Pseudomonas chlororaphis A54 Enhances Drought Tolerance in Pinus sylvestris var. mongolica Through Coordinated Plant Physiological, Rhizosphere Microbial, and Soil Functional Responses

Drought severely restricts the growth and establishment of Pinus sylvestris var. mongolica seedlings, whereas the mechanisms by which plant growth-promoting rhizobacteria improve host drought tolerance remain incompletely understood. In this study, strain A54 was evaluated under four drought gradients (ND, LD, MD, and SD) in a greenhouse pot experiment. Seedling growth, nutrient accumulation, physiological traits, rhizosphere bacterial communities, soil functional variables, genome annotation, and qRT-PCR were integrated to clarify the drought-alleviating effects of A54. At the strain level, A54 maintained growth and ACC deaminase-associated functional performance under PEG-induced osmotic stress. A54 inoculation alleviated drought-induced growth suppression, with seedling height increasing by 69.7% under MD and 87.7% under SD relative to the corresponding controls. A54 also improved nutrient maintenance, especially Stem TN and Leaf TK, enhanced antioxidant capacity, and reduced osmotic stress, membrane lipid peroxidation, and stress-hormone accumulation. In the rhizosphere, A54 reshaped bacterial community structure by increasing the proportion of persistent taxa and selectively enriching drought-associated taxa, especially Pseudarthrobacter. A54-treated soils also maintained higher levels of available nutrients and enzyme activities under drought. Genome annotation and representative gene expression further supported the functional potential of A54 in nitrogen metabolism, ACC deaminase-associated ethylene regulation, oxidative defense, and osmotic or ion homeostasis. supporting its role in enhancing drought tolerance. These findings support the potential application of A54 as a bio-inoculant to improve afforestation performance under water-limited conditions.