Combined Effects of a Bacterial–Fungal Composite Inoculant Derived from the Rhizosphere of Gentiana macrophylla on Seed Germination, Seedling Growth, and Biomass Allocation

Gentiana macrophylla (Gentianaceae) is a valuable medicinal herb facing severe wild resource depletion, but its sustainable cultivation is constrained by poor germination and root-shoot imbalances caused by synthetic regulators like 1-naphthaleneacetic acid (NAA). A cross-kingdom microbial consortium was constructed using indigenous rhizosphere isolates: the bacterium Brevibacillus laterosporus X9 and the fungus Trichoderma harzianum Z20. Its effects on seed germination kinetics were evaluated in a 30-day assay conducted in a constant-temperature incubator at 30 °C in the dark, as well as on subsequent seedling growth and physiological and biochemical parameters in a 21-day pot experiment conducted in a controlled growth chamber. The effects were compared with those of single-strain inoculations, synthetic auxin (NAA) treatments, and an uninoculated control (CK). Co-inoculation with X9 and Z20 resulted in the highest final germination rate (78.33%, 19.9% higher than CK) and germination potential (48.33%), whereas NAA treatments were associated with less synchronized germination. Furthermore, X9 + Z20 enhanced root development, increasing belowground dry weight by 36.4%, relative growth rate by 97.9%, and root-to-shoot ratio by 15.2%. Conversely, high-concentration synthetic auxin (H-NAA, 1.0 mg L−1) reduced the root-to-shoot ratio by 34.8% relative to CK, inducing severe root stunting. In addition, X9 + Z20 improved several physiological and biochemical parameters in the seedlings, with total chlorophyll, soluble sugar, soluble protein, and free amino acid contents increasing by 48.3%, 30.6%, 10.1%, and 41.5%, respectively, relative to the uninoculated control. PCA, correlation, and log response ratio analyses suggested cross-kingdom complementary biostimulatory effects. In conclusion, co-inoculating indigenous X9 and Z20 promoted seedling growth without the root-shoot imbalance seen with NAA, providing an effective biological strategy for sustainable G. macrophylla production.