Effects And Mechanisms Of Phytoalexins On Degradation Of PAHs By Endophytic Bacteria

The use of endophytic bacteria in bioremediation is a promising environmental remediation technology.However,the interactions,symbiotic mechanisms and influencing factors between plants and endophytes have not been fully understood,especially the significance of the chemical environment in plants to the degradation of organic pollutants by endophytic bacteria is unclear.Since endophytes are always found in the intercellular-space of plant(apoplast),it is necessary to further analyze the effects and mechanisms of special components in plant apoplast on the degradation of organic pollutants by endophyes.In this paper,the effects and mechanisms of phytoalexin,a special component in plant apoplast,on the degradation of PAHs by endophytes were studied.A strain of PAHs degrading bacteria with high degrading activity was isolated from ryegrass that grown in PAHs-contaminated soil.The effects and mechanisms of coumarin,resveratrol and rutin,which were representative of different phytoalexins,on the growth and degradation of PAHs by the strain were studied.The main conclusions are shown as follows:(1)An endophytic strain could utilize acenaphthene and phenanthrene as the sole carbon source was isolated from ryegrass(Loliumperenne L.)grown in PAHs-contaminated soil.The endophytic strain was identified as Methylobacterium extorquens based on physiological characteristics and 16S rDNA sequence homology analysis,and named as Methylobacterium extorquens C1(M.extorquens C1).The M.extorquens C1 was a gram-negative spore-free bacterium that could effectively degrade PAHs.Under suitable growth conditions,the 3-day degradation ratios.of acenaphthene and phenanthrene were 50.9%and 42.8%when the initial concentration of acenaphthene and phenanthrene were 3.50 mg/L and 1.00 mg/L.(2)Coumarin,resveratrol and rutin promoted the degradation of PAHs by M.extorquens C1 at low concentration(0?2.00 mg/L,0?0.30 mg/Land 0?0.50 mg/L for oumarin,resveratrol and rutin,respectively);but inhibited the degradation of PAHs by M.extorquens C1 at high concentration(2.00?20.0 mg/L,0.30?3.00 mg/L and 0.50?5.00 mg/L for oumarin,resveratrol and rutin,respectively).The most effective concentrations of the phytoalexins on the degradation of PAHs by M.extorquens C1 were:coumarin 0.20 mg/L,resveratrol 0.15 mg/L and rutin 0.25 mg/L,respectively.(3)The promoting effects of the phytoalexins on PAH-remove can be attributed to their facilitation of PAH-sorption on the cell surface and the enhancement of intracellular degradation of PAHs by M.extorquens C1.On one hand,coumarin in 0?2.00 mg/L,resveratrol in 0?3.00 mg/L and rutin in 0?5.00 mg/L significantly promoted the sorption of PAHs by M.extorquens C1.And the degree of sorption-promotion increased with the concentration of phytoalexins.On the other hand,the activity of the key enzymes for intracellular degradation of PAHs(dehydrogenase,catechol-1,2-dioxygenase and catechol-2,3-dioxygenase)were significantly enhanced when coumarin,resveratrol and rutin were at low concentrations(0?0.20 mg/L,0?0.30 mg/L and 0?0.25 mg/L for oumarin,resveratrol and rutin,respectively).