Response Mechanism Of Colonized Bacteria In Rice Root To Phenanthrene And/or Cd In Rhizospheric Soil

The combined pollution of polycyclic aromatic hydrocarbons(PAHs)farmland and heavy metals(HMs)seriously affects the yield and quality of agricultural products.The colonization of rhizosphere soil microorganisms in the root system will affect the migration of pollutants from soil phases to plants.Therefore,a strain of PAH01 was screened from PAHs and HMs contaminated paddy soil(Mycolicibacterium sp.PAH01)that has the ability to repair phenanthrene,cadmium and their combined pollution.We explored the effect of the bacteria on phenanthrene in the water phase system and studied the inhibition and control mechanism of phenanthrene and/or cadmium and its compound pollution by the colonization of this fungus in rice roots under hydroponic and soil culture systems.We aim to develop rice rhizosphere colonization which reduce the accumulation of pollutants in rice.This paper has made the following progress:(1)A strain of PAH01 that can degrade multiple PAHs and can tolerate cadmium which was screened from PAHs and HMs contaminated paddy soil.In the phenanthrene and cadmium single system,phenanthrene(1.0 mg/L)can be completely degraded within 68 hours,3.8% to 18.3% of cadmium can be adsorbed on the cell wall in 96 hours,and 1.7% to 10.0% of cadmium can be accumulated in the cell;In the compound pollution system,the bacteria can completely degrade phenanthrene(1.0 mg/L)within 7 days and can simultaneously adsorb 8.6%～9.0% of cadmium.(2)The degradation pathway of phenanthrene and the mechanism of cadmium tolerance by PAH01 bacteria are clarified.The PAH01 bacteria contains the PAHs degradation gene pah E4,which can degrade phenanthrene via the phthalic acid pathway;It has a cadmium-tolerant gene czc D,and the-OH and amide II bands on the cell wall and exudates(EPS)are resistant to cadmium.(3)It is proved that PAH01 can prevent the transfer and accumulation of phenanthrene and/or cadmium compound pollution into rice after the root system is colonized at the tillering stage of rice.The phthalic acid in rice root exudates can be used as the carbon source of the PAH01 bacteria to promote the colonization of the bacteria in the rice roots.After the colonization,the EPS of the bacteria can promote the tillering and elongation of the rice roots;In the water culture system,the contents of phenanthrene and cadmium in rice decreased by 60.9% ～ 83.3% and 13.9% ～27.8% respectively after colonization compared with the non-colonization group;In the pot culture system,the phenanthrene content in rice roots and leaves decreased by65.6% and 57.1% respectively,and the cadmium content in rice roots and leaves decreased by 7.7% and 34.7% respectively.(4)It was confirmed that PAH01 can reduce the content of malondialdehyde(MDA)and increase the activity of superoxide dismutase(SOD)in rice,and regulate rice genes OsHMA2,OsHMA3 and Os Nramp5 at the tillering stage under the conditions of hydroponic and pot experiment.In the hydroponic culture system,after colonization,the content of SOD increased by 9.3%,the content of MDA decreased by 34.4%,the expression of oshma2 and osnramp5 decreased by 75.5% and 48.1%,and the expression of oshma3 increased by 17.8%;In the pot culture system,the expression of oshma2 and osnramp5 decreased by 58.1%,47.2%,20.5% and 30.0%respectively in the sterilized group and the unsterilized group after colonization;The amount of oshma3 increased by 52.4% and 54.3% respectively.