The endosomal recycling of FgSnc1 by FgSnx41-FgSnx4 heterodimer is essential for polarized growth and pathogenicity in Fusarium graminearum

Two distinct SNARE complexes mediate vesicle fusion with the plasma membrane to ensure effective development and pathogenesis of Fusarium oxysporum f. sp. cubense

SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) facilitate docking and fusion of vesicles with their target membranes, playing a crucial role in vesicle trafficking and exocytosis. However, the spatial assembly and roles of plasma membrane (PM)-associated SNAREs in phytopathogen development and pathogenicity are not clearly understood. In this study, we analysed the roles and molecular mechanisms of PM-associated SNARE complexes in the banana Fusarium wilt fungus Fusarium oxysporum f. sp. cubense tropical race 4 (FocTR4). Our findings demonstrate that FocSso1 is important for the fungal growth, conidiation, host penetration and colonization. Mechanistically, FocSso1 regulates protein secretion by mediating vesicle docking and fusion with the PM and hyphal apex. Interestingly, a FocSso1-FocSec9-FocSnc1 complex was observed to assemble not only at the fungal PM but also on the growing hyphal apex, facilitating exocytosis. FocSso2, a paralogue of FocSso1, was also found to form a ternary SNARE complex with FocSec9 and FocSnc1, but it mainly localizes to the PM in old hyphae. The functional analysis of this protein demonstrated that it is dispensable for the fungal growth but necessary for host penetration and colonization. The other subunits, FocSec9 and FocSnc1, are involved in the fungal development and facilitate host penetration. Furthermore, FocSso1 and FocSnc1 are functionally interdependent, as loss of FocSso1 leads to mis-sorting and degradation of FocSnc1 in the vacuole and vice versa. Overall, this study provides insight into the formation of two spatially and functionally distinct PM SNARE complexes and their involvement in vesicle exocytosis to regulate development and pathogenicity of FocTR4.

Interplay of transport vesicles during plant-fungal pathogen interaction

Vesicle trafficking is an essential cellular process upon which many physiological processes of eukaryotic cells rely. It is usually the 'language' of communication among the components of the endomembrane system within a cell, between cells and between a cell and its external environment. Generally, cells have the potential to internalize membrane-bound vesicles from external sources by endocytosis. Plants constantly interact with both mutualistic and pathogenic microbes. A large part of this interaction involves the exchange of transport vesicles between the plant cells and the microbes. Usually, in a pathogenic interaction, the pathogen releases vesicles containing bioactive molecules that can modulate the host immunity when absorbed by the host cells. In response to this attack, the host cells similarly mobilize some vesicles containing pathogenesis-related compounds to the pathogen infection site to destroy the pathogen, prevent it from penetrating the host cell or annul its influence. In fact, vesicle trafficking is involved in nearly all the strategies of phytopathogen attack subsequent plant immune responses. However, this field of plant-pathogen interaction is still at its infancy when narrowed down to plant-fungal pathogen interaction in relation to exchange of transport vesicles. Herein, we summarized some recent and novel findings unveiling the involvement of transport vesicles as a crosstalk in plant-fungal phytopathogen interaction, discussed their significance and identified some knowledge gaps to direct future research in the field. The roles of vesicles trafficking in the development of both organisms are also established.

Special Issue "Genomics of Fungal Plant Pathogens"

Plant diseases can be classified according to pathogenic organisms, and 70-80% of them are fungal diseases [...].

Genome-Wide Characterization of the RNA Exosome Complex in Relation to Growth, Development, and Pathogenicity of Fusarium graminearum

The RNA exosome complex is a conserved, multisubunit RNase complex that contributes to the processing and degradation of RNAs in mammalian cells. However, the roles of the RNA exosome in phytopathogenic fungi and how it relates to fungal development and pathogenicity remain unclear. Herein, we identified 12 components of the RNA exosome in the wheat fungal pathogen Fusarium graminearum. Live-cell imaging showed that all the components of the RNA exosome complex are localized in the nucleus. FgEXOSC1 and FgEXOSCA were successfully knocked out; they are both involved in the vegetative growth, sexual reproduction, and pathogenicity of F. graminearum. Moreover, deletion of FgEXOSC1 resulted in abnormal toxisomes, decreased deoxynivalenol (DON) production, and downregulation of the expression levels of DON biosynthesis genes. The RNA-binding domain and N-terminal region of FgExosc1 are required for its normal localization and functions. Transcriptome sequencing (RNA-seq) showed that the disruption of FgEXOSC1 resulted in differential expression of 3,439 genes. Genes involved in processing of noncoding RNA (ncRNA), rRNA and ncRNA metabolism, ribosome biogenesis, and ribonucleoprotein complex biogenesis were significantly upregulated. Furthermore, subcellular localization, green fluorescent protein (GFP) pulldown, and coimmunoprecipitation (co-IP) assays demonstrated that FgExosc1 associates with the other components of the RNA exosome to form the RNA exosome complex in F. graminearum. Deletion of FgEXOSC1 and FgEXOSCA reduced the relative expression of some of the other subunits of the RNA exosome. Deletion of FgEXOSC1 affected the localization of FgExosc4, FgExosc6, and FgExosc7. In summary, our study reveals that the RNA exosome is involved in vegetative growth, sexual reproduction, DON production, and pathogenicity of F. graminearum. IMPORTANCE The RNA exosome complex is the most versatile RNA degradation machinery in eukaryotes. However, little is known about how this complex regulates the development and pathogenicity of plant-pathogenic fungi. In this study, we systematically identified 12 components of the RNA exosome complex in Fusarium head blight fungus Fusarium graminearum and first unveiled their subcellular localizations and established their biological functions in relation to the fungal development and pathogenesis. All the RNA exosome components are localized in the nucleus. FgExosc1 and FgExoscA are both required for the vegetative growth, sexual reproduction, DON production and pathogenicity in F. graminearum. FgExosc1 is involved in ncRNA processing, rRNA and ncRNA metabolism process, ribosome biogenesis and ribonucleoprotein complex biogenesis. FgExosc1 associates with the other components of RNA exosome complex and form the exosome complex in F. graminearum. Our study provides new insights into the role of the RNA exosome in regulating RNA metabolism, which is associated with fungal development and pathogenicity.

