Abstract
Metarhizium species fungi are able to produce resistant structures termed microsclerotia, formed by compact and melanized threads of hyphae. These propagules are tolerant to desiccation and produce infective conidia; thus, they are promising candidates to use in biological control programs. In this study, we investigated the tolerance to both ultraviolet B (UV-B) radiation and heat of microsclerotia of Metarhizium robertsii strain ARSEF 2575. We also adapted the liquid medium and culture conditions to obtain mycelial pellets from the same isolate in order to compare these characteristics between both types of propagules. We followed the peroxisome biogenesis and studied the oxidative stress during differentiation from conidia to microsclerotia by transmission electron microscopy after staining with a peroxidase activity marker and by the expression pattern of genes potentially involved in these processes. We found that despite their twice smaller size, microsclerotia exhibited higher dry biomass, yield, and conidial productivity than mycelial pellets, both with and without UV-B and heat stresses. From the 16 genes measured, we found an induction after 96-h differentiation in the oxidative stress marker genes MrcatA, MrcatP, and Mrgpx; the peroxisome biogenesis factors Mrpex5 and Mrpex14/17; and the photoprotection genes Mrlac1 and Mrlac2; and Mrlac3. We concluded that an oxidative stress scenario is induced during microsclerotia differentiation in M. robertsii and confirmed that because of its tolerance to desiccation, heat, and UV-B, this fungal structure could be an excellent candidate for use in biological control of pests under tropical and subtropical climates where heat and UV radiation are detrimental to entomopathogenic fungi survival and persistence.
Highlights
Fungal microsclerotia are hardened masses of pigmented hyphal aggregates (50– 600 μm), serving as survival structures and first described in the entomopathogenic fungi Metarhizium species when grown dimorphically in submerged liquid cultures
As scarce information is available regarding microsclerotial tolerance to abiotic factors (Corval et al, 2021), the aims of this study were to compare the effect of ultraviolet B (UV-B) radiation and heat on both MS and P tolerance and to characterize for the first time the expression pattern of genes potentially involved in oxidative stress, pigmentation, and peroxisome biogenesis during M. robertsii microsclerotial differentiation
Fungi exposed to UV-B radiation (1,283.38 mW m−2) had their conidial production decreased in a time-dependent manner; both MS and P resulted to be tolerant after exposure to heat (45◦C)
Summary
Fungal microsclerotia (from hereafter referred to as MS) are hardened masses of pigmented hyphal aggregates (50– 600 μm), serving as survival structures and first described in the entomopathogenic fungi Metarhizium species when grown dimorphically in submerged liquid cultures This dimorphism in Metarhizium lies in the transition from conidia to myceliogenic growth followed by the development of dense and compact hyphal threads forming MS. Filamentous fungi can grow as pellets by mycelial formation in submerged cultures (from hereafter referred to as P) (Nair et al, 2016; Zhang and Zhang, 2016; Veiter et al, 2018), which can be slightly distinct in morphology and probably less tolerant to environmental stresses in relation to MS Abiotic factors such as ultraviolet (UV) radiation and heat stress determine fungal propagule survival and persistence, and both are more pronounced in tropical and subtropical regions. Heat stress can delay and reduce the effectiveness of conidia germination, sporulation, and growth of thermosensitive entomopathogenic fungi (Fernandes et al, 2008, 2010; Paixão et al, 2019)
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