Identification of NoxD/Pro41 as the homologue of the p22phox NADPH oxidase subunit in fungi

Evolutionary and structural analyses of the NADPH oxidase family in eukaryotes reveal an initial calcium dependency

Reactive oxygen species are unstable molecules generated by the partial reduction of dioxygen. NADPH oxidases are a ubiquitous family of enzymes devoted to ROS production. They fuel an array of physiological roles in different species and are chemically demanding enzymes requiring FAD, NADPH and heme prosthetic groups in addition to either calcium or a various number of cytosolic mediators for activity. These activating partners are exclusive components that partition and distinguish the NOX members from one another. To gain insight into the evolution of these activating mechanisms, and in general in their evolutionary history, we conducted an in-depth phylogenetic analysis of the NADPH oxidase family in eukaryotes. We show that all characterized NOXs share a common ancestor, which comprised a fully formed catalytic unit. Regarding the activation mode, we identified calcium-dependency as the earliest form of NOX regulation. The protein-protein mode of regulation would have evolved more recently by gene-duplication with the concomitant loss of the EF-hands motif region. These more recent events generated the diversely activated NOX systems as observed in extant animals and fungi.

ClNOX1/ClNOXR-mediated MAPK and cAMP-PKA signalling pathways and ROS metabolism are involved in Curvularia lunata sexual reproduction and host infection

NADPH oxidases (NOXs) and hydrogen peroxide (H₂ O₂ ) are involved in physiological and pathological processes, and cell fate decisions in organisms. However, regulatory mechanism of NOXs and the role of H₂ O₂ on fungal sexual reproduction and host infection remain largely unexplored. Here, we identified ROS metabolic genes and key signalling genes of MAPK and cAMP-PKA pathways in Curvularia lunata, which were NOX ClNOX1 and ClNOXR, superoxide dismutase ClSOD1 and catalase ClCAT4, redox-regulated transcription factor ClAP1, Ras small GTPases Clg2P, pheromone-response MAPK ClK1 and cAMP-PKA ClSCHA, and characterized the functions of these genes. The results showed that ClNOX1 localized to the plasma membrane. ClNOX1 and ClNOXR were involved in sexual reproduction and host infection via ClNOX1/ClNOXR-derived H₂ O₂ as well as MAPK and cAMP-PKA signalling pathways. H₂ O₂ acted as a signalling molecule to regulate sexual reproduction and host infection in C. lunata.

A gene cluster with positive and negative elements controls bistability and hysteresis of the Crippled versus Normal growth in the fungus Podospora anserina

The Crippled Growth (CG) cell degeneration of the model ascomycete Podospora anserina (strain S) is controlled by a prion-like element and has been linked to the self-activation of the PaMpk1 MAP kinase cascade. Here, we report on the identification of the "86-11" locus containing twelve genes, ten of which are involved either in setting up the self-activation loop of CG or in inhibiting this loop, as demonstrated by targeted gene deletion. Interestingly, deletion of the whole locus results only in the elimination of CG and in no detectable additional physiological defect. Sequence comparison shows that these ten genes belong to four different families, each one endowed with a specific activity: two encode factors activating the loop, a third one encodes a factor crucial for inhibition of the loop and the fourth one participates in inhibiting the loop in a pathway parallel to the one controlled by the previously described PDC1 gene. Intriguingly, a very distant homologue of this "86-11" locus is present at the syntenic position in Podospora comata (strain T) that do not present Crippled Growth. Introgression of the P. comata strain T locus in P. anserina strain S and the P. anserina strain S in P. comata strain T showed that both drive CG in the P. anserina strain S genetic background, but not in the genetic background of strain P. comata T, indicating that genetic determinants outside the twelve-gene locus are responsible for lack of CG in P. comata strain T. Our data question the role of this twelve-gene locus in the physiology of P. anserina.

NADPH Oxidase Gene, FgNoxD, Plays a Critical Role in Development and Virulence in Fusarium graminearum

NADPH oxidase is an enzyme that generates reactive oxygen species from oxygen and NADPH and is highly conserved in eukaryotes. In Fusarium graminearum, a series of different Nox enzymes have been identified. NoxA is involved in sexual development and ascospore production and, like NoxB, also contributes to pathogenicity. Both NoxA and NoxB are regulated by the subunit NoxR, whereas NoxC is usually self-regulated by EF-hand motifs found on the enzyme. In this study, we characterized another NADPH oxidase in F. graminearum, FgNoxD. In the FgNoxD deletion mutant, vegetative growth and conidia production were reduced, while sexual development was totally abolished. The FgNoxD deletion mutant also showed reduced resistance to cell wall perturbing agents; cell membrane inhibitors; and osmotic, fungicide, cold, and extracellular oxidative stress, when compared to the wild type. Moreover, in comparison to the wild type, the FgNoxD deletion mutant exhibited reduced virulence against the host plant. The FgNoxD deletion mutant produced less deoxynivalenol than the wild type, and the Tri5 and Tri6 gene expression was also downregulated. In conclusion, our findings show that FgNoxD is involved in the survival against various stresses, conidiation, sexual development, and virulence, highlighting this enzyme as a new target to control the disease caused by F. graminearum.