SNARE Protein Snc1 Is Essential for Vesicle Trafficking, Membrane Fusion and Protein Secretion in Fungi

Fungi are an important group of microorganisms that play crucial roles in a variety of ecological and biotechnological processes. Fungi depend on intracellular protein trafficking, which involves moving proteins from their site of synthesis to the final destination within or outside the cell. The soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNARE) proteins are vital components of vesicle trafficking and membrane fusion, ultimately leading to the release of cargos to the target destination. The v-SNARE (vesicle-associated SNARE) Snc1 is responsible for anterograde and retrograde vesicle trafficking between the plasma membrane (PM) and Golgi. It allows for the fusion of exocytic vesicles to the PM and the subsequent recycling of Golgi-localized proteins back to the Golgi via three distinct and parallel recycling pathways. This recycling process requires several components, including a phospholipid flippase (Drs2-Cdc50), an F-box protein (Rcy1), a sorting nexin (Snx4-Atg20), a retromer submit, and the COPI coat complex. Snc1 interacts with exocytic SNAREs (Sso1/2, Sec9) and the exocytic complex to complete the process of exocytosis. It also interacts with endocytic SNAREs (Tlg1 and Tlg2) during endocytic trafficking. Snc1 has been extensively investigated in fungi and has been found to play crucial roles in various aspects of intracellular protein trafficking. When Snc1 is overexpressed alone or in combination with some key secretory components, it results in enhanced protein production. This article will cover the role of Snc1 in the anterograde and retrograde trafficking of fungi and its interactions with other proteins for efficient cellular transportation.

FgAP1σ Is Critical for Vegetative Growth, Conidiation, Virulence, and DON Biosynthesis in Fusarium graminearum

The AP1 complex is a highly conserved clathrin adaptor that plays important roles in regulating cargo protein sorting and intracellular vesicle trafficking in eukaryotes. However, the functions of the AP1 complex in the plant pathogenic fungi including the devastating wheat pathogen Fusarium graminearum are still unclear. In this study, we investigated the biological functions of FgAP1^σ, a subunit of the AP1 complex in F. graminearum. Disruption of FgAP1^σ causes seriously impaired fungal vegetative growth, conidiogenesis, sexual development, pathogenesis, and deoxynivalenol (DON) production. The ΔFgap1^σ mutants were found to be less sensitive to KCl- and sorbitol-induced osmotic stresses but more sensitive to SDS-induced stress than the wild-type PH-1. Although the growth inhibition rate of the ΔFgap1^σ mutants was not significantly changed under calcofluor white (CFW) and Congo red (CR) stresses, the protoplasts released from ΔFgap1^σ hyphae were decreased compared with the wild-type PH-1, suggesting that FgAP1^σ is necessary for cell wall integrity and osmotic stresses in F. graminearum. Subcellular localization assays showed that FgAP1^σ was predominantly localized to endosomes and the Golgi apparatus. In addition, FgAP1^β-GFP, FgAP1^γ-GFP, and FgAP1^μ-GFP also localize to the Golgi apparatus. FgAP1^β interacts with FgAP1^σ, FgAP1^γ, and FgAP1^μ, while FgAP1^σ regulates the expression of FgAP1^β, FgAP1^γ, and FgAP1^μ in F. graminearum. Furthermore, the loss of FgAP1^σ blocks the transportation of the v-SNARE protein FgSnc1 from the Golgi to the plasma membrane and delays the internalization of FM4-64 dye into the vacuole. Taken together, our results demonstrate that FgAP1^σ plays vital roles in vegetative growth, conidiogenesis, sexual reproduction, DON production, pathogenicity, cell wall integrity, osmotic stress, exocytosis, and endocytosis in F. graminearum. These findings unveil the functions of the AP1 complex in filamentous fungi, most notably in F. graminearum, and lay solid foundations for effective prevention and control of Fusarium head blight (FHB).

FgSnt1 of the Set3 HDAC complex plays a key role in mediating the regulation of histone acetylation by the cAMP-PKA pathway in Fusarium graminearum

The cAMP-PKA pathway is critical for regulating growth, differentiation, and pathogenesis in fungal pathogens. In Fusarium graminearum, mutants deleted of PKR regulatory-subunit of PKA had severe defects but often produced spontaneous suppressors. In this study eleven pkr suppressors were found to have mutations in FgSNT1, a component of the Set3C histone deacetylase (HDAC) complex, that result in the truncation of its C-terminal region. Targeted deletion of the C-terminal 98 aa (CT98) in FgSNT1 suppressed the defects of pkr in growth and H4 acetylation. CT98 truncation also increased the interaction of FgSnt1 with Hdf1, a major HDAC in the Set3 complex. The pkr mutant had no detectable expression of the Cpk1 catalytic subunit and PKA activities, which was not suppressed by mutations in FgSNT1. Cpk1 directly interacted with the N-terminal region of FgSnt1 and phosphorylated it at S443, a conserved PKA-phosphorylation site. CT98 of FgSnt1 carrying the S443D mutation interacted with its own N-terminal region. Expression of FgSNT1S443D rescued the defects of pkr in growth and H4 acetylation. Therefore, phosphorylation at S443 and suppressor mutations may relieve self-inhibitory binding of FgSnt1 and increase its interaction with Hdf1 and H4 acetylation, indicating a key role of FgSnt1 in crosstalk between cAMP signaling and Set3 complex.

