O-Mannosyltransferase CfPmt4 Regulates the Growth, Development and Pathogenicity of Colletotrichum fructicola

Camellia oleifera is a woody, edible-oil plant native to China. Anthracnose is the major disease of Ca. oleifera, and Colletotrichum fructicola is the main epidemic pathogen. Our previous research indicated that CfHac1 (homologous to ATF/CREB1) and CfGcn5 (general control nonderepressible 5, Gcn5) are integral to key cellular processes that govern fungal development and pathogenesis. Further transcriptomic analyses of the CfHac1 and CfGcn5 mutants, particularly under conditions of endoplasmic reticulum (ER) stress, hold the potential to unveil additional genes implicated in this critical cellular response. We identified all OST/PMT (oligosaccharyltransferase/Protein O-Mannosyltransferases) genes in C. fructicola and analyzed their expression levels. To elucidate novel glycosylation-related genes that may be important for the virulence of C. fructicola, we took an unbiased transcriptomic approach comparing wild-type and the ∆Cfhac1 mutant. Notably, all OST/PMT genes were induced by dithiothreitol and down-regulated in the ΔCfhac1 mutant, yet only the CfPMT4 (Protein O-Mannosyltransferases 4) gene (A04626) was unaffected in the ΔCfgcn5. The results of targeted gene deletion experiments indicate that CfPMT4 plays a crucial role in both vegetative growth and conidiation. Additionally, our investigation revealed that the ΔCfpmt4 exhibits deficiencies in appressorium formation, as well as in its response to cell wall integrity and endoplasmic reticulum stresses. Furthermore, the mutant displayed impaired glycogen metabolism, which may contribute to reduced penetration ability. Overall, CfPmt4, an O-mannosyltransferase, controls the growth, development, and pathogenicity of Colletotrichum fructicola. Understanding the function of the CfPMT4 homolog could provide a potential molecular target for controlling Ca. oleifera anthracnose.

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.

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

Class I/II hydrophobins constitute a family of small amphiphilic proteins that mediate cell hydrophobicity and adhesion to host or substrata and have pleiotropic effects in filamentous fungi. Here we report that only class I Hyd1 is essential for conidial hydrophobicity and insect pathogenicity among three hydrophobins (Hyd1-3) characterized in Metarhizium robertsii, an insect-pathogenic fungus. Aerial conidiation levels of three Δhyd1 mutants were much more reduced in 5-day-old cultures than in 7-day-old cultures, which were wettable (hydrophilic), but restored to a wild-type level in 15-day-old cultures. The Δhyd1 mutants were compromised in conidial quality, including significant decreases in hydrophobicity (58%), adhesion to insect cuticle (36%), insect pathogenicity via normal cuticle infection (37%), UVB resistance (20%), and heat tolerance (10%). In contrast, none of all examined phenotypes were affected in the null mutants of hyd2 and hyd3. Intriguingly, micromorphology and integrity of hydrophobin rodlet bundles on conidial coat were not affected in all mutant and wild-type strains, but the rodlet bundles were disordered in the absence of hyd1, suggesting a link of the disorder to the decreased hydrophobicity. Therefore, Hyd1 mediates the fungal hydrophobicity and plays an important role in conidial quality control and insect-pathogenic lifecycle. Class I Hyd2 and class II Hyd3 seem functionally redundant in M. robertsii.

Comparative proteomic analysis of two divergent strains provides insights into thermotolerance mechanisms of Ganoderma lingzhi

