Vital role for cyclophilin B (CypB) in asexual development, dimorphic transition and virulence of Beauveria bassiana

Characterization and regulation of salt upregulated cyclophilin from a halotolerant strain of Penicillium oxalicum

Penicillium species are an industrially important group of fungi. Cyclophilins are ubiquitous proteins and several members of this family exhibit peptidyl-prolyl cis-trans isomerase (PPIase) activity. We had earlier demonstrated that the salt-induced PPIase activity in a halotolerant strain of P. oxalicum was associated with enhanced expression of a cyclophilin gene, PoxCYP18. Cloning and characterization of PoxCYP18 revealed that its cDNA consists of 522 bp encoding a protein of 173 amino acid residues, with predicted molecular mass and pI values of 18.91 kDa and 8.87, respectively. The recombinant PoxCYP18 can catalyze cis-trans isomerization of peptidyl-prolyl bond with a catalytic efficiency of 1.46 × 107 M-1 s-1 and is inhibited specifically only by cyclosporin A, with an inhibition constant of 5.04 ± 1.13 nM. PoxCYP18 consists of two cysteine residues at positions - 45 and - 170, and loses its activity under oxidizing conditions. Substitution of these residues alone or together by site-directed mutagenesis revealed that the PPIase activity of PoxCYP18 is regulated through a redox mechanism involving the formation of disulfide linkages. Heterologous expression of PoxCYP18 conferred enhanced tolerance to salt stress in transgenic E. coli cells, implying that this protein imparts protection to cellular processes against salt-induced damage.

FluG and FluG-like FlrA Coregulate Manifold Gene Sets Vital for Fungal Insect-Pathogenic Lifestyle but Not Involved in Asexual Development

The central developmental pathway (CDP) activator gene brlA is activated by the upstream genes fluG and flbA–flbE in Aspergillus nidulans. Increasing evidences of fungal genome divergence make it necessary to clarify whether such genetic principles fit Pezizomycotina. Previously, fluG disruption resulted in limited conidiation defect and little effect on the expression of brlA and flbA–flbE in Beauveria bassiana possessing the other FluG-like regulator FlrA. Here, single-disruption (SD) mutants of flrA and double-disruption (DD) mutants of flrA and fluG were analyzed to clarify whether FlrA and FluG are upstream regulators of key CDP genes. Despite similar subcellular localization, no protein-protein interaction was detected between FlrA and FluG, suggesting mutual independence. Three flrA SD mutants showed phenotypes similar to those previously described for ΔfluG, including limited conidiation defect, facilitated blastospore production, impaired spore quality, blocked host infection, delayed proliferation in vivo, attenuated virulence, and increased sensitivities to multiple stresses. Three DD mutants resembled the SD mutants in all phenotypes except more compromised pathogenicity and tolerance to heat shock- or calcofluor white-induced stress. No CDP gene appeared in 1,622 and 2,234 genes dysregulated in the ΔflrA and ΔfluG mutants, respectively. The majority (up/down ratio: 540:875) of those dysregulated genes were co-upregulated or co-downregulated at similar levels in the two mutants. These findings unravel novel roles for flrA and fluG in coregulating manifold gene sets vital for fungal adaptation to insect-pathogenic lifestyle and environment but not involved in CDP activation.

IMPORTANCE FluG is a core regulator upstream of central developmental pathway (CDP) in Aspergillus nidulans but multiple FluG-like regulators (FLRs) remain functionally uncharacterized in ascomycetes. Our previous study revealed no role for FluG in the CDP activation and an existence of sole FLR (FlrA) in an insect-pathogenic fungus. This study reveals a similarity of FlrA to FluG in domain architecture and subcellular localization. Experimental data from analyses of targeted single- and double-gene knockout mutants demonstrate similar roles of FrlA and FluG in stress tolerance and infection cycle but no role of either in CDP activation. Transcriptomic analyses reveal that FlrA and FluG coregulate a large number of same genes at similar levels. However, the regulated genes include no key CDP gene. These findings uncover that FlrA and FluG play similar roles in the fungal adaptation to insect-pathogenic lifestyle and environment but no role in the activation of CDP.

