Biosynthesis and function of rhizobacterial secondary metabolites in plant abiotic stress tolerance

Plant growth-promoting rhizobacteria enhance plant growth and stress resilience, but the metabolite-based mechanisms behind these effects remain insufficiently characterized. This study aimed to assess the metabolite profiles of three rhizobacterial treatments (RK1, RT2 and RT3) and evaluate their effects on drought tolerance in lettuce (\textit{Lactuca sativa}). The strains were cultured under standard laboratory conditions, and their intracellular metabolites were analyzed using gas chromatography-mass spectrometry. Results showed production of key compounds such as proline, glycine, glutamine, niacin, riboflavin, biotin, pantothenic acid, luteolin and apigenin 7-glucoside, with proline being the most abundant across strains. Lettuce plants were grown under controlled conditions and inoculated by soil drenching at transplantation. Drought stress was imposed 5 days after inoculation by withholding water for 7 days. Survival rate and fresh weight were measured after rewatering. Plants treated with rhizobacterial strains, particularly RT3, had significantly higher survival rates and fresh weight compared to the uninoculated control. These findings highlight the distinct contribution of specific rhizobacterial metabolites to drought tolerance and demonstrate their potential as microbial bioinoculants for improving plant performance under water-limited conditions.