Plant Growth-Promoting Rhizobacteria Paenibacillus polymyxa 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 Paenibacillus polymyxa HL14–3 with high drought-tolerance potential and efficient colonization ability. P. polymyxa HL14–3 inoculation effectively alleviated drought-induced growth inhibition and oxidative stress and improved the root system architecture in cucumber. Furthermore, P. polymyxa HL14–3 improved stomatal closure and leaf relative water content, reducing water loss in cucumber under drought stress. Importantly, P. polymyxa 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 P. polymyxa 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.