Heat stress (HS) is a major abiotic factor influencing fungal growth and metabolism. However, the genetic basis of thermotolerance in Ganoderma lingzhi (G. lingzhi) remains largely unknown. In this study, we investigated the thermotolerance capacities of 21 G. lingzhi strains and screened the thermo-tolerant (S566) and heat-sensitive (Z381) strains. The mycelia of S566 and Z381 were collected and subjected to a tandem mass tag (TMT)-based proteome assay. We identified 1493 differentially expressed proteins (DEPs), with 376 and 395 DEPs specific to the heat-tolerant and heat-susceptible genotypes, respectively. In the heat-tolerant genotype, upregulated proteins were linked to stimulus regulation and response. Proteins related to oxidative phosphorylation, glycosylphosphatidylinositol-anchor biosynthesis, and cell wall macromolecule metabolism were downregulated in susceptible genotypes. After HS, the mycelial growth of the heat-sensitive Z381 strain was inhibited, and mitochondrial cristae and cell wall integrity of this strain were severely impaired, suggesting that HS may inhibit mycelial growth of Z381 by damaging the cell wall and mitochondrial structure. Furthermore, thermotolerance-related regulatory pathways were explored by analyzing the protein-protein interaction network of DEPs considered to participate in the controlling the thermotolerance capacity. This study provides insights into G. lingzhi thermotolerance mechanisms and a basis for breeding a thermotolerant germplasm bank for G. lingzhi and other fungi.

Differential Roles of Five Fluffy Genes (flbA–flbE) in the Lifecycle In Vitro and In Vivo of the Insect–Pathogenic Fungus Beauveria bassiana

The fluffy genes flbA–flbE are well-known players in the upstream developmental activation pathway that activates the key gene brlA of central developmental pathway (CDP) to initiate conidiation in Aspergillus nidulans. Here, we report insignificant roles of their orthologs in radial growth of Beauveria bassiana under normal culture conditions and different stresses although flbA and flbD were involved in respective responses to heat shock and H₂O₂. Aerial conidiation level was lowered in the deletion mutants of flbB and flbE (~15%) less than of flbA and flbC (~30%), in which the key CDP genes brlA and abaA were repressed consistently during normal incubation. The CDP-controlled blastospore production in submerged cultures mimicking insect hemolymph was abolished in the flbA mutant with brlA and abaA being sharply repressed, and decreased by 55% in the flbC mutant with only abaA being downregulated. The fungal virulence against a model insect was attenuated in the absence of flbA more than of flbC irrespective of normal cuticle infection or cuticle-bypassing infection (intrahemocoel injection). These findings unravel more important role of flbA than of flbC, but null roles of flbB/D/E, in B. bassiana’s insect–pathogenic lifecycle and a scenario distinctive from that in A.nidulans.

Molecular basis and regulatory mechanisms underlying fungal insecticides' resistance to solar ultraviolet irradiation

Resistance to solar ultraviolet (UV) irradiation is crucial for field-persistent control efficacies of fungal formulations against arthropod pests, because their active ingredients are formulated conidia very sensitive to solar UV wavelengths. This review seeks to summarize advances in studies aiming to quantify, understand and improve conidial UV resistance. One focus of studies has been on the many sets of genes that have been revealed in the postgenomic era to contribute to or mediate UV resistance in the insect pathogens serving as main sources of fungal insecticides. Such genetic studies have unveiled the broad basis of UV-resistant molecules including cytosolic solutes, cell wall components, various antioxidant enzymes, and numerous effectors and signaling proteins, that function in developmental, biosynthetic and stress-responsive pathways. Another focus has been on the molecular basis and regulatory mechanisms underlying photorepair of UV-induced DNA lesions and photoreactivation of UV-impaired conidia. Studies have shed light upon a photoprotective mechanism depending on not only one or two photorepair-required photolyases, but also two white collar proteins and other partners that play similar or more important roles in photorepair via interactions with photolyases. Research hotspots are suggested to explore a regulatory network of fungal photoprotection and to improve the development and application strategies of UV-resistant fungal insecticides. © 2021 Society of Chemical Industry.