Fusarium graminearum GGA protein is critical for fungal development, virulence and ascospore discharge through its involvement in vesicular trafficking

Vesicular trafficking is a conserved material transport process in eukaryotic cells. The GGA family proteins are clathrin adaptors that are involved in eukaryotic vesicle transport, but their functions in phytopathogenic filamentous fungi remain unexplored. Here, we examined the only GGA family protein in Fusarium graminearum, FgGga1, which localizes to both the late Golgi and endosomes. In the absence of FgGga1, the fungal mutant exhibited defects in vegetative growth, DON biosynthesis, ascospore discharge and virulence. Fluorescence microscopy analysis revealed that FgGga1 is associated with trans-Golgi network (TGN)-to-plasma membrane, endosome-to-TGN and endosome-to-vacuole transport. Mutational analysis on the five domains of FgGga1 showed that the VHS domain was required for endosome-to-TGN transport while the GAT^167-248 and the hinge domains were required for both endosome-to-TGN and endosome-to-vacuole transport. Importantly, the deletion of the FgGga1 domains that are required in vesicular trafficking also inhibited vegetative growth and virulence of F. graminearum. In addition, FgGga1 interacted with the ascospore discharge regulator Ca²⁺ ATPase FgNeo1, whose transport to the vacuole is dependent on FgGga1-mediated endosome-to-vacuole transport. Our results suggest that FgGga1 is required for fungal development and virulence via FgGga1-mediated vesicular trafficking, and FgGga1-mediated endosome-to-vacuole transport facilitates ascospore discharge in F. graminearum.

Proximity-dependent biotinylation identifies a suite of candidate effector proteins from Fusarium graminearum

Fusarium graminearum is a fungal pathogen that causes Fusarium head blight in cereal crops. The identification of proteins secreted from pathogens to overcome plant defenses and cause disease, collectively known as effectors, can reveal the etiology of a disease process. Proximity-dependent biotin identification (BioID) was used to identify potential effector proteins secreted in planta by F. graminearum during the infection of Arabidopsis. Mass spectrometry analysis of streptavidin affinity-purified proteins revealed over 300 proteins from F. graminearum, of which 62 were candidate effector proteins (CEPs). An independent analysis of secreted proteins from axenic cultures of F. graminearum showed a 42% overlap with CEPs, thereby assuring confidence in the BioID methodology. The analysis also revealed that 19 out of 62 CEPs (approx. 30%) had been previously characterized with virulence function in fungi. The functional characterization of additional CEPs was undertaken through deletion analysis by the CRISPR/Cas9 method, and by overexpression into Triticum aestivum (wheat) leaves by the Ustilago hordei delivery system. Deletion studies of 12 CEPs confirmed the effector function of three previously characterized CEPs and validated the function of another four CEPs on wheat inflorescence or vegetative tissues. Lastly, overexpression in wheat showed that all seven CEPs enhanced resistance against the bacterial pathogen Pseudomonas syringae DC3000.

Fusarium verticillioides Pex7/20 mediates peroxisomal PTS2 pathway import, pathogenicity, and fumonisin B1 biosynthesis

Fusarium verticillioides, a well-known fungal pathogen that causes severe disease in maize and contaminates the grains with fumonisin B1 (FB1) mycotoxin, affects the yield and quality of maize worldwide. The intrinsic roles of peroxisome targeting signal (PTS)-containing proteins in phytopathogens remain elusive. We therefore explored the regulatory role and other biological functions of the components of PTS2 receptor complex, FvPex7 and FvPex20, in F. verticillioides. We found that FvPex7 directly interacts with the carboxyl terminus of FvPex20 in F. verticillioides. PTS2-containing proteins are recognized and bound by the FvPex7 receptor or the FvPex7-Pex20 receptor complex in the cytoplasm, but the peroxisome localization of the PTS2-Pex7-Pex20 complex is only determined by Pex20 in F. verticillioides. However, we observed that some putative PTS2 proteins that interact with Pex7 are not transported into the peroxisomes, but a PTS1 protein that interacts with Pex5 was detected in the peroxisomes. Furthermore, ΔFvpex7pex20 as well as ΔFvpex7pex5 double mutants exhibited reduced pathogenicity and FB1 biosynthesis, along with defects in conidiation. The PTS2 receptor complex mutants (ΔFvpex7pex20) grew slowly on minimal media and showed reduced sensitivity to cell wall and cell membrane stress-inducing agents compared to the wild type. Taken together, we conclude that the PTS2 receptor complex mediates peroxisome matrix proteins import and contributes to pathogenicity and FB1 biosynthesis in F. verticillioides. KEY POINTS: • FvPex7 directly interacts with FvPex20 in F. verticillioides. • vThe PTS2 receptor complex is essential for the importation of PTS2-containing matrix protein into peroxisomes in F. verticillioides. • Fvpex7/pex20 is involved in pathogenicity and FB1 biosynthesis in F. verticillioides.