Fungal insecticidal activity elevated by non-risky markerless overexpression of an endogenous cysteine-free protein gene in Beauveria bassiana

BACKGROUND

Fungal insecticides are notorious for slow kill action, an intrinsic trait that can be improved by the genetic engineering of an exogenous or endogenous virulence factor. However, transgenic insecticides expressing exogenous toxin and herbicide-resistant marker genes may cause unexpected ecological risks and are hardly permitted for field release due to strict regulatory hurdles. It is necessary to improve biotechnology that can speed up fungal insect-killing action and exclude ecological risk source.

RESULTS

A markerless transformation system of Beauveria bassiana, a main source of wide-spectrum fungal insecticides, was reconstructed based on the fungal uridine auxotrophy (Δura3). The system was applied for overexpression of the small cysteine-free protein (120 amino acids) gene cfp previously characterized as a regulator of the fungal virulence and gene expression. Three cfp-overexpressed strains showed much faster kill action to Galleria mellonella larvae than the parental wild-type via normal cuticle infection but no change in vegetative growth and aerial condition. The faster kill action was achieved due to not only significant increases in conidial adherence to insect cuticle and total activity of secreted cuticle-degrading Pr1 proteases and of antioxidant enzymes crucial for collapse of insect immune defense but acceleration of hemocoel localization, proliferation in vivo and host death from mummification.

CONCLUSION

The markerless system is free of any foreign DNA fragment as a source of ecologic risk and provides a novel biotechnological approach to enhancing fungal insecticidal activity with non-risky endogenous genes and striding over the regulatory hurdles. © 2022 Society of Chemical Industry.

Three Small Cysteine-Free Proteins (CFP1–3) Are Required for Insect-Pathogenic Lifestyle of Metarhizium robertsii

Unique CFP (cysteine-free protein; 120 aa) has been identified as an extraordinary virulence factor in Beauveria bassiana (Cordycipitaceae), a main source of wide-spectrum fungal insecticides. Its homologs exclusively exist in wide-spectrum insect pathogens of Hypocreales, suggesting their importance for a fungal insect-pathogenic lifestyle. In this study, all three CFP homologs (CFP1–3, 128–145 aa) were proven essential virulence factors in Metarhizium robertsii (Clavicipitaceae). Despite limited effects on asexual cycles in vitro, knockout mutants of cfp1,cfp2 and cfp3 were severely compromised in their capability for normal cuticle infection, in which most tested Galleria mellonella larvae survived. The blocked cuticle infection concurred with reduced secretion of extracellular enzymes, including Pr1 proteases required cuticle penetration. Cuticle-bypassing infection by intrahemocoel injection of ~250 conidia per larva resulted in a greater reduction in virulence in the mutant of cfp1 (82%) than of cfp2 (21%) or cfp3 (25%) versus the parental wild-type. Transcriptomic analysis revealed dysregulation of 604 genes (up/down ratio: 251:353) in the Δcfp1 mutant. Many of them were involved in virulence-related cellular processes and events aside from 154 functionally unknown genes (up/down ratio: 56:98). These results reinforce the essential roles of small CFP homologs in hypocrealean fungal adaptation to insect-pathogenic lifestyle and their exploitability for the genetic improvement of fungal insecticidal activity.

Cyclophilin acts as a ribosome biogenesis factor by chaperoning the ribosomal protein (PlRPS15) in filamentous fungi

