Biochemistry of microsclerotia differentiation in entomopathogenic fungi: from stress to colour, and implications for insect biocontrol.

Microsclerotia (MS) are compact, pigmented propagules of entomopathogenic fungi that are resistant to desiccation and capable of producing infective conidia, positioning them as promising agents for biological control. The transition from conidia to microsclerotia in liquid media is triggered by oxidative stress and involves intricate signalling pathways that induce a range of cellular and molecular changes, including the activation of antioxidant defence systems, iron signalling, peroxisome biogenesis, and pigmentation. The biochemical processes influenced by MS-specific culture media are crucial for the development of fungal structures that exhibit thermotolerance, resilience, and high conidiogenesis. This review delves into these processes, examining the dynamics of aggregation, the role of reactive oxygen species in metabolic regulation, and the stress responses that drive melanin biosynthesis and, ultimately, MS maturation. Furthermore, a comprehensive overview of the bioinsecticidal activity of MS against arthropod pests, as reported in the literature, is also presented.