Small GTPase Rab7-mediated FgAtg9 trafficking is essential for autophagy-dependent development and pathogenicity in Fusarium graminearum

The small GTPase Ypt7 of Penicillium expansum is required for growth, patulin biosynthesis and virulence

Ypt GTPases are the largest subfamily of small GTPases involved in membrane transport. Here, a PeYpt7 gene deletion mutant of P. expansum was constructed. The ΔPeYpt7 mutant showed reduced colony growth with abnormal mycelial growth, reduced conidiation, and insufficient spore development. The mutation rendered the pathogen susceptible to osmotic stress and cell wall stressors. In addition, the absence of PeYpt7 reduced patulin production in P. expansum and significantly limited gene expression (PatG, PatH, PatI, PatD, PatF, and PatL). In addition, the mutant showed attenuated virulence in infected fruit and reduced expression of pathogenic factors was (PMG, PG, PL, and GH1). Thus, PeYpt7 modulates the growth, morphology, patulin accumulation, and pathogenicity of P. expansum by limiting the expression of related genes.

Nucleoside Diphosphate Kinase FgNdpk Is Required for DON Production and Pathogenicity by Regulating the Growth and Toxisome Formation of Fusarium graminearum

Nucleoside diphosphate kinases (NDPKs) are nucleotide metabolism enzymes that play different physiological functions in different species. However, the roles of NDPK in phytopathogen and mycotoxin production are not well understood. In this study, we showed that Fusarium graminearum FgNdpk is important for vegetative growth, conidiation, sexual development, and pathogenicity. Furthermore, FgNdpk is required for deoxynivalenol (DON) production; deletion of FgNDPK downregulates the expression of DON biosynthesis genes and disrupts the formation of FgTri4-GFP-labeled toxisomes, while overexpression of FgNDPK significantly increases DON production. Interestingly, FgNdpk colocalizes with the DON biosynthesis proteins FgTri1 and FgTri4 in the toxisome, and coimmunoprecipitation (Co-IP) assays show that FgNdpk associates with FgTri1 and FgTri4 in vivo and regulates their localizations and expressions, respectively. Taken together, these data demonstrate that FgNdpk is important for vegetative growth, conidiation, and pathogenicity and acts as a key protein that regulates toxisome formation and DON biosynthesis in F. graminearum.

Small GTPase Rab7 is involved in stress adaptation to carbon starvation to ensure the induced cellulase biosynthesis in Trichoderma reesei

BACKGROUND

The saprophytic filamentous fungus Trichoderma reesei represents one of the most prolific cellulase producers. The bulk production of lignocellulolytic enzymes by T. reesei not only relies on the efficient transcription of cellulase genes but also their efficient secretion after being translated. However, little attention has been paid to the functional roles of the involved secretory pathway in the high-level production of cellulases in T. reesei. Rab GTPases are key regulators in coordinating various vesicle trafficking associated with the eukaryotic secretory pathway. Specifically, Rab7 is a representative GTPase regulating the transition of the early endosome to the late endosome followed by its fusion to the vacuole as well as homotypic vacuole fusion. Although crosstalk between the endosomal/vacuolar pathway and the secretion pathway has been reported, the functional role of Rab7 in cellulase production in T. reesei remains unknown.

RESULTS

A TrRab7 was identified and characterized in T. reesei. TrRab7 was shown to play important roles in T. reesei vegetative growth and vacuole morphology. Whereas knock-down of Trrab7 significantly compromised the induced production of T. reesei cellulases, overexpression of the key transcriptional activator, Xyr1, restored the production of cellulases in the Trrab7 knock-down strain (Ptcu-rab7KD) on glucose, indicating that the observed defective cellulase biosynthesis results from the compromised cellulase gene transcription. Down-regulation of Trrab7 was also found to make T. reesei more sensitive to various stresses including carbon starvation. Interestingly, overexpression of Snf1, a serine/threonine protein kinase known as an energetic sensor, partially restored the cellulase production of Ptcu-rab7KD on Avicel, implicating that TrRab7 is involved in an energetic adaptation to carbon starvation which contributes to the successful cellulase gene expression when T. reesei is transferred from glucose to cellulose.

CONCLUSIONS

TrRab7 was shown to play important roles in T. reesei development and a stress response to carbon starvation resulting from nutrient shift. This adaptation may allow T. reesei to successfully initiate the inducing process leading to efficient cellulase production. The present study provides useful insights into the functional involvement of the endosomal/vacuolar pathway in T. reesei development and hydrolytic enzyme production.