The rapid transport of ribosomal proteins (RPs) into the nucleus and their efficient assembly into pre-ribosomal particles are prerequisites for ribosome biogenesis. Proteins that act as dedicated chaperones for RPs to maintain their stability and facilitate their assembly have not been identified in filamentous fungi. PlCYP5 is a nuclear cyclophilin in the nematophagous fungus Purpureocillium lilacinum, whose expression is up-regulated during abiotic stress and nematode egg-parasitism. Here, we found that PlCYP5 co-translationally interacted with the unassembled small ribosomal subunit protein, PlRPS15 (uS19). PlRPS15 contained an eukaryote-specific N-terminal extension that mediated the interaction with PlCYP5. PlCYP5 increased the solubility of PlRPS15 independent of its catalytic peptide-prolyl isomerase function and supported the integration of PlRPS15 into pre-ribosomes. Consistently, the phenotypes of the PlCYP5 loss-of-function mutant were similar to those of the PlRPS15 knockdown mutant (e.g. growth and ribosome biogenesis defects). PlCYP5 homologs in Arabidopsis thaliana, Homo sapiens, Schizosaccharomyces pombe, Sclerotinia sclerotiorum, Botrytis cinerea and Metarhizium anisopliae were identified. Notably, PlCYP5-PlRPS15 homologs from three filamentous fungi interacted with each other but not those from other species. In summary, our data disclosed a unique dedicated chaperone system for RPs by cyclophilin in filamentous fungi.

A Small Cysteine-Free Protein Acts as a Novel Regulator of Fungal Insect-Pathogenic Lifecycle and Genomic Expression

Small secreted proteins (SSPs), particularly cysteine-rich proteins secreted during fungal infection, comprise virulence effectors in plant-pathogenic fungi but remain unknown in insect-pathogenic fungi. We report here that only a small cysteine-free protein (CFP) is indispensable for insect pathogenicity of Beauveria bassiana among 10 studied SSPs (99 to 274 amino acids [aa]), including seven hypothetical proteins containing 0 to 12 Cys residues. CFP (120 aa) features an N-terminal signal peptide (residues 1 to 17), a nuclear localization signal motif (residues 24 to 57), and no predictable domain. Its homologs exist exclusively in insect-pathogenic Cordycipitaceae and Clavicipitaceae. Fluorescence-tagged CFP fusion protein was localized in the nucleus but extracellularly undetectable, suggesting an inability for CFP to be secreted out. Disruption of cfp resulted in abolished pathogenicity via normal cuticle infection, attenuated virulence via hemocoel injection, compromised conidiation capacity versus little growth defect, impaired conidial coat, blocked secretion of cuticle-degrading enzymes, impeded proliferation in vivo, disturbed cell cycle, reduced stress tolerance, and 1,818 dysregulated genes (genomic 17.54%). Hundreds of those genes correlated with phenotypic changes observed in the disruption mutant. Intriguingly, nearly 40% of those dysregulated genes encode hypothetical or unknown proteins, and another 13% encode transcription factors and enzymes or proteins collectively involved in genome-wide gene regulation. However, purified CFP showed no DNA-binding activity in an electrophoretic mobility shift assay. These findings unveil that CFP is a novel regulator of fungal insect-pathogenic life cycle and genomic expression and that cysteine richness is dispensable for distinguishing virulence effectors from putative SSPs in B. bassiana IMPORTANCE Small cysteine-rich proteins secreted during plant-pathogenic fungal infection comprise virulence effectors. Our study confirms that only a cysteine-free protein (CFP) is determinant to insect-pathogenic fungal virulence among 10 small putatively secreted proteins containing 0 to 12 Cys residues. Disruption of cfp abolished insect pathogenicity and caused not only a series of compromised cellular events associated with host infection and disease development but also dysregulation of 1,818 genes, although no DNA-binding activity was detected in purified CFP samples. Nearly 13% of those genes encode transcription factors and enzymes or proteins collectively involved in transcriptional regulation. Altogether, CFP serves as a novel regulator of the fungal insect-pathogenic life cycle and genomic expression. Cysteine richness is dispensable for distinguishing virulence effectors from the fungal SSPs.

