Specificity of the effect of different endophytic Bacillus velezensis strains on soybean cultivars with contrasting growth habits

This study evaluates the influence of the source of isolation of an endophytic seed-borne isolate within a single bacterial species on its subsequent compatibility with a new host. Two Bacillus velezensis strains were tested: one (KS 38 AU) isolated from barley seeds and the other (KS 63 AU) from soybean seeds. Seed treatments based on these strains were applied to soybean (Glycine max L.). Two soybean cultivars differing in growth habit served as experimental material: cv. Lidiya (indeterminate type) and cv. Apis (determinate type). Plants were grown in a climate chamber under controlled conditions. At the V3 growth stage, leaf photosynthetic pigment content and free proline accumulation were assessed. Strain KS 63 AU was compatible with both soybean cultivars, increasing chlorophyll *a* content and reducing proline levels. In contrast, in the determinate cv. Apis, strain KS 38 AU caused a sharp decline in chlorophyll *a* content (to 36% of the control), yet paradoxically reduced proline to levels comparable with those of the compatible strain. The Cultivar × Strain interaction for proline was highly significant (p = 0.004). The results obtained for the determinate cv. Apis indicate that strain KS 38 AU may trigger accelerated leaf senescence with concomitant nitrogen remobilization. These findings demonstrate that endophyte origin and plant growth habit jointly determine the outcome of inoculation, which has direct implications for the rational selection of microbial inoculants based on seed-borne endophytic bacteria in legume production.