AoRab7A interacts with AoVps35 and AoVps41 to regulate vacuole assembly, trap formation, conidiation, and functions of proteasomes and ribosomes in Arthrobotrys oligospora

Rab GTPases regulate vesicle trafficking in organisms and play crucial roles in growth and development. Arthrobotrys oligospora is a ubiquitous nematode-trapping (NT) fungus, it can form elaborate traps to capture nematodes. Our previous study found that deletion of Aorab7A abolished the trap formation and sporulation. Here, we investigated the regulatory mechanism of AoRab7A using transcriptomic, biochemical, and phenotypic comparisons. Transcriptome analysis, yeast library screening, and yeast two-hybrid assay identified two vacuolar protein sorting (Vps) proteins, AoVps41 and AoVps35, as putative targets of AoRab7A. The deletion of Aovps41 and Aovps35 caused considerable defects in multiple phenotypic traits, such as conidiation and trap formation. We further found a close connection between AoRab7A and Vps proteins in vesicle-vacuole fusion, which triggered vacuolar fragmentation. Further transcriptome analysis showed that AoRab7A and AoVps35 play essential roles in many cellular processes and components including proteasomes, autophagy, fatty acid degradation, and ribosomes in A. oligospora. Furthermore, we verified that AoRab7A, AoVps41, and AoVps35 are involved in ribosome and proteasome functions. The absence of these proteins inhibited the biosynthesis of nascent proteins and enhanced ubiquitination. Our findings suggest that AoRab7A interacts with AoVps41 and AoVps35 to mediate vacuolar fusion and influence lipid droplet accumulation, autophagy, and stress response. These proteins are especially required for the conidiation and trap development of A. oligospora.

A feedback regulation of FgHtf1-FgCon7 loop in conidiogenesis and development of Fusarium graminearum

The transcription factor FgHtf1 is important for conidiogenesis in Fusarium graminearum and it positively regulates the expression of the sporulation-related gene FgCON7. However, the regulatory mechanism underlying its functions is still unclear. The present study intends to uncover the functional mechanism of FgHtf1 in relation to FgCon7 in F. graminearum. We demonstrated that FgCON7 serves as a target gene for FgHtf1. Interestingly, FgCon7 also binds the promoter region of FgHTF1 to negatively regulate its expression, thus forming a negative-feedback loop. We demonstrated that FgHtf1 and FgCon7 have functional redundancy in fungal development. FgCon7 localizes in the nucleus and has transcriptional activation activity. Deletion of FgCON7 significantly reduces conidia production. 4444 genes were regulated by FgCon7 in ChIP-Seq, and RNA-Seq revealed 4430 differentially expressed genes in FgCON7 deletion mutant, with CCAAT serving as a consensus binding motif of FgCon7 to the target genes. FgCon7 directly binds the promoter regions of FgMSN2, FgABAA, FgVEA and FgSMT3 genes and regulates their expression. These genes were found to be important for conidiogenesis. To our knowledge, this is the first study that unveiled the mutual regulatory functions of FgCON7 and FgHTF1 to form a negative-feedback loop, and how the loop mediates sporulation in F. graminearum.

A novel partitivirus conferring hypovirulence by affecting vesicle transport in the fungus Colletotrichum

Colletotrichum spp. are economically important phytopathogenic fungi that cause anthracnose in a variety of plant species worldwide. Hypovirulence-associated mycoviruses provide new options for the biological control of plant fungal diseases. Here, we found a novel partitivirus from Colletotrichum alienum and named it Colletotrichum alienum partitivirus 1 (CaPV1). CaPV1 contained two dsRNA segments encoding an RNA-dependent RNA polymerase and a capsid protein and was classified under the genus Gammapartitivirus of the family Partitiviridae. CaPV1 significantly decreased host virulence, mycelial growth, appressorial development, and appressorium turgor but increased conidial production with abnormal morphology. In addition, CaPV1 could be successfully transfected into other Colletotrichum species, including C. fructicola, C. spaethianum, and C. gloeosporioides, and caused hypovirulence, indicating the broad application potential of this virus. CaPV1 caused significant transcriptional rewiring of the host fungus C. alienum. Notably, some genes related to vesicle transport in the CaPV1-infected strain were downregulated, consistent with the impaired endocytosis pathway in this fungus. When the Rab gene CaRab7, which is associated with endocytosis in vesicle transport, was knocked out, the virulence of the mutants was reduced. Overall, our findings demonstrated that CaPV1 has the potential to control anthracnose caused by Colletotrichum, and the mechanism by which Colletotrichum induces hypovirulence is caused by affecting vesicle transport.IMPORTANCEColletotrichum is a kind of economically important phytopathogenic fungi that cause anthracnose disease in a variety of plant species worldwide. We found a novel mycovirus of the Gammapartitivirus genus and Partitiviridae family from the phytopathogenic fungus Colletotrichum alienum and named it CaPV1. This study revealed that CaPV1 infection significantly decreased host virulence and fitness by affecting mycelial growth, appressorial development, and appressorium turgor. In addition, CaPV1 could also infect other Colletotrichum species, including C. fructicola, C. spaethianum, and C. gloeosporioides, by viral particle transfection and resulting in hypovirulence of these Colletotrichum species. Transcriptomic analysis showed that CaPV1 caused significant transcriptional rewiring of the host fungus C. alienum, especially the genes involved in vesicle transport. Moreover, endocytosis and gene knockout assays demonstrated that the mechanism underlying CaPV1-induced hypovirulence is, at least in part, caused by affecting the vesicle transport of the host fungus. This study provided insights into the mechanisms underlying the pathogenesis of Colletotrichum species and mycovirus-fungus interactions, linking the role of mycovirus and fungus vesicle transport systems in shaping fungal pathogenicity.