PKR Protects the Major Catalytic Subunit of PKA Cpk1 from FgBlm10-Mediated Proteasome Degradation in Fusarium graminearum

For optimal proteolytic function, the proteasome core (CP or 20S) must associate with activators. The cAMP-PKA pathway is reported to affect the activity of the proteasome in humans. However, the relationship between the proteasome and PKA is not well characterized. Our results showed that the major catalytic subunit Cpk1 was degraded without the protection of Pkr. Eleven (out of 67) pkr suppressors had FgBlm10 C-terminal truncation, one suppressor had an amino acid change mutation in the PRE6 ortholog (FGRRES_07282), and one in the PRE5 ortholog (FGRRES_05222). These mutations rescued the defects in growth and conidial morphology, Cpk1 stability, and PKA activities in the pkr mutant. The interaction of FgBlm10 with FgPre5 and FgPre6 were detected by co-immunoprecipitation, and the essential elements for their interaction were characterized, including the FgBlm10 C-terminus, amino acid D82 of FgPre6 and K62 of FgPre5. Additional FgBlm10-interacting proteins were identified in the wild type and pkr mutant, suggesting that PKA regulates the preference of FgBlm10-mediated proteasome assembly. In addition, PKA indirectly affected the phosphorylation of FgBlm10, and its localization in the nucleus. The truncation of the FgBlm10 C terminus also enhanced nuclear import and bleomycin resistance, suggesting its role in proteasome assembly at DNA damage sites. Collectively, our data demonstrated that regulation between PKA and proteasome degradation is critical for the vegetative growth of F. graminearum.

The Sorting Nexin Genes ChSNX4 and ChSNX41 Are Required for Reproductive Development, Stress Adaption and Virulence in Cochliobolus heterostrophus

Sorting nexins are a conserved protein family involved in many cellular processes in fungi, and the function of sorting nexin Snx4 (Atg24) and Snx41 (Atg20) in Cochliobolus heterostrophus was not clear. The ΔChsnx4 and ΔChsnx41 mutants were generated by a PCR-based marker method to determine the roles of Snx4 and Snx41 in reproductive development, stress adaption, and virulence in C. heterostrophus. Compared with the wild-type strain, the ΔChsnx4 and ΔChsnx41 mutants exhibited obvious changes in vegetative growth and in morphology of conidia. In addition, the conidiation, appressorium formation, and virulence of snx4 and snx41 mutants were dramatically reduced. Moreover, ΔChsnx4 and ΔChsnx41 mutants were more sensitive to oxidative stress (menadione and H₂O₂), cell wall integrity stress (Congo red and calcofluor white), fungicides, and isothiocyanates. All the phenotypes mentioned above were restored in complemented strains. In addition, ChSnx4 and ChSnx41 were proven to interact with each other through yeast two-hybrid. Taken together, these findings indicated that ChSNX4 and ChSNX41 were important for fungal growth, asexual development, stress adaption, and virulence in C. heterostrophus.

Mitochondrial prohibitin complex regulates fungal virulence via ATG24-assisted mitophagy

Prohibitins are highly conserved eukaryotic proteins in mitochondria that function in various cellular processes. The roles of prohibitins in fungal virulence and their regulatory mechanisms are still unknown. Here, we identified the prohibitins ChPhb1 and ChPhb2 in a plant pathogenic fungus Colletotrichum higginsianum and investigated their roles in the virulence of this anthracnose fungus attacking crucifers. We demonstrate that ChPhb1 and ChPhb2 are required for the proper functioning of mitochondria, mitophagy and virulence. ChPhb1 and ChPhb2 interact with the autophagy-related protein ChATG24 in mitochondria, and ChATG24 shares similar functions with these proteins in mitophagy and virulence, suggesting that ChATG24 is involved in prohibitin-dependent mitophagy. ChPhb1 and ChPhb2 modulate the translocation of ChATG24 into mitochondria during mitophagy. The role of ChATG24 in mitophagy is further confirmed to be conserved in plant pathogenic fungi. Our study presents that prohibitins regulate fungal virulence by mediating ATG24-assisted mitophagy.

Prohibitins recruit ChATG24 into the mitochondria to modulate mitophagy, thereby affecting the virulence of Colletotrichum higginsianum.

The development of extracellular vesicle markers for the fungal phytopathogen Colletotrichum higginsianum

Fungal phytopathogens secrete extracellular vesicles (EVs) associated with enzymes and phytotoxic metabolites. While these vesicles are thought to promote infection, defining the true contents and functions of fungal EVs, as well as suitable protein markers, is an ongoing process. To expand our understanding of fungal EVs and their possible roles during infection, we purified EVs from the hemibiotrophic phytopathogen Colletotrichum higginsianum, the causative agent of anthracnose disease in multiple plant species, including Arabidopsis thaliana. EVs were purified in large numbers from the supernatant of protoplasts but not the supernatant of intact mycelial cultures. We purified two separate populations of EVs, each associated with over 700 detected proteins, including proteins involved in vesicle transport, cell wall biogenesis and the synthesis of secondary metabolites. We selected two SNARE proteins (Snc1 and Sso2) and one 14‐3‐3 protein (Bmh1) as potential EV markers and generated transgenic strains expressing fluorescent fusions. Each marker was confirmed to be protected inside EVs. Fluorescence microscopy was used to examine the localization of each marker during infection on Arabidopsis leaves. These findings further our understanding of EVs in fungal phytopathogens and will help build an experimental system to study EV interkingdom communication between plants and fungi.

The Small GTPase FgRab1 Plays Indispensable Roles in the Vegetative Growth, Vesicle Fusion, Autophagy and Pathogenicity of Fusarium graminearum

Rab GTPases are key regulators of membrane and intracellular vesicle transports. However, the biological functions of FgRab1 are still unclear in the devastating wheat pathogen Fusarium graminearum. In this study, we generated constitutively active (CA) and dominant-negative (DN) forms of FgRAB1 from the wild-type PH-1 background for functional analyses. Phenotypic analyses of these mutants showed that FgRab1 is important for vegetative growth, cell wall integrity and hyphal branching. Compared to the PH-1 strain, the number of spores produced by the Fgrab1DN strain was significantly reduced, with obviously abnormal conidial morphology. The number of septa in the conidia of the Fgrab1DN mutant was fewer than that observed in the PH-1 conidia. Fgrab1DN was dramatically reduced in its ability to cause Fusarium head blight symptoms on wheat heads. GFP-FgRab1 was observed to partly localize to the Golgi apparatus, endoplasmic reticulum and Spitzenkörper. Furthermore, we found that FgRab1 inactivation blocks not only the transport of the v-SNARE protein FgSnc1 from the Golgi to the plasma membrane but also the fusion of endocytic vesicles with their target membranes and general autophagy. In summary, our results indicate that FgRab1 plays vital roles in vegetative growth, conidiogenesis, pathogenicity, autophagy, vesicle fusion and trafficking in F. graminearum.

