BcIqg1, a fungal IQGAP homolog, interacts with NADPH oxidase, MAP kinase and calcium signaling proteins and regulates virulence and development inBotrytis cinerea

RAS signalling genes can be used as host-induced gene silencing targets to control fungal diseases caused by Sclerotinia sclerotiorum and Botrytis cinerea

Sclerotinia sclerotiorum causes white mold (also called stem rot, Sclerotinia blight, etc.) in many economically important plants. It is a notorious soilborne fungal pathogen due to its wide host range and ability to survive in soil for long periods of time as sclerotia. Although host-induced gene silencing (HIGS) was recently demonstrated to be an effective method for controlling white mold, limited gene targets are available. Here, using a forward genetics approach, we identified a RAS-GTPase activating protein, SsGAP1, which plays essential roles in sclerotia formation, compound appressoria production and virulence. In parallel, as revealed by our knockout analysis, the SsGAP1 ortholog in Botrytis cinerea, BcGAP1, plays similar roles in fungal development and virulence. By knocking down SsRAS1 and SsRAS2, we also revealed that both SsRAS1 and SsRAS2 are required for vegetative growth, sclerotia development, compound appressoria production and virulence in S. sclerotiorum. Due to the major roles these RAS signalling components play in Sclerotiniaceae biology, they can be used as HIGS targets to control diseases caused by both S. sclerotiorum and B. cinerea. Indeed, when we introduced HIGS constructs targeting SsGAP1, SsRAS1 and SsRAS2 in Nicotiana benthamiana and Arabidopsis thaliana, we observed reduced virulence. Taken together, our forward genetics gene discovery pipeline in S. sclerotiorum is highly effective in identifying novel HIGS targets to control S. sclerotiorum and B. cinerea.

The infection cushion of Botrytis cinerea: a fungal 'weapon' of plant-biomass destruction

The necrotrophic plant-pathogen fungus Botrytis cinerea produces multicellular appressoria dedicated to plant penetration, named infection cushions (IC). A microarray analysis was performed to identify genes upregulated in mature IC. The expression data were validated by RT-qPCR analysis performed in vitro and in planta, proteomic analysis of the IC secretome and biochemical assays. 1231 upregulated genes and 79 up-accumulated proteins were identified. The data support the secretion of effectors by IC: phytotoxins, ROS, proteases, cutinases, plant cell wall-degrading enzymes and plant cell death-inducing proteins. Parallel upregulation of sugar transport and sugar catabolism-encoding genes would indicate a role of IC in nutrition. The data also reveal a substantial remodelling of the IC cell wall and suggest a role for melanin and chitosan in IC function. Lastly, mutagenesis of two upregulated genes in IC identified secreted fasciclin-like proteins as actors in the pathogenesis of B. cinerea. These results support the role of IC in plant penetration and also introduce other unexpected functions for this fungal organ, in colonization, necrotrophy and nutrition of the pathogen.

The Destructive Fungal Pathogen Botrytis cinerea-Insights from Genes Studied with Mutant Analysis

Botrytis cinerea is one of the most destructive fungal pathogens affecting numerous plant hosts, including many important crop species. As a molecularly under-studied organism, its genome was only sequenced at the beginning of this century and it was recently updated with improved gene annotation and completeness. In this review, we summarize key molecular studies on B. cinerea developmental and pathogenesis processes, specifically on genes studied comprehensively with mutant analysis. Analyses of these studies have unveiled key genes in the biological processes of this pathogen, including hyphal growth, sclerotial formation, conidiation, pathogenicity and melanization. In addition, our synthesis has uncovered gaps in the present knowledge regarding development and virulence mechanisms. We hope this review will serve to enhance the knowledge of the biological mechanisms behind this notorious fungal pathogen.

Fungicidal Effect of Pyraclostrobin against Botrytis cinerea in Relation to Its Crystal Structure

Pyraclostrobin (PYR) is a commonly used strobilurin fungicide, which inhibits mitochondrial respiration at the ubiquinol oxidation center site of the cytochrome bc1 complex. Little information is available regarding the crystal structure of PYR on its fungicidal effect. In this study, the crystal structures of eight PYRs (PYR-A to H) from different sources are determined by using high-resolution X-ray powder diffraction (XRPD) and model construction with the Pawley refinement module. The effects of PYRs on mycelium growth, the kinetics of mycelial growth, conidial germination, and tube elongation of conidia of Botrytis cinerea from tomato are compared. The level of organic acids in the mitochondrial tricarboxylic acid cycle of PYR-treated B. cinerea is analyzed. The results show that PYR-A to PYR-H have their own unique character of XRPD patterns, but the crystal morphology of eight PYRs presents in the triclinic crystal system and space group P1̅. PYR-D with the eclipsed conformation and rational edge angles α (72.599°) and β (98.612°) in the crystal cell shows the highest inhibitory effect against mycelium growth with EC₅₀ as 3.383 μg mL^-1, the best time-dependent effects on the mycelium growth kinetics, and the strongest inhibition on tube elongation of conidia, whereas PYR-E with anticonformation is the worst. Moreover, a significant accumulation of fumarate, malate, and oxalate in the PYR-D-treated mycelium is observed. These findings reinforce the need for a definite crystal structure of PYR to limit usage and mitigate future selection pressure for gray mold management.