Trichoderma virens Big Ras GTPase-1, a molecular switch for transforming a mutualistic fungus to plants in a deleterious microbe

Trichoderma spp. are ascomycete filamentous fungi widely distributed worldwide that establish mutualistic relationships with plants by antagonizing phytopathogens in the rhizosphere and colonizing the plant roots, hence, promoting plant growth and triggering the systemic resistance against phytopathogens. During the first stages of root colonization by Trichoderma, plants recognize the fungus as an invader by inducing the plant defense system, including the production of reactive oxygen species (ROS). Some members of the small Ras GTPases regulate NADPH oxidases and, therefore, ROS production. However, their role in mutualistic microorganisms that colonize plant roots is poorly understood. It has been demonstrated that Trichoderma virens strains lacking TBRG-1, a member of a new family of the Ras GTPases, impair their biocontrol capability and behave like a pathogen on tomato seeds and seedlings. Here, we found that TBRG-1 is essential in T. virens to maintain the mutualistic relationship with plants because a mutant-lacking tbrg-1 gen could not induce plant growth in Arabidopsis and tomatoes. Furthermore, treatment of Arabidopsis seedlings with Δtbrg-1 induced strongly PR-1a, the systemic acquired resistance (SAR) marker gene at early times of the interaction, which correlated with enhanced foliar damage by Botrytis cinerea, resembling the behavior of a biotrophic phytopathogen. Additionally, leaves of plants treated with either T. virens wild-type (wt) or Δtbrg-1 and challenged or not with Botrytis showed ROS production to a different extent, as well as differential expression of cell detoxification-related genes, CAT1, and APX1. Root colonization assays of sid-2 and jar1 mutant lines affected in SAR and induced systemic resistance (ISR) by Δtbrg-1 and the wt strain, suggest an important role of both pathways in the recognition of the fungus and that TBRG-1 plays a pivotal role in root colonization. Furthermore, we found that TBRG-1 is a negative regulator of NoxR expression, which may impact the mutualistic interaction.

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.

Magnaporthe oryzae endoplasmic reticulum membrane complex regulates the biogenesis of membrane proteins for pathogenicity

In eukaryotes, the majority of newly synthesized integral membrane proteins are inserted into the endoplasmic reticulum (ER) membrane before transferred to their functional sites. The conserved ER membrane complex (EMC) takes part in the insertion process for tail-anchored membrane proteins. However, the function of EMC in phytopathogenic fungi has not been characterized. Here, we report the identification and functional characterization of two EMC subunits MoEmc5 and MoEmc2 in Magnaporthe oryzae. The knockout mutants ΔMoemc5 and ΔMoemc2 exhibit substantial defect in autophagy, pathogenicity, cell wall integrity, and magnesium ion sensitivity. We demonstrate that the autophagy process was severely impaired in the ΔMoemc5 and ΔMoemc2 mutants because of the low-protein steady-state level of Atg9, the sole membrane-associated autophagy protein. Furthermore, the protein level of membrane proteins Chs4, Fks1, and MoMnr2 is also significantly reduced in the ΔMoemc5 and ΔMoemc2 strains, leading to their supersensitivity to Calcofluor white, Congo red, and magnesium. In addition, MoEmc5, but not MoEmc2, acts as a magnesium transporter independent of its EMC function. Magnaporthe oryzae EMC regulates the biogenesis of membrane proteins for autophagy and virulence; therefore, EMC subunits could be potential targets for fungicide design in the future.

