Seed Endophyte <em>Bacillus atrophaeus</em> Colonizes Root and Shoot Tissues Providing Antifungal Activity During Wheat Seedling Establishment

Seed-associated endophytes become active during germination, playing important roles as early colonizers of plant tissues, contributing to plant health while residing in a protective niche. In this study, we characterized a wheat-derived bacterial isolate, JunSE1L, to determine its functional traits and ecological role in the plant microbiome. The isolate was identified as Bacillus atrophaeus based on 16S rRNA analysis. JunSE1L exhibited nutrient-dependent plasticity in colony architecture, forming robust hydrophobic biofilms and pellicles under rich nutrient availability, while swarming and forming thin, often dendritic colonies under defined nutrition. JunSE1L produced highly surface-active compounds that lowered the surface tension of water to 30 mN/m and released potent proteolytic and hemolytic compounds, thus equipping JunSE1L for offensive roles, as examined against several fungal pathogens. JunSE1L inhibited Fusarium proliferatum and Mucor hiemalis in live-cell assays, while cell-free supernatant selectively inhibited M. hiemalis. JunSE1L was recovered from multiple plant compartments, including rhizosphere, rhizoplane, and aerial tissues, and was observed to emerge from cut plant tissues, supporting seed-endophyte mobilization upon germination to colonize distal tissues. Seed surface inoculation experiments with JunSE1L showed limited attachment at low cell densities and reduced seedling vigor at higher inoculum levels, supporting agricultural approaches nurturing the existing seed microbiome.