The Aspergillus nidulans IQGAP orthologue SepG is required for constriction of the contractile actomyosin ring

In this research we report that the sepG1 mutation in Aspergillus nidulans resides in gene AN9463, which is predicted to encode an IQGAP orthologue. The genetic lesion is predicted to result in a G-to-R substitution at residue 1637 of the 1737-residue protein in a highly conserved region of the RasGAP-C-terminal (RGCT) domain. When grown at restrictive temperature, strains expressing the sepG^G1637R (sepG1) allele are aseptate, with reduced colony growth and aberrantly formed conidiophores. The aseptate condition can be replicated by deletion of AN9463 or by downregulating its expression via introduced promoters. The mutation does not prevent assembly of a cortical contractile actomyosin ring (CAR) at putative septation sites, but tight compaction of the rings is impaired and the rings fail to constrict. Both GFP::SepG wild type and the GFP-tagged product of the sepG1 allele localize to the CAR at both permissive and restrictive temperatures. Downregulation of myoB (encoding the A. nidulans type-II myosin heavy chain) does not prevent formation of SepG rings at septation sites, but filamentous actin is required for CAR localization of SepG and MyoB. We identify fourteen probable IQ-motifs (EF-hand protein binding sites) in the predicted SepG sequence. Two of the A. nidulans EF-hand proteins, myosin essential light chain (AnCdc4) and myosin regulatory light chain (MrlC), colocalize with SepG and MyoB at all stages of CAR formation and constriction. However, calmodulin (CamA) appears at septation sites only after the CAR has become fully compacted. When expression of sepG is downregulated, leaving MyoB as the sole IQ-motif protein in the pre-compaction CAR, both MrlC and AnCdc4 continue to associate with the forming CAR. When myoB expression is downregulated, leaving SepG as the sole IQ-motif protein in the CAR, AnCdc4 association with the forming CAR continues but MrlC fails to associate. This supports a model in which the IQ motifs of MyoB bind both MrlC and AnCdc4, while the IQ motifs of SepG bind only AnCdc4. Downregulation of either mrlC or Ancdc4 results in an aseptate phenotype, but has no effect on association of either SepG or MyoB with the CAR.

Transcriptome analysis and functional validation reveal a novel gene, BcCGF1, that enhances fungal virulence by promoting infection-related development and host penetration

Simultaneous transcriptome analyses of both host plants and pathogens, and functional validation of the identified differentially expressed genes (DEGs) allow us to better understand the mechanisms underlying their interactions. Here, we analyse the mixed transcriptome derived from Botrytis cinerea (the causal agent of grey mould) infected tomato leaves at 24 hr after inoculation, a critical time point at which the pathogen has penetrated and developed in the leaf epidermis, whereas necrotic symptoms have not yet appeared. Our analyses identified a complex network of genes involved in the tomato-B. cinerea interaction. The expression of fungal transcripts encoding candidate effectors, enzymes for secondary metabolite biosynthesis, hormone and reactive oxygen species (ROS) production, and autophagy-related proteins was up-regulated, suggesting that these genes may be involved in the initial infection processes. Specifically, tomato genes involved in phytoalexin production, stress responses, ATP-binding cassette transporters, pathogenesis-related proteins, and WRKY DNA-binding transcription factors were up-regulated. We functionally investigated several B. cinerea DEGs via gene replacement and pathogenicity assays, and demonstrated that BcCGF1 was a novel virulence-associated factor that mediates fungal development and virulence via regulation of conidial germination, conidiation, infection structure formation, host penetration, and stress adaptation. The fungal infection-related development was controlled by BcCGF-mediated ROS production and exogenous cAMP restored the mutant infection-related development. Our findings provide new insights into the elucidation of the simultaneous tactics of pathogen attack and host defence. Our systematic elucidation of BcCGF1 in mediating fungal pathogenesis may open up new targets for fungal disease control.

