Hydrophobicity-Related Protein Contents and Surface Areas of Aerial Conidia are Useful Traits for Formulation Design of Fungal Biocontrol Agents

The Spt10 GNAT Superfamily Protein Modulates Development, Cell Cycle Progression and Virulence in the Fungal Insect Pathogen, Beauveria bassiana

Chromatin remodeling is mediated in part by post-translational acetylation/deacetylation modifications of histones. Histone acetyltransferases (HATs), e.g., members of the GNAT/MYST superfamily, activate gene transcription via promotion of euchromatin formation. Here, we characterized a GNAT family HAT, Spt10 (BbSpt10), in the environmentally and economically important fungal insect pathogen, Beauveria bassiana. Targeted gene knockout of BbSpt10 resulted in impaired asexual development and morphogenesis; reduced abilities to utilize various carbon/nitrogen sources; reduced tolerance to heat, fungicides, and DNA damage stress; and attenuated virulence. The ΔBbSpt10 mutant showed disrupted cell cycle development and abnormal hyphal septation patterns. Transcriptome analyses of wild type and ΔBbSpt10 cells revealed the differential expression of 373 genes, including 153 downregulated and 220 upregulated genes. Bioinformatic analyses revealed downregulated genes to be enriched in pathways involved in amino acid metabolism, cellular transportation, cell type differentiation, and virulence, while upregulated genes were enriched in carbon/nitrogen metabolism, lipid metabolism, DNA process, and cell rescue, defense, and virulence. Downregulated virulence genes included hydrophobins, cellular transporters (ABC and MFS multidrug transporters) and cytochrome P450 detoxification genes. These data indicated broad effects of BbSpt10 on fungal development, multi-stress response, and virulence.

Principal component analysis of the biological characteristics of entomopathogenic fungi in nutrient-limited and cuticle-based media

Media formulated with insect cuticle (0.5% and 1%; Sphequit Sph®), with a reduction in nutrients (¼ Sabouraud dextrose agar + yeast [SDAY]) and commercial media (potato dextrose agar, Sabouraud dextrose agar) were evaluated for the cultivation of Beauveria bassiana, Cordyceps javanica (Isaria javanica [Bally] Samson & Hywel-Jones), and Metarhizium robertsii. By using principal component analysis, it was determined that the ¼ SDAY and Sph formulations have greater advantages than commercial media for the development of fungi. The ¼ SDAY and Sph (0.5% and 1%) improved hydrophobicity, radial growth rate, germination, conidia yield, and virulence in B. bassiana; in M. robertsii, they favored conidia yield, germination, and virulence, and in C. javanica, the ¼ SDAY and Sph 0.5% media enhanced conidia yield, germination, radial growth rate, and virulence. We suggest that these formulations are an alternative to commercial culture media as they are cheaper and appropriate to improve the growth characteristics and virulence of the three strains evaluated. Some applications of culture media are suggested, and the importance of multivariate analysis as an exploratory tool to carry out the choice of culture media in a suitable way for the development of mycoinsecticides is also discussed.

Pleiotropic effects of the histone deacetylase Hos2 linked to H4-K16 deacetylation, H3-K56 acetylation, and H2A-S129 phosphorylation in Beauveria bassiana

