Whole-genome analysis of two-component signal transduction genes in fungal pathogens

PGPBS, a mitogen-activated protein kinase kinase, is required for vegetative differentiation, cell wall integrity, and pathogenicity of the barley leaf stripe fungus Pyrenophora graminea

The high-osmolarity glycerol (HOG) signaling pathway regulates the adaptation of fungi to environmental stressors. The mitogen-activated protein kinase kinase (MAPKK) PBS2 of Saccharomyces cerevisiae serves as a scaffold protein in the HOG pathway. We characterized the pgpbs gene of Pyrenophora graminea, which encodes a MAPKK that is 56% orthologous to PBS2 of S. cerevisiae. A cloning technique based on homology was applied to amplify the pgpbs gene. Specific silent mutations then were generated in pgpbs. We evaluated the potential roles of PGPBS in the osmotic response, vegetative differentiation, cell wall integrity, drug resistance, and pathogenicity. Our findings indicated that the pgpbs coding region comprises 2075 base pairs and encodes a protein of 676 amino acids. Mutants deficient in pgpbs expression had significant reductions in vegetative growth and were sensitive to calcofluor white (CFW), an inhibitor of cell wall synthesis. Mutants also lost pathogenicity and were sensitive to an osmotic stress-inducing medium containing NaCl and sorbitol. Moreover, mutants had increased resistance to the dicarboximide fungicide iprodione and the triazole fungicide tebuconazole. These findings suggest that pgpbs is involved in the osmotic and ionic stress responses, vegetative differentiation, cell wall integrity, virulence, and tolerance to iprodione and tebuconazole. We expect that our findings will help elucidate the pathogenesis of barley leaf stripe and will inform strategies for breeding resistance to this disease.

Novel Histidine Kinase Gene HisK2301 from Rhodosporidium kratochvilovae Contributes to Cold Adaption by Promoting Biosynthesis of Polyunsaturated Fatty Acids and Glycerol

Hybrid histidine kinase (HHKs) are widespread in fungi, but their roles in the regulation of fungal adaptation to environmental stresses remain largely unclear. To elucidate this, we cloned HisK2301 from Rhodosporidium kratochvilovae strain YM25235, characterized HisK2301 as a novel HHK, and further investigated the role of HisK2301 by overexpressing it in YM25235. Our results revealed that HisK2301 can promote adaptation of YM25235 to cold, osmotic, and salt stresses. During cold stress, HisK2301 significantly enhanced the biosynthesis of polyunsaturated fatty acids (PUFA) and intracellular glycerol. HisK2301 also augmented the expression levels of Δ¹²/Δ¹⁵ fatty acid desaturase ( RKD12) and glycerol-3-phosphate dehydrogenase1 ( GPD1), which are responsible for PUFA and glycerol biosynthesis, respectively. To conclude, our findings give the first insight into the defense and mechanisms of HisK2301 in fungi against cold stress and suggest the potential use of the novel cold-adapted HHK HisK2301 in industrial processes, such as large-scale production of PUFA.

Diversity and Evolution of Sensor Histidine Kinases in Eukaryotes

Histidine kinases (HKs) are primary sensor proteins that act in cell signaling pathways generically referred to as "two-component systems" (TCSs). TCSs are among the most widely distributed transduction systems used by both prokaryotic and eukaryotic organisms to detect and respond to a broad range of environmental cues. The structure and distribution of HK proteins are now well documented in prokaryotes, but information is still fragmentary for eukaryotes. Here, we have taken advantage of recent genomic resources to explore the structural diversity and the phylogenetic distribution of HKs in the prominent eukaryotic supergroups. Searches of the genomes of 67 eukaryotic species spread evenly throughout the phylogenetic tree of life identified 748 predicted HK proteins. Independent phylogenetic analyses of predicted HK proteins were carried out for each of the major eukaryotic supergroups. This allowed most of the compiled sequences to be categorized into previously described HK groups. Beyond the phylogenetic analysis of eukaryotic HKs, this study revealed some interesting findings: 1) characterization of some previously undescribed eukaryotic HK groups with predicted functions putatively related to physiological traits; 2) discovery of HK groups that were previously believed to be restricted to a single kingdom in additional supergroups, and 3) indications that some evolutionary paths have led to the appearance, transfer, duplication, and loss of HK genes in some phylogenetic lineages. This study provides an unprecedented overview of the structure and distribution of HKs in the Eukaryota and represents a first step toward deciphering the evolution of TCS signaling in living organisms.

Insights into regulatory roles of MAPK-cascaded pathways in multiple stress responses and life cycles of insect and nematode mycopathogens

Fungal entomopathogenicity may have evolved at least 200 million years later than carnivorism of nematophagous fungi on Earth. This mini-review focuses on the composition and regulatory roles of mitogen-activated protein kinase (MAPK) cascades, which act as stress-responsive signaling pathways. Unveiled by genomic comparison, three MAPK cascades of these mycopathogens consist of singular MAPKs (Fus3/Hog1/Slt2), MAPK kinases (Ste7/Pbs2/Mkk1), and MAPK kinase kinases (Ste11/Ssk2/Bck1). All cascaded components characterized in fungal entomopathogens play conserved and special roles in regulating multiple stress responses and phenotypes associated with biological control potential. Fus3-cascaded components are indispensable for fungal growth on oligotrophic substrata and virulence, and mediate cell tolerance to Na⁺/K⁺ toxicity, which is often misinterpreted as hyperosmotic effect but readily clarified by transcriptional changes of Na⁺/K⁺ ATPase genes and/or cell responses to osmotic polyols. Hog1-cascaded components regulate osmotolerance positively and phenylpyrrole-type fungicide resistance negatively, and also play differential roles in cell growth, conidiation, virulence, and responses to other stress cues. Ste11 has no stress-responsive role in the Beauveria Hog1 cascade despite an essential role in branched yeast Hog1 cascade. Slt2-cascaded components are required for mediation of cell wall integrity and repair of cell wall damage. A crosstalk between Hog1 and Slt2 cascades ensures fungal osmotolerance inside or outside insect. In nematode-trapping fungi, Slt2 is indispensable for cell wall integrity, conidiation, and mycelial trap formation, suggesting that the Slt2 cascade could have evolved along a distinct trajectory required for fungal carnivorism and dispersal/survival in nematode habitats. Altogether, the MAPK cascades are major parts of signaling network that regulate fungal adaptation to insects and nematodes and their habitats.