The N-mannosyltransferase gene BbAlg9 contributes to cell wall integrity, fungal development and the pathogenicity of Beauveria bassiana

The mannosyltransferase Alg9 plays a vital role in N-linked protein glycosylation in Saccharomyces cerevisiae, but its function in most filamentous fungi is not clear. The present study characterized BbAlg9 (an ortholog of S. cerevisiae Alg9) in Beauveria bassiana to determine the roles of N-mannosyltransferase in biological control potential of the filamentous entomopathogenic fungus. The disruption of BbAlg9 led to slower fungal growth in media with various nutrition compositions. The conidiation of ΔBbAlg9 was less than that of the wild type from the third to the fifth day but showed no significant difference on the sixth day, suggesting that BbAlg9 affects the development of conidia rather than conidial yield of late stage. ΔBbAlg9 showed defects in conidial germination, multiple stress tolerances and the yield of blastospores, with altered size and density, and virulence in hosts infected via the immersion and injection methods. The deletion of BbAlg9 resulted in defects in cell wall integrity, including increased mannoprotein and glucan content and decreased chitin content, which were accompanied by transcriptional activation or suppression of genes related to cell wall component biosynthesis. Notably, deletion of the N-mannosyltransferase BbAlg9 altered the transcription levels of O-mannosyltransferase genes (Pmt and Ktr family). These data show that BbAlg9 is involved in the fungal development, conidial stress tolerance, cell wall integrity and virulence of B. bassiana.

The Tudor Domain-Containing Protein BbTdp1 Contributes to Fungal Cell Development, the Cell Cycle, Virulence, and Transcriptional Regulation in the Insect Pathogenic Fungus Beauveria bassiana

Beauveria bassiana is an insect pathogenic fungus that serves as a model system for exploring the mechanisms of fungal development and host-pathogen interactions. Clinical and experimental studies have indicated that SND1 is closely correlated with the progression and invasiveness of common cancers as a potential oncogene, but this gene has rarely been studied in fungi. Here, we characterized the contributions of an SND1 ortholog (Tdp1) by constructing a BbTdp1 deletion strain and a complemented strain of B. bassiana. Compared with the wild-type (WT) strain, the ΔBbTdp1 mutant lost conidiation capacity (∼87.7%) and blastospore (∼96.3%) yields, increased sensitivity to chemical stress (4.4 to 54.3%) and heat shock (∼44.2%), and decreased virulence following topical application (∼24.7%) and hemocoel injection (∼40.0%). Flow cytometry readings showed smaller sizes of both conidia and blastospores for ΔBbTdp1 mutants. Transcriptomic data revealed 4,094 differentially expressed genes (|log2 ratio| > 2 and a q value of <0.05) between ΔBbTdp1 mutants and the WT strain, which accounted for 41.6% of the total genes, indicating that extreme fluctuation in the global gene expression pattern had occurred. Moreover, deletion of BbTdp1 led to an abnormal cell cycle with a longer S phase and shorter G2/M and G0/G1 phases of blastospores, and enzyme-linked immunosorbent assay confirmed that the level of phosphorylated cyclin-dependent kinase 1 (Cdk1) in the ΔBbTdp1 strain was ∼31.5% lower than in the WT strain. In summary, our study is the first to report that BbTdp1 plays a vital role in regulating conidia and blastospore yields, fungal morphological changes, and pathogenicity in entomopathogenic fungi. IMPORTANCE In this study, we used Beauveria bassiana as a biological model to report the role of BbTdp1 in entomopathogenic fungi. Our findings indicated that BbTdp1 contributed significantly to cell development, the cell cycle, and virulence in B. bassiana. In addition, deletion of BbTdp1 led to drastic fluctuations in the transcriptional profile. BbTdp1 can be developed as a novel target for B. bassiana development and pathogenicity, which also provides a framework for the study of Tdp1 in other fungi.

O-mannosyltransferase MaPmt2 contributes to stress tolerance, cell wall integrity and virulence in Metarhizium acridum

As a conserved post-translational modification, O-mannosyltransferase families play important roles in many cellular processes. Three subfamilies (MaPmt1, MaPmt2 and MaPmt4) are grouped in Metarhizium acridum according to sequence homology. The functions of MaPmt1 and MaPmt4 have been characterized in M. acridum previously. In this study, the functions of another member belonging to the Pmt2 subfamily, MaPmt2, were identified through RNAi strategy. The three MaPmt2 knockdown mutants showed dramatically decreased expression of MaPmt2. Phenotypic analyses showed that the mutants exhibited decreased tolerances to wet-heat, UV-B irradiation and cell wall perturbing chemicals. Further studies revealed that the mutants presented thinner cell walls observed by transmission electron microscope combined with changed cell wall components. Besides, knockdown of MaPmt2 decelerated conidial germination and decreased conidial yield. Compared with the wild-type strain, the MaPmt2 knockdown mutants caused impaired virulence only by topical inoculation. Results illustrated that the decreased virulence by inoculation could result from the delayed conidial germination on locust wings, reduced appressorium formation, as well as reduced turgor pressure in MaPmt2 knockdown mutants.