Lifespan Increase of Podospora anserina by Oleic Acid Is Linked to Alterations in Energy Metabolism, Membrane Trafficking and Autophagy

The maintenance of cellular homeostasis over time is essential to avoid the degeneration of biological systems leading to aging and disease. Several interconnected pathways are active in this kind of quality control. One of them is autophagy, the vacuolar degradation of cellular components. The absence of the sorting nexin PaATG24 (SNX4 in other organisms) has been demonstrated to result in impairments in different types of autophagy and lead to a shortened lifespan. In addition, the growth rate and the size of vacuoles are strongly reduced. Here, we report how an oleic acid diet leads to longevity of the wild type and a PaAtg24 deletion mutant (ΔPaAtg24). The lifespan extension is linked to altered membrane trafficking, which abrogates the observed autophagy defects in ΔPaAtg24 by restoring vacuole size and the proper localization of SNARE protein PaSNC1. In addition, an oleic acid diet leads to an altered use of the mitochondrial respiratory chain: complex I and II are bypassed, leading to reduced reactive oxygen species (ROS) production. Overall, our study uncovers multiple effects of an oleic acid diet, which extends the lifespan of P. anserina and provides perspectives to explain the positive nutritional effects on human aging.

Cdc42 regulates reactive oxygen species production in the pathogenic yeast Candida albicans

Across eukaryotes, Rho GTPases such as Rac and Cdc42 play important roles in establishing cell polarity, which is a key feature of cell growth. In mammals and filamentous fungi, Rac targets large protein complexes containing NADPH oxidases (NOX) that produce reactive oxygen species (ROS). In comparison, Rho GTPases of unicellular eukaryotes were believed to signal cell polarity without ROS, and it was unclear whether Rho GTPases were required for ROS production in these organisms. We document here the first example of Rho GTPase-mediated post-transcriptional control of ROS in a unicellular microbe. Specifically, Cdc42 is required for ROS production by the NOX Fre8 of the opportunistic fungal pathogen Candida albicans. During morphogenesis to a hyphal form, a filamentous growth state, C. albicans FRE8 mRNA is induced, which leads to a burst in ROS. Fre8-ROS is also induced during morphogenesis when FRE8 is driven by an ectopic promoter; hence, Fre8 ROS production is in addition controlled at the post-transcriptional level. Using fluorescently tagged Fre8, we observe that the majority of the protein is associated with the vacuolar system. Interestingly, much of Fre8 in the vacuolar system appears inactive, and Fre8-induced ROS is only produced at sites near the hyphal tip, where Cdc42 is also localized during morphogenesis. We observe that Cdc42 is necessary to activate Fre8-mediated ROS production during morphogenesis. Cdc42 regulation of Fre8 occurs without the large NOX protein complexes typical of higher eukaryotes and therefore represents a novel form of ROS control by Rho GTPases.

Surviving the odds: From perception to survival of fungal phytopathogens under host-generated oxidative burst

Fungal phytopathogens pose a serious threat to global crop production. Only a handful of strategies are available to combat these fungal infections, and the increasing incidence of fungicide resistance is making the situation worse. Hence, the molecular understanding of plant-fungus interactions remains a primary focus of plant pathology. One of the hallmarks of host-pathogen interactions is the overproduction of reactive oxygen species (ROS) as a plant defense mechanism, collectively termed the oxidative burst. In general, high accumulation of ROS restricts the growth of pathogenic organisms by causing localized cell death around the site of infection. To survive the oxidative burst and achieve successful host colonization, fungal phytopathogens employ intricate mechanisms for ROS perception, ROS neutralization, and protection from ROS-mediated damage. Together, these countermeasures maintain the physiological redox homeostasis that is essential for cell viability. In addition to intracellular antioxidant systems, phytopathogenic fungi also deploy interesting effector-mediated mechanisms for extracellular ROS modulation. This aspect of plant-pathogen interactions is significantly under-studied and provides enormous scope for future research. These adaptive responses, broadly categorized into "escape" and "exploitation" mechanisms, are poorly understood. In this review, we discuss the oxidative stress response of filamentous fungi, their perception signaling, and recent insights that provide a comprehensive understanding of the distinct survival mechanisms of fungal pathogens in response to the host-generated oxidative burst.

Reactive oxygen species: A generalist in regulating development and pathogenicity of phytopathogenic fungi

Reactive oxygen species (ROS) are small molecules with high oxidative activity, and are usually produced as byproducts of metabolic processes in organisms. ROS play an important role during the interaction between plant hosts and pathogenic fungi. Phytopathogenic fungi have evolved sophisticated ROS producing and scavenging systems to achieve redox homeostasis. Emerging evidences suggest that ROS derived from fungi are involved in various important aspects of the development and pathogenesis, including formation of conidia, sclerotia, conidial anastomosis tubes (CATs) and infectious structures. In this mini-review, we summarize the research progress on the redox homeostasis systems, the versatile functions of ROS in the development and pathogenesis of phytopathogenic fungi, and the regulation effects of exogenous factors on intercellular ROS and virulence of the fungal pathogens.

