Root-associated endophytic bacterial community composition and structure of three medicinal licorices and their changes with the growing year

BACKGROUND: The dried roots and rhizomes of medicinal licorices are widely used worldwide as a traditional medicinal herb, which are mainly attributed to a variety of bioactive compounds that can be extracted from licorice root. Endophytes and plants form a symbiotic relationship, which is an important source of host secondary metabolites. RESULTS: In this study, we used high-throughput sequencing technology and high-performance liquid chromatography to explore the composition and structure of the endophytic bacterial community and the content of bioactive compounds (glycyrrhizic acid, liquiritin and total flavonoids) in different species of medicinal licorices (Glycyrrhiza uralensis, Glycyrrhiza glabra, and Glycyrrhiza inflata) and in different planting years (1–3 years). Our results showed that the contents of the bioactive compounds in the roots of medicinal licorices were not affected by the species, but were significantly affected by the main effect growing year (1–3) (P < 0.05), and with a trend of stable increase in the contents observed with each growing year. In 27 samples, a total of 1,979,531 effective sequences were obtained after quality control, and 2432 effective operational taxonomic units (OTUs) were obtained at 97% identity. The phylum Proteobacteria, Actinobacteria, Bacteroidetes and Firmicutes, and the genera unified-Rhizobiaceae, Pseudomonas, Novosphingobium, and Pantoea were significantly dominant in the 27 samples. Distance-based redundancy analysis (db-RDA) showed that the content of total flavonoids explained the differences in composition and distribution of endophytic bacterial communities in roots of cultivated medicinal liquorices to the greatest extent. Total soil salt was the most important factor that significantly affected the endophytic bacterial community in soil factors, followed by ammonium nitrogen and nitrate nitrogen. Among the leaf nutrition factors, leaf water content had the most significant effect on the endophytic bacterial community, followed by total phosphorus and total potassium. CONCLUSIONS: This study not only provides information on the composition and distribution of endophytic bacteria in the roots of medicinal licorices, but also reveals the influence of abiotic factors on the community of endophytic bacteria and bioactive compounds, which provides a reference for improving the quality of licorice.