Isolation and Identification of IAA-Producing Rhizobacteria from Alfalfa and Their Strain-Specific Growth-Promoting Effects in Arid Regions

In this study, we aimed to isolate indigenous plant-growth-promoting rhizobacteria (PGPR) with high indole-3-acetic acid (IAA)-producing capacity from alfalfa rhizospheres in arid regions of Northwest China and systematically evaluate their bioacceleration effects on alfalfa growth. Fifteen bacterial strains were isolated from rhizosphere soils collected in Ningxia and Inner Mongolia. Among them, four high-IAA-producing strains were selected and identified as Brevundimonas sp. B3, Pantoea sp. P10, and Microbacterium sp. M1 and M7 based on 16S rDNA sequencing. Pot experiments showed strain-specific growth-promoting effects: P10 significantly increased plant biomass (increasing fresh weight by 10.04% and dry weight by 11.76%, with p < 0.05), while M7 notably enhanced plant height (by 16.48%, with p < 0.05) and branching. Physiological and cytological analyses revealed that the tested strains improved chlorophyll content (30–45% above the control), reduced malondialdehyde (MDA) levels (20–40% below the control), and differentially regulated root-tip cell elongation. Principal component analysis further supported the comprehensive promotive effects of these strains, with P10 exhibiting the highest overall performance (PC1–PC4 cumulative variance: 83.1%). Within the limitations of controlled pot experiments, these findings highlight the potential of native PGPR strains, particularly P10 and M7, as promising candidates for developing region-specific microbial inoculants with which to enhance alfalfa productivity in arid and semi-arid environments.