Identification and characterization of parvalbumin-like protein in Trichophyton violaceum

Parvalbumins play crucial physiological roles in neuromuscular systems of vertebrates, such as cell-cycle, development of neurons, contraction of muscles, and regulation of intracellular calcium. To perform these neuromuscular functions, parvalbumin may be in associated with other proteins including calbindin, carbonic anhydrase, and cytochrome oxidase. Humans may show an IgE-specific hypersensitivity to parvalbumins after consumption of some distinct fish species. While this protein is abundant in fish muscles, literature review of publications related to fish parvalbumins, do not point to the presence of parvalbumins in eukaryotic microbes. In this study, we propose that distantly related parvalbumins may be found in some non-fish species. Bioinformatics studies such as multiple sequence alignment (MSA), phylogenetic analysis as well as molecular-based experiments indicate that, at least two parvalbumins sequences (UniProt IDs: A0A178F775 and A0A178F7E4) with EF-hand domains and Ca²⁺-binding sites could be identified in Trichophyton violaceum, a pathogenic fungal species. It was determined that both genes consisted of a single exon and encoded for parvalbumin proteins possessing conserved amino acid motifs. Antigenicity prediction revealed antigenic sites located in both sides of the Ca²⁺-binding site of the first EF-hand domain. Our phylogenetic analysis revealed that one of parvalbumins (UniProt ID: 0A178F775) can be evolved to other parvalbumins in T. violaceum (UniProt ID: A0A178F7E4) and fish species through evolutionary phenomenon. To confirm our in-silico findings, we designed three primer pairs to detect one of the T. violaceum parvalbumins (UniProt ID: A0A178F7E4) by polymerase chain reaction (PCR); one primer pair showed a strong and specific band in agarose gel electrophoresis. To evaluate the specificity of the method, the primers were tested on extracted DNA from Trichophyton rubrum and T. mentagrophytes. The results demonstrated that the evaluated parvalbumin gene (UniProt ID: A0A178F7E4) was T. violaceum-specific and this pathogenic fungus can be differentiated from T. rubrum and T. mentagrophytes through identification of parvalbumin genes. Further studies are necessary to unravel the biochemical and physiological functions of parvalbumins in T. violaceum.

The Subtilisin-Like Protease Bcser2 Affects the Sclerotial Formation, Conidiation and Virulence of Botrytis cinerea

Botrytis cinerea, a ubiquitous necrotrophic plant-pathogenic fungus, is responsible for grey mold and rot disease in a very wide range of plant species. Subtilisin-like proteases (or subtilases) are a very diverse family of serine proteases present in many organisms and are reported to have a broad spectrum of biological functions. Here, we identified two genes encoding subtilisin-like proteases (Bcser1 and Bcser2) in the genome of B. cinerea, both of which contain an inhibitor I9 domain and a peptidase S8 domain. The expression levels of Bcser1 and Bcser2 increased during the sclerotial forming stage, as well as during a later stage of hyphal infection on Arabidopsis thaliana leaves, but the up-regulation of Bcser1 was significantly higher than that of Bcser2. Interestingly, deletion of Bcser1 had no effect on the fungal development or virulence of B. cinerea. However, deletion of Bcser2 or double deletion of Bcser1 and Bcser2 severely impaired the hyphal growth, sclerotial formation and conidiation of B. cinerea. We also found that ∆Bcser2 and ∆Bcser1/2 could not form complete infection cushions and then lost the ability to infect intact plant leaves of Arabidopsis and tomato but could infect wounded plant tissues. Taken together, our results indicate that the subtilisin-like protease Bcser2 is crucial for the sclerotial formation, conidiation, and virulence of B. cinerea.

The H3K4 demethylase Jar1 orchestrates ROS production and expression of pathogenesis-related genes to facilitate Botrytis cinerea virulence

Histone 3 Lysine 4 (H3K4) demethylation is ubiquitous in organisms, however the roles of H3K4 demethylase JARID1(Jar1)/KDM5 in fungal development and pathogenesis remain largely unexplored. Here, we demonstrate that Jar1/KDM5 in Botrytis cinerea, the grey mould fungus, plays a crucial role in these processes. The BcJAR1 gene was deleted and its roles in fungal development and pathogenesis were investigated using approaches including genetics, molecular/cell biology, pathogenicity and transcriptomic profiling. BcJar1 regulates H3K4me3 and both H3K4me2 and H3K4me3 methylation levels during vegetative and pathogenic development, respectively. Loss of BcJAR1 impairs conidiation, appressorium formation and stress adaptation; abolishes infection cushion (IC) formation and virulence, but promotes sclerotium production in the ΔBcjar1 mutants. BcJar1 controls reactive oxygen species (ROS) production and proper assembly of Sep4, a core septin protein and virulence determinant, to initiate infection structure (IFS) formation and host penetration. Exogenous cAMP partially restored the mutant appressorium, but not IC, formation. BcJar1 orchestrates global expression of genes for ROS production, stress response, carbohydrate transmembrane transport, secondary metabolites, etc., which may be required for conidiation, IFS formation, host penetration and virulence of the pathogen. Our work systematically elucidates BcJar1 functions and provides novel insights into Jar1/KDM5-mediated H3K4 demethylation in regulating fungal development and pathogenesis.