Overexpression of the CORVET complex alleviates the fungicidal effects of fludioxonil on the yeast Saccharomyces cerevisiae expressing hybrid histidine kinase 3

The hybrid histidine kinase 3 (HHK3) is a highly conserved sensor kinase in fungi that regulates the downstream HOG/p38 mitogen-activated protein kinase (MAPK). In addition to its role in osmoadaptation, HHK3 is involved in hyphal morphogenesis, conidiation, virulence, and cellular adaptation to oxidative stress. However, the molecular mechanisms by which it controls these processes remain obscure. Moreover, HHK3 is a molecular target for antifungal agents such as fludioxonil, which thereby interferes with the HOG/p38 pathway, leading to the abnormal accumulation of glycerol and subsequent cell lysis. Here, we used a chemical genomics approach with the yeast Saccharomyces cerevisiae to better understand the fungicidal action of fludioxonil and the role of HHK3 in fungal growth and physiology. Our results indicated that the abnormal accumulation of glycerol is not the primary cause of fludioxonil toxicity. Fludioxonil appears to impair endosomal trafficking in the fungal cells. We found that the components of class C core vacuole/endosome tethering (CORVET) complex are essential for yeast viability in the presence of a subthreshold dose of fludioxonil and that their overexpression alleviates fludioxonil toxicity. We also noted that by impeding secretory vesicle trafficking, fludioxonil inhibits hyphal growth in the opportunistic fungal pathogen Candida albicans Our results suggest that HHK3 regulates fungal hyphal growth by affecting vesicle trafficking. Together, our results reveal an important role of CORVET complex in the fungicidal action of fludioxonil downstream of HHK3.

The histidine kinase slnCl1 of Colletotrichum lindemuthianum as a pathogenicity factor against Phaseolus vulgaris L

Colletotrichum lindemuthianum, the causal agent of anthracnose, is responsible for significant damage in the common bean (Phaseolus vulgaris L.). Unraveling the genetic mechanisms involved in the plant/pathogen interaction is a powerful approach for devising efficient methods to control this disease. In the present study, we employed the Restriction Enzyme-Mediated Integration (REMI) methodology to identify the gene slnCl1, encoding a histidine kinase protein, as involved in pathogenicity. The mutant strain, MutCl1, generated by REMI, showed an insertion in the slnCl1 gene, deficiency of the production and melanization of appressoria, as well as the absence of pathogenicity on bean leaves when compared with the wild-type strain. The slnCl1 gene encodes a histidine kinase class IV called SlnCl1 showing identity of 97% and 83% with histidine kinases from Colletotrichum orbiculare and Colletotrichum gloesporioides, respectively. RNA interference was used for silencing the histidine kinase gene and confirm slnCl1 as a pathogenicity factor. Furthermore, we identified four major genes involved in the RNA interference-mediated gene silencing in Colletotrichum spp. and demonstrated the functionality of this process in C. lindemuthianum. Silencing of the EGFP reporter gene and slnCl1 were demonstrated using qPCR. This work reports for the first time the isolation and characterization of a HK in C. lindemuthianum and the occurrence of gene silencing mediated by RNA interference in this organism, demonstrating its potential use in the functional characterization of pathogenicity genes.

Molecular Characterization, Fitness, and Mycotoxin Production of Fusarium asiaticum Strains Resistant to Fludioxonil

Fludioxonil is used in seedborne disease management of various fungal pathogens, including Fusarium asiaticum, the predominant causal agent of Fusarium head blight in China. In this study, we screened resistant strains from a large number of F. asiaticum strains collected from 2012 to 2016 and found that 4 of 1,000 field strains were highly resistant to fludioxonil. The 50% effective concentration values of the resistant strains and induced mutants ranged from 80 to >400 μg/ml. Compared with field-sensitive strains, all field-collected and laboratory-induced resistant strains exhibited fitness defects in traits including mycelial growth, conidial production, pathogenicity, and sensitivity to osmotic conditions. In the presence of fludioxonil, significantly higher glycerol accumulation was found in sensitive strains but not in resistant individuals. The fludioxonil-resistant strains produced lower amounts of glycerol in liquid culture and lower amounts of trichothecene mycotoxins in rice culture and inoculated wheat spikelets than the fludioxonil-sensitive strains. Sequence analyses of the key genes of the two-component histidine kinase signaling pathway showed various amino acid substitutions in the Os1, Os4, and Os5 genes between field-sensitive and resistant strains or mutants. The results of this study suggest a potential risk of fludioxonil resistance development and a possible influence of resistance mutations on fitness parameters and toxin production in F. asiaticum.

Kinome Expansion in the Fusarium oxysporum Species Complex Driven by Accessory Chromosomes

Isolates of Fusarium oxysporum are adapted to survive a wide range of host and nonhost conditions. In addition, F. oxysporum was recently recognized as the top emerging opportunistic fungal pathogen infecting immunocompromised humans. The sensory and response networks of these fungi undoubtedly play a fundamental role in establishing the adaptability of this group. We have examined the kinomes of 12 F. oxysporum isolates and highlighted kinase families that distinguish F. oxysporum from other fungi, as well as different isolates from one another. The amplification of kinases involved in environmental signal relay and regulating downstream cellular responses clearly sets Fusarium apart from other Ascomycetes. Although the functions of many of these kinases are still unclear, their specific proliferation highlights them as a result of the evolutionary forces that have shaped this species complex and clearly marks them as targets for exploitation in order to combat disease.

The Fusarium oxysporum species complex (FOSC) is a group of soilborne pathogens causing severe disease in more than 100 plant hosts, while individual strains exhibit strong host specificity. Both chromosome transfer and comparative genomics experiments have demonstrated that lineage-specific (LS) chromosomes contribute to the host-specific pathogenicity. However, little is known about the functional importance of genes encoded in these LS chromosomes. Focusing on signaling transduction, this study compared the kinomes of 12 F. oxysporum isolates, including both plant and human pathogens and 1 nonpathogenic biocontrol strain, with 7 additional publicly available ascomycete genomes. Overall, F. oxysporum kinomes are the largest, facilitated in part by the acquisitions of the LS chromosomes. The comparative study identified 99 kinases that are present in almost all examined fungal genomes, forming the core signaling network of ascomycete fungi. Compared to the conserved ascomycete kinome, the expansion of the F. oxysporum kinome occurs in several kinase families such as histidine kinases that are involved in environmental signal sensing and target of rapamycin (TOR) kinase that mediates cellular responses. Comparative kinome analysis suggests a convergent evolution that shapes individual F. oxysporum isolates with an enhanced and unique capacity for environmental perception and associated downstream responses.

IMPORTANCE Isolates of Fusarium oxysporum are adapted to survive a wide range of host and nonhost conditions. In addition, F. oxysporum was recently recognized as the top emerging opportunistic fungal pathogen infecting immunocompromised humans. The sensory and response networks of these fungi undoubtedly play a fundamental role in establishing the adaptability of this group. We have examined the kinomes of 12 F. oxysporum isolates and highlighted kinase families that distinguish F. oxysporum from other fungi, as well as different isolates from one another. The amplification of kinases involved in environmental signal relay and regulating downstream cellular responses clearly sets Fusarium apart from other Ascomycetes. Although the functions of many of these kinases are still unclear, their specific proliferation highlights them as a result of the evolutionary forces that have shaped this species complex and clearly marks them as targets for exploitation in order to combat disease.

