Medicago sativa L. Root Exudation of Phenolic Compounds and Effect of Flavonoids on Phenanthrene Degradation by Two Rhizobacteria.

Abstract

Background: Plant-microbial degradation of organic pollutants occurs in the rhizosphere under the influence of plant root exudates. Similarities in chemical structure to polycyclic aromatic hydrocarbons (PAHs), phenolic compounds and flavonoids released with exudates can determine the ability of rhizosphere microorganisms to degrade hazardous aromatic pollutants.

Methods: Here, we analyzed phenolic compounds in the root exudates of alfalfa (Medicago sativa L.) grown in quartz sand uncontaminated and phenanthrene-contaminated quartz sand, a model PAH pollutant, under axenic conditions. The effect of six flavonoids (naringenin, rutin, morin, quercetin, apigenin, and luteolin) on phenanthrene degradation by two PAH-degrading bacteria, Ensifer meliloti P221 and Mycolicibacterium gilvum PAM1, previously isolated from the rhizosphere of alfalfa was also investigated. Ultraviolet (UV)-vis spectroscopy and high-performance liquid chromatography (HPLC) were applied to assay flavonoid and phenanthrene content in cultivation media.

Results: The quantitative and qualitative characteristics of the root-exuded phenolic compounds changed under the influence of phenanthrene. The impact of the flavonoids on PAH biodegradation varied from neutral or even inhibitory to stimulatory. The same flavonoid (quercetin) had opposite effects on the growth of the two bacteria and on phenanthrene degradation. The effect of the flavonoids on bacterial growth did not depend on the presence of PAHs. Using naringenin as an example, we showed that increased PAH degradations could not accompany bacterial growth promotion by any flavonoid. Except for rutin, all flavonoids were subject to bacterial degradation. Inoculation of alfalfa with the competent rhizobacterium Ensifer meliloti increased the contents phenolic compounds in the plant root exudate, promoted qualitative changes in their profile, and increased the rhizodegradation of phenanthrene from 6% and 22% to 57% and 34% at initial phenanthrene concentrations of 50 and 100 mg/L respectively.

Conclusion: Our data suggest a the role for plant flavonoids in the rhizome-mediated degradation of PAHs. The microbe-induced qualitative and quantitative changes in root exudation illustrate the induction of PAH-mediated catabolic activity in the rhizosphere.