Genome-Wide Characterization of PX Domain-Containing Proteins Involved in Membrane Trafficking-Dependent Growth and Pathogenicity of Fusarium graminearum

The Phox homology (PX) domain is a membrane recruitment module that binds to phosphoinositides (PI) mediating the selective sorting and transport of transmembrane proteins, lipids, and other critical cargo molecules via membrane trafficking processes. However, the mechanism of vesicular trafficking mediated by PX domain-containing proteins in phytopathogenic fungi and how this relates to the fungal development and pathogenicity remain unclear. Here, we systematically identified and characterized the functions of PX domain-containing proteins in the plant fungal pathogen Fusarium graminearum. Our data identified 14 PX domain-containing proteins in F. graminearum, all of which were required for plant infection and deoxynivalenol (DON) production, with the exception of FgMvp1 and FgYkr078. Furthermore, all the PX domain-containing proteins showed distinct localization patterns that were limited to the endosomes, vacuolar membrane, endoplasmic reticulum, cytoplasm, and hyphal septa/tips. Remarkably, among these proteins, FgBem1 targeted to surface crescent and septal pores and was retained at the septum pores even after actin constriction during septum development. Further analyses demonstrated that the surface crescent targeting of FgBem1 solely depended on its SH3 domains, while its septum and apex anchoring localization relied on its PX domain, which was also indispensable for reactive oxygen species (ROS) production, sexual development, and pathogenicity in F. graminearum. In summary, our study is the first detailed and comprehensive functional analysis of PX domain-containing proteins in filamentous fungi, and it provides new insight into the mechanism of FgBem1 involved in septum and apex anchorage mediated by its PX domain, which is necessary for sexual development and pathogenicity of F. graminearum.

FgRab5 and FgRab7 are essential for endosomes biogenesis and non-redundantly recruit the retromer complex to the endosomes in Fusarium graminearum

The retromer complex, composed of the cargo-selective complex (CSC) Vps35-Vps29-Vps26 in complex with the sorting nexin dimer Vps5-Vps17, mediates the sorting and retrograde transport of cargo proteins from the endosomes to the trans-Golgi network in eukaryotic cells. Rab proteins belong to the Ras superfamily of small GTPases and regulate many trafficking events including vesicle formation, budding, transport, tethering, docking and fusion with target membranes. Herein, we investigated the potential functional relationship between the retromer complex and the 11 Rab proteins that exist in Fusarium graminearum using genetic and high-resolution laser confocal microscopic approaches. We found that only FgRab5 (FgRab5A and FgRab5B) and FgRab7 associate with the retromer complex. Both FgVps35-GFP and FgVps17-GFP are mis-localized and appear diffused in the cytoplasm of ΔFgrab5A, ΔFgrab5B and ΔFgrab7 mutants as compared to their punctate localization within the endosomes of the wild-type. FgRab7 and FgRab5B were found to co-localize with the retromer on endosomal membranes. Most strikingly, we found that these three Rab GTPases are indispensable for endosome biogenesis as both early and late endosomes could not be detected in the cells of the mutants after FM4-64 staining of the cells, while they were very clearly seen in the wild-type PH-1. Furthermore, FgRab7 was found to recruit FgVps35 but not FgVps17 to the endosomal membranes, whereas FgRab5B recruits both FgVps35 and FgVps17 to the membranes. Thus, we conclude that the Rab proteins FgRab5A, FgRab5B and FgRab7 play critical roles in the biogenesis of endosomes and in regulating retromer-mediated trafficking in F. graminearum.

Trehalose Phosphate Synthase Complex-Mediated Regulation of Trehalose 6-Phosphate Homeostasis Is Critical for Development and Pathogenesis in Magnaporthe oryzae

Trehalose biosynthesis pathway is a potential target for antifungal drug development, and trehalose 6-phosphate (T6P) accumulation is widely known to have toxic effects on cells. However, how organisms maintain a safe T6P level and cope with its cytotoxicity effects when accumulated have not been reported. Herein, we unveil the mechanism by which the rice blast fungus Magnaporthe oryzae avoids T6P accumulation and the genetic and physiological adjustments it undergoes to self-adjust the metabolite level when it is unavoidably accumulated. We found that T6P accumulation leads to defects in fugal development and pathogenicity. The accumulated T6P impairs cell wall assembly by disrupting actin organization. The disorganization of actin impairs the distribution of chitin synthases, thereby disrupting cell wall polymer distribution. Additionally, accumulation of T6P compromise energy metabolism. M. oryzae was able to overcome the effects of T6P accumulation by self-mutation of its MoTPS3 gene at two different mutation sites. We further show that mutation of MoTPS3 suppresses MoTps1 activity to reduce the intracellular level of T6P and partially restore ΔMotps2 defects. Overall, our results provide insights into the cytotoxicity effects of T6P accumulation and uncover a spontaneous mutation strategy to rebalance accumulated T6P in M. oryzae.