Structural, Evolutionary, and Functional Analysis of the Protein O-Mannosyltransferase Family in Pathogenic Fungi

Protein O-mannosyltransferases (Pmts) comprise a group of proteins that add mannoses to substrate proteins at the endoplasmic reticulum. This post-translational modification is important for the faithful transfer of nascent glycoproteins throughout the secretory pathway. Most fungi genomes encode three O-mannosyltransferases, usually named Pmt1, Pmt2, and Pmt4. In pathogenic fungi, Pmts, especially Pmt4, are key factors for virulence. Although the importance of Pmts for fungal pathogenesis is well established in a wide range of pathogens, questions remain regarding certain features of Pmts. For example, why does the single deletion of each pmt gene have an asymmetrical impact on host colonization? Here, we analyse the origin of Pmts in fungi and review the most important phenotypes associated with Pmt mutants in pathogenic fungi. Hence, we highlight the enormous relevance of these glycotransferases for fungal pathogenic development.

Cyclodepsipeptide Biosynthesis in Hypocreales Fungi and Sequence Divergence of The

Fungi from the Hypocreales order synthesize a range of toxic non-ribosomal cyclic peptides with antimicrobial, insecticidal and cytotoxic activities. Entomopathogenic Beauveria, Isaria and Cordyceps as well as phytopathogenic Fusarium spp. are known producers of beauvericins (BEAs), beauvenniatins (BEAEs) or enniatins (ENNs). The compounds are synthesized by beauvericin/enniatin synthase (BEAS/ESYN1), which shows significant sequence divergence among Hypocreales members. We investigated ENN, BEA and BEAE production among entomopathogenic (Beauveria, Cordyceps, Isaria) and phytopathogenic (Fusarium) fungi; BEA and ENNs were quantified using an LC-MS/MS method. Phylogenetic analysis of partial sequences of putative BEAS/ESYN1 amplicons was also made. Nineteen fungal strains were identified based on sequence analysis of amplified ITS and tef-1α regions. BEA was produced by all investigated fungi, with F. proliferatum and F. concentricum being the most efficient producers. ENNs were synthesized mostly by F. acuminatum, F. avenaceum and C. confragosa. The phylogeny reconstruction suggests that ancestral BEA biosynthesis independently diverged into biosynthesis of other compounds. The divergent positioning of three Fusarium isolates raises the possibility of parallel acquisition of cyclic depsipeptide synthases in ancient complexes within Fusarium genus. Different fungi have independently evolved NRPS genes involved in depsipeptide biosynthesis, with functional adaptation towards biosynthesis of overlapping yet diversified metabolite profiles.

Phenotypic and molecular insights into heat tolerance of formulated cells as active ingredients of fungal insecticides