Functional characterization of the upstream components of the Hog1-like kinase cascade in hyperosmotic and carbon sensing in Trichoderma reesei

BACKGROUND

Trichoderma reesei holds a high capacity for protein secretion and represents the most important cellulase producer in industry. However, the external signal sensing and intracellular signal transduction during cellulose induction remain unclear. As one of the most pervasive signal transduction pathways in all eukaryotic species, the mitogen-activated protein kinase (MAPK) pathway and its upstream sensing and signaling components are involved in various physiological processes including stress and nutrient sensing. Particularly, the Hog1-type MAPK Tmk3 has been reported to be involved in the cellulase production in T. reesei.

RESULTS

Here we established the physiological role of two upstream regulatory branches, the Sho1 branch and the Sln1 branch, of the Hog1-type Tmk3 pathway in T. reesei. Deletion of Trste20 of the Sho1 branch or repression of Trypd1 of the Sln1 branch reduced the resistance to high salt stress, whereas TrSho1 showed an opposing effect to that of TrSte20 and the identified TrSln1 seemed to be dispensable in the osmotic regulation. The Sho1 and Sln1 branches also participated in the cell wall integrity maintenance and other stress responses (i.e. oxidative and thermo stresses). Notably, TrSho1 and TrSte20 of the Sho1 branch and TrYpd1 of the Sln1 branch were shown to be differentially involved in the cellulase production of T. reesei. Repression of Trypd1 hardly affected cellulase induction, whereas overexpression of Trypd1 resulted in the reduced production of cellulases. Contrary to the case of Trypd1, repression of Trsho1 or deletion of Trste20 significantly reduced the transcription of cellulase genes.

CONCLUSIONS

TrSho1 and TrSte20 of the Sho1 branch and TrYpd1 of the Sln1 branch are all involved in general stress responses including hyperosmotic regulation and cell wall integrity maintenance. Moreover, our study revealed that the Sho1 and Sln1 osmosensing pathways are differentially involved in the regulation of cellulase production in T. reesei. The Sho1 branch positively regulated the production of cellulases and the transcription of cellulase genes while TrYpd1 of the Sln1 branch negatively controlled the cellulase production, supporting the crosstalks of osmosensing and nutrient sensing.

The phosphorelay signal transduction system in Candida glabrata: an in silico analysis

Signaling systems allow microorganisms to sense and respond to different stimuli through the modification of gene expression. The phosphorelay signal transduction system in eukaryotes involves three proteins: a sensor protein, an intermediate protein and a response regulator, and requires the transfer of a phosphate group between two histidine-aspartic residues. The SLN1-YPD1-SSK1 system enables yeast to adapt to hyperosmotic stress through the activation of the HOG1-MAPK pathway. The genetic sequences available from Saccharomyces cerevisiae were used to identify orthologous sequences in Candida glabrata, and putative genes were identified and characterized by in silico assays. An interactome analysis was carried out with the complete genome of C. glabrata and the putative proteins of the phosphorelay signal transduction system. Next, we modeled the complex formed between the sensor protein CgSln1p and the intermediate CgYpd1p. Finally, phosphate transfer was examined by a molecular dynamic assay. Our in silico analysis showed that the putative proteins of the C. glabrata phosphorelay signal transduction system present the functional domains of histidine kinase, a downstream response regulator protein, and an intermediate histidine phosphotransfer protein. All the sequences are phylogenetically more related to S. cerevisiae than to C. albicans. The interactome suggests that the C. glabrata phosphorelay signal transduction system interacts with different proteins that regulate cell wall biosynthesis and responds to oxidative and osmotic stress the same way as similar systems in S. cerevisiae and C. albicans. Molecular dynamics simulations showed complex formation between the response regulator domain of histidine kinase CgSln1 and intermediate protein CgYpd1 in the presence of a phosphate group and interactions between the aspartic residue and the histidine residue. Overall, our research showed that C. glabrata harbors a functional SLN1-YPD1-SSK1 phosphorelay system.

Methylmalonate-semialdehyde dehydrogenase mediated metabolite homeostasis essentially regulate conidiation, polarized germination and pathogenesis in Magnaporthe oryzae

Plants generate multitude of aldehydes under abiotic and biotic stress conditions. Ample demonstrations have shown that rice-derived aldehydes enhance the resistance of rice against the rice-blast fungus Magnaporthe oryzae. However, how the fungal pathogen nullifies the inhibitory effects of host aldehydes to establish compatible interaction remains unknown. Here we identified and evaluated the in vivo transcriptional activities of M. oryzae aldehyde dehydrogenase (ALDH) genes. Transcriptional analysis of M. oryzae ALDH genes revealed that the acetylating enzyme Methylmalonate-Semialdehyde Dehydrogenase (MoMsdh/MoMmsdh) elevated activities during host invasion and colonization of the fungus. We further examined the pathophysiological importance of MoMSDH by deploying integrated functional genetics, and biochemical approaches. MoMSDH deletion mutant ΔMomsdh exhibited germination defect, hyper-branching of germ tube and failed to form appressoria on hydrophobic and hydrophilic surface. The MoMSDH disruption caused accumulation of small branch-chain amino acids, pyridoxine and AMP/cAMP in the ΔMomsdh mutant and altered Spitzenkörper organization in the conidia. We concluded that MoMSDH contribute significantly to the pathogenesis of M. oryzae by regulating the mobilization of Spitzenkörper during germ tube morphogenesis, appressoria formation by acting as metabolic switch regulating small branch-chain amino acids, inositol, pyridoxine and AMP/cAMP homeostasis.

Resistance risk assessment for fludioxonil in Bipolaris maydis

Bipolaris maydis (anamorph: Cochliobolus heterostrophus) is the causal agent of Southern Corn Leaf Blight (SCLB), leading to huge annually losses worldwide. Although fludioxonil, a phenylpyrrole fungicide with a broad spectrum of activity, was introduced in the 1990s, no baseline sensitivity has been established for B. maydis. One hundred field isolates were used to establish a baseline sensitivity of B. maydis against fludioxonil during 2015-2016. The results showed that the baseline sensitivity was distributed as a unimodal curve with a mean EC₅₀ value of 0.044±0.022μgmL^-1. With repeated exposure to fludioxonil, a total of five fludioxonil-resistant mutants (RF>100, RF=Resistance factor) were obtained in the laboratory. Compared with the parental isolates, the five fludioxonil-resistant mutants showed decreased fitness in sporulation and virulence, and exhibited different features of sensitivity to various stresses (oxidation and osmotic pressure, cell membrane and cell wall inhibitors), but not in mycelial growth on PDA without stress amendation. The five fludioxonil-resistant mutants showed a positive cross-resistance between fludioxonil and the dicarboximide fungicide procymidone, but not between fludioxonil and boscalid or fluazinam. All mutants exhibited stable resistance to fludioxonil after 10 transfers, as indicated by resistance factor values that ranged from 116.82 to 445.59. When treated with 1.0 M NaCl, all the fludioxonil-resistant mutants showed greater mycelial glycerol content than corresponding parental isolates. Sequencing alignment results of Bmos1 indicated that mutant R27-5 had a single point mutation (Z1125K), while the mutant R104 had a 34-bp deletion fragment between the codons of amino acid residues 1125 to 1236 and encodes a putative attenuated 1133-AA protein. The 34-bp deletion fragment led to not only a 11-AA deletion(DNAVNQKLAVR), but also the resulting frameshift mutation and early stop. The mutations of R27-5 and R104 were located in the Rec domain of the Bmos1 gene. No mutations at the Bmos1 were detected in the other three resistant mutants R27-1, R27-2 and R32.