NADPH Oxidase ClNOX2 Regulates Melanin-Mediated Development and Virulence in Curvularia lunata

The role of NADPH oxidases (NOXs) in pathogenesis and development in the Curvularia leaf spot agent Curvularia lunata remains poorly understood. In this study, we identified C. lunata ClNOX2, which localized to the plasma membrane and was responsible for reactive oxygen species (ROS) generation. Scavenging the ROS production inhibited the conidial germination and appressorial formation. The ClNOX2 and ClBRN1 deletion mutants were defective in 1,8-dihydroxynaphthalene (DHN) melanin accumulation, appressorial formation, and cellulase synthesis and exhibited lower virulence. However, disruption of the ClNOX2 and ClBRN1 genes facilitated hyphal growth, enhanced stress adaptation to cell-wall-disrupting agents, and promoted developmental processes such as conidiation, conidial germination, and pseudothecium and ascus formation. Interestingly, loss of ClM1, the cell wall integrity (CWI) mitogen-activated protein kinase gene in C. lunata, led to morphology and pathogenicity phenotypes similar to ClNOX2 and ClBRN1 deletion mutants such as abnormal conidia, fewer appressoria, less melanin, increased hyphal growth, and enhanced tolerance to Congo red (CR). These results indicated that the ClNOX2 gene plays an important role in C. lunata development and virulence via regulating intracellular DHN melanin biosynthesis. Quantitative reverse-transcription PCR revealed that the ClNOX2-related ROS signaling pathway and ClM1-mediated CWI signaling pathway are cross-linked in regulating DHN melanin biosynthesis. Our findings provide new insights into how ClNOX2 participates in pathogenesis and development in hemibiotrophic plant fungal pathogens.[Formula: see text] Copyright © 2020 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Sordaria macrospora: 25 years as a model organism for studying the molecular mechanisms of fruiting body development

Abstract

Fruiting bodies are among the most complex multicellular structures formed by fungi, and the molecular mechanisms that regulate their development are far from understood. However, studies with a number of fungal model organisms have started to shed light on this developmental process. One of these model organisms is Sordaria macrospora, a filamentous ascomycete from the order Sordariales. This fungus has been a genetic model organism since the 1950s, but its career as a model organism for molecular genetics really took off in the 1990s, when the establishment of a transformation protocol, a mutant collection, and an indexed cosmid library provided the methods and resources to start revealing the molecular mechanisms of fruiting body development. In the 2000s, “omics” methods were added to the S. macrospora tool box, and by 2020, 58 developmental genes have been identified in this fungus. This review gives a brief overview of major method developments for S. macrospora, and then focuses on recent results characterizing different processes involved in regulating development including several regulatory protein complexes, autophagy, transcriptional and chromatin regulation, and RNA editing.

Key points

•Sordaria macrospora is a model system for analyzing fungal fruiting body development.

•More than 100 developmental mutants are available for S. macrospora.

•More than 50 developmental genes have been characterized in S. macrospora.

NADPH oxidase regulates chemotropic growth of the fungal pathogen Fusarium oxysporum towards the host plant

Soil-inhabiting fungal pathogens use chemical signals to locate and colonise the host plant. In the vascular wilt fungus Fusarium oxysporum, hyphal chemotropism towards tomato roots is triggered by secreted plant peroxidases (Prx), which catalyse the reductive cleavage of reactive oxygen species (ROS). Here we show that this chemotropic response requires the regulated synthesis of ROS by the conserved fungal NADPH oxidase B (NoxB) complex, and their transformation into hydrogen peroxide (H₂ O₂ ) by superoxide dismutase (SOD). Deletion of NoxB or the regulatory subunit NoxR, or pharmacological inhibition of SOD, specifically abolished chemotropism of F. oxysporum towards Prx gradients. Addition of isotropic concentrations of H₂ O₂ rescued chemotropic growth in the noxBΔ and noxRΔ mutants, but not in a mutant lacking the G protein-coupled receptor Ste2. Prx-triggered rapid Nox- and Ste2-dependent phosphorylation of the cell wall integrity mitogen-activated protein kinase (CWI MAPK) Mpk1, an essential component of the chemotropic response. These results suggest that Ste2 and the CWI MAPK cascade function downstream of NoxB in Prx chemosensing. Our findings reveal a new role for Nox enzymes in directed hyphal growth of a filamentous pathogen towards its host and might be of broad interest for chemotropic interactions between plants and root-colonising fungi.

The NADPH oxidase in Volvariella volvacea and its differential expression in response to mycelial ageing and mechanical injury

NADPH oxidases are enzymes that have been reported to generate reactive oxygen species (ROS) in animals, plants and many multicellular fungi in response to environmental stresses. Six genes of the NADPH oxidase complex components, including vvnoxa, vvnoxb, vvnoxr, vvbema, vvrac1 and vvcdc24, were identified based on the complete genomic sequence of the edible fungus Volvariella volvacea. The number of vvnoxa, vvrac1, vvbema and vvcdc24 transcripts fluctuated with ageing, and the gene expression patterns of vvnoxa, vvrac1 and vvbema were significantly positively correlated. However, the expression of vvnoxb and vvnoxr showed no significant difference during ageing. In hyphae subjected to mechanical injury stress, both O₂^- and H₂O₂ concentrations were increased. The expression of vvnoxa, vvrac1, vvbema and vvcdc24 was substantially upregulated, but vvnoxb and vvnoxr showed no response to mechanical injury stress at the transcriptional level. Additionally, the transcription of vvnoxa, vvrac1, vvbema and vvcdc24 could be repressed when the intracellular ROS were eliminated by diphenyleneiodonium (DPI) chloride and reduced glutathione (GSH) treatments. These results indicated a positive feedback loop involving NADPH oxidase and intracellular ROS, which might be the reason for the oxidative burst during injury stress.