Histone acetyltransferases and deacetylases maintain dynamics of lysine acetylation/deacetylation on histones and nonhistone substrates involved in gene regulation and cellular events. Hos2 is a Class I histone deacetylases that deacetylates unique histone H4-K16 site in yeasts. Here, we report that orthologous Hos2 deacetylates H4-K16 and is also involved in the acetylation of histone H3-K56 and the phosphorylation of histone H2A-S129 and cyclin-dependent kinase 1 CDK1-Y15 in Beauveria bassiana, a filamentous fungal insect pathogen. These site-specific modifications are evidenced with hyperacetylated H4-K16, hypoacetylated H3-K56, and both hypophosphorylated H2A-S129 and CDK1-Y15 in absence of hos2. Consequently, the Δhos2 mutant suffered increased sensitivities to DNA-damaging and oxidative stresses, disturbed cell cycle, impeded cytokinesis, increased cell size or length, reduced conidiation capacity, altered conidial properties, and attenuated virulence. These phenotypic changes correlated well with dramatic repression of many genes that are essential for DNA damage repair, G₁ /S transition and DNA synthesis, hyphal septation, and asexual development. The uncovered ability for Hos2 to directly deacetylate H4-K16 and to indirectly modify H3-K56, H2A-S129, and CDK1-Y15 provides novel insight into more subtle regulatory role for Hos2 in genomic stability and diverse cellular events in the fungal insect pathogen than those revealed previously in nonentomophathogenic fungi.

A conidial protein (CP15) of Beauveria bassiana contributes to the conidial tolerance of the entomopathogenic fungus to thermal and oxidative stresses

Aerial conidia are central dispersing structures for most fungi and represent the infectious propagule for entomopathogenic fungus Beauveria bassiana, thus the active ingredients of commercial mycoinsecticides. Although a number of formic-acid-extractable (FAE) cell wall proteins from conidia have been characterized, the functions of many such proteins remain obscure. We report that a conidial FAE protein, termed CP15, isolated from B. bassiana is related to fungal tolerance to thermal and oxidative stresses. The full-length genomic sequence of CP15 was shown to lack introns, encoding for a 131 amino acid protein (15.0 kDa) with no sequence identity to any known proteins in the NCBI database. The function of this new gene with two genomic copies was examined using the antisense-RNA method. Five transgenic strains displayed various degrees of silenced CP15 expression, resulting in significantly reduced conidial FAE protein profiles. The FAE protein contents of the strains were linearly correlated to the survival indices of their conidia when exposed to 30-min wet stress at 48°C (r (2) = 0.93). Under prolonged 75-min heat stress, the median lethal times (LT(50)s) of their conidia were significantly reduced by 13.6-29.5%. The CP15 silenced strains were also 20-50% less resistant to oxidative stress but were not affected with respect to UV-B or hyperosmotic stress. Our data indicate that discrete conidial proteins may mediate resistance to some abiotic stresses, and that manipulation of such proteins may be a viable approach to enhancing the environmental fitness of B. bassiana for more persisting control of insect pests in warmer climates.

In vivo passages of heterologous Beauveria bassiana isolates improve conidial surface properties and pathogenicity to Nilaparvata lugens (Homoptera: Delphacidae)

In entomopathogenic hyphomycetes, desired candidates against the brown planthopper, Nilaparvata lugens (a sap-sucking rice pest in Asia), are lacking. In this study, 21 Beauveria bassiana isolates from heterologous host insects showed low pathogenicity to third-instar nymphs sprayed at the high concentration of ∼ 1000 conidia/mm(2), causing only 2-23% mortalities. Of those, three isolates killed significantly more nymphs (up to 45-62%) after two in vivo passages but no more after further passage. Conidial hydrophobicity rates (H(r)), zeta potentials (P(z)), and subtilisin-like protease (Pr1) activities (A(p)) of these isolates showed the same trends in the three host passages (N: 0-3). In multivariate correlation, the variables N, H(r) and P(z) were found contributing 89% to the mortality variation (r(2)=0.89). Significant positive correlations were also found between H(r) and N (r(2)=0.64), P(z) and N (r(2)=0.52), A(p) and N (r(2)=0.51), H(r) and A(p) (r(2)=0.45), and P(z) and A(p) (r(2)=0.57), respectively. However, irregular changes of H(r) and P(z) occurred in four other isolates, whose pathogenicity to N. lugens was not enhanced by repeated host passages, resulting in no correlation between the variables. Our data indicate that the conidial surface properties H(r) and P(z) associated with cuticle adhesion reflect the heterologous host-induced adaptation and help to select fungal candidates against N. lugens from repeated in vivo passages.