Molecular characterization of Aspergillus fumigatus TcsC, a characteristic type III hybrid histidine kinase of filamentous fungi harboring six HAMP domains

The type III hybrid histidine kinase (HHK) TcsC enables the pathogenic mold Aspergillus fumigatus to thrive under hyperosmotic conditions. It is, moreover, of particular interest, since it is the target of certain antifungal agents, such as fludioxonil. This study was aimed at a functional characterization of the domains that constitute the sensing and the kinase module of TcsC. The sensing module consists of six HAMP domains, an architecture that is commonly found in type III HHKs of filamentous fungi. To dissect the functional role of the individual domains, we have analyzed a set of truncated derivatives of TcsC with respect to their impact on fungal growth and their ability to respond to hyperosmotic stress and fludioxonil. Our data demonstrate that the TcsC kinase module per se is constitutively active and under the control of the sensing module. We furthermore found that the sixth HAMP domain alone is sufficient to arrest the kinase module in an inactive state. This effect can be partially lifted by the presence of the fifth HAMP domain. Constructs harboring more than these two HAMP domains are per se inactive and all six HAMP domains are required to enable a response to fludioxonil or hyperosmotic stress. When expressed in an A. fumigatus wild type strain, the construct harboring only the sixth HAMP domain exerts a strong dominant negative effect on the native TcsC. This effect is successively reduced in other constructs harboring increasing numbers of HAMP domains. To our knowledge, this is the first molecular characterization of a type III HHK containing six HAMP domains. Our data strongly suggest that TcsC is a positive regulator of its MAPK SakA and thereby differs fundamentally from the prototypic yeast type III HHK DhNik1 of Debaryomyces hansenii, which harbors only five HAMP domains and acts as a negative regulator of its MAPK.

Molecular and biochemical characterization of dimethachlone resistant isolates of Sclerotinia sclerotiorum

Sclerotinia sclerotiorum is a necrotrophic fungal plant pathogen with a broad host range. The dicarboximide fungicide dimethachlone has been used to control this pathogen for more than a decade and resistance to dimethachlone has recently been reported in China. Compared with sensitive isolates, the three dimethachlone resistant isolates with resistance ratios of 78.3, 85.5, and 94.8 exhibited significantly (P<0.05) higher cell membrane permeability and peroxidase and polyphenol oxidase activities. Dimethachlone at 0.25μg/mL significantly increased cell membrane permeability and enhanced activity of the two enzymes in both resistant and sensitive isolates. There were no significant differences in glycerol or oxalate content between the resistant and sensitive isolates. Dimethachlone treatment increased glycerol content in the resistant isolates and reduced in the sensitive isolates (P<0.01). Sequencing of three genes involved in two-component signal pathway and of three genes in mitogen-activated protein (MAP) kinase cascade demonstrated that the dimethachlone resistant isolates HLJ4 and HLJ6 harbored point mutations of I232T and G1087D, respectively, in the deduce amino acid sequence of the histidine kinase (HK) gene Sshk. HLJ4 had a point mutation of P96L in the deduced amino acid sequence of the MAP kinase-kinase gene SsPbs. The expression levels of the Sshk gene were higher in HLJ4 and HLJ6 than in HLJ3 and the sensitive isolate HLJMG2, and transcription of the Sshk gene was up-regulated by dimethachlone for the three resistant isolates.

Characterization of the Hog1 MAPK pathway in the entomopathogenic fungus Beauveria bassiana

High-osmolarity glycerol (HOG) pathway required for yeast osmoregulation relies upon the mitogen-activated protein kinase (MAPK) Hog1 cascade that comprise the MAPKKKs Ssk2/Ssk22 and Ste11 converging on the MAPKK Pbs2. Here we show a Hog1 cascade with the unique MAPKKK Ssk2 acting in Beauveria bassiana. Hypersensitivity to high osmolarity and high resistance to fludioxonil fungicide appeared in Δssk2, Δpbs2 and Δhog1 mutants whereas the two hallmark phenotypes were reversed in Δste11. Increased sensitivity to heat shock and decreased sensitivity to cell wall perturbation also occurred in the three mutants but not in Δste11 although antioxidant phenotypes were different in all deletion mutants. Intriguingly, signals of Hog1 phosphorylation induced by osmotic, oxidative and thermal cues were present in Δste11 but absent in Δssk2 and Δpbs2. Moreover, vegetative growth on minimal media with different carbon/nitrogen sources was much more suppressed in Δste11 and Δssk2 than in Δpbs2 and Δhog1 although all mutants suffered similar, but severe, conidiation defects on a standard medium. Normal host infection was abolished in Δste11 while virulence was differentially attenuated in other mutants. Our findings exclude Ste11 from the Hog1 cascade that regulates multiple stress responses and environmental adaptation of B. bassiana and perhaps other filamentous fungi.

The Identification of Phytohormone Receptor Homologs in Early Diverging Fungi Suggests a Role for Plant Sensing in Land Colonization by Fungi

Histidine kinases (HKs) are among the most prominent sensing proteins studied in the kingdom Fungi. Their distribution and biological functions in early diverging fungi (EDF), however, remain elusive. We have taken advantage of recent genomic resources to elucidate whether relationships between the occurrence of specific HKs in some EDF and their respective habitat/lifestyle could be established. This led to the unexpected discovery of fungal HKs that share a high degree of similarity with receptors for plant hormones (ethylene and cytokinin). Importantly, these phytohormone receptor homologs are found not only in EDF that behave as plant root symbionts or endophytes but also in EDF species that colonize decaying plant material. We hypothesize that these particular sensing proteins promoted the interaction of EDF with plants, leading to the conquest of land by these ancestral fungi.

