Enhanced drought tolerance in Arabidopsis thaliana is mediated by Pseudomonas sp. M25 linked to root hair growth.

Plants establish intimate associations with rhizosphere microorganisms that profoundly influence their growth, development, and stress resilience. Among these, plant-growth-promoting rhizobacteria (PGPR) enhance nutrient acquisition, modulate phytohormone homeostasis and reshape root system architecture, thereby improving plant fitness. Despite extensive evidence supporting their beneficial effects, the molecular and cellular mechanisms underlying microbe-driven modulation of specific root traits remain incompletely understood. Through extensive phenotypic investigation, we established that Pseudomonas sp. M25, a previously described PGPR strain, produces a significant increase in leaf relative water content and evapotranspiration of Arabidopsis thaliana without impacting on rosette growth or photosynthetic parameters. Inoculation with M25 leads to enhanced drought tolerance, and this is associated not with changes in root architecture but with a marked increase in root hair (RH) abundance and length. The stimulation of RH development by this Pseudomonas strain is based on the genetic requirement for RH-related basic helix-loop-helix family transcription factors, including ROOT HAIR DEFECTIVE 6 (RHD6) and RHD6-LIKE 1 (RSL1), which regulate RH development via RHD6-LIKE 4 (RSL4) and RHD6-LIKE 2 (RSL2). Pseudomonas sp. M25 can partially circumvent the lack of RHD6 but requires RSL1 and the downstream transcription factors RSL2 and RSL4 to induce RH growth. These findings indicate that this bacterium can circumvent RHD6 to activate RSL4, which subsequently promotes RH growth. Our investigation identifies some essential signaling components regulated by Pseudomonas sp. M25 to optimize RH responses.