.The bacterium' suspension of strain S113 was uniformly mixed in soils with 0.02 and 0.05 mg·kg-1 soil dry weight chlorimuron-ethyl,planting cucumber seedlings for cultivation.Tthe growth indexes of cucumber were firstly measured.At 14 d,chlorimuron-ethyl caused obvious phytotoxicity to cucumber,but strain S113 had obvious removal effect on its phytotoxicity.Then,the colonization of the S113 strain with gfp on the root surface of cucumber was observed by confocal laser scanning microscopy(CLSM).The results showed that the strain S113 could colonize on the meristem zone,elongation zone and mature zone of cucumber roots surface.The specific esterase gene sulE,which is unique to S113 strain,was specifically amplified by qPCR.The results indicated that the final concentration(107 CFU·g-1 soil dry weight)of strain S113 could colonize on cucumber roots surface,rhizosphere and non-rhizosphere soils for at least 14 days,and the number of S113 reached at 104-105 cells·g-1 cucumber roots and 104-107 cells·g-1 rhizosphere soil,which is more than that of the non-rhizospheric soil.Cucumber root exudates were extracted by water-soluble method,and the organic acids in cucumber root exudates were identified by high performance liquid chromatography(HPLC).The preliminary investigation of the effect of cucumber root exudates in the process of removing the chlorsulfuron-methyl toxicity to cucumber by strain S113 was conducted.These results showed that cucumber root exudates could promote the growth of the strain S113 and its degradation of chlorimuron-methyl.Chlorsulfuron-methyl inhibited the secretion of organic acids in cucumber roots,and the addition of strain S113 had a restorative effect on it.Strain S113 showed obvious chemotactic response to cucumber root exudates and its organic acids,and the chemotactic response to citric acid was strongest.At the same time,cucumber root exudates,citric acid and fumaric acid could significantly promote the colonization of strain S113 on cucumber root surface.The bacterium suspension of strain S113 was uniformly mixed in soils with 0.02 and 1 mg·kg-1 soil dry weight chlorimuron-ethyl,and cucumber seedlings were planted for cultivation.The colonization of strain S113 in cucumber roots surface,rhizosphere,and non-rhizosphere soils at 0,7,14 and 21 d were tested,and the degradation rate of chlorimuron-ethyl in rhizosphere and non-rhizosphere soil were measured.The amino acid composition and content in cucumber root exudates of each treatment were determined by automatic amino acid analyzer.These results indicated that the optimum inoculation amount of strain S113 was 108 CFU·g-1 soil dry weight,and it had good viability in the root,rhizosphere and non-rhizosphere soil treated with chlorimuron-ethyl,and could survive to at least 21 d.At 14 d,chlorimuron-methyl was not detected in the rhizosphere and non-rhizosphere soil under the treatment with the addition of strain S113.Chlorimuron-methyl had an inhibitory effect on the secretion of leucine,isoleucine and valine from cucumber roots,but it did not completely inhibit these three amino acids secretion.Through high-throughput sequencing,the microecological effects of cucumber rhizosphere and non-rhizosphere soils were studied.The results showed that chlorimuron-ethyl could reduce the diversity and abundance of bacterial community structure in the soil.The addition of strain S113 restored the community structure.There were significant differences in bacterial community structure between rhizosphere and non-rhizosphere soil of cucumber,indicating that root exudates were also the key factors affecting bacterial community structure.