Mechanism Of Biofilm Formation By Phenanthrene-degrading Bacteria On Rice Root Surface

Polycyclic aromatic hydrocarbons(PAHs)are ubiquitous recalcitrant and hypertoxic organic pollutants.They had been identified as one of the main organic pollutants in China's farmland.Crops grown in PAHs-contaminated soil uptake PAHs that accumulate in the edible parts,posing a substantial threat to human health.Recent studies have highlighted that certain PAH-degrading microorganisms are able to form a biofilm on the surfaces of plant roots,leading to the accumulation of high biomass and its long-term survival for continuous degradation in rhizosphere soil,thus reducing the absorption of PAHs by roots and ensuring the safety of crop products.However,to our knowledge,in situ PAH-degrading biofilm on the root surfaces has not been observed.The parameters which the mechanism of biofilm formation on the stablity and continuous function of biodegradation still need to be optimized.In this work,phenanthrene represents the PAHs,the mechanisms on the biofilm formation by phenanthrene-degrading bacteria on the root surface of rices were discussed,including the strain cell characteristics,plant root surface structure and root exudates.The aim is to establish a biodegradation barrier on the crop root surface,thereby reducing or blocking the absorption and accumulation of PAHs in plant roots and ensuring food security.The main results are as follows:(1)Strong kinematics,high initial adhesion rate,self-aggregation ability and strong hydrophobicity of cell wall endow JM2-gfp with strong biofilm formation.JM2-gfp can form biofilm at 48 h with a thickness of 32.8 ?m.The strains of JM2-gfp were encapsulated in the extracellular matrix(EPS)secreted by bacterial cells.The content of carbohydrate and protein were 74.68 and 211.9 ?g/mL,respectively.For the degradation function of JM2-gfp biofilm,the biofilm can degrade 52.8% phenanthrene within 48 h,which is 2.5 times as much as that of planktonic bacteria.The biofilm is more efficient for PAHs degradation than planktonic bacteria.(2)The rice species of ‘Liaojing 401',‘Koshihikari' and ‘Zhenzhuhong',which were mainly cultivated in Liaoning Province,were used as the sample of plants.The effects of their root surface structure and physiological characteristics on the biofilm formation of JM2-gfp were investigated under Hogland medium culture conditions.The hydrophobicity of rice root surfaces were ‘Zhenzhuhong' >‘Koshihikari' > ‘Liaojing 401' The biofilm formation ability of strains JM2-gfp on the rice roots surfaces of ‘Liaojing401',‘Koshihikari',‘Zhenzhuhong' increased with the hydrophobicity of substrate,on the root surfaces of ‘Zhenzhuhong' the biofilm showed the highest density of tightly intertwined bacteria among the three rice species in particular,‘Koshihikari' followed,and finally ‘Liaojing401'.The determination of low molecular weight organic acids in rice root exudates showed that the most abundant oxalic acid was beneficial to the formation of biofilm on rice root surface.(3)In this study,the phenanthrene-degrading and barriering function of biofilm on rice root surfaces were carried out.The residual phenanthrene in plant roots from ‘Zhengzhuhong',‘Koshihikari' and ‘Liaojing401' with biofilm mediated were significantly decreased by 71.9%,69.3% and 58.7% than those without biofilm groups after 10 days of exposure.Thus,the biofilm colonized on roots play an important role on degradation to reduce the level of phenanthrene uptake into plants,thereby the present work provides significant new insights into lowering environmental risks of PAHs in crops product from contaminated agriculture soils.At the same time,the effects of biofilm on rice root surface were studied.The results showed that biofilm on root surface could significantly increase the activity of antioxidase in rice,which indicated that enhanced the ability of rice to resist oxidative stress.Biofilm on rice root surface also has a certain effect on the metabolism of chlorophyll,which promotes the increase of chlorophyll a and total chlorophyll,thereby enhancing the photosynthesis intensity of plants.