IMPORTANCEM. oryzae, the causative agent of the rice blast disease, threatens rice production worldwide. Our results revealed that T6P accumulation, caused by the disruption of MoTPS2, has toxic effects on fugal development and pathogenesis in M. oryzae. The accumulated T6P impairs the distribution of cell wall polymers via actin organization and therefore disrupts cell wall structure. M. oryzae uses a spontaneous mutation to restore T6P cytotoxicity. Seven spontaneous mutation sites were found, and a mutation in MoTPS3 was further identified. The spontaneous mutation in MoTPS3 can partially rescue ΔMotps2 defects by suppressing MoTps1 activity to alleviate T6P cytotoxicity. This study provides clear evidence for better understanding of T6P cytotoxicity and how the fungus protects itself from T6P’s toxic effects when it has accumulated to severely high levels.

Compartmentalization of Melanin Biosynthetic Enzymes Contributes to Self-Defense against Intermediate Compound Scytalone in Botrytis cinerea

In filamentous fungi, 1,8-dihydroxynaphthalene (DHN) melanin is a major component of the extracellular matrix, endowing fungi with environmental tolerance and some pathogenic species with pathogenicity. However, the subcellular location of the melanin biosynthesis pathway components remains obscure. Using the gray mold pathogen Botrytis cinerea, the DHN melanin intermediate scytalone was characterized via phenotypic and chemical analysis of mutants, and the key enzymes participating in melanin synthesis were fused with fluorescent proteins to observe their subcellular localizations. The Δbcscd1 mutant accumulated scytalone in the culture filtrate rather than in mycelium. Excessive scytalone appears to be self-inhibitory to the fungus, leading to repressed sclerotial germination and sporulation in the Δbcscd1 mutant. The BcBRN1/2 enzymes responsible for synthesizing scytalone were localized in endosomes and found to be trafficked to the cell surface, accompanied by the accumulation of BcSCD1 proteins in the cell wall. In contrast, the early-stage melanin synthesis enzymes BcPKS12/13 and BcYGH1 were localized in peroxisomes. Taken together, the results of this study revealed the subcellular distribution of melanin biosynthetic enzymes in B. cinerea, indicating that the encapsulation and externalization of the melanin synthetic enzymes need to be delicately orchestrated to ensure enzymatic efficiency and protect itself from the adverse effect of the toxic intermediate metabolite.IMPORTANCE The devastating gray mold pathogen Botrytis cinerea propagates via melanized conidia and sclerotia. This study reveals that the sclerotial germination of B. cinerea is differentially affected by different enzymes in the melanin synthesis pathway. Using gene knockout mutants and chemical analysis, we found that excessive accumulation of the melanin intermediate scytalone is inhibitory to B. cinerea. Subcellular localization analysis of the melanin synthesis enzymes of B. cinerea suggested two-stage partitioning of the melanogenesis pathway: the intracellular stage involves the steps until the intermediate scytalone was translocated to the cell surface, whereas the extracellular stage comprises all the steps occurring in the wall from scytalone to final melanin formation. These strategies make the fungus avert self-poisoning during melanin production. This study opens avenues for better understanding the mechanisms of secondary metabolite production in filamentous fungi.

The Golgin Protein RUD3 Regulates Fusarium graminearum Growth and Virulence

Golgins are coiled-coil proteins that play prominent roles in maintaining the structure and function of the Golgi complex. However, the role of golgin proteins in phytopathogenic fungi remains poorly understood. In this study, we functionally characterized the Fusarium graminearum golgin protein RUD3, a homolog of _ScRUD3/GMAP-210 in Saccharomyces cerevisiae and mammalian cells. Cellular localization observation revealed that RUD3 is located in the cis-Golgi. Deletion of RUD3 caused defects in vegetative growth, ascospore discharge, deoxynivalenol (DON) production, and virulence. Moreover, the Δrud3 mutant showed reduced expression of tri genes and impairment of the formation of toxisomes, both of which play essential roles in DON biosynthesis. We further used green fluorescent protein (GFP)-tagged SNARE protein SEC22 (SEC22-GFP) as a tool to study the transport between the endoplasmic reticulum (ER) and Golgi and observed that SEC22-GFP was retained in the cis-Golgi in the Δrud3 mutant. RUD3 contains the coiled coil (CC), GRAB-associated 2 (GA2), GRIP-related Arf binding (GRAB), and GRAB-associated 1 (GA1) domains, which except for GA1, are indispensable for normal localization and function of RUD3, whereas only CC is essential for normal RUD3-RUD3 interaction. Together, these results demonstrate how the golgin protein RUD3 mediates retrograde trafficking in the ER-to-Golgi pathway and is necessary for growth, ascospore discharge, DON biosynthesis, and pathogenicity in F. graminearum IMPORTANCE Fusarium head blight (FHB) caused by the fungal pathogen Fusarium graminearum is an economically important disease of wheat and other small grain cereal crops worldwide, and limited effective control strategies are available. A better understanding of the regulation mechanisms of F. graminearum development, deoxynivalenol (DON) biosynthesis, and pathogenicity is therefore important for the development of effective control management of this disease. Golgins are attached via their extreme carboxy terminus to the Golgi membrane and are involved in vesicle trafficking and organelle maintenance in eukaryotic cells. In this study, we systematically characterized a highly conserved Golgin protein, RUD3, and found that it is required for vegetative growth, ascospore discharge, DON production, and pathogenicity in F. graminearum Our findings provide a comprehensive characterization of the golgin family protein RUD3 in plant-pathogenic fungus, which could help to identify a new potential target for effective control of this devastating disease.

