Promotion of growth and secondary metabolism in Cyclocarya paliurus by Bacillus velezensis FZB42: Insights from rhizosphere nutrient, hormones and microbiome

Cyclocarya paliurus (Batal.) Iljinskaja is a prominent medicinal plant for edible and pharmaceuticals, yet its practical application is highly constrained due to the limited production of secondary metabolites in the leaves and the growth-secondary metabolism trade-off. Inoculation with soil beneficial microorganisms is a sustainable strategy to stimulate the production of secondary metabolites and balance such trade-off. The pot experiment and field trials were assigned to investigate how Bacillus velezensis FZB42 affects the growth and production of secondary metabolites in C. paliurus by integrated underground profiles (native microbial community, nutrient availability, and hormone levels in the rhizosphere) and aboveground responses (nutrient acquisition, biomass, photosynthesis, and secondary metabolites in plants). The results from pot experiment presented the improvements not only in the plant biomass but also in secondary metabolites driven by FZB42. The field trials further preliminary substantiated the effects of inoculant on the growth-promotion and secondary-metabolism-enhancement. Based on potted plants, significant increments in root and leaf biomass of the inoculated plants were 1.75 and 1.52 times of the control, while the contents of flavonoids and triterpenoids reached 10.89 mg g−1 and 43.66 mg g−1, respectively, apparently higher than 7.90 mg g−1 and 35.91 mg g−1 in the control. Inoculation enhanced soil nutrient availability through increased extracellular enzyme activities, resulting in 190 % and 62.98 % increases in available nitrogen and phosphorus, respectively. Moreover, FZB42 application enriched specific native bacterial taxa (e.g., Gammaproteobacteria) and reshaped rhizosphere hormone profiles, including indole-3-acetic acid (IAA), salicylic acid (SA), and jasmonates (JA). Structural equation modeling further demonstrated that alterations in rhizosphere hormone levels mediated by specific native bacterial taxa exerted significant effects on plant secondary metabolism. These findings offer insights into the regulations of FZB42 on growth–metabolism trade-off in medicinal plants, and its potential application as a bio-inoculant in plantations of C. paliurus for medicinal use.