NADPH Oxidase Is Crucial for the Cellular Redox Homeostasis in Fungal Pathogen Botrytis cinerea

During interactions, both plants and pathogens produce reactive oxygen species (ROS). Plants generate ROS for defense induction, while pathogens synthesize ROS for growth, sporulation, and virulence. NADPH oxidase (NOX) complex in the plasma membrane represents a main protein complex for ROS production in pathogens. Although NOX plays a crucial role in pathogenicity of pathogens, the underlying molecular mechanisms of NOX, especially the proteins regulated by NOX, remain largely unknown. Here, we applied an iodoacetyl tandem mass tag-based redox proteomic assay to investigate the protein redox dynamics in deletion mutant of bcnoxR, which encodes a regulatory subunit of NOX in the fungal pathogen Botrytis cinerea. In total, 214 unique peptidyl cysteine (Cys) thiols from 168 proteins were identified and quantified in both the wild type and ∆bcnoxR mutant. The Cys thiols in the ∆bcnoxR mutant were generally more oxidized than those in the wild type, suggesting that BcNoxR is essential for maintaining the equilibrium of the redox state in B. cinerea. Site-specific thiol oxidation analysis indicated that 142 peptides containing the oxidized thiols changed abundance significantly in the ∆bcnoxR mutant. Proteins containing these differential peptides are classified into various functional categories. Functional analysis revealed that one of these proteins, 6-phosphate dehydrogenase, played roles in oxidative stress response and pathogenesis of B. cinerea. These results provide insight into the potential target proteins and the ROS signal transduction pathway regulated by NOX.

A homologue of the fungal tetraspanin Pls1 is required for Epichloë festucae expressorium formation and establishment of a mutualistic interaction with Lolium perenne

Epichloë festucae is an endophytic fungus that forms a mutualistic symbiotic association with the grass host Lolium perenne. Endophytic hyphae exit the host by an appressorium-like structure known as an expressorium. In plant-pathogenic fungi, the tetraspanin Pls1 and the NADPH oxidase component Nox2 are required for appressorium development. Previously we showed that the homologue of Nox2, NoxB, is required for E. festucae expressorium development and establishment of a mutualistic symbiotic interaction with the grass host. Here we used a reverse genetics approach to functionally characterize the role of the E. festucae homologue of Pls1, PlsA. The morphology and growth of ΔplsA in axenic culture was comparable to wild-type. The tiller length of plants infected with ΔplsA was significantly reduced. Hyphae of ΔplsA had a proliferative pattern of growth within the leaves of L. perenne with increased colonization of the intercellular spaces and the vascular bundles. The ΔplsA mutant was also defective in expressorium development although the phenotype was not as severe as for ΔnoxB, highlighting potentially distinct roles for PlsA and NoxB in signalling through the NoxB complex. Hyphae of ΔplsA proliferate below the cuticle surface but still occasionally form an expressorium-like structure that enables the mutant hyphae to exit the leaf to grow on the surface. These expressoria still form a septin ring-like structure at the point of cuticle exit as found in the wild-type strain. These results establish that E. festucae PlsA has an important, but distinct, role to NoxB in expressorium development and plant symbiosis.

Functional analysis of the exocyst subunit BcExo70 in Botrytis cinerea

Botrytis cinerea is one of the most important saprophytic plant pathogenic fungi. The exocyst complex and exocytosis was demonstrated to be involved in fungal development and plant infection. Here, we investigated the function of an exocyst subunit gene Bcexo70 in B. cinerea. The results show that knockout of the Bcexo70 gene significantly reduced the fungal growth and hindered the production of conidia and sclerotia. The Bcexo70 deletion strains showed a severe decrease in virulence toward tomato leaves and reduced secretion of cell wall-degrading enzyme. Confocal and electronic microscopic observation showed that the vesicles in the Bcexo70 mutants were enlarged and scattered in the cytoplasm compared to the regular distribution in the hyphal tip in wild-type strain. This study showed that the exocyst gene Bcexo70 is crucial for fungal growth, conidiation and pathogenicity in B. cinerea.

Communicate and Fuse: How Filamentous Fungi Establish and Maintain an Interconnected Mycelial Network