Formulated conidia of insect-pathogenic fungi, such as Beauveria and Metarhizium, serve as the active ingredients of fungal insecticides but are highly sensitive to persistent high temperatures (32-35 °C) that can be beyond their upper thermal limits especially in tropical areas and during summer months. Fungal heat tolerance and inter- or intra-specific variability are critical factors and limitations to field applications of fungal pesticides during seasons favoring outbreaks of pest populations. The past decades have witnessed tremendous advances in improving fungal pesticides through selection of heat-tolerant strains from natural isolates, improvements and innovations in terms of solid-state fermentation technologies for the production of more heat-tolerant conidia, and the use of genetic engineering of candidate strains for enhancing heat tolerance. More recently, with the entry into a post-genomic era, a large number of signaling and effector genes have been characterized as important sustainers of heat tolerance in both Beauveria and Metarhizium, which represent the main species used as fungal pesticides worldwide. This review focuses on recent advances and provides an overview into the broad molecular basis of fungal heat tolerance and its multiple regulatory pathways. Emphases are placed on approaches for screening of heat-tolerant strains, methods for optimizing conidial quality linked to virulence and heat tolerance particularly involving cell wall architecture and optimized trehalose/mannitol contents, and how molecular determinants can be exploited for genetic improvement of heat tolerance and pest-control potential. Examples of fungal pesticides with different host spectra and their appropriateness for use in apiculture are given. KEY POINTS: • Heat tolerance is critical for field stability and efficacy of fungal insecticides. • Inter- and intra-specific variability exists in insect-pathogenic fungi. • Optimized production technology and biotechnology can improve heat tolerance. • Fungal heat tolerance is orchestrated by multiple molecular pathways.

Nuclear Ssr4 Is Required for the In Vitro and In Vivo Asexual Cycles and Global Gene Activity of Beauveria bassiana

Ssr4 is known to serve as a cosubunit of chromatin-remodeling SWI/SNF and RSC complexes in yeasts but has not been functionally characterized in fungi. This study unveils for the first time the pleiotropic effects caused by deletion of ssr4 and its role in mediating global gene expression in a fungal insect pathogen. Our findings confirm an essential role of Ssr4 in hydrophobin biosynthesis and assembly required for growth, differentiation, and development of aerial hyphae for conidiation and conidial adhesion to insect surface and its essentiality for insect pathogenicity and virulence-related cellular events. Importantly, Ssr4 can regulate nearly one-fourth of all genes in the fungal genome in direct and indirect manners, including dozens involved in gene activity and hundreds involved in metabolism and/or transport of carbohydrates, amino acids, lipids, and/or inorganic ions. These findings highlight a significance of Ssr4 for filamentous fungal lifestyle.

Ssr4 serves as a cosubunit of chromatin-remodeling SWI/SNF and RSC complexes in yeasts but remains functionally uncharacterized due to its essentiality for yeast viability. Here, we report pleiotropic effects of the deletion of the ssr4 ortholog nonessential for cell viability in Beauveria bassiana, an asexual insect mycopathogen. The deletion of ssr4 resulted in severe growth defects on different carbon/nitrogen sources, increased hyphal hydrophilicity, blocked hyphal differentiation, and 98% reduced conidiation capacity compared to a wild-type standard. The limited Δssr4 conidia featured an impaired coat with disordered or obscure hydrophobin rodlet bundles, decreased hydrophobicity, increased size, and lost insect pathogenicity via normal cuticle infection and 90% of virulence via intrahemocoel injection. The expression of genes required for hydrophobin biosynthesis and assembly of the rodlet layer was drastically repressed in more hydrophilic Δssr4 cells. Transcriptomic analysis revealed 2,517 genes differentially expressed in the Δssr4 mutant, including 1,505 downregulated genes and 1,012 upregulated genes. The proteins encoded by hundreds of repressed genes were involved in metabolism and/or transport of carbohydrates, amino acids, and lipids, inorganic ion transport and energy production or conversion, including dozens involved in DNA replication, transcription, translation, and posttranslational modifications. However, purified Ssr4 samples showed no DNA-binding activity, implying that the role of Ssr4 in genome-wide gene regulation could rely upon its acting as a cosubunit of the two complexes. These findings provide the first insight into an essential role of Ssr4 in the asexual cycle in vitro and in vivo of B. bassiana and highlights its importance for the filamentous fungal lifestyle.