Fludioxonil Induces Drk1, a Fungal Group III Hybrid Histidine Kinase, To Dephosphorylate Its Downstream Target, Ypd1

Novel antifungal drugs and targets are urgently needed. Group III hybrid histidine kinases (HHKs) represent an appealing new therapeutic drug target because they are widely expressed in fungi but absent from humans. We investigated the mode of action of the widely utilized, effective fungicide fludioxonil. The drug acts in an HHK-dependent manner by constitutive activation of the HOG (high-osmolarity glycerol) pathway, but its mechanism of action is poorly understood. Here, we report a new mode of drug action that entails conversion of the HHK from a kinase into a phosphatase. We expressed Drk1 (dimorphism-regulating kinase), which is an intracellular group III HHK from the fungal pathogen Blastomyces dermatitidis, in Saccharomyces cerevisiae Drk1 engendered drug sensitivity in B. dermatitidis and conferred sensitivity upon S. cerevisiae In response to fludioxonil, Drk1 behaved as a phosphatase rather than as a kinase, leading to dephosphorylation of its downstream target, Ypd1, constitutive activation of the HOG pathway, and yeast cell death. Aspartic acid residue 1140 in the Drk1 receiver domain was required for in vivo phosphatase activity on Ypd1, and Hog1 was required for drug effect, indicating fidelity in HHK-dependent drug action. In in vitro assays with purified protein, intact Drk1 demonstrated intrinsic kinase activity, and the Drk1 receiver domain exhibited intrinsic phosphatase activity. However, fludioxonil failed to induce intact Drk1 to dephosphorylate Ypd1. We conclude that fludioxonil treatment in vivo likely acts on an upstream target that triggers HHK to become a phosphatase, which dephosphorylates its downstream target, Ypd1.

Phenylpyrroles: 30 Years, Two Molecules and (Nearly) No Resistance

Phenylpyrroles are chemical analogs of the natural antifungal compound pyrrolnitrin. Fenpiclonil, but mainly fludioxonil are registered against multiple fungal crop diseases since over 25 years for seed or foliar treatment. They have severe physiological impacts on the pathogen, including membrane hyperpolarization, changes in carbon metabolism and the accumulation of metabolites leading to hyphal swelling and burst. The selection and characterization of mutants resistant to phenylpyrroles have revealed that these fungicides activate the fungal osmotic signal transduction pathway through their perception by a typical fungal hybrid histidine kinase (HHK). The HHK is prone to point mutations that confer fungicide resistance and affect its sensor domain, composed of tandem repeats of HAMP motifs. Fludioxonil resistant mutants have been selected in many fungal species under laboratory conditions. Generally they present severe impacts on fitness parameters. Since only few cases of field resistance specific to phenylpyrroles have been reported one may suspect that the fitness penalty of phenylpyrrole resistance is the reason for the lack of field resistance.

Molecular and Biochemical Characterization of Laboratory and Field Mutants of Botrytis cinerea Resistant to Fludioxonil

Botrytis cinerea is a filamentous phytopathogen with a high risk of developing resistance to fungicides. The phenylpyrrole fungicide fludioxonil has been reported to have excellent activity against B. cinerea and increasingly has been applied to control gray mold in China. In this study, molecular and biochemical characteristics of laboratory and field mutants of B. cinerea resistant to fludioxonil has been investigated. During 2012 to 2014, B. cinerea isolates collected from Jiangsu and Shandong Provinces in China were tested in vitro for sensitivity to fungicides commonly used to suppress gray mold of cucumber and tomato. Among the 75 isolates collected from cucumber in 2013, two were highly resistant (HR) to fludioxonil. Of the 308 isolates collected from tomato in 2014, four were fludioxonil-HR. This was the first time that B. cinerea isolates HR to fludioxonil had been detected in the field. Six fludioxonil-resistant mutants were obtained in the laboratory by selection on fungicide-amended media. These mutants exhibited stable resistance to fludioxonil, as indicated by resistance factor values that ranged from 34.38 to >10,000. In comparison with fludioxonil-sensitive isolates of B. cinerea, all field and laboratory mutants showed reduced fitness, as defined by mycelial growth, sporulation, virulence, and sensitivity to osmotic stress. When treated with fludioxonil at 1 μg/ml, sensitive isolates showed increased glycerol contents in mycelium and expression levels of Bchog1, while levels in field and laboratory HR mutants increased only slightly. Sequences of the Bos1 gene of field and laboratory fludioxonil-HR mutants showed that mutations in field mutants were located in the histidine kinase, adenylyl cyclase, methyl-accepting chemotaxis protein, and phosphatase (HAMP) domains of the N-terminal region, whereas mutations in the laboratory mutants were distributed in HAMP domains or in the HATPase_c domain of the C-terminal region. These results will enhance our understanding of the resistance mechanism of B. cinerea to fludioxonil.

Reconstruction of the High-Osmolarity Glycerol (HOG) Signaling Pathway from the Halophilic Fungus Wallemia ichthyophaga in Saccharomyces cerevisiae

The basidiomycetous fungus Wallemia ichthyophaga grows between 1.7 and 5.1 M NaCl and is the most halophilic eukaryote described to date. Like other fungi, W. ichthyophaga detects changes in environmental salinity mainly by the evolutionarily conserved high-osmolarity glycerol (HOG) signaling pathway. In Saccharomyces cerevisiae, the HOG pathway has been extensively studied in connection to osmotic regulation, with a valuable knock-out strain collection established. In the present study, we reconstructed the architecture of the HOG pathway of W. ichthyophaga in suitable S. cerevisiae knock-out strains, through heterologous expression of the W. ichthyophaga HOG pathway proteins. Compared to S. cerevisiae, where the Pbs2 (ScPbs2) kinase of the HOG pathway is activated via the SHO1 and SLN1 branches, the interactions between the W. ichthyophaga Pbs2 (WiPbs2) kinase and the W. ichthyophaga SHO1 branch orthologs are not conserved: as well as evidence of poor interactions between the WiSho1 Src-homology 3 (SH3) domain and the WiPbs2 proline-rich motif, the absence of a considerable part of the osmosensing apparatus in the genome of W. ichthyophaga suggests that the SHO1 branch components are not involved in HOG signaling in this halophilic fungus. In contrast, the conserved activation of WiPbs2 by the S. cerevisiae ScSsk2/ScSsk22 kinase and the sensitivity of W. ichthyophaga cells to fludioxonil, emphasize the significance of two-component (SLN1-like) signaling via Group III histidine kinase. Combined with protein modeling data, our study reveals conserved and non-conserved protein interactions in the HOG signaling pathway of W. ichthyophaga and therefore significantly improves the knowledge of hyperosmotic signal processing in this halophilic fungus.