Cell-to-cell communication and cell fusion are fundamental biological processes across the tree of life. Survival is often dependent upon being able to identify nearby individuals and respond appropriately. Communication between genetically different individuals allows for the identification of potential mating partners, symbionts, prey, or predators. In contrast, communication between genetically similar (or identical) individuals is important for mediating the development of multicellular organisms or for coordinating density-dependent behaviors (i.e., quorum sensing). This review describes the molecular and genetic mechanisms that mediate cell-to-cell communication and cell fusion between cells of Ascomycete filamentous fungi, with a focus on Neurospora crassa. Filamentous fungi exist as a multicellular, multinuclear network of hyphae, and communication-mediated cell fusion is an important aspect of colony development at each stage of the life cycle. Asexual spore germination occurs in a density-dependent manner. Germinated spores (germlings) avoid cells that are genetically different at specific loci, while chemotropically engaging with cells that share identity at these recognition loci. Germlings with genetic identity at recognition loci undergo cell fusion when in close proximity, a fitness attribute that contributes to more rapid colony establishment. Communication and cell fusion also occur between hyphae in a colony, which are important for reinforcing colony architecture and supporting the development of complex structures such as aerial hyphae and sexual reproductive structures. Over 70 genes have been identified in filamentous fungi (primarily N. crassa) that are involved in kind recognition, chemotropic interactions, and cell fusion. While the hypothetical signal(s) and receptor(s) remain to be described, a dynamic molecular signaling network that regulates cell-cell interactions has been revealed, including two conserved MAP-Kinase cascades, a conserved STRIPAK complex, transcription factors, a NOX complex involved in the generation of reactive oxygen species, cell-integrity sensors, actin, components of the secretory pathway, and several other proteins. Together these pathways facilitate the integration of extracellular signals, direct polarized growth, and initiate a transcriptional program that reinforces signaling and prepares cells for downstream processes, such as membrane merger, cell fusion and adaptation to heterokaryon formation.

Redox signalling from NADPH oxidase targets metabolic enzymes and developmental proteins in Fusarium graminearum

NADPH oxidase (NOX) is one of the sources of reactive oxygen species (ROS) that modulates the activity of proteins through modifications of their cysteine residues. In a previous study, we demonstrated the importance of NOX in both the development and pathogenicity of the phytopathogen Fusarium graminearum. In this article, comparative proteomics between the wild-type and a Nox mutant of F. graminearum was used to identify active cysteine residues on candidate redox-sensing proteins. A two-dimensional gel approach based on labelling with monobromobimane (mBBR) identified 19 candidate proteins, and was complemented with a gel-free shotgun approach based on a biotin switch method, which yielded 99 candidates. The results indicated that, in addition to temporal regulation, a large number of primary metabolic enzymes are potentially targeted by NoxAB-generated ROS. Targeted disruption of these metabolic genes showed that, although some are dispensable, others are essential. In addition to metabolic enzymes, developmental proteins, such as the Woronin body major protein (FGSG_08737) and a glycosylphosphatidylinositol (GPI)-anchored protein (FGSG_10089), were also identified. Deletion of either of these genes reduced the virulence of F. graminearum. Furthermore, changing the redox-modified cysteine (Cys325 ) residue in FGSG_10089 to either serine or phenylalanine resulted in a similar phenotype to the FGSG_10089 knockout strain, which displayed reduced virulence and altered cell wall morphology; this underscores the importance of Cys325 to the function of the protein. Our results indicate that NOX-generated ROS act as intracellular signals in F. graminearum and modulate the activity of proteins affecting development and virulence in planta.

The MAPK kinase BcMkk1 suppresses oxalic acid biosynthesis via impeding phosphorylation of BcRim15 by BcSch9 in Botrytis cinerea

The mitogen-activated protein kinase (MAPK) cassette of the cell wall integrity (CWI) pathway is primarily responsible for orchestrating changes of cell wall. However, functions of this cassette in other cellular processes are not well understood. Here, we found that the Botrytis cinerea mutant of MAPK kinase (BcMkk1) displays more serious defects in mycelial growth, conidiation, responses to cell wall and oxidative stresses, but possesses less reduced virulence than the mutants of its upstream (BcBck1) and downstream (BcBmp3) kinases. Interestingly, BcMkk1, but not BcBck1 and BcBmp3, negatively regulates production of oxalic acid (OA) and activity of extracellular hydrolases (EHs) that are proposed to be virulence factors of B. cinerea. Moreover, we obtained evidence that BcMkk1 negatively controls OA production via impeding phosphorylation of the Per-Arnt-Sim (PAS) kinase BcRim15 by the Ser/Thr kinase BcSch9. In addition, the fungal Pro40 homolog BcPro40 was found to interact simultaneously with three MAPKs, implying that BcPro40 is a scaffold protein of the CWI pathway in B. cinerea. Taken together, results of this study reveal that BcMkk1 negatively modulates virulence via suppressing OA biosynthesis in B. cinerea, which provides novel insight into conserved and species-specific functions of the MAPK kinase in fungi.

Botrytis cinerea causes pre- and postharvest diseases in more than 200 economically important crops. In this study, the roles of cell wall integrity (CWI)-related MAPK kinase BcMkk1in regulating B. cinerea virulence were investigated using genetic and biochemical approaches. We found that the MAPK kinase BcMkk1 positively regulates virulence via the CWI pathway. Unexpectedly, BcMkk1 also negatively regulates fungal virulence via restraining oxalic acid production, by impeding phosphorylation of the PAS kinase BcRim15 mediated by the kinase BcSch9. To our knowledge, this is the first report that a MAPK kinase can negatively modulate fungal virulence on host plants. Our results provide novel insight into biological functions of a MAPK kinase in fungal pathogenesis.

