Microbial mechanisms controlling the rhizosphere effect of ryegrass on degradation of polycyclic aromatic hydrocarbons in an aged-contaminated agricultural soil

The phytoremediation of polycyclic aromatic hydrocarbons (PAHs) depends on plant-microbe interactions in the rhizosphere, but the extent and mechanisms of these effects are still unclear. The aim of this study was to investigate the effects of plants on PAHs degradation and on associated changes in transcriptionally active bacterial community structures, degrading bacteria, and especially functional gene expression in a PAH-contaminated agricultural soil. We conducted a rhizo-box experiment planted with ryegrass for 60 days under glasshouse conditions. The degradation of 12PAHs in rhizosphere and non-rhizosphere soils during the experiment was quantified using HPLC analysis. The dynamics of active bacterial community structure in soil were assessed using the molecular tool of cDNA-based denaturing gradient gel electrophoresis (DGGE). Functional PAH-ring hydroxylating dioxygenase (PAH-RHDα) genes were cloned using the Gram-positive (GP) and Gram-negative (GN) primers. The abundance and activity of total bacteria and GP/GN PAH-degraders were monitored by real-time PCR. Results demonstrate that the agricultural soil contained known PAH degraders and RHDα genes, such as nidA3, pdoA, nahAc and phnAc. The presence of ryegrass promoted the dissipation of PAHs and changed the structures of active bacterial communities in soil. In general, the abundance of total bacteria and GP PAH-degrader increased during the inoculation time. The ryegrass root enhanced the functional bacterial diversity in the early stages (0–10 days). However, the effect of ryegrass roots may stimulate the expression of GP and GN PAH-RHDα genes after 40 days. Our results indicate that ryegrass increased the degradation of PAHs by promoting bacteria diversity, increasing the abundance of total bacteria and PAH degraders, and stimulating RHDα gene expression.