Gene diversity explains variation in biological features of insect killing fungus, Beauveria bassiana

Beauveria bassiana is a species complex whose isolates show considerable natural genetic variability. However, little is known about how this genetic diversity affects the fungus performance. Herein, we characterized the diversity of genes involved in various mechanisms of the infective cycle of 42 isolates that have different growth rates, thermotolerance and virulence. The analysed genes showed general genetic diversity measured as non-synonymous changes (NSC) and copy number variation (CNV), with most of them being subjected to positive episodic diversifying selection. Correlation analyses between NSC or CNV and the isolate virulence, thermotolerance and growth rate revealed that various genes shaped the biological features of the fungus. Lectin-like, mucin signalling, Biotrophy associated and chitinase genes NSCs correlated with the three biological features of B. bassiana. In addition, other genes (i.e. DNA photolyase and cyclophilin B) that had relatively conserved sequences, had variable CNs across the isolates which were correlated with the variability of either virulence or thermotolerance of B. bassiana isolates. The data obtained is important for a better understanding of population structure, ecological and potential impact when isolates are used as mycoinsecticides and can justify industrialization of new isolates.

The MaCreA Gene Regulates Normal Conidiation and Microcycle Conidiation in Metarhizium acridum

As a C₂H₂ type zinc finger transcription factor, CreA is the key in Carbon Catabolism Repression (CCR) pathway, which negatively regulates the genes in carbon sources utilization. As conidiation in filamentous fungi is affected by nutritional conditions, CreA may contribute to fungal conidiation, which has been well studied in filamentous fungi, especially Aspergillus spp., but researches on entomopathogenic fungi are not enough. In this study, we found a homologous gene MaCreA in Metarhizium acridum, and the MaCreA deletion strain showed delayed conidiation, significant decrease in conidial yield, and 96.88% lower conidial production, when compared with the wild-type strain, and the normal conidiation and microcycle conidiation pattern shift was blocked. RT-qPCR showed that the transcription levels of the genes FlbD and LaeA (related to asexual development) were significantly altered, and those of most of the conidiation-related genes were higher in ΔMaCreA strain. The results of RNA-Seq revealed that MaCreA regulated the two conidiation patterns by mediating genes related to cell cycle, cell division, cell wall, and cell polarity. In conclusion, CreA, as a core regulatory gene in conidiation, provides new insight into the mechanism of conidiation in entomopathogenic fungi.

BrlA and AbaA Govern Virulence-Required Dimorphic Switch, Conidiation, and Pathogenicity in a Fungal Insect Pathogen

Dimorphic plant and human mycopathogens require a switch from the usual yeast growth to filamentous growth for host tissue penetration, and the switch is controlled by multiple signaling systems other than the central developmental pathway. Unlike these fungi, dimorphic insect mycopathogens usually grow by hyphal extension, infect the host by hyphal penetration through the insect cuticle, and switch to unicellular blastospores from the penetrating hyphae only after entry into the host hemocoel, where blastospore propagation by yeast-like budding accelerates host mummification. Here, we report a dependence of the virulence-required dimorphic transition on the central pathway activators BrlA and AbaA in Beauveria bassiana Deletion of brlA or abaA abolished both aerial conidiation and submerged blastospore formation in vitro despite no negative impact on hyphal growth in various media, including a broth mimic of insect hemolymph. The hyphae of either deletion mutant lost insect pathogenicity through normal cuticle penetration, contrasting with a high infectivity of wild-type hyphae. The mutant hyphae injected into the host hemocoel failed to form blastospores, resulting in slower lethal action. Uncovered by transcriptomic analysis, several genes involved in host adhesion and cuticle degradation were sharply repressed in both deletion mutants versus wild type. However, almost all signaling genes homologous to those acting in the dimorphic switch of other fungi were not differentially expressed at a significant level and hence unlikely to be involved in shutting down the dimorphic switch of each deletion mutant. Therefore, like aerial conidiation, the submerged dimorphic switch in vitro and in vivo is a process of asexual development governed by the two central pathway activators in B. bassiana IMPORTANCE Dimorphic insect mycopathogens infect the host by hyphal penetration through the host cuticle and switch from the penetrating hyphae to unicellular blastospores after entry into the host hemocoel, where blastospore propagation by yeast-like budding accelerates host mummification to death. The fungal virulence-required dimorphic switch is confirmed as a process of asexual development directly regulated by BrlA and AbaA, two key activators of the central developmental pathway in an insect mycopathogen. This finding unveils a novel mechanism distinct from the control of the dimorphic switch by multiple signaling systems other than the central developmental pathway in dimorphic plant and human mycopathogens, which switch from the usual yeast growth to filamentous growth required for pathogenicity through host tissue penetration.

