Cross-Host Growth Promotion and Colonization of Tomato (Solanum lycopersicum L.) by a Sweet Potato Rhizosphere-Derived Bacillus sp. RF-12: Single and Consortium Inoculation with Endophytic Bacteria

The effectiveness of rhizosphere bacteria isolated from sweet potato (Ipomoea batatas (L.) Lam.) in consortium inoculations and cross-host applications remains poorly understood. Therefore, in this study, we evaluated the cross-host plant growth-promoting efficacy and colonization dynamics of Bacillus sp. RF-12, isolated from the sweet potato rhizosphere, in tomato (Solanum lycopersicum L.) compared with three endophytic bacterial strains isolated from sweet potato applied individually or as a consortium and investigated the influence of microbial interactions on bacterial colonization and plant growth. Tomato seedlings were grown under controlled conditions, inoculated with the respective bacterial treatments, and evaluated for plant growth and bacterial populations in the rhizosphere, roots, and shoots 16 d after inoculation. B. RF-12 exhibited the greatest overall plant growth-promoting effect, increasing root and shoot fresh weights by 101.9% and 134.1%, respectively, and shoot length by 28.5% relative to the uninoculated control. The consortium inoculation also significantly promoted plant growth under the experimental conditions tested. B. RF-12 successfully colonized the rhizosphere and internal tissues of tomato, despite being originally isolated from the sweet potato rhizosphere; however, its population remained 0.9–2.7 log units lower than those of the endophytic strains and further decreased under consortium inoculation. These findings suggest that colonization abundance alone may not determine plant growth-promoting effectiveness. Overall, the results highlight the potential of evaluating rhizosphere-associated bacteria isolated from one crop species for their ability to colonize and promote growth in a different host plant. Further studies under field conditions and across additional crop species are needed to evaluate their persistence, host range, and potential agricultural applications.