A conserved dimorphism-regulating histidine kinase controls the dimorphic switching in Paracoccidioides brasiliensis

Paracoccidioides brasiliensis and P. lutzii, thermally dimorphic fungi, are the causative agents of paracoccidioidomycosis (PCM). Paracoccidioides infection occurs when conidia or mycelium fragments are inhaled by the host, which causes the Paracoccidioides cells to transition to the yeast form. The development of disease requires conidia inside the host alveoli to differentiate into yeast cells in a temperature-dependent manner. We describe the presence of a two-component signal transduction system in P. brasiliensis, which we investigated by expression analysis of a hypothetical protein gene (PADG_07579) that showed high similarity with the dimorphism-regulating histidine kinase (DRK1) gene of Blastomyces dermatitidis and Histoplasma capsulatum This gene was sensitive to environmental redox changes, which was demonstrated by a dose-dependent decrease in transcript levels after peroxide stimulation and a subtler decrease in transcript levels after NO stimulation. Furthermore, the higher PbDRK1 levels after treatment with increasing NaCl concentrations suggest that this histidine kinase can play a role as osmosensing. In the mycelium-yeast (M→Y) transition, PbDRK1 mRNA expression increased 14-fold after 24 h incubation at 37°C, consistent with similar observations in other virulent fungi. These results demonstrate that the PbDRK1 gene is differentially expressed during the dimorphic M→Y transition. Finally, when P. brasiliensis mycelium cells were exposed to a histidine kinase inhibitor and incubated at 37°C, there was a delay in the dimorphic M→Y transition, suggesting that histidine kinases could be targets of interest for PCM therapy.

Assessing the relevance of light for fungi: Implications and insights into the network of signal transmission

Light represents an important environmental cue, which provides information enabling fungi to prepare and react to the different ambient conditions between day and night. This adaptation requires both anticipation of the changing conditions, which is accomplished by daily rhythmicity of gene expression brought about by the circadian clock, and reaction to sudden illumination. Besides perception of the light signal, also integration of this signal with other environmental cues, most importantly nutrient availability, necessitates light-dependent regulation of signal transduction pathways and metabolic pathways. An influence of light and/or the circadian clock is known for the cAMP pathway, heterotrimeric G-protein signaling, mitogen-activated protein kinases, two-component phosphorelays, and Ca(2+) signaling. Moreover, also the target of rapamycin signaling pathway and reactive oxygen species as signal transducing elements are assumed to be connected to the light-response pathway. The interplay of the light-response pathway with signaling cascades results in light-dependent regulation of primary and secondary metabolism, morphology, development, biocontrol activity, and virulence. The frequent use of fungi in biotechnology as well as analysis of fungi in the artificial environment of a laboratory therefore requires careful consideration of still operative evolutionary heritage of these organisms. This review summarizes the diverse effects of light on fungi and the mechanisms they apply to deal both with the information content and with the harmful properties of light. Additionally, the implications of the reaction of fungi to light in a laboratory environment for experimental work and industrial applications are discussed.

Dissecting the role of histidine kinase and HOG1 mitogen-activated protein kinase signalling in stress tolerance and pathogenicity of Parastagonospora nodorum on wheat

The HOG1 mitogen-activated protein kinase (MAPK) pathway is activated through two-component histidine kinase (HK) signalling. This pathway was first characterized in the budding yeast Saccharomyces cerevisiae as a regulator of osmotolerance. The fungus Parastagonospora nodorum is the causal agent of septoria nodorum blotch of wheat. This pathogen uses host-specific effectors in tandem with general pathogenicity mechanisms to carry out its infection process. Genes showing strong sequence homology to S. cerevisiae HOG1 signalling pathway genes have been identified in the genome of P. nodorum. In this study, we examined the role of the pathway in the virulence of P. nodorum on wheat by disrupting putative pathway component genes: HOG1 (SNOG_13296) MAPK and NIK1 (SNOG_11631) hybrid HK. Mutants deleted in NIK1 and HOG1 were insensitive to dicarboximide and phenylpyrrole fungicides, but not a fungicide that targets ergosterol biosynthesis. Furthermore, both Δnik1 and Δhog1 mutants showed increased sensitivity to hyperosmotic stress. However, HOG1, but not NIK1, is required for tolerance to elevated temperatures. HOG1 deletion conferred increased tolerance to 6-methoxy-2-benzoxazolinone, a cereal phytoalexin. This suggests that the HOG1 signalling pathway is not exclusively associated with NIK1. Both Δnik1 and Δhog1 mutants retained the ability to infect and cause necrotic lesions on wheat. However, we observed that the Δhog1 mutation resulted in reduced production of pycnidia, asexual fruiting bodies that facilitate spore dispersal during late infection. Our study demonstrated the overlapping and distinct roles of a HOG1 MAPK and two-component HK signalling in P. nodorum growth and pathogenicity.

The Fast-Evolving phy-2 Gene Modulates Sexual Development in Response to Light in the Model Fungus Neurospora crassa

Rapid responses to changes in incident light are critical to the guidance of behavior and development in most species. Phytochrome light receptors in particular play key roles in bacterial physiology and plant development, but their functions and regulation are less well understood in fungi. Nevertheless, genome-wide expression measurements provide key information that can guide experiments that reveal how genes respond to environmental signals and clarify their role in development. We performed functional genomic and phenotypic analyses of the two phytochromes in Neurospora crassa, a fungal model adapted to a postfire environment that experiences dramatically variable light conditions. Expression of phy-1 and phy-2 was low in early sexual development and in the case of phy-2 increased in late sexual development. Under light stimulation, strains with the phytochromes deleted exhibited increased expression of sexual development-related genes. Moreover, under red light, the phy-2 knockout strain commenced sexual development early. In the evolution of phytochromes within ascomycetes, at least two duplications have occurred, and the faster-evolving phy-2 gene has frequently been lost. Additionally, the three key cysteine sites that are critical for bacterial and plant phytochrome function are not conserved within fungal phy-2 homologs. Through the action of phytochromes, transitions between asexual and sexual reproduction are modulated by light level and light quality, presumably as an adaptation for fast asexual growth and initiation of sexual reproduction of N. crassa in exposed postfire ecosystems.

Environmental signals, including light, play critical roles in regulating fungal growth and pathogenicity, and balance of asexual and sexual reproduction is critical in fungal pathogens’ incidence, virulence, and distribution. Red light sensing by phytochromes is well known to play critical roles in bacterial physiology and plant development. Homologs of phytochromes were first discovered in the fungal model Neurospora crassa and then subsequently in diverse other fungi, including many plant pathogens. Our study investigated the evolution of red light sensors in ascomycetes and confirmed—using the model fungus Neurospora crassa—their roles in modulating the asexual-sexual reproduction balance in fungi. Our findings also provide a key insight into one of the most poorly understood aspects of fungal biology, suggesting that further study of the function of phytochromes in fungi is critical to reveal the genetic basis of the asexual-sexual switch responsible for fungal growth and distribution, including diverse and destructive plant pathogens.