Two Novel Hypovirulence-Associated Mycoviruses in the Phytopathogenic Fungus Botrytis cinerea: Molecular Characterization and Suppression of Infection Cushion Formation

Botrytis cinerea is a necrotrophic fungus causing disease on many important agricultural crops. Two novel mycoviruses, namely Botrytis cinerea hypovirus 1 (BcHV1) and Botrytis cinerea fusarivirus 1 (BcFV1), were fully sequenced. The genome of BcHV1 is 10,214 nt long excluding a poly-A tail and possesses one large open reading frame (ORF) encoding a polyprotein possessing several conserved domains including RNA-dependent RNA polymerase (RdRp), showing homology to hypovirus-encoded polyproteins. Phylogenetic analysis indicated that BcHV1 may belong to the proposed genus Betahypovirus in the viral family Hypoviridae. The genome of BcFV1 is 8411 nt in length excluding the poly A tail and theoretically processes two major ORFs, namely ORF1 and ORF2. The larger ORF1 encoded polypeptide contains protein domains of an RdRp and a viral helicase, whereas the function of smaller ORF2 remains unknown. The BcFV1 was phylogenetically clustered with other fusariviruses forming an independent branch, indicating BcFV1 was a member in Fusariviridae. Both BcHV1 and BcFV1 were capable of being transmitted horizontally through hyphal anastomosis. Infection by BcHV1 alone caused attenuated virulence without affecting mycelial growth, significantly inhibited infection cushion (IC) formation, and altered expression of several IC-formation-associated genes. However, wound inoculation could fully rescue the virulence phenotype of the BcHV1 infected isolate. These results indicate the BcHV1-associated hypovirulence is caused by the viral influence on IC-formation-associated pathways.

The pre-rRNA processing factor Nop53 regulates fungal development and pathogenesis via mediating production of reactive oxygen species

Botrytis cinerea is a necrotrophic plant fungal pathogen that annually causes enormous economic losses worldwide. The ribosome is an organelle for cellular protein biosynthesis. However, little is known about how the ribosome operates as a machine to mediate microbial pathogenesis. Here, we demonstrate that Nop53, a late-acting factor for 60S ribosomal subunit maturation, is crucial for the pathogen's development and virulence. BcNop53 is functionally equivalent to yeast nop53p. Complementation of BcNOP53 completely restored the growth defect of the yeast Δnop53 mutant. BcNop53 is located in nuclei and disruption of BcNOP53 also dramatically impaired pathogen growth. Deletion of BcNOP53 blocked infection structure formation and abolished virulence of the pathogen, possibly due to reduced production of reactive oxygen species. Moreover, loss of BcNOP53 impaired pathogen conidiation and stress adaptation, altered conidial and sclerotial morphology, retarded conidium and sclerotium germination as well as reduced the activities of cell-wall degradation-associated enzymes. Sclerotium production was, however, increased. Complementation with the wild-type BcNOP53 allele rescued defects found in the ΔBcnop53 mutant. Our work establishes a systematic elucidation of Nop53 in regulating microbial development and pathogenesis, provides novel insights into ribosomal processes that regulate fungal pathogenesis, and may open up new targets for addressing fungal diseases.

Tpc1 is an important Zn(II)2Cys6 transcriptional regulator required for polarized growth and virulence in the rice blast fungus

The establishment of polarity is a critical process in pathogenic fungi, mediating infection-related morphogenesis and host tissue invasion. Here, we report the identification of TPC1 (Transcription factor for Polarity Control 1), which regulates invasive polarized growth in the rice blast fungus Magnaporthe oryzae. TPC1 encodes a putative transcription factor of the fungal Zn(II)₂Cys₆ family, exclusive to filamentous fungi. Tpc1-deficient mutants show severe defects in conidiogenesis, infection-associated autophagy, glycogen and lipid metabolism, and plant tissue colonisation. By tracking actin-binding proteins, septin-5 and autophagosome components, we show that Tpc1 regulates cytoskeletal dynamics and infection-associated autophagy during appressorium-mediated plant penetration. We found that Tpc1 interacts with Mst12 and modulates its DNA-binding activity, while Tpc1 nuclear localisation also depends on the MAP kinase Pmk1, consistent with the involvement of Tpc1 in this signalling pathway, which is critical for appressorium development. Importantly, Tpc1 directly regulates NOXD expression, the p22^phox subunit of the fungal NADPH oxidase complex via an interaction with Mst12. Tpc1 therefore controls spatial and temporal regulation of cortical F-actin through regulation of the NADPH oxidase complex during appressorium re-polarisation. Consequently, Tpc1 is a core developmental regulator in filamentous fungi, linking the regulated synthesis of reactive oxygen species and the Pmk1 pathway, with polarity control during host invasion.

