Soybean Growth Promotion by the Euryhaline Marine Bacterium Cobetia amphilecti KMM 296 Is Associated with Hormonal and Transcriptional Responses

Marine bacteria remain insufficiently explored as potential plant bioinoculants. This study evaluated the plant growth-promoting potential of the euryhaline marine bacterium Cobetia amphilecti KMM 296 by comparing the effects of viable cells and cell-free culture supernatant on Glycine max. Compared with control plants, viable C. amphilecti cells increased seed germination by approximately 11%, while root length and root weight were more than 2.3 and 1.8 times higher, respectively. At the reproductive stage, bacterial treatment nearly doubled the seed number and increased yield per plant by approximately 2.4-fold. These changes were accompanied by increases in indole-3-acetic acid, gibberellin, and photosynthetic pigment levels. The treatment also upregulated genes associated with auxin biosynthesis and signaling, gibberellin metabolism, and root elongation by 1.58–4.97-fold. The supernatant produced weaker or intermediate responses, including increases in soybean root length, seed number, and yield. Comparative reconstruction of 23 metabolic pathways across 34 Cobetia genomes predicted conserved metabolic potential for auxin-related and volatile-compound metabolism, phosphate acquisition, polyamine biosynthesis, and stress adaptation. Overall, viable KMM 296 cells exerted substantially stronger plant responses than the cell-free culture supernatant, supporting further evaluation of this strain as a candidate euryhaline marine bioinoculant.