FIG 1.

The transition of Beauveria bassiana along the saprobe/mutualist/parasite continuum. B. bassiana is naturally present in the soil, where it lives as a saprotroph exploiting leaf litter and other organic matter for energy and nutrients. As a mutualist, B. bassiana establishes associations with plants by growing as an endophyte (i.e., growing without inducing drastic changes in plant immunity or causing harm to the plant). In exchange for carbon, B. bassiana benefits its host plants in varied ways, including protecting them from insect pests. B. bassiana-mediated resistance toward insects in plants may encompass direct parasitism and production of toxins and volatile organic compounds (VOCs), deterring insect pests, or interacting with beneficial insects. B. bassiana is a generalist insect pathogen. Spores attach to the insect cuticle, germinating and penetrating the insect exoskeleton and invading and proliferating in the hemocoel. Lipid assimilation and hydrolysis are central for this fungus to breach the insect cuticle. As a parasite, B. bassiana switches from biotrophic to a necrotrophic growth (growing and sporulating on the cadaver). Production of toxins during infection may accelerate the death of the insect and signal the biotrophic-necrotrophic switch. Also, some toxic proteins/metabolites could antagonize microbial competitors during necrotrophic/saprophytic growth.