Plant Growth-Promoting Rhizobacteria <i>Paenibacillus polymyxa</i> HL14–3 Inoculation Enhances Drought Tolerance in Cucumber by Triggering Abscisic Acid-Mediated Stomatal Closure

Drought limits crop growth and yield. Inoculation with plant growth-promoting rhizobacteria (PGPR) emerges as a promising strategy to protect crops against drought. However, the number of drought-tolerant PGPR is limited, and the regulation mechanisms remain elusive. Here, we screened a novel drought-tolerant PGPR strain <i>Paenibacillus polymyxa</i> HL14-3 with high drought-tolerance potential and efficient colonization ability. <i>P. polymyxa</i> HL14-3 inoculation effectively alleviated drought-induced growth inhibition and oxidative stress and improved the root system architecture in cucumber. Furthermore, <i>P. polymyxa</i> HL14-3 improved stomatal closure and leaf relative water content, reducing water loss in cucumber under drought stress. Importantly, <i>P. polymyxa</i> HL14-3 inoculation enhanced drought tolerance in cucumber by inducing abscisic acid synthesis, which was counteracted by root irrigation with the ABA synthesis inhibitor fluridone. Together, our results demonstrate that <i>P. polymyxa</i> HL14-3 inoculation enhances drought tolerance in cucumber by triggering ABA-mediated stomatal closure, providing an effective drought-tolerant PGPR for promoting agricultural production in arid areas.