Cellular polarity is an intrinsic feature of filamentous fungal growth and pathogenesis. In this study, we identified a gene required for fungal polar growth and virulence in the rice blast fungus Magnaporthe oryzae. This gene has been named TPC1 (Transcription factor for Polarity Control 1). The Tpc1 protein belongs to the fungal Zn(II)₂Cys₆ binuclear cluster family. This DNA-binding motif is present exclusively in the fungal kingdom. We have characterised defects associated with lack of Tpc1 in M. oryzae. We show that Tpc1 is involved in polarised growth and virulence. The M. oryzae Δtpc1 mutant shows a delay in glycogen and lipid metabolism, and infection-associated autophagy–processes that regulate appressorium-mediated M. oryzae plant infection. The saprophytic fungus Neurospora crassa contains a Tpc1 homolog (NcTpc1) involved in vegetative growth and sustained tip elongation, suggesting that Tpc1-like proteins act as core regulators of polarised growth and development in filamentous fungi. A comparative transcriptome analysis has allowed us to identify genes regulated by Tpc1 in M. oryzae including NoxD, an important component of the fungal NADPH complex. Significantly, Tpc1 interacts with Mst12, a component of the Pmk1 signalling pathway essential for appressorium development, and modulates Mst12 binding affinity to NOXD promoter region. We conclude that Tpc1 is a key regulator of polarity in M. oryzae that regulates growth, autophagy and septin-mediated reorientation of the F-actin cytoskeleton to facilitate plant cell invasion.

The Protein Disulfide Isomerase of Botrytis cinerea: An ER Protein Involved in Protein Folding and Redox Homeostasis Influences NADPH Oxidase Signaling Processes

Botrytis cinerea is a filamentous plant pathogen, which infects hundreds of plant species; within its lifestyle, the production of reactive oxygen species (ROS) and a balanced redox homeostasis are essential parameters. The pathogen is capable of coping with the plant’s oxidative burst and even produces its own ROS to enhance the plant’s oxidative burst. Highly conserved NADPH oxidase (Nox) complexes produce the reactive molecules. The membrane-associated complexes regulate a large variety of vegetative and pathogenic processes. Besides their commonly accepted function at the plasma membrane, recent studies reveal that Nox complexes are also active at the membrane of the endoplasmic reticulum. In this study, we identified the essential ER protein BcPdi1 as new interaction partner of the NoxA complex in B. cinerea. Mutants that lack this ER chaperone display overlapping phenotypes to mutants of the NoxA signaling pathway. The protein appears to be involved in all major developmental processes, such as the formation of sclerotia, conidial anastomosis tubes and infection cushions (IC’s) and is needed for full virulence. Moreover, expression analyses and reporter gene studies indicate that BcPdi1 affects the redox homeostasis and unfolded protein response (UPR)-related genes. Besides the close association between BcPdi1 and BcNoxA, interaction studies provide evidence that the ER protein might likewise be involved in Ca²⁺ regulated processes. Finally, we were able to show that the potential key functions of the protein BcPdi1 might be affected by its phosphorylation state.

The Botrytis cinerea PAK kinase BcCla4 mediates morphogenesis, growth and cell cycle regulating processes downstream of BcRac

Rac proteins are involved in a variety of cellular processes. Effector proteins that interact with active Rac convey the GTPase-generated signal to downstream developmental cascades and processes. Here we report on the analysis of the main effector and signal cascade downstream of BcRac, the Rac homolog of the grey mold fungus Botrytis cinerea. Several lines of evidence highlighted the p21-activated kinase Cla4 as an important effector of Rac in fungi. Analysis of Δbccla4 strains revealed that the BcCla4 protein was sufficient to mediate all of the examined BcRac-driven processes, including hyphal growth and morphogenesis, conidia production and pathogenicity. In addition, the Δbccla4 strains had altered nuclei content, a phenomenon that was previously observed in Δbcrac isolates, thus connecting the BcRac/BcCla4 module with cell cycle control. Further analyses revealed that BcRac/BcCla4 control mitotic entry through changes in phosphorylation status of the cyclin dependent kinase BcCdk1. The complete cascade includes the kinase BcWee1, which is downstream of BcCla4 and upstream of BcCdk1. These results provide a mechanistic insight on the connection of cell cycle, morphogenesis and pathogenicity in fungi, and position BcCla4 as the most essential effector and central regulator of all of these processes downstream of BcRac.

