Endophytic bacteria Priestia megaterium 170T-4 improves soybean salt tolerance through regulation of ion homeostasis and phytohormone signaling pathways

Salt stress in coastal saline-alkali soils impairs plant survival and growth. Plant growth-promoting rhizosphere bacteria (PGPR) and endophytic bacteria can enhance salinity tolerance via stable host associations. This study used culture-based and transcriptomic methods to investigate culturable endophytic bacteria in soybean and their salt-tolerance mechanisms. A total of 154 strains were isolated from the roots of 10 soybean varieties cultivated in coastal saline-alkali soil, spanning 4 phyla, 35 genera, and 76 species. <i>Microbacterium phyllosphaerae</i> and <i>Priestia megaterium</i> were identified as dominant species, from which two representative strains were selected to assess their growth-promoting effects under salt stress. Strain 170T-4 was identified as <i>P. megaterium</i> via multilocus sequence analysis and showed high salt tolerance, growing in up to 6% NaCl. Pot experiments showed that strain 170T-4 significantly improved plant height, root elongation, Na⁺/K⁺ homeostasis, proline, and chlorophyll content. Transcriptome profiling and RT-qPCR revealed that strain 170T-4 regulates K⁺ transport-related genes (<i>GORK</i> and <i>SKOR</i>), ethylene signaling related genes (<i>PTI5</i>, <i>EIN3</i>, and <i>ERF1</i>), and the allene oxide cyclase gene (<i>AOC</i>). Overall, strain 170T-4 improved soybean growth under salt stress by modulating ion transport, osmotic responses, and hormone signaling, showing strong potential as a microbial inoculant for saline-alkali soils.