Phytochemical effects of Apium graveolens on the abundances of functional genes associated with PAH degradation in soil

The biodegradation of polycyclic aromatic hydrocarbons in soil is enhanced by chemical constituents of various plant species that support the growth and activity of PAH degrading bacteria in the rhizosphere. Here, we investigated the phytochemical effects of celeriac (Apium graveolens var. rapaceum) root tissues, a known stimulator of PAH degradation, on the copy numbers of nahAc, xylE, and 16S rRNA genes to determine the relative contributions of growth-linked degradation versus selective enrichment of PAH degraders. Real-time quantitative PCR analysis showed that the copy numbers of 16S rRNA genes, as a reporter of general bacterial population size, increased approximately 2- to 5-fold in soil amended with celeriac, but were not affected by exposure to naphthalene alone. In contrast, copy numbers of two functional genes (nahAc and xylE), commonly used as reporters for monitoring the population size of PAH degraders, were significantly increased following exposure to naphthalene but were differently affected by soil amendment with celeriac. As compared with the copy numbers of nahAc gene in unamended control soil, the nahAc copy numbers were increased by 460-fold in soil amended with naphthalene alone and by 610-fold in soil amended with both naphthalene and celeriac after 10 days of incubation. The copy numbers of xylE were increased by 20-fold in soil amended with naphthalene alone and increased by 200-fold in soil amended with both naphthalene and celeriac after 10 days of incubation. The result suggests that increased degradation rates of PAHs in the presence of celeriac involves both growth linked processes and selective enrichment of the degrader population.

土壤中多环芳烃（polycyclic aromatic hydrocarbons, PAH）的生物降解可通过多种植物的化学成分得到增强，此类成分能够促进根际（rhizosphere）微环境中多环芳烃降解菌的生长与代谢活性。本研究以已被证实可促进多环芳烃降解的块根芹（Apium graveolens var. rapaceum）根组织为研究对象，探究其植物化学组分对nahAc、xylE及16S rRNA基因拷贝数的影响，以此明确生长偶联型降解过程，与多环芳烃降解菌选择性富集这两种途径的相对贡献。
实时定量PCR（real-time quantitative PCR）分析结果显示：作为通用细菌种群规模指示因子的16S rRNA基因拷贝数，在施加块根芹的土壤中提升了约2~5倍，但仅暴露于萘（naphthalene）时未发生显著变化。与之形成鲜明对比的是，两种常被用于监测多环芳烃降解菌种群规模的功能基因（nahAc与xylE），其拷贝数仅在暴露于萘后显著上升，且受块根芹土壤改良处理的影响存在显著差异。
与未施加任何处理的对照土壤相比，仅施加萘的土壤在培养10天后，其nahAc基因拷贝数提升了460倍；而同时施加萘与块根芹的土壤中，该基因拷贝数则提升了610倍。此外，仅施加萘的土壤中xylE基因拷贝数提升了20倍，同时施加萘与块根芹的土壤中该基因拷贝数提升了200倍。
上述结果表明，块根芹存在条件下多环芳烃降解速率的提升，同时涉及生长偶联型降解过程，以及降解菌种群的选择性富集这两种核心机制。