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.

Neurospora crassa NADPH Oxidase NOX-1 Is Localized in the Vacuolar System and the Plasma Membrane

The NADPH oxidases (NOX) catalyze the production of superoxide by transferring electrons from NADPH to O₂, in a regulated manner. In Neurospora crassa NOX-1 is required for normal growth of hyphae, development of aerial mycelium and asexual spores, and it is essential for sexual differentiation and cell-cell fusion. Determining the subcellular localization of NOX-1 is a critical step in understanding the mechanisms by which this enzyme can regulate all these different processes. Using fully functional versions of NOX-1 tagged with mCherry, we show that in growing hyphae NOX-1 shows only a minor association with the endoplasmic reticulum (ER) markers Ca²⁺-ATPase NCA-1 and an ER lumen-targeted GFP. Likewise, NOX-1 shows minor co-localization with early endosomes labeled with YPT-52, a GTPase of the Rab5 family. In contrast, NOX-1 shows extensive co-localization with two independent markers of the entire vacuolar system; the vacuolar ATPase subunit VMA-1 and the fluorescent molecule carboxy-DFFDA. In addition, part of NOX-1 was detected at the plasma membrane. The NOX-1 regulatory subunit NOR-1 displays a very different pattern of localization, showing a fine granular distribution along the entire hypha and some accumulation at the hyphal tip. In older hyphal regions, germinating conidia, and conidiophores it forms larger and discrete puncta some of which appear localized at the plasma membrane and septa. Notably, co-localization of NOX-1 and NOR-1 was mainly observed under conidial cell-cell fusion conditions in discrete vesicular structures. NOX functions in fungi have been evaluated mainly in mutants that completely lacked this protein, also eliminating interactions between hyphal growth regulatory proteins NOR-1, the GTPase RAC-1 and the scaffold protein BEM-1. To dissect NOX-1 roles as scaffold and as ROS-producing enzyme, we analyzed the function of NOX-1::mCherry proteins carrying proline 382 by histidine (P382H) or cysteine 524 by arginine (C524R) substitutions, predicted to only affect NADPH-binding. Without notably affecting NOX-1 localization or protein levels, each of these substitutions resulted in lack of function phenotypes, indicating that NOX-1 multiple functions are all dependent on its oxidase activity. Our results open new interpretations to possible NOX functions, as components of the fungal vacuolar system and the plasma membrane, as well as to new vacuolar functions.

The mitochondrial translocase of the inner membrane PaTim54 is involved in defense response and longevity in Podospora anserina

Fungi are very successful microorganisms capable of colonizing virtually any ecological niche where they must constantly cope with competitors including fungi, bacteria and nematodes. We have shown previously that the ascomycete Podopora anserina exhibits Hyphal Interference (HI), an antagonistic response triggered by direct contact of competing fungal hyphae. When challenged with Penicillium chrysogenum, P. anserina produces hydrogen peroxide at the confrontation and kills the hyphae of P. chrysogenum. Here, we report the characterization of the PDC²²¹⁸ mutant affected in HI. When challenged with P. chrysogenum, the PDC²²¹⁸ mutant produces a massive oxidative burst at the confrontation. However, this increased production of hydrogen peroxide is not correlated to increased cell death in P. chrysogenum. Hence, the oxidative burst and cell death in the challenger are uncoupled in PDC²²¹⁸. The gene affected in PDC²²¹⁸ is PaTim54, encoding the homologue of the budding yeast mitochondrial inner membrane import machinery component Tim54p. We show that PaTim54 is essential in P. anserina and that the phenotypes displayed by the PDC²²¹⁸ mutant, renamed PaTim54²²¹⁸, are the consequence of a drastic reduction in the expression of PaTim54. Among these pleiotropic phenotypes, PDC²²¹⁸-PaTim54²²¹⁸- displays increased lifespan, a phenotype in line with the observed mitochondrial defects in the mutant.

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.

Production, Signaling, and Scavenging Mechanisms of Reactive Oxygen Species in Fruit-Pathogen Interactions

Reactive oxygen species (ROS) play a dual role in fruit–pathogen interaction, which largely depends on their different levels in cells. Fruit recognition of a pathogen immediately triggers an oxidative burst that is considered an integral part of the fruit defense response. ROS are also necessary for the virulence of pathogenic fungi. However, the accumulation of ROS in cells causes molecular damage and finally leads to cell death. In this review, on the basis of data regarding ROS production and the scavenging systems determining ROS homeostasis, we focus on the role of ROS in fruit defense reactions against pathogens and in fungi pathogenicity during fruit–pathogen interaction.

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.

