Enhancement of Drought Stress Tolerance in Lettuce by Pseudomonas fluorescens FSO7 Isolated from Artemisia ordosica Rhizosphere

Drought is a major abiotic constraint on plant productivity, and plant growth-promoting rhizobacteria (PGPR) provide a sustainable strategy for improving crop drought tolerance. The rhizosphere of the dominant Mu Us Desert plant Artemisia ordosica may harbor drought-adapted microorganisms, but their functions remain insufficiently characterized. In this study, drought-tolerant rhizobacteria isolated from the A. ordosica rhizosphere were screened for growth-promoting effects on lettuce (Lactuca sativa L.) under controlled pot conditions. The dominant strain FSO7, identified as Pseudomonas fluorescens, exhibited the strongest effects. Under drought stress, FSO7 inoculation significantly increased plant height, stem width, fresh weight, and dry weight by 26.62%, 111.29%, 310.19%, and 44.02%, respectively, relative to uninoculated controls. Integrated transcriptomic and untargeted metabolomic analyses showed that FSO7 inoculation was associated with marked changes in lettuce carbon metabolism under drought, involving genes and metabolites related to starch and sucrose metabolism, glycolysis, and the TCA cycle, concurrently with up-regulation of genes in the flavonoid biosynthetic pathway. These findings suggest that P. fluorescens FSO7 may alleviate drought-induced growth inhibition in lettuce through coordinated modulation of carbon metabolism and flavonoid-mediated antioxidant metabolism. This strain represents a promising microbial resource for developing bioinoculants for water-limited agriculture.