IMPORTANCE Ssr4 is known to serve as a cosubunit of chromatin-remodeling SWI/SNF and RSC complexes in yeasts but has not been functionally characterized in fungi. This study unveils for the first time the pleiotropic effects caused by deletion of ssr4 and its role in mediating global gene expression in a fungal insect pathogen. Our findings confirm an essential role of Ssr4 in hydrophobin biosynthesis and assembly required for growth, differentiation, and development of aerial hyphae for conidiation and conidial adhesion to insect surface and its essentiality for insect pathogenicity and virulence-related cellular events. Importantly, Ssr4 can regulate nearly one-fourth of all genes in the fungal genome in direct and indirect manners, including dozens involved in gene activity and hundreds involved in metabolism and/or transport of carbohydrates, amino acids, lipids, and/or inorganic ions. These findings highlight a significance of Ssr4 for filamentous fungal lifestyle.

The velvet protein VeA functions in asexual cycle, stress tolerance and transcriptional regulation of Beauveria bassiana

VeA is a key velvet protein that regulates sexual/asexual development and secondary metabolism in filamentous fungi, particularly Aspergilli, but has not been explored yet in asexual insect mycopathogens, such as Beauveria bassiana. Here, we report a localization of B. bassiana VeA in the cytoplasm of hyphal cells exposed to either light or dark cue and its migration to the nucleus only in darkness. Deletion of veA resulted in facilitated hyphal growth and decreased cell length on rich media, light growth defects on scant media, and increased sensitivities to oxidation, high osmolarity and prolonged heat shock during colony growth. Compared to wild-type, the deletion mutant was much more triggered in conidiation at optimal 25 °C in darkness than in a light/dark (L:D) cycle of 12:12, indicating the role of VeA acting as a negative regulator of conidiation in a light-dependent manner. The mutant conidia produced at L:D 12:12 showed defects in germination, thermotolerance and UVB resistance but no change in virulence, contrasting to attenuated virulence for the mutant conidia produced in darkness. Intriguingly, fungal outgrowth and conidiation were markedly suppressed on the surfaces of the mutant-mummified insect cadavers, suggesting a significant role of VeA in fungal survival, dispersal and prevalence in host habitats. Transcriptomic analysis revealed 1248 and 1183 differentially expressed genes in the deletion mutant versus wild-type grown at L:D 0:24 and 12:12 respectively, including those involved in central developmental pathway and secondary metabolism. Altogether, VeA is functionally involved in asexual cycle, stress tolerance and transcriptional regulation of B. bassiana.

Global Insight into Lysine Acetylation Events and Their Links to Biological Aspects in Beauveria bassiana, a Fungal Insect Pathogen

Lysine acetylation (Kac) events in filamentous fungi are poorly explored. Here we show a lysine acetylome generated by LC-MS/MS analysis of immunoaffinity-based Kac peptides from normal hyphal cells of Beauveria bassiana, a fungal entomopathogen. The acetylome comprised 283 Kac proteins and 464 Kac sites. These proteins were enriched to eight molecular functions, 20 cellular components, 27 biological processes, 20 KEGG pathways and 12 subcellular localizations. All Kac sites were characterized as six Kac motifs, including a novel motif (KacW) for 26 Kac sites of 17 unknown proteins. Many Kac sites were predicted to be multifunctional, largely expanding the fungal Kac events. Biological importance of identified Kac sites was confirmed through functional analysis of Kac sites on Pmt1 and Pmt4, two O-mannosyltransferases. Singular site mutations (K88R and K482R) of Pmt1 resulted in impaired conidiation, attenuated virulence and decreased tolerance to oxidation and cell wall perturbation. These defects were close to or more severe than those caused by the deletion of pmt1. The Pmt4 K360R mutation facilitated colony growth under normal and stressful conditions and enhanced the fungal virulence. Our findings provide the first insight into the Kac events of B. bassiana and their links to the fungal potential against insect pests.