Phenotypic instability in fungi

Fungi are prone to phenotypic instability, that is, the vegetative phase of these organisms, be they yeasts or molds, undergoes frequent switching between two or more behaviors, often with different morphologies, but also sometime having different physiologies without any obvious morphological outcome. In the context of industrial utilization of fungi, this can have a negative impact on the maintenance of strains and/or on their productivity. Instabilities have been shown to result from various mechanisms, either genetic or epigenetic. This chapter will review different types of instabilities and discuss some lesser-known ones, mostly in filamentous fungi, while it will direct readers to additional literature in the case of well-known phenomena such as the amyloid prions or fungal senescence. It will present in depth the "white/opaque" switch of Candida albicans and the "crippled growth" degeneration of the model fungus Podospora anserina. These are two of the most thoroughly studied epigenetic phenotypic switches. I will also discuss the "sectors" presented by many filamentous ascomycetes, for which a prion-based model exists but is not demonstrated. Finally, I will also describe intriguing examples of phenotypic instability for which an explanation has yet to be provided.

Identification and characterization of PDC1, a novel protein involved in the epigenetic cell degeneration Crippled Growth in Podospora anserina

The model fungus Podospora anserina exhibits Crippled Growth (CG), a cell degeneration process linked to the spreading of a prion-like hereditary element. Previous work has shown that the PaMpk1 MAP kinase and the PaNox1 NADPH oxidase are key player in setting up CG. Here, we identified PDC1, a new gene that negatively regulates the PaMpk1 pathway, by identifying the gene mutated in the PDC²²⁰⁵ mutant. This mutant exhibits strong CG in conditions where the wild-type does not. PDC1 encodes a small protein conserved in other Pezizomycotina. The protein contains four evolutionary-conserved cysteines, a tryptophan and a histidine; all six amino-acid are essential for function. PDC1 is located in the cytosol and is present in lower amounts in stationary hyphae in accordance with its role as a repressor. Epistasis analyses place PDC1 between PaMpk1 and PaNox1.

PaPro1 and IDC4, Two Genes Controlling Stationary Phase, Sexual Development and Cell Degeneration in Podospora anserina

Filamentous fungi frequently undergo bistable phenotypic switches. Crippled Growth of Podospora anserina is one such bistable switch, which seems to rely upon the mis-activation of a self-regulated PaMpk1 MAP kinase regulatory pathway. Here, we identify two new partners of this pathway: PaPro1, a transcription factor orthologous to Sordaria macrospora pro1 and Neurospora crassa ADV-1, and IDC4, a protein with an AIM24 domain. Both PaPro1 and IDC4 regulate stationary phase features, as described for the other actors of the PaMpk1 signaling pathway. However, PaPro1 is also involved in the control of fertilization by activating the transcription of the HMG8 and the mating type transcription factors, as well as the sexual pheromones and receptor genes. The roles of two components of the STRIPAK complex were also investigated by inactivating their encoding genes: PaPro22 and PaPro45. The mutants of these genes were found to have the same phenotypes as PaPro1 and IDC4 mutants as well as additional phenotypes including slow growth, abnormally shaped hyphae, pigment accumulation and blockage of the zygotic tissue development, indicating that the STRIPAK complex regulates, in addition to the PaMpk1 one, other pathways in P. anserina. Overall, the mutants of these four genes confirm the model by which Crippled Growth is due to the abnormal activation of the PaMpk1 MAP kinase cascade.

Two closely related Rho GTPases, Cdc42 and RacA, of the en-dophytic fungus Epichloë festucae have contrasting roles for ROS production and symbiotic infection synchronized with the host plant

Epichloë festucae is an endophytic fungus which systemically colonizes temperate grasses to establish symbiotic associations. Maintaining symptomless infection is a key requirement for endophytes, a feature that distinguishes them from pathogenic fungi. While pathogenic fungi extend their hyphae by tip growth, hyphae of E. festucae systemically colonize the intercellular space of expanding host leaves via a unique mechanism of hyphal intercalary growth. This study reports that two homologous Rho GTPases, Cdc42 and RacA, have distinctive roles in the regulation of E. festucae growth in planta. Here we highlight the vital role of Cdc42 for intercalary hyphal growth, as well as involvement of RacA in regulation of hyphal network formation, and demonstrate the consequences of mutations in these genes on plant tissue infection. Functions of Cdc42 and RacA are mediated via interactions with BemA and NoxR respectively, which are expected components of the ROS producing NOX complex. Symbiotic defects found in the racA mutant were rescued by introduction of a Cdc42 with key amino acids substitutions crucial for RacA function, highlighting the significance of the specific interactions of these GTPases with BemA and NoxR for their functional differentiation in symbiotic infection.