The GTPase-Activating Protein FgGyp1 Is Important for Vegetative Growth, Conidiation, and Virulence and Negatively Regulates DON Biosynthesis in Fusarium graminearium

Ypt1 is a small Rab GTPase in yeast, Gyp1 functions at the Golgi as a negative regulator of Ypt1. Gyp1 homologs are conserved in filamentous fungi. However, the roles of Gyp1 in phytopathogenic fungi are still unclear. Herein, we investigated the functions of FgGyp1 in the wheat pathogen Fusarium graminearum by live-cell imaging, genetic, and pathological analyses. Targeted gene replacement method was used to delete FgGYP1 in F. graminearum. Phenotypic analyses showed that FgGyp1 is critically important not only for the vegetative growth of F. graminearum but also its conidiation. The mutant’s vegetative growth was significantly reduced by 70% compared to the wild type PH-1. The virulence of FgGYP1 deletion mutant was significantly decreased when compared with the wild type PH-1. We further found that FgGyp1 negatively regulates DON production of the fungus. Live-cell imaging clearly demonstrated that FgGyp1 mainly localizes to the Golgi apparatus. Moreover, the TBC domain, C-terminal, and N-terminal regions of FgGyp1 are found to be indispensable for its biological functions and normal localization. The Arg357 residue of FgGyp1 is essential for its functions but dispensable for the normal localization of the protein, while the Arg284 residue is not required for both the functions and normal localization of the protein. Furthermore, we showed that FgGyp1 essentially hydrolyzes the GTP-bound FgRab1 (activated form) to its corresponding GDP-bound (inactive) form in vitro, suggesting that FgGyp1 is a GTPase-activating protein (GAP) for FgRab1. Finally, FgGyp1 was found to be important for FgSnc1-mediated fusion of secretory vesicles from the Golgi with the plasma membrane in F. graminearum. Put together, these data demonstrate that FgGyp1 functions as a GAP for FgRab1 and is important for vegetative growth, conidiation and virulence, and negatively regulates DON biosynthesis in F. graminearum.

FgSpa2 recruits FgMsb3, a Rab8 GAP, to the polarisome to regulate polarized trafficking, growth and pathogenicity in Fusarium graminearum

In filamentous fungi, hyphal growth depends on the continuous delivery of vesicles to the growing tips. It is unclear how fast-growing hyphae coordinate simultaneous cell extension and expansion in the tip cells. We have functionally characterized 12 TBC (Tre-2/Bub2/Cdc16) domain-containing proteins in Fusarium graminearum. Among them, FgMsb3 is found to regulate hyphal tip expansion and to be required for pathogenicity. The regulatory mechanism of FgMsb3 has been further investigated by genetic, high-resolution microscopy and high-throughput co-immunoprecipitation strategies. The FgMsb3 protein localizes at the polarisome and the hyphal apical dome (HAD) where it acts as a GTPase-activating protein for FgRab8 which is required for apical secretion-mediated growth and pathogenicity. Deletion of FgMSB3 causes excessive polarized trafficking but blocks the fusion of FgSnc1-associated vesicles to the plasma membrane. Moreover, we establish that FgSpa2 interacts with FgMsb3, enabling FgMsb3 tethering to the polarisome. Loss of FgSpa2 or other polarisome components (FgBud6 and FgPea2) causes complete shifting of FgMsb3 to the HAD and this affects the polarized growth and pathogenicity of the fungus. In summary, we conclude that FgSpa2 regulates FgMsb3-FgRab8 cascade and this is crucial for creating a steady-state equilibrium that maintains continuous polarized growth and contributes to the pathogenicity of F. graminearum.

eIF3k Domain-Containing Protein Regulates Conidiogenesis, Appressorium Turgor, Virulence, Stress Tolerance, and Physiological and Pathogenic Development of Magnaporthe oryzae Oryzae

The eukaryotic translation initiation factor 3 (eIF3) complex consists of essential and non-essential sub-complexes. Non-essential eIF3 complex subunits, such as eIF3e, eIF3j, eIF3k, and eIF3l, modulate stress tolerance and enhance the lifespan of Neurospora crassa and Caenorhabditis elegans. However, there is limited knowledge of the role of the non-essential eIF3 sub-complex in the pathophysiological development of plant fungal pathogens. Here, we deployed genetic and biochemical techniques to explore the influence of a hypothetical protein containing eIF3k domain in Magnaporthe oryzae Oryzae (MoOeIF3k) on reproduction, hyphae morphogenesis, stress tolerance, and pathogenesis. Also, the targeted disruption of MoOeIF3k suppressed vegetative growth and asexual sporulation in ΔMoOeif3k strains significantly. We demonstrated that MoOeIF3k promotes the initiation and development of the rice blast disease by positively regulating the mobilization and degradation of glycogen, appressorium integrity, host penetration, and colonization during host-pathogen interaction. For the first time, we demonstrated that the eIF3k subunit supports the survival of the blast fungus by suppressing vegetative growth and possibly regulating the conversions and utilization of stored cellular energy reserves under starvation conditions. We also observed that the deletion of MoOeIF3k accelerated ribosomal RNA (rRNA) generation in the ΔMoOeif3k strains with a corresponding increase in total protein output. In summary, this study unravels the pathophysiological significance of eIF3k filamentous fungi. The findings also underscored the need to systematically evaluate the individual subunits of the non-essential eIF3 sub-complex during host-pathogen interaction. Further studies are required to unravel the influence of synergetic coordination between translation and transcriptional regulatory machinery on the pathogenesis of filamentous fungi pathogens.