The Pmt2p-Mediated Protein O-Mannosylation Is Required for Morphogenesis, Adhesive Properties, Cell Wall Integrity and Full Virulence of Magnaporthe oryzae

Protein O-mannosylation is a type of O-glycosylation that is characterized by the addition of mannose residues to target proteins, and is initially catalyzed by evolutionarily conserved protein O-mannosyltransferases (PMTs). In this study, three members of PMT were identified in Magnaporthe oryzae, and the pathogenic roles of MoPmt2, a member of PMT2 subfamily, were analyzed. We found that MoPmt2 is a homolog of Saccharomyces cerevisiae Pmt2 and could complement yeast Pmt2 function in resistance to CFW. Quantitative RT-PCR revealed that MoPmt2 is highly expressed during conidiation, and targeted disruption of MoPmt2 resulted in defects in conidiation and conidia morphology. The MoPmt2 mutants also showed a distinct reduction in fungal growth, which was associated with severe alterations in hyphal polarity. In addition, we found that the MoPmt2 mutants severely reduced virulence on both rice plants and barley leaves. The subsequent examination revealed that the fungal adhesion, conidial germination, CWI and invasive hyphae growth in host cells are responsible for defects on appressorium mediated penetration, and thus attenuated the pathogenicity of MoPmt2 mutants. Taken together, our results suggest that protein O-mannosyltransferase MoPmt2 plays essential roles in fungal growth and development, and is required for the full pathogenicity of M. oryzae.

Transcriptional control of fungal cell cycle and cellular events by Fkh2, a forkhead transcription factor in an insect pathogen

Transcriptional control of the cell cycle by forkhead (Fkh) transcription factors is likely associated with fungal adaptation to host and environment. Here we show that Fkh2, an ortholog of yeast Fkh1/2, orchestrates cell cycle and many cellular events of Beauveria bassiana, a filamentous fungal insect pathogen. Deletion of Fkh2 in B. bassiana resulted in dramatic down-regulation of the cyclin-B gene cluster and hence altered cell cycle (longer G₂/M and S, but shorter G₀/G₁, phases) in unicellular blastospores. Consequently, ΔFkh2 produced twice as many, but smaller, blastospores than wild-type under submerged conditions, and formed denser septa and shorter/broader cells in aberrantly branched hyphae. In these hyphae, clustered genes required for septation and conidiation were remarkedly up-regulated, followed by higher yield and slower germination of aerial conidia. Moreover, ΔFkh2 displayed attenuated virulence and decreased tolerance to chemical and environmental stresses, accompanied with altered transcripts and activities of phenotype-influencing proteins or enzymes. All the changes in ΔFkh2 were restored by Fkh2 complementation. All together, Fkh2-dependent transcriptional control is vital for the adaptation of B. bassiana to diverse habitats of host insects and hence contributes to its biological control potential against arthropod pests.

Three α-1,2-mannosyltransferases contribute differentially to conidiation, cell wall integrity, multistress tolerance and virulence of Beauveria bassiana

Members of α-1,2-mannosyltransferase (Ktr) family are required for protein O-mannosylation for the elongation of Ser/Thr mannose residues in yeasts but functionally unknown in most filamentous fungi. Here we characterized the functions of the Ktr orthologues Ktr1, Ktr4 and Kre2/Mnt1 in Beauveria bassiana, a filamentous enotmopathogen, and found that they were positive, but differential, mediators of many biological traits. Inactivation of Ktr4 and Kre2 resulted in 92% reduction of conidial yield on a standard medium and growth defects on substrates with altered carbon or nitrogen sources and availability, accompanied with reduced conidial size and complexity. This contrasts to the dispensability of Ktr1 for fungal growth and conidiation. More cell wall damage occurred in Δktr4 and Δkre2 than in Δktr1, including altered contents of the cell wall components mannoproteins, α-glucans and chitin, more carbohydrate epitopes changed on conidial surfaces, much lower conidial hydrophobicity, and thinner cell walls. Consequently, Δktr4 and Δkre2 became more sensitive to oxidation and cell wall perturbation than Δktr1 during colony growth or conidial germination despite less difference in their sensitivities to two osmotic agents. Conidial thermotolerance, UV-B resistance and virulence were all lowered greatly in Δktr4 and Δkre2 but only the thermotolerance decreased in Δktr1. All the phenotypical changes were well restored to wild-type levels by the complementation of each target gene. Our results indicate that Ktr4 and Kre2 contribute more to the biocontrol potential of B. bassiana than Ktr1 although all of them are significant contributors.