Functional diversity and ecological consequences of endophytic Bacillus-plant interactions

The genus Bacillus, particularly endophytic species, has been widely studied as a source of plant growth-promoting bacteria in agricultural systems. These microorganisms contribute to plant performance through nutrient acquisition, phytohormone production, pathogen suppression, microbiome modulation, and enhanced tolerance to biotic and abiotic stresses. However, their ecological roles, functional plasticity, and genomic diversity remain poorly integrated into conceptual frameworks that extend beyond crop-based applications. Functional plasticity is reflected in their ability to colonize diverse plant hosts and tissues and to promote similar plant responses through distinct molecular mechanisms. Likewise, genomic diversity is evidenced by variation in accessory genomes, biosynthetic gene clusters, and regulatory networks that shape ecological functions and metabolite production. This review examines endophytic Bacillus as a model for understanding how metabolically versatile and genomically plastic bacteria establish functional, but context-dependent, associations with plants. Drawing on evidence from functional genomics, pangenomics, metabolomics, and microbial ecology, we discuss mechanisms associated with plant growth promotion and emphasize their dependence on host identity, environmental conditions, and microbial interactions. We address functional convergence arising from distinct genetic and metabolic routes, the contribution of accessory genomes and regulatory variation, and the ecological consequences of microbial inoculation in resident plant-associated microbiomes. We also highlight the limitations of in vitro screening approaches and the need for experimental validation across multiple biological scales to establish robust genotype-phenotype relationships. Finally, we extend the discussion beyond agricultural systems to consider the use of endophytic Bacillus in wild plant systems and ecological restoration, emphasizing the importance of evaluating both functional outcomes and ecological impacts.