The Genomes of Three Uneven Siblings: Footprints of the Lifestyles of Three Trichoderma Species

The genus Trichoderma contains fungi with high relevance for humans, with applications in enzyme production for plant cell wall degradation and use in biocontrol. Here, we provide a broad, comprehensive overview of the genomic content of these species for "hot topic" research aspects, including CAZymes, transport, transcription factors, and development, along with a detailed analysis and annotation of less-studied topics, such as signal transduction, genome integrity, chromatin, photobiology, or lipid, sulfur, and nitrogen metabolism in T. reesei, T. atroviride, and T. virens, and we open up new perspectives to those topics discussed previously. In total, we covered more than 2,000 of the predicted 9,000 to 11,000 genes of each Trichoderma species discussed, which is >20% of the respective gene content. Additionally, we considered available transcriptome data for the annotated genes. Highlights of our analyses include overall carbohydrate cleavage preferences due to the different genomic contents and regulation of the respective genes. We found light regulation of many sulfur metabolic genes. Additionally, a new Golgi 1,2-mannosidase likely involved in N-linked glycosylation was detected, as were indications for the ability of Trichoderma spp. to generate hybrid galactose-containing N-linked glycans. The genomic inventory of effector proteins revealed numerous compounds unique to Trichoderma, and these warrant further investigation. We found interesting expansions in the Trichoderma genus in several signaling pathways, such as G-protein-coupled receptors, RAS GTPases, and casein kinases. A particularly interesting feature absolutely unique to T. atroviride is the duplication of the alternative sulfur amino acid synthesis pathway.

Friends or foes? Emerging insights from fungal interactions with plants

Fungi interact with plants in various ways, with each interaction giving rise to different alterations in both partners. While fungal pathogens have detrimental effects on plant physiology, mutualistic fungi augment host defence responses to pathogens and/or improve plant nutrient uptake. Tropic growth towards plant roots or stomata, mediated by chemical and topographical signals, has been described for several fungi, with evidence of species-specific signals and sensing mechanisms. Fungal partners secrete bioactive molecules such as small peptide effectors, enzymes and secondary metabolites which facilitate colonization and contribute to both symbiotic and pathogenic relationships. There has been tremendous advancement in fungal molecular biology, omics sciences and microscopy in recent years, opening up new possibilities for the identification of key molecular mechanisms in plant-fungal interactions, the power of which is often borne out in their combination. Our fragmentary knowledge on the interactions between plants and fungi must be made whole to understand the potential of fungi in preventing plant diseases, improving plant productivity and understanding ecosystem stability. Here, we review innovative methods and the associated new insights into plant-fungal interactions.

Two-Component Signaling Regulates Osmotic Stress Adaptation via SskA and the High-Osmolarity Glycerol MAPK Pathway in the Human Pathogen Talaromyces marneffei

For successful infection to occur, a pathogen must be able to evade or tolerate the host's defense systems. This requires the pathogen to first recognize the host environment and then signal this response to elicit a complex adaptive program in order to activate its own defense strategies. In both prokaryotes and eukaryotes, two-component signaling systems are utilized to sense and respond to changes in the external environment. The hybrid histidine kinases (HHKs) at the start of the two-component signaling pathway have been well characterized in human pathogens. However, how these HHKs regulate processes downstream currently remains unclear. This study describes the role of a response regulator downstream of these HHKs, sskA, in Talaromyces marneffei, a dimorphic human pathogen. sskA is required for asexual reproduction, hyphal morphogenesis, cell wall integrity, osmotic adaptation, and the morphogenesis of yeast cells both in vitro at 37°C and during macrophage infection, but not during dimorphic switching. Comparison of the ΔsskA mutant with a strain in which the mitogen-activated protein kinase (MAPK) of the high-osmolarity glycerol pathway (SakA) has been deleted suggests that SskA acts upstream of this pathway in T. marneffei to regulate these morphogenetic processes. This was confirmed by assessing the amount of phosphorylated SakA in the ΔsskA mutant, antifungal resistance due to a lack of SakA activation, and the ability of a constitutively active sakA allele (sakA(F316L) ) to suppress the ΔsskA mutant phenotypes. We conclude that SskA regulates morphogenesis and osmotic stress adaptation in T. marneffei via phosphorylation of the SakA MAPK of the high-osmolarity glycerol pathway. IMPORTANCE This is the first study in a dimorphic fungal pathogen to investigate the role of a response regulator downstream of two-component signaling systems and its connection to the high-osmolarity glycerol pathway. This study will inspire further research into the downstream components of two-component signaling systems and their role during pathogenic growth.

Quantitative Site-Specific Phosphoproteomics of Trichoderma reesei Signaling Pathways upon Induction of Hydrolytic Enzyme Production

The filamentous fungus Trichoderma reesei is used for industrial production of secreted enzymes including carbohydrate active enzymes, such as cellulases and hemicellulases. The production of many of these enzymes by T. reesei is influenced by the carbon source it grows on, where the regulation system controlling hydrolase genes involves various signaling pathways. T. reesei was cultivated in the presence of sorbitol, a carbon source that does not induce the production of cellulases and hemicellulases, and then exposed to either sophorose or spent-grain extract, which are efficient inducers of the enzyme production. Specific changes at phosphorylation sites were investigated in relation to the production of cellulases and hemicellulases using an MS-based framework. Proteome-wide phosphorylation following carbon source exchange was investigated in the early stages of induction: 0, 2, 5, and 10 min. The workflow involved sequential trypsin digestion, TiO2 enrichment, and MS analysis using a Q Exactive mass spectrometer. We report on the identification and quantitation of 1721 phosphorylation sites. Investigation of the data revealed a complex signaling network activated upon induction involving components related to light-mediated cellulase induction, osmoregulation, and carbon sensing. Changes in protein phosphorylation were detected in the glycolytic pathway, suggesting an inhibition of glucose catabolism at 10 min after the addition of sophorose and as early as 2 min after the addition of spent-grain extract. Differential phosphorylation of factors related to carbon storage, intracellular trafficking, cytoskeleton, and cellulase gene regulation were also observed.

Systematic gene deletion and functional characterization of histidine kinase phosphorelay receptors (HKRs) in the human pathogenic fungus Aspergillus fumigatus

Histidine kinase receptors (HKRs) appear to be a common strategy for model and pathogenic fungi to sense and respond to environmental stresses. In the human pathogen Aspergillus fumigatus, which is responsible for invasive aspergillosis, 13 genes potentially encoding HKRs have been identified. Until now, only three HKRs have been functionally characterized. The aim of this study was to perform the systematic invalidation of A. fumigatus HKR genes and the careful phenotypic characterization of the relevant mutants. This study notably allowed to gain new important insights into the role of HKRs in physiology of A. fumigatus. Actually, we showed that (i) NikA/TcsC could be involved in the cell wall integrity pathway, (ii) Fhk6 and PhkA were involved in the regulation of the "fluffy" developmental program, (iii) PhkB could participate in the regulation of conidiation and (iv) PhkA was implied in the resistance of oxidative stresses.

