Only one of three hydrophobins (Hyd1-3) contributes to conidial hydrophobicity and insect pathogenicity of Metarhizium robertsii

Metarhizium acridum exhibits a different conidial hydrophobicity than other insect-pathogenic fungi

Metarhizium acridum is an insect-pathogenic fungus with a narrow host range that is used to control grasshoppers, locusts, and crickets. Its conidia show impressive resilience against UV-B radiation and heat compared to other insect pathogens. Despite this high tolerance, M. acridum is notably susceptible to various chemical stressors. The conidial surface is a monolayer formed by hydrophobins and the first barrier against stressors. However, little is known about the conidial surface properties of this fungus. This work aimed to evaluate the hydrophobic properties of the conidial surface in M. acridum ARSEF 324 and to compare them with those of other entomopathogen fungi. For this, drop profiles of water and diiodomethane on fungal cultures were analyzed, and the conidial hydrophobicity was estimated from a contact angle following the sessile drop method. The analysis of droplet profiles revealed distinct hydrophobic properties among the conidia of various insect-pathogenic fungi. The conidia of Beauveria bassiana, M. acridum, and Metarhizium robertsii exhibited high hydrophobicity, as indicated by water contact angles greater than 140°. In contrast, the conidia of Tolypocladium inflatum showed slightly lower hydrophobicity with a water contact angle below 130°. The behavior of diiodomethane further highlighted the variation in surface interactions, particularly for M. acridum, where the drop flattened instantly, indicating its low affinity for nonpolar compounds, which correlated with the extremely low tolerance of ARSEF 324 to chemicals. Overall, the study provides a deeper understanding of fungal surface properties, which could have implications for its tolerance to chemical stressors.

Deciphering roles of nine hydrophobins (Hyd1A-F and Hyd2A-C) in the asexual and insect-pathogenic lifecycles of Beauveria bassiana

Hydrophobins are small amphiphilic proteins that confer filamentous fungal hydrophobicity needed for hyphal growth, development, dispersal and adhesion to host and substrata. In insect-pathogenic Beauveria bassiana, nine hydrophobins (class I Hyd1A-F and class II Hyd2A-C) were proven to localize on the cell walls of aerial hyphae and conidia but accumulate in the vacuoles and vesicles of submerged hyphae and blastospores, respectively. Conidial hydrophobicity, adhesion to insect cuticle, virulence via normal cuticle infection and dispersal potential were significantly more reduced by the hyd1A deletion leading to complete ablation of slender rodlets on conidial coat than the hyd1B deletion, which caused a failure to assemble morphologically irregular rodlets into orderly bundles. Aerial conidiation and submerged blastospore production were compromised in Δhyd2A and Δhyd2C. The deletion of hyd1D stimulated conidial germination and virulence via insect hemocoel colonization, which was accelerated in Δhyd2A but decelerated in Δhyd2B. However, these deletion mutants were unaffected in radial growth on rich/minimal media and responses to osmotic, oxidative, cell wall-perturbing and heat-shock stresses except for an increase in conidial thermotolerance of Δhyd1A or cell sensitivity of Δhyd1B to Congo red-induced stress. None of examined phenotypes was altered in Δhyd1C, Δhyd1E and Δhyd1F. Conclusively, Hyd1A and Hyd1B co-regulate the formation, morphology and orderly assembly of rodlet bundles required for conidial hydrophobicity and infectivity, which are independent of Hyd1C-F and Hyd2A-C in B. bassiana. These results unveil a necessity to distinguish major, minor and dispensable roles among multiple class I/II hydrophobin genes in an ascomycetous pathogen.

Light enhances the production of conidia and influences their hydrophobicity in Tolypocladium inflatum

Insect fungal pathogens such as Beauveria bassiana, Metarhizium robertsii, and Tolypocladium inflatum have been used as insect biocontrol agents. Their infection mechanism involves non-specific adhesion to the host cuticle, which is controlled by hydrophobins, small proteins that form an amphipathic monolayer with rodlet morphology on diverse fungal structures. Light is an abiotic factor that may influence a wide range of cellular processes, including conidiogenesis, stress tolerance, and metabolite biosynthesis. Although its effects have been studied in many fungi, little is known about the effects of light on the hydrophobic properties of conidia. The aim of this work was to investigate the influence of visible light on the conidial hydrophobicity of three entomopathogenic fungal species. For this, conidia of B. bassiana, M. robertsii, and T. inflatum were grown either under light or in the dark, drop profiles of water and diiodomethane on conidial surfaces were analyzed, and conidial hydrophobicity was estimated from contact angle measurements. Moreover, conidial production was determined, and their genome was screened with sequences for hydrophobins. Conidia of B. bassiana and M. robertsii are more hydrophobic than conidia of T. inflatum. The light modified the surface tension of T. inflatum; therefore, conidia of T. inflatum became hydrophilic. However, light did not affect the conidial hydrophobicity of B. bassiana and M. robertsii. In addition, light modified the conidial production of B. bassiana and T. inflatum cultures, but it had no effect on the conidial production of M. robertsii. The T. inflatum genome contains two predicted proteins whose sequence is akin to that of proven class II hydrophobins from other ascomycetes. Presumably, these proteins are responsible for the conidial hydrophobicity properties in this fungus. Our study helps elucidate how light affects the conidial hydrophobicity of entomopathogenic fungi.

Rad6, a ubiquitin conjugator required for insect-pathogenic lifestyle, UV damage repair, and genomic expression of Beauveria bassiana

The E2 ubiquitin conjugator Rad6 is required for DNA damage bypass in budding yeast but remain functionally unknown in filamentous fungi. Here, we report pleiotropic effect of Rad6 ortholog in Beauveria bassiana, a wide-spectrum fungal insecticide. Global ubiquitination signal was greatly attenuated in the absence of rad6. The blocked ubiquitination led to severe growth defect, blocked asexual development, and abolished infectivity/insect pathogenicity, which correlated with compromised conidial quality (including viability, hydrophobicity, adherence to insect cuticle, and thermotolerance) and blocked secretion of cuticle-degrading enzymes including Pr1 family proteases. Importantly, Rad6 played much greater role in photoreactivation of UVB-impaired conidia by a 3- or 5-h light plus 9- or 7-h dark incubation than in dark reactivation of those impaired conidia by a 12-h dark incubation. The high activity of Rad6 in photoreactivation in vivo was derived from its link to a protein complex cored by the photolyase regulators WC1 and WC2 via the strong interactions of Rad6 with the E3 partner Rad18 and Rad18 with WC2 revealed in yeast two-hybrid assays. Transcriptomic analysis resulted in identification of 2700 differentially regulated genes involved in various function categories and metabolism pathways, indicating a regulatory role of Rad6-mediated ubiquitination in gene expression networks and genomic stability. Conclusively, Rad6 is required for asexual and insect-pathogenic lifecycles, solar UV damage repair, and genomic expression of B. bassiana. The primary dependence of its strong anti-UV role on photoreactivation in vivo unveils a scenario distinct from the core role of its yeast ortholog in DNA damage bypass.