A Cellular Fusion Cascade Regulated by LaeA Is Required for Sclerotial Development in Aspergillus flavus

Aspergillus flavus is a saprophytic soil fungus that poses a serious threat worldwide as it contaminates many food and feed crops with the carcinogenic mycotoxin called aflatoxin. This pathogen persists as sclerotia in the soil which enables fungal survival in harsh environmental conditions. Sclerotia formation by A. flavus depends on successful cell communication and hyphal fusion events. Loss of LaeA, a conserved developmental regulator in fungi, abolishes sclerotia formation in this species whereas overexpression (OE) of laeA results in enhanced sclerotia production. Here we demonstrate that sclerotia loss and inability to form heterokaryons in A. flavusΔlaeA is mediated by homologs of the Neurospora crassa ham (hyphal anastomosis) genes termed hamE-I in A. flavus. LaeA positively regulates ham gene expression and deletion of hamF, G, H, or I phenocopies ΔlaeA as demonstrated by heterokaryon and sclerotia loss and reduced aflatoxin synthesis and virulence of these mutants. Deletion of hamE showed a less severe phenotype. hamE-I homologs are positively regulated by the clock controlled transcription factor ADV-1 in N. crassa. Similarly, the ADV-1 homolog NosA regulates hamE-I expression in A. flavus, is required for sclerotial development and is itself positively regulated by LaeA. We speculate that a putative LaeA>NosA>fusion cascade underlies the previously described circadian clock regulation of sclerotia production in A. flavus.

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.

Innovation and constraint leading to complex multicellularity in the Ascomycota

The advent of complex multicellularity (CM) was a pivotal event in the evolution of animals, plants and fungi. In the fungal Ascomycota, CM is based on hyphal filaments and arose in the Pezizomycotina. The genus Neolecta defines an enigma: phylogenetically placed in a related group containing mostly yeasts, Neolecta nevertheless possesses Pezizomycotina-like CM. Here we sequence the Neolecta irregularis genome and identify CM-associated functions by searching for genes conserved in Neolecta and the Pezizomycotina, which are absent or divergent in budding or fission yeasts. This group of 1,050 genes is enriched for functions related to diverse endomembrane systems and their organization. Remarkably, most show evidence for divergence in both yeasts. Using functional genomics, we identify new genes involved in fungal complexification. Together, these data show that rudimentary multicellularity is deeply rooted in the Ascomycota. Extensive parallel gene divergence during simplification and constraint leading to CM suggest a deterministic process where shared modes of cellular organization select for similarly configured organelle- and transport-related machineries.

The fungal Ascomycota provide a model phylum to investigate the evolution of complex multicellularity. Here, the authors combine genome sequencing with comparative and functional genomics to identify diverse endomembrane related machineries associated with the gain and loss of fungal complexity.

The Neurospora Transcription Factor ADV-1 Transduces Light Signals and Temporal Information to Control Rhythmic Expression of Genes Involved in Cell Fusion

Light and the circadian clock have a profound effect on the biology of organisms through the regulation of large sets of genes. Toward understanding how light and the circadian clock regulate gene expression, we used genome-wide approaches to identify the direct and indirect targets of the light-responsive and clock-controlled transcription factor ADV-1 in Neurospora crassa A large proportion of ADV-1 targets were found to be light- and/or clock-controlled, and enriched for genes involved in development, metabolism, cell growth, and cell fusion. We show that ADV-1 is necessary for transducing light and/or temporal information to its immediate downstream targets, including controlling rhythms in genes critical to somatic cell fusion. However, while ADV-1 targets are altered in predictable ways in Δadv-1 cells in response to light, this is not always the case for rhythmic target gene expression. These data suggest that a complex regulatory network downstream of ADV-1 functions to generate distinct temporal dynamics of target gene expression relative to the central clock mechanism.

IDC2 and IDC3, two genes involved in cell non-autonomous signaling of fruiting body development in the model fungus Podospora anserina

Filamentous ascomycetes produce complex multicellular structures during sexual reproduction. Little is known about the genetic pathways enabling the construction of such structures. Here, with a combination of classical and reverse genetic methods, as well as genetic mosaic and graft analyses, we identify and provide evidence for key roles for two genes during the formation of perithecia, the sexual fruiting bodies, of the filamentous fungus Podospora anserina. Data indicate that the proteins coded by these two genes function cell-non-autonomously and that their activity depends upon conserved cysteines, making them good candidate for being involved in the transmission of a reactive oxygen species (ROS) signal generated by the PaNox1 NADPH oxidase inside the maturing fruiting body towards the PaMpk1 MAP kinase, which is located inside the underlying mycelium, in which nutrients are stored. These data provide important new insights to our understanding of how fungi build multicellular structures.

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.

Transcription factor PRO1 targets genes encoding conserved components of fungal developmental signaling pathways

The filamentous fungus Sordaria macrospora is a model system to study multicellular development during fruiting body formation. Previously, we demonstrated that this major process in the sexual life cycle is controlled by the Zn(II)₂ Cys₆ zinc cluster transcription factor PRO1. Here, we further investigated the genome-wide regulatory network controlled by PRO1 by employing chromatin immunoprecipitation combined with next-generation sequencing (ChIP-seq) to identify binding sites for PRO1. We identified several target regions that occur in the promoter regions of genes encoding components of diverse signaling pathways. Furthermore, we identified a conserved DNA-binding motif that is bound specifically by PRO1 in vitro. In addition, PRO1 controls in vivo the expression of a DsRed reporter gene under the control of the esdC target gene promoter. Our ChIP-seq data suggest that PRO1 also controls target genes previously shown to be involved in regulating the pathways controlling cell wall integrity, NADPH oxidase and pheromone signaling. Our data point to PRO1 acting as a master regulator of genes for signaling components that comprise a developmental cascade controlling fruiting body formation.