Genome-Wide Identification and Characterization of the Cyclophilin Gene Family in the Nematophagous Fungus Purpureocillium lilacinum

Purpureocillium lilacinum has been widely used as a commercial biocontrol agent for the control of plant parasitic nematodes. Whole genome analysis promotes the identification of functional genes and the exploration of their molecular mechanisms. The Cyclophilin (CYP) gene family belongs to the immunophillin superfamily, and has a conserved cyclophilin-like domain (CLD). CYPs are widely identified in prokaryotes and eukaryotes, and can be divided into single- and multi-domain proteins. In the present study, 10 CYP genes possessing the CLD, named PlCYP1-P10, were identified from the genome of P. lilacinum strain 36-1. Those 10 PlCYPs were predicted to have different cellular localizations in P. lilacinum. Phylogenetic and gene structure analysis revealed the evolutionary differentiation of CYPs between Ascomycotina and Saccharomycotina fungi, but conservation within the Ascomycotina fungi. Motif and gene structure distributions further support the result of phylogenetic analysis. Each PlCYP gene had a specific expression pattern in different development stages of P. lilacinum and its parasitism stage on eggs of Meloidogyne incognita. In addition, the 10 PlCYP genes exhibited different expression abundances in response to abiotic stresses, among which PlCYP4 was highly expressed at a high temperature (35 °C), while PlCYP6 was up-regulated under 5 mM of H₂O₂ stress. Furthermore, the heterologous expression of PlCYP4 and PlCYP6 in Escherichia coli enhanced the cellular tolerance against a high temperature and H₂O₂. In summary, our study indicates the potential functions of PlCYPs in virulence and the stress response, and also provides a frame for further analysis of the CYP gene family in Ascomycotina fungi.

Genomic Analysis of the Insect-Killing Fungus Beauveria bassiana JEF-007 as a Biopesticide

Insect-killing fungi have high potential in pest management. A deeper insight into the fungal genes at the whole genome level is necessary to understand the inter-species or intra-species genetic diversity of fungal genes, and to select excellent isolates. In this work, we conducted a whole genome sequencing of Beauveria bassiana (Bb) JEF-007 and characterized pathogenesis-related features and compared with other isolates including Bb ARSEF2860. A large number of Bb JEF-007 genes showed high identity with Bb ARSEF2860, but some genes showed moderate or low identity. The two Bb isolates showed a significant difference in vegetative growth, antibiotic-susceptibility, and virulence against Tenebrio molitor larvae. When highly identical genes between the two Bb isolates were subjected to real-time PCR, their transcription levels were different, particularly in heat shock protein 30 (hsp30) gene which is related to conidial thermotolerance. In several B. bassiana isolates, chitinases and trypsin-like protease genes involved in pathogenesis were highly conserved, but other genes showed noticeable sequence variation within the same species. Given the transcriptional and genetic diversity in B. bassiana, a selection of virulent isolates with industrial advantages is a pre-requisite, and this genetic approach could support the development of excellent biopesticides with intellectual property protection.

Microbial cyclophilins: specialized functions in virulence and beyond

Cyclophilins belong to the superfamily of peptidyl-prolyl cis/trans isomerases (PPIases, EC: 5.2.1.8), the enzymes that catalyze the cis/trans isomerization of peptidyl-prolyl peptide bonds in unfolded and partially folded polypeptide chains and native state proteins. Cyclophilins have been extensively studied, since they are involved in multiple cellular processes related to human pathologies, such as neurodegenerative disorders, infectious diseases, and cancer. However, the presence of cyclophilins in all domains of life indicates a broader biological importance. In this mini-review, we summarize current advances in the study of microbial cyclophilins. Apart from their anticipated role in protein folding and chaperoning, cyclophilins are involved in several other biological processes, such as cellular signal transduction, adaptation to stress, control of pathogens virulence, and modulation of host immune response. Since many existing family members do not have well-defined functions and novel ones are being characterized, the requirement for further studies on their biological role and molecular mechanism of action is apparent.