The type II phosphoinositide 4-kinase FgLsb6 is important for the development and virulence of Fusarium graminearum

Fusarium graminearum is the main pathogenic fungus causing Fusarium head blight (FHB), which is a wheat disease with a worldwide prevalence. In eukaryotes, phosphatidylinositol 4-phosphate (PI4P), which participates in many physiological processes, is located primarily in different organelles, including the trans-Golgi network (TGN), plasma membrane and endosomes. Type II phosphatidylinositol 4-kinases (PI4Ks) are involved in regulating the production of PI4P in yeast, plants and mammalian cells. However, the role of these proteins in phytopathogenic fungi is not well understood. In this study, we characterized the type II PI4K protein FgLsb6 in F. graminearum, a homolog of Lsb6 in Saccharomyces cerevisiae. Unlike Lsb6, FgLsb6 localizes to the vacuoles and endosomes. The ΔFglsb6 mutant displayed defects in vegetative growth, deoxynivalenol (DON) production and pathogenicity. Furthermore, the ΔFglsb6 deletion mutant also exhibited increased resistance to osmotic, oxidative and cell wall stresses. Further analyses of the ΔFglsb6 mutant showed that it was defective in the generation of PI4P on endosomes and endocytosis. Collectively, our data suggest that the decreased vegetative growth and pathogenicity of ΔFglsb6 was due to the conservative roles of FgLsb6 in the generation of PI4P on endosomes and endocytosis.

The sorting nexin FgAtg20 is involved in the Cvt pathway, non-selective macroautophagy, pexophagy and pathogenesis in Fusarium graminearum

The sorting nexin Atg20/Snx42 plays an important role in autophagy. The wheat head blight pathogen Fusarium graminearum contains an FgAtg20 protein orthologous to Saccharomyces cerevisiae Atg20/Snx42, but its function remains largely unknown. Here, we report a role for FgAtg20 in regulating morphogenesis and fungal pathogenicity. Cytological observation and Western blot analysis revealed that ΔFgAtg20 mutants are defective in vacuolar transport and proteolysis of GFP-FgAtg8, indicating that FgAtg20 is required for non-selective macroautophagy. Furthermore, we found that FgATG20 is necessary for the maturation of FgApe1, an indicator of the cytoplasm-to-vacuole targeting (Cvt) pathway. Immunoblot analysis displayed lower level of FgPex14, a peroxisomal integral membrane protein in ΔFgAtg20 mutants, suggesting that pexophagy is impaired. Furthermore, we demonstrate that FgAtg20 forms a complex with FgAtg1, FgAtg11, FgAtg17 and FgAtg24. When considered together, we conclude that FgAtg20 plays a critical role in vegetative growth, conidiation and pathogenicity of the head blight pathogen, and is involved in the Cvt pathway, non-selective macroautophagy and pexophagy.

Current views on endocytosis in filamentous fungi

Filamentous fungi grow by adding cell wall and membrane exclusively at the apex of tubular structures called hyphae. Growth was previously believed to occur only through exocytosis at the Spitzenkörper, an organised body of secretory macro- and microvesicles found only in growing hyphae. More recent work has indicated that an area deemed the sub-apical collar is enriched for endocytosis and is also required for hyphal growth. It is now generally believed that polarity of filamentous fungi is achieved through the balancing of the processes of endocytosis and exocytosis at these two areas. This review is an update on the current progress and understanding surrounding the occurrence of endocytosis and its spatial regulation as they pertain to growth and pathogenicity in filamentous fungi.

Role of sorting nexin PaATG24 in autophagy, aging and development of Podospora anserina

Sorting nexins are a conserved protein family involved in vesicle transport, membrane trafficking and protein sorting. The sorting nexin ATG24/SNX4 has been demonstrated to be involved in different autophagy pathways and in endosomal trafficking. However, its impact on cellular quality control and on aging and development is still elusive. Here we report studies analyzing the function of PaATG24 in the aging model Podospora anserina. Ablation of PaATG24 leads to a reduced growth rate, infertility, and to a pronounced lifespan reduction. These characteristics are accompanied by alterations of the morphology and size distribution of vacuoles and severe impairments in non-selective and selective autophagy of peroxisomes (pexophagy) and mitochondria (mitophagy). While general autophagy and pexophagy are almost completely blocked, a PaATG24-independent form of mitophagy is induced during aging. In the ΔPaAtg24 mutant a strong accumulation of peroxisomes occurs while mitochondrial abundance is only slightly increased. These mitochondria are partially affected in function. Most strikingly, although some PaATG24-independent mitophagy exists, it appears that this is not sufficient to remove dysfunctional mitochondria efficiently enough to prevent premature aging. Overall our data emphasize the key role of mitochondria in aging and of mitophagy in quality control to keep a population of "healthy" mitochondria during aging.

The putative deubiquitinating enzyme MoUbp4 is required for infection-related morphogenesis and pathogenicity in the rice blast fungus Magnaporthe oryzae

Ubiquitination is a key regulatory mechanism that affects numerous important biological processes, including cellular differentiation and pathogenesis in eukaryotic cells. Attachment of proteins to ubiquitin is reversed by specialized proteases, deubiquitinating enzymes (DUBs), which are essential for precursor processing, maintaining ubiquitin homeostasis and promoting protein degradation by recycling ubiquitins. Here, we report the identification of a novel non-pathogenic T-DNA-tagged mutant T612 of Magnaporthe oryzae with a single insertion in the second exon of MoUBP4, which encodes a putative ubiquitin carboxyl-terminal hydrolase. Targeted gene deletion mutants of MoUBP4 are significantly reduced in mycelial growth, conidiation, and increased in tolerance to SDS and CR (Congo red) cell-wall damage. The ΔMoubp4 mutants are blocked in penetration and invasive growth, which results in the loss of pathogenicity. Many conidia produced by the ΔMoubp4 mutants are unable to form appressoria and mobilization and degradation of glycogen and lipid droplets are significantly delayed. Moreover, immunohybridization analysis revealed that total protein ubiquitination levels of the null mutants were significantly increased, indicating that MoUbp4 functions as a deubiquitination enzyme. Taken together, we conclude that MoUbp4 is required for deubiquitination, infection-related morphogenesis and pathogenicity in M. oryzae.