Hyphopodium-Specific VdNoxB/VdPls1-Dependent ROS-Ca2+ Signaling Is Required for Plant Infection by Verticillium dahliae

Verticillium dahliae is a phytopathogenic fungus obligate in root infection. A few hyphopodia differentiate from large numbers of hyphae after conidia germination on the root surface for further infection. However, the molecular features and role of hyphopodia in the pathogenicity of V. dahliae remain elusive. In this study, we found that the VdPls1, a tetraspanin, and the VdNoxB, a catalytic subunit of membrane-bound NADPH oxidases for reactive oxygen species (ROS) production, were specifically expressed in hyphopodia. VdPls1 and VdNoxB highly co-localize with the plasma membrane at the base of hyphopodia, where ROS and penetration pegs are generated. Mutant strains, VdΔnoxb and VdΔpls1, in which VdPls1 and VdNoxB were deleted, respectively, developed defective hyphpodia incapable of producing ROS and penetration pegs. Defective plasma membrane localization of VdNoxB in VdΔpls1 demonstrates that VdPls1 functions as an adaptor protein for the recruitment and activation of the VdNoxB. Furthermore, in VdΔnoxb and VdΔpls1, tip-high Ca2+ accumulation was impaired in hyphopodia, but not in vegetative hyphal tips. Moreover, nuclear targeting of VdCrz1 and activation of calcineurin-Crz1 signaling upon hyphopodium induction in wild-type V. dahliae was impaired in both knockout mutants, indicating that VdPls1/VdNoxB-dependent ROS was specifically required for tip-high Ca2+ elevation in hyphopodia to activate the transcription factor VdCrz1 in the regulation of penetration peg formation. Together with the loss of virulence of VdΔnoxb and VdΔpls1, which are unable to initiate colonization in cotton plants, our data demonstrate that VdNoxB/VdPls1-mediated ROS production activates VdCrz1 signaling through Ca2+ elevation in hyphopodia, infectious structures of V. dahliae, to regulate penetration peg formation during the initial colonization of cotton roots.

The PaPsr1 and PaWhi2 genes are members of the regulatory network that connect stationary phase to mycelium differentiation and reproduction in Podospora anserina

In filamentous fungi, entrance into stationary phase is complex as it is accompanied by several differentiation and developmental processes, including the synthesis of pigments, aerial hyphae, anastomoses and sporophores. The regulatory networks that control these processes are still incompletely known. The analysis of the "Impaired in the development of Crippled Growth (IDC)" mutants of the model filamentous ascomycete Podospora anserina has already yielded important information regarding the pathway regulating entrance into stationary phase. Here, the genes affected in two additional IDC mutants are identified as orthologues of the Saccharomyces cerevisiae WHI2 and PSR1 genes, known to regulate stationary phase in this yeast, arguing for a conserved role of these proteins throughout the evolution of ascomycetes.

Plant biomass degrading ability of the coprophilic ascomycete fungus Podospora anserina

The degradation of plant biomass is a major challenge towards the production of bio-based compounds and materials. As key lignocellulolytic enzyme producers, filamentous fungi represent a promising reservoir to tackle this challenge. Among them, the coprophilous ascomycete Podospora anserina has been used as a model organism to study various biological mechanisms because its genetics are well understood and controlled. In 2008, the sequencing of its genome revealed a great diversity of enzymes targeting plant carbohydrates and lignin. Since then, a large array of lignocellulose-acting enzymes has been characterized and genetic analyses have enabled the understanding of P. anserina metabolism and development on plant biomass. Overall, these research efforts shed light on P. anserina strategy to unlock recalcitrant lignocellulose deconstruction.

NADPH oxidases as electrochemical generators to produce ion fluxes and turgor in fungi, plants and humans

The NOXs are a family of flavocytochromes whose basic structure has been largely conserved from algae to man. This is a very simple system. NADPH is generally available, in plants it is a direct product of photosynthesis, and oxygen is a largely ubiquitous electron acceptor, and the electron-transporting core of an FAD and two haems is the minimal required to pass electrons across the plasma membrane. These NOXs have been shown to be essential for diverse functions throughout the biological world and, lacking a clear mechanism of action, their effects have generally been attributed to free radical reactions. Investigation into the function of neutrophil leucocytes has demonstrated that electron transport through the prototype NOX2 is accompanied by the generation of a charge across the membrane that provides the driving force propelling protons and other ions across the plasma membrane. The contention is that the primary function of the NOXs is to supply the driving force to transport ions, the nature of which will depend upon the composition and characteristics of the local ion channels, to undertake a host of diverse functions. These include the generation of turgor in fungi and plants for the growth of filaments and invasion by appressoria in the former, and extension of pollen tubes and root hairs, and stomatal closure, in the latter. In neutrophils, they elevate the pH in the phagocytic vacuole coupled to other ion fluxes. In endothelial cells of blood vessels, they could alter luminal volume to regulate blood pressure and tissue perfusion.

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.

