Effect And Mechanisms Of Biofilm-producing Bacillus Amyloliquefaciens P29 On Reduced Lead Accumulation In Leaf Vegetable

With rapid industrialization and urbanization in China,a large amount of agricultural soils is threatened by heavy metals,especially by lead(Pb).With the development of vegetable planting industry in China,excessive accumulation of Pb in vegetables will pose a great threat to human health.Technology of metal immobilization by bacteria may help to reduce heavy metal uptake by vegetables and to practically remediate farmland soils.Biofilms have adsorption on heavy metals such as lead,cadmium and chromium.Therefore,in this study,the lead-resistant bacteria Bacillus amyloliquefaciens P29 was selected to investigate the biofilm formation and characteristics and mechanism of Pb immobilization by strain P29.Moreover,the effects and mechanisms of B.amyloliquefaciens P29 and different biofilm-forming ability mutants on reducing Pb uptake in leaf vegetables and immobilization remediation of Pb-contaminated soil were explored.Research results are as follows:Compared with the Pb-free control,the amount of biofilm produced by strain P29 under the stress of 250 and 450?M Pb increased by 18-142%,and the contents of extracellular protein and exopolysaccharide in biofilm respectively increased by 13-84%and 26-83%.Pb stress also delayed strain P29 biofilm detachment.Moreover,functional groups such as hydroxyl group,carbonyl group,amino group,phosphate group and carboxyl group on strain P29 biofilm and its extracellular polymer could bind to Pb.A biofilm-increasing mutant strain P29?sinR::spe was constructed.Compared with the wild strain P29,the amount and extracellular polymer content of biofilm produced by the mutant strain P29?sinR::spe increased significantly by 28%and 24-59%,respectively.At the same time,the amount of Pb immobilized by the mutant strainP29?sinR::spe biofilm increased significantly by 100-180%.Under the condition of hydroponic culture,the Pb content in lettuce and Chinese cabbage decreased by 66%and 37%in the mutant strain P29?sinR::spe treatment compared with the wild strain P29 treatment,respectively.And the colonization of mutant strain P29?sinR::spe on vegetable root surface was significantly higher than that of wild strain P29.The results of pot experiment showed that mutant strain P29?sinR::spe treatment significantly reduced Pb content in the upper and lower parts of lettuce by 26%and 30%respectively compared with the WT treatment.And the application of mutant strain P29?sinR::spe significantly reduced DTPA-extractable Pb content in rhizosphere soil(16%).Compared with the WT,application of mutant strain P29?sinR::spe significantly increased the colonization amount of strain P29 on root surface and rhizosphere soil.Moreover,extracellular polymer content and the proportion of biofilm-producing bacteria in rhizosphere soil increased greatly by 20-32%and 13-15%in the mutant strain P29?sinR::spe treatment.At the same time,compared with the WT.the mutant strain P29?sinR::spe treatment significantly increased the proportion of aggregates in vegetable rhizosphere soil of 2?0.2 mm by 22-26%.The results of correlation analysis showed that there was a significant negative correlation between DTPA-extracted Pb content and colonization of strain P29 in rhizosphere soil.