The septin protein Sep4 facilitates host infection by plant fungal pathogens via mediating initiation of infection structure formation

Many phytopathogenic fungi use infection structures (IFSs, i.e., appressoria and infection cushions) to penetrate host cuticles. However, the conserved mechanisms that mediate initiation of IFS formation in divergent pathogens upon sensing the presence of host plants remain obscure. Here, we demonstrate that a conserved septin gene SEP4 plays crucial roles in this process. Disruption of SEP4 in the plant grey mould fungus Botrytis cinerea completely blocked IFS formation and abolished the virulence of ΔBcsep4 mutants on unwounded hosts. During IFS formation, mutants lacking SEP4 could produce reactive oxygen species (ROS) normally. Inhibition of ROS production in strains harbouring the SEP4 gene resulted in disordered assembly of Sep4 and the subsequent failure to form infection cushions, suggesting that proper Sep4 assembly regulated by ROS is required for initiation of IFS formation and infection. Moreover, loss of SEP4 severely impaired mutant conidiation, melanin and chitin accumulation in hyphal tips and lesion expansion on wounded hosts, but significantly promoted germ tube elongation and sclerotium production. SEP4-mediated fungal pathogenic development, including IFS formation, was validated in the hemibiotroph Magnaporthe oryzae. Our findings indicate that Sep4 plays pleiotropic roles in B. cinerea development and specifically facilities host infection by mediating initiation of IFS formation in divergent plant fungal pathogens in response to ROS signaling.

Update on Nox function, site of action and regulation in Botrytis cinerea

Background

The production of reactive oxygen species (ROS) and a balanced redox homeostasis are essential parameters, which control the infection process of the plant pathogen Botrytis cinerea. The necrotrophic fungus is able to cope with the plants’ oxidative burst and even produces its own ROS to overcome the plants’ defense barrier. Major enzyme complexes, which are responsible for the production of superoxide, are NADPH oxidase (Nox) complexes. They play a central role in various growth, differentiation and pathogenic processes. However, information about their regulation and the integration in the complex signaling network of filamentous fungi is still scarce.

Results

In this work, we give an update on Nox structure, function, site of action and regulation. We show that functionality of the catalytic Nox-subunits seems to be independent from their transcriptional regulation and that the membrane orientation of BcNoxA would allow electron transport inside the ER. Following previous studies, which provided evidence for distinct functions of the NoxA complex inside the ER, we highlight in this work that the N-terminus of BcNoxA is essential for these functions. Finally, we elucidate the role of BcNoxD and BcNoxB inside the ER by complementing the deletion mutants with ER bound alleles.

Conclusions

This study provides a deeper analysis of the Nox complexes in B. cinerea. Besides new insights in the overall regulation of the complexes, we provide further evidence that the NoxA complex has a predominant role inside the ER, while the NoxB complex is mainly important outside the ER, likely at the plasma membrane. By considering all other putative Nox complex members, we propose a putative model, which describes the distinct complex pattern upon certain differentiation processes.

Electronic supplementary material

The online version of this article (doi:10.1186/s40694-016-0026-6) contains supplementary material, which is available to authorized users.

Colletotrichum higginsianum Mitogen-Activated Protein Kinase ChMK1: Role in Growth, Cell Wall Integrity, Colony Melanization, and Pathogenicity

Colletotrichum higginsianum is an economically important pathogen that causes anthracnose disease in a wide range of cruciferous crops. To facilitate the efficient control of anthracnose disease, it will be important to understand the mechanism by which the cruciferous crops and C. higginsianum interact. A key step in understanding this interaction is characterizing the mitogen-activated protein kinases (MAPK) signaling pathway of C. higginsianum. MAPK plays important roles in diverse physiological processes of multiple pathogens. In this study, a Fus3/Kss1-related MAPK gene, ChMK1, from C. higginsianum was analyzed. The results showed that the Fus3/Kss1-related MAPK ChMK1 plays a significant role in cell wall integrity. Targeted deletion of ChMK1 resulted in a hypersensitivity to cell wall inhibitors, reduced conidiation and albinistic colonies. Further, the deletion mutant was also unable to form melanized appressorium, a specialized infection structure that is necessary for successful infection. Therefore, the deletion mutant loses pathogenicity on A. thaliana leaves, demonstrating that ChMK1 plays an essential role in the early infection step. In addition, the ChMK1 deletion mutant showed an attenuated growth rate that is different from that of its homolog in Colletotrichum lagenarium, indicating the diverse roles that Fus3/Kss1-related MAPKs plays in phytopathogenic fungi. Furthermore, the expression level of three melanin synthesis associated genes were clearly decreased in the albinistic ChMK1 mutant compared to that of the wild type strain, suggesting that ChMK1 is also required for colony melanization in C. higginsianum.

Reactive oxygen species in development and infection processes

Reactive oxygen species (ROS) are important signaling molecules that affect vegetative and pathogenic processes in pathogenic fungi. There is growing evidence that ROS are not only secreted during the interaction of host and pathogen but also involved in tightly controlled intracellular processes. The major ROS producing enzymes are NADPH oxidases (Nox). Recent investigations in fungi revealed that Nox-activity is responsible for the formation of infection structures, cytoskeleton architecture as well as interhyphal communication. However, information about the localization and site of action of the Nox complexes in fungi is limited and signaling pathways and intracellular processes affected by ROS have not been fully elucidated. This review focuses on the role of ROS as signaling molecules in fungal "model" organisms: it examines the role of ROS in vegetative and pathogenic processes and gives special attention to Nox complexes and their function as important signaling hubs.