Chasing stress signals - Exposure to extracellular stimuli differentially affects the redox state of cell compartments in the wild type and signaling mutants of Botrytis cinerea

Reactive oxygen species (ROS) are important molecules influencing intracellular developmental processes as well as plant pathogen interactions. They are produced at the infection site and affect the intracellular redox homeostasis. However, knowledge of ROS signaling pathways, their connection to other signaling cascades, and tools for the visualization of intra- and extracellular ROS levels and their impact on the redox state are scarce. By using the genetically encoded biosensor roGFP2 we studied for the first time the differences between the redox states of the cytosol, the intermembrane space of mitochondria and the ER in the filamentous fungus Botrytis cinerea. We showed that the ratio of oxidized to reduced glutathione inside of the cellular compartments differ and that the addition of hydrogen peroxide (H2O2), calcium chloride (CaCl2) and the fluorescent dye calcofluor white (CFW) have a direct impact on the cellular redox states. Dependent on the type of stress agents applied, the redox states were affected in the different cellular compartments in a temporally shifted manner. By integrating the biosensor in deletion mutants of bcnoxA, bcnoxB, bctrx1 and bcltf1 we further elucidated the putative roles of the different proteins in distinct stress-response pathways. We showed that the redox states of ΔbcnoxA and ΔbcnoxB display a wild-type pattern upon exposure to H2O2, but appear to be strongly affected by CaCl2 and CFW. Moreover, we demonstrated the involvement of the light-responsive transcription factor BcLtf1 in the maintenance of the redox state in the intermembrane space of the mitochondria. Finally, we report that CaCl2 as well as cell wall stress-inducing agents stimulate ROS production and that ΔbcnoxB produces significantly less ROS than the wild type and ΔbcnoxA.

Functional Analysis of Developmentally Regulated Genes chs7 and sec22 in the Ascomycete Sordaria macrospora

During sexual development, filamentous ascomycetes form complex, three-dimensional fruiting bodies for the generation and dispersal of spores. In previous studies, we identified genes with evolutionary conserved expression patterns during fruiting body formation in several fungal species. Here, we present the functional analysis of two developmentally up-regulated genes, chs7 and sec22, in the ascomycete Sordaria macrospora. The genes encode a class VII (division III) chitin synthase and a soluble N-ethylmaleimide-sensitive-factor attachment protein receptor (SNARE) protein, respectively. Deletion mutants of chs7 had normal vegetative growth and were fully fertile but showed sensitivity toward cell wall stress. Deletion of sec22 resulted in a reduced number of ascospores and in defects in ascospore pigmentation and germination, whereas vegetative growth was normal in the mutant. A SEC22-EGFP fusion construct under control of the native sec22 promoter and terminator regions was expressed during different stages of sexual development. Expression of several development-related genes was deregulated in the sec22 mutant, including three genes involved in melanin biosynthesis. Our data indicate that chs7 is dispensable for fruiting body formation in S. macrospora, whereas sec22 is required for ascospore maturation and germination and thus involved in late stages of sexual development.

Conservation of fungal and animal nicotinamide adenine dinucleotide phosphate oxidase complexes

Nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (Nox) are a group of eukaryotic flavoenzymes that catalyse the reduction of dioxygen to the superoxide anion using electrons provided by NADPH. An integral membrane flavocytochrome b558 heterodimer, composed of the catalytic subunit gp91(phox) and the adaptor protein p22(phox), is essential for catalytic activity of the mammalian Nox2 complex. Two homologues of the mammalian gp91(phox), NoxA and NoxB, have been identified in fungi and shown to be crucial for distinct fungal cell differentiation and developmental processes, but to date, no homologue of the p22(phox) adaptor protein has been identified. Isolation of a mutant from Podospora anserina with a phenotype identical to a previously characterised PaNox1 mutant, combined with phylogenetic analysis, identified a fungal homologue of p22(phox) called PaNoxD. The same adaptor protein was shown to be a component of the Botrytis cinerea NoxA complex as supported by the identical phenotypes of the bcnoxA and bcnoxD mutants and direct physical interaction between BcNoxA and BcNoxD. These results suggest that NoxA/NoxD is the fungal equivalent of the mammalian gp91(phox)/p22(phox) flavocytochrome complex. Tetraspanin (Pls1) mutants of P. anserina and B. cinerea have identical phenotypes to noxB mutants, suggesting that Pls1 is the corresponding integral membrane adaptor for assembly of the NoxB complex.

BcNoxD, a putative ER protein, is a new component of the NADPH oxidase complex in Botrytis cinerea

NADPH oxidases (Nox) are major enzymatic producer of reactive oxygen species (ROS). In fungi these multi-enzyme complexes are involved in sexual differentiation and pathogenicity. However, in contrast to mammalian systems, the composition and recruitment of the fungal Nox complexes are unresolved. Here we introduce a new Nox component, the membrane protein NoxD in the grey mold fungus Botrytis cinerea. It has high homology to the ER protein Pro41 from Sordaria macrospora, similar functions to the catalytic Nox subunit BcNoxA in differentiation and pathogenicity, and shows similarities to phagocytic p22phox. BcNoxA and BcNoxD interact with each other. Both proteins are involved in pathogenicity, fusion of conidial anastomosis tubes (CAT) and formation of sclerotia and conidia. These data support our earlier view based on localization studies, for an ER-related function of the Nox complex. We present the first evidence that some functions of the BcNoxA complex are indeed linked to the ER, while others clearly require export from the ER.