Analysis of Microbial Diversity in Lagotis Brevituba Maxim. from Different Production Areas on the Qinghai-Xizang Plateau and Its Correlation with Secondary Metabolic Products

Lagotis brevituba Maxim. (Lagotis brevituba), belonging to the genus Lagotis within the family Scrophulariaceae. The content of its secondary metabolites exhibits significant variation across different provenances, yet the underlying mechanisms linking these variations to microbial associations remain unclear. In this study, 12 samples of rhizosphere soil, root systems, and plant specimens of Lagotis brevituba were collected from various locations on the Qinghai-Xizang Plateau, and the physicochemical properties of the soil were systematically determined. Using high-throughput 16S and ITS rRNA sequencing technologies, the diversity and composition of bacterial and fungal communities were analyzed; simultaneously, the concentrations of 10 key secondary metabolites were quantitatively determined using high-performance liquid chromatography (HPLC). Furthermore, correlation network analysis and redundancy analysis were used to investigate the relationships among soil physicochemical factors, microbial communities, and secondary metabolites. The results indicate that an elevation of 4500 m serves as a critical threshold, marked by significant changes in soil physicochemical properties. Specifically, compared to the HA group (3500–4500 m), the EA group (elevation > 4500 m) exhibited significantly lower soil organic matter and available phosphorus content, while pH and total nitrogen content were significantly higher. These soil changes indicate the presence of nutrient-poor and alkaline stress conditions, which in turn led to the differentiation of rhizosphere bacterial communities and stimulated the synthesis of more antioxidant metabolites (e.g., significantly elevated β-Sitosterol, Quercetin, and Plantamajoside) in the EA group. Microbial community analysis revealed that bacterial community structure exhibited a significant response to changes in elevation, whereas fungal communities showed no significant differences. Soil physicochemical properties (particularly pH, SOM, and AP) are key mediating factors driving microbial community differentiation and the accumulation of secondary metabolites. A combined analysis of correlation networks and Zi-Pi values identified 19 key OTUs (Operational Taxonomic Units) mediating the relationships between microorganisms and metabolites. Notably, Aquabacterium exhibited significant negative correlations with Luteolin and Acteoside, while Streptomyces showed significant positive correlations with Hyperoside (P < 0.05). This study demonstrates that soil environmental factors structure microbial communities, which in turn play a crucial role in regulating the secondary metabolites of medicinal plants. These findings provide theoretical foundations for elucidating the environment-microbiome-metabolite nexus in Lagotis brevituba.