Phenanthrene-degrading bacteria on root surfaces: a natural defense that protects plants from phenanthrene contamination

Abstract

Bacteria on root surfaces (BRS) have tremendous practical importance in agriculture because of their various ecological functions and ability to degrade recalcitrant organic pollutants. However, little is known on the influence of polycyclic aromatic hydrocarbon (PAH)-degrading BRS on the uptake and accumulation of PAHs by plants. Thus, we sought to investigate whether the PAH-degrading BRS would protect plants from PAH contamination. A phenanthrene-degrading bacterium was isolated from the root surface of a plant growing in a PAH-contaminated site using enrichment culture. After bud soaking or root inoculation with the isolated bacterium, greenhouse container experiments were performed. Phenanthrene residues in soil and plant samples were analyzed by high-performance liquid chromatography (HPLC). A phenanthrene-degrading bacterium, Sphingobium sp. RS2, was isolated from the root surface of Conyza canadensis L. Cronq. and could degrade more than 99% of phenanthrene (100 mg·L−1) within 72 h. Inoculating alfalfa (Medicago sativa L.) plants with strain RS2-gfp (gfp-labeled RS2) revealed that alfalfa plants colonized with strain RS2-gfp exhibited a lower capacity for phenanthrene accumulation in both roots and shoots. Compared with the RS2-gfp-free treatment, the concentration of phenanthrene in RS2-gfp-inoculated plants was reduced by 39.9% and 62.5% in roots and shoots via bud soaking and by 25.1% and 35.8% via root inoculation, respectively. We propose that PAH-degrading BRS can protect plants from PAH contamination. The results of this work should benefit food safety and human health in PAH-contaminated sites.