Introduction
Microorganisms play a fundamental role in shaping the biology, ecology, and evolutionary trajectories of most organisms, and fungi are no exception. Fungal species exist within complex microbial networks involving plants, bacteria, viruses, and other fungi, with interactions occurring both externally around the mycelial surface and internally within the hyphae. Recent studies have revealed that many fungi harbor endobacteria (EB), which can profoundly influence their physiology, development, and ecological interactions. Members of the genus Metarhizium, common soil fungi and important entomopathogens, have only recently been recognized as hosts to EB, particularly Bacillus subtilis. Understanding how these endobacteria modulate fungal life cycles and virulence is crucial for improving the biological control potential of Metarhizium against insect pests.
Endobacteria–Fungi Symbiosis: Molecular Basis and Ecological Significance
Endobacteria residing within fungal hyphae establish a unique symbiotic system where intracellular microorganisms can directly influence host gene expression, metabolic pathways, and stress responses. In Metarhizium spp., these symbionts may modulate signaling pathways related to sporulation, nutrient acquisition, and host infection. The ecological significance of these interactions extends beyond the fungus itself, as EB-mediated changes can determine fungal competitiveness in the rhizosphere, colonization efficiency in plant roots, and interactions with other soil microbes. Research into the molecular basis of this symbiosis could provide insights into microbial evolution and potential biotechnological applications.
Influence of Bacillus subtilis Endobacteria on Metarhizium Physiology
Identifying Bacillus subtilis as an internal bacterium in Metarhizium strains opens new avenues to explore how these bacteria alter fungal physiology. EB may modulate hyphal growth, conidiation, stress tolerance, and secondary metabolite production. These physiological changes could be driven by bacterial metabolites, effector-like molecules, or nutrient exchanges occurring within the hyphae. Understanding these mechanisms is essential for unraveling the full metabolic and developmental impact of EB on fungal hosts and could help optimize fungal strains for agricultural applications.
Negative Impact of Endobacteria on Metarhizium Virulence
Experimental evidence indicates that endobacteria can reduce the entomopathogenic capacity of Metarhizium spp. against insect hosts such as Galleria mellonella and Tenebrio molitor. EB may disrupt the fungal infection process by attenuating the production of virulence factors, degrading insect cuticle-degrading enzymes, or interfering with the fungus’s ability to proliferate inside the insect hemocoel. This negative modulation challenges the assumption that intracellular bacteria always provide mutualistic benefits to their hosts, highlighting the complexity of fungal–bacterial relationships and their consequences for biological pest control.
Rhizosphere-Derived Metarhizium Strains and Their Microbial Associations
Rhizosphere-associated Metarhizium species represent a biologically rich yet understudied ecological niche. These fungi interact closely with plant roots, responding to root exudates, competing with soil microbes, and utilizing bacterial partners for nutrient cycling. The discovery of endobacteria within Metarhizium strains isolated from the rhizosphere suggests that soil and plant-associated environments may promote stable endosymbiotic relationships. Studying these microbial networks may help elucidate how rhizosphere conditions influence fungal behavior, persistence, and virulence.
Future Research Directions in Metarhizium–Endobacteria Interactions
Future studies should prioritize genomic, transcriptomic, and metabolomic analyses to uncover the mechanisms by which endobacteria alter Metarhizium virulence and development. Experimental manipulation—such as curing fungi of their EB and reintroducing them—could clarify causal relationships between bacterial presence and fungal phenotypes. Additionally, exploring EB diversity across global Metarhizium populations will help determine whether negative virulence effects are universal or strain-specific. This knowledge has practical implications for developing improved bioinsecticides with optimized efficacy and stability.
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