A survey of HK, HPt, and RR domains and their organization in two-component systems and phosphorelay proteins of organisms with fully sequenced genomes

Two Component Systems and Phosphorelays (TCS/PR) are environmental signal transduction cascades in prokaryotes and, less frequently, in eukaryotes. The internal domain organization of proteins and the topology of TCS/PR cascades play an important role in shaping the responses of the circuits. It is thus important to maintain updated censuses of TCS/PR proteins in order to identify the various topologies used by nature and enable a systematic study of the dynamics associated with those topologies. To create such a census, we analyzed the proteomes of 7,609 organisms from all domains of life with fully sequenced and annotated genomes. To begin, we survey each proteome searching for proteins containing domains that are associated with internal signal transmission within TCS/PR: Histidine Kinase (HK), Response Regulator (RR) and Histidine Phosphotranfer (HPt) domains, and analyze how these domains are arranged in the individual proteins. Then, we find all types of operon organization and calculate how much more likely are proteins that contain TCS/PR domains to be coded by neighboring genes than one would expect from the genome background of each organism. Finally, we analyze if the fusion of domains into single TCS/PR proteins is more frequently observed than one might expect from the background of each proteome. We find 50 alternative ways in which the HK, HPt, and RR domains are observed to organize into single proteins. In prokaryotes, TCS/PR coding genes tend to be clustered in operons. 90% of all proteins identified in this study contain just one of the three domains, while 8% of the remaining proteins combine one copy of an HK, a RR, and/or an HPt domain. In eukaryotes, 25% of all TCS/PR proteins have more than one domain. These results might have implications for how signals are internally transmitted within TCS/PR cascades. These implications could explain the selection of the various designs in alternative circumstances.

Unraveling the Function of the Response Regulator BcSkn7 in the Stress Signaling Network of Botrytis cinerea

Important for the lifestyle and survival of every organism is the ability to respond to changing environmental conditions. The necrotrophic plant pathogen Botrytis cinerea triggers an oxidative burst in the course of plant infection and therefore needs efficient signal transduction to cope with this stress. The factors involved in this process and their precise roles are still not well known. Here, we show that the transcription factor Bap1 and the response regulator (RR) B. cinerea Skn7 (BcSkn7) are two key players in the oxidative stress response (OSR) of B. cinerea; both have a major influence on the regulation of classical OSR genes. A yeast-one-hybrid (Y1H) approach proved direct binding to the promoters of gsh1 and grx1 by Bap1 and of glr1 by BcSkn7. While the function of Bap1 is restricted to the regulation of oxidative stress, analyses of Δbcskn7 mutants revealed functions beyond the OSR. Involvement of BcSkn7 in development and virulence could be demonstrated, indicated by reduced vegetative growth, impaired formation of reproductive structures, and reduced infection cushion-mediated penetration of the host by the mutants. Furthermore, Δbcskn7 mutants were highly sensitive to oxidative, osmotic, and cell wall stress. Analyses of Δbap1 bcskn7 double mutants indicated that loss of BcSkn7 uncovers an underlying phenotype of Bap1. In contrast to Saccharomyces cerevisiae, the ortholog of the glutathione peroxidase Gpx3p is not required for nuclear translocation of Bap1. The presented results contribute to the understanding of the OSR in B. cinerea and prove that it differs substantially from that of yeast, demonstrating the complexity and versatility of components involved in signaling pathways.

Genomewide analysis of phytochrome proteins in the phylum Basidiomycota

Phytochromes are photoreceptor proteins involved in the detection of the red and far-red regions of the visible light spectrum. Fungal phytochromes are hybrid histidine kinases with a conserved domain architecture composed of an N-terminal photosensory module and a C-terminal regulatory output module that includes the histidine kinase and response regulator receiver domains. In this study, we have analyzed the distribution, domain architecture, and phylogenetic analysis of phytochrome proteins in 47 published genome sequences among the phylum Basidiomycota. Genome analysis revealed that almost every genome of basidiomycetes contained at least one gene encoding a phytochrome protein. Domain architecture of fungal phytochromes was completely conserved in the identified phytochromes of basidiomycetes, and phylogenetic analysis clustered these proteins into clades related with the phylogenetic classification of this fungal phylum.

MrSkn7 controls sporulation, cell wall integrity, autolysis, and virulence in Metarhizium robertsii

Two-component signaling pathways generally include sensor histidine kinases and response regulators. We identified an ortholog of the response regulator protein Skn7 in the insect-pathogenic fungus Metarhizium robertsii, which we named MrSkn7. Gene deletion assays and functional characterizations indicated that MrSkn7 functions as a transcription factor. The MrSkn7 null mutant of M. robertsii lost the ability to sporulate and had defects in cell wall biosynthesis but was not sensitive to oxidative and osmotic stresses compared to the wild type. However, the mutant was able to produce spores under salt stress. Insect bioassays using these spores showed that the virulence of the mutant was significantly impaired compared to that of the wild type due to the failures to form the infection structure appressorium and evade host immunity. In particular, deletion of MrSkn7 triggered cell autolysis with typical features such as cell vacuolization, downregulation of repressor genes, and upregulation of autolysis-related genes such as extracellular chitinases and proteases. Promoter binding assays confirmed that MrSkn7 could directly or indirectly control different putative target genes. Taken together, the results of this study help us understand the functional divergence of Skn7 orthologs as well as the mechanisms underlying the development and control of virulence in insect-pathogenic fungi.

The role of AtfA and HOG MAPK pathway in stress tolerance in conidia of Aspergillus fumigatus

Aspergillus fumigatus is a life-threatening pathogenic fungus, whose conidium is the infectious agent of aspergillosis. To better understand the mechanism underlying the long-term viability of conidia, we characterized a bZip transcription factor, AtfA, with special reference to stress-tolerance in conidia. The atfA deletion mutant conidia showed significant sensitivity to high temperature and oxidative stress. The trehalose content that accumulated in conidia was reduced in the mutant conidia. Transcriptome analysis revealed that AtfA regulated several stress-protection-related genes such as catA, dprA, scf1, and conJ at the conidiation stage. The upstream high-osmolarity glycerol pathway was also involved in conferring stress tolerance in conidia because ΔpbsB showed stress sensitivity and reduced trehalose in conidia. However, a mutant lacking the SakA mitogen-activated protein kinase (MAPK) produced normal conidia. We investigated another MAPK, MpkC, in relation with SakA, and the double deletion mutant, ΔsakA,mpkC, was defective in conidia stress tolerance. We concluded that MpkC is able to bypass SakA, and the two MAPKs redundantly regulate the conidia-related function of AtfA in A. fumigatus.