Endophytic Bacillus pumilus G5 with silicon alleviates drought stress and enhances glycyrrhizic acid in Glycyrrhiza uralensis Fisch. via ABA and JA signaling

Drought stress is a major challenge for global agriculture, impacting the production of medicinal plants. Plant growth-promoting bacteria (PGPB) and silicon (Si), an environmentally friendly element, are both known to support plant adaptation to drought and enhance the accumulation of bioactive metabolites. In this study, we investigated how Bacillus pumilus G5 (G5) and Si regulate hormone biosynthesis, signal transduction, and glycyrrhizic acid formation in Glycyrrhiza uralensis under drought conditions. Physiological measurements combined with transcriptomic profiling were used to clarify the relationship between hormone pathways and glycyrrhizic acid production. Drought treatment decreased glycyrrhizic acid levels by suppressing the expression of HMGR, HDR, and FPPS. In contrast, the combined G5 + Si treatment markedly improved glycyrrhizic acid accumulation by stimulating the enzymatic activities of FPPS, SQS, and UGT. Although G5 + Si enhanced Crtz and NCED activities, it did not change ABA concentrations in drought-stressed plants, indicating that this treatment maintains ABA homeostasis. Instead, it accelerates the turnover between ABA biosynthesis and degradation, thereby activating PYR/PYL and SnRK2 and improving drought resistance. Furthermore, G5 + Si mitigated drought damage by promoting JA signaling and up-regulating MYC2, ultimately increasing both drought tolerance and glycyrrhizic acid accumulation. These findings emphasise the strong synergy between G5 and Si in supporting stress mitigation and enhancing the medicinal quality of G. uralensis . • G5 and Si synergistically alleviate drought stress. • G5 + Si boosts glycyrrhizic acid by enhancing FPPS, SQS, UGT activities. • ABA homeostasis is maintained via accelerated turnover. • JA signaling and MYC2 are enhanced by G5 + Si. • Drought tolerance and medicinal quality are improved.