The mitotic exit mediated by small GTPase Tem1 is essential for the pathogenicity of Fusarium graminearum

The mitotic exit is a key step in cell cycle, but the mechanism of mitotic exit network in the wheat head blight fungus Fusarium graminearum remains unclear. F. graminearum infects wheat spikelets and colonizes the entire head by growing through the rachis node at the bottom of each spikelet. In this study, we found that a small GTPase FgTem1 plays an important role in F. graminearum pathogenicity and functions in regulating the formation of infection structures and invasive hyphal growth on wheat spikelets and wheat coleoptiles, but plays only little roles in vegetative growth and conidiation of the phytopathogen. FgTem1 localizes to both the inner nuclear periphery and the spindle pole bodies, and negatively regulates mitotic exit in F. graminearum. Furthermore, the regulatory mechanisms of FgTem1 have been further investigated by high-throughput co-immunoprecipitation and genetic strategies. The septins FgCdc10 and FgCdc11 were demonstrated to interact with the dominant negative form of FgTem1, and FgCdc11 was found to regulate the localization of FgTem1. The cell cycle arrest protein FgBub2-FgBfa1 complex was shown to act as the GTPase-activating protein (GAP) for FgTem1. We further demonstrated that a direct interaction exists between FgBub2 and FgBfa1 which crucially promotes conidiation, pathogenicity and DON production, and negatively regulates septum formation and nuclear division in F. graminearum. Deletions of FgBUB2 and FgBFA1 genes caused fewer perithecia and immature asci formations, and dramatically down-regulated trichothecene biosynthesis (TRI) gene expressions. Double deletion of FgBUB2/FgBFA1 genes showed that FgBUB2 and FgBFA1 have little functional redundancy in F. graminearum. In summary, we systemically demonstrated that FgTem1 and its GAP FgBub2-FgBfa1 complex are required for fungal development and pathogenicity in F. graminearum.

The crucial role of the regulatory mechanism of the Atg1/ULK1 complex in fungi

Autophagy, an evolutionarily conserved cellular degradation pathway in eukaryotes, is hierarchically regulated by autophagy-related genes (Atgs). The Atg1/ULK1 complex is the most upstream factor involved in autophagy initiation. Here，we summarize the recent studies on the structure and molecular mechanism of the Atg1/ULK1 complex in autophagy initiation, with a special focus on upstream regulation and downstream effectors of Atg1/ULK1. The roles of pathogenicity and autophagy aspects in Atg1/ULK1 complexes of various pathogenic hosts, including plants, insects, and humans, are also discussed in this work based on recent research findings. We establish a framework to study how the Atg1/ULK1 complex integrates the signals that induce autophagy in accordance with fungus to mammalian autophagy regulation pathways. This framework lays the foundation for studying the deeper molecular mechanisms of the Atg1 complex in pathogenic fungi.

Proteomic Profiling of Plant and Pathogen Interaction on the Leaf Epidermis

The plant epidermis is the first line of plant defense against pathogen invasion, and likely contains important regulatory proteins related to the plant–pathogen interaction. This study aims to identify the candidates of these regulatory proteins expressed in the plant epidermis. We performed comparative proteomic studies to identify rapidly and locally expressed proteins in the leaf epidermis inoculated with fungal phytopathogen. The conidia solutions were dropped onto the Arabidopsis leaf surface, and then, we collected the epidermal tissues from inoculated and mock-treated leaves at 4 and 24 hpi. The label-free quantification methods showed that expressions of Arabidopsis proteins, which are related to defense signals, such as BAK1, MKK5, receptor-like protein kinases, transcription factors, and stomatal functions, were rapidly induced in the epidermal tissues of inoculated leaves. In contrast, most of them were not differentially regulated by fugal inoculation in the whole leaves. These findings clearly indicate that epidermal proteomics can monitor locally expressed proteins in inoculated areas of plant tissues. We also identified the 61 fungal proteins, including effector-like proteins specifically expressed on the Arabidopsis epidermis. Our new findings suggested that epidermal proteomics is useful for understanding the local expressions of plant and fungal proteins related to their interactions.

The CfSnt2-Dependent Deacetylation of Histone H3 Mediates Autophagy and Pathogenicity of Colletotrichum fructicola

Camellia oleifera is one of the most valuable woody edible-oil crops, and anthracnose seriously afflicts its yield and quality. We recently showed that the CfSnt2 regulates the pathogenicity of Colletotrichum fructicola, the dominant causal agent of anthracnose on C. oleifera. However, the molecular mechanisms of CfSnt2-mediated pathogenesis remain largely unknown. Here, we found that CfSnt2 is localized to the nucleus to regulate the deacetylation of histone H3. The further transcriptomic analysis revealed that CfSnt2 mediates the expression of global genes, including most autophagy-related genes. Furthermore, we provided evidence showing that CfSnt2 negatively regulates autophagy and is involved in the responses to host-derived ROS and ER stresses. These combined functions contribute to the pivotal roles of CfSnt2 on pathogenicity. Taken together, our studies not only illustrate how CfSnt2 functions in the nucleus, but also link its roles on the autophagy and responses to host-derived stresses with pathogenicity in C. fructicola.

Histone Acetyltransferase CfGcn5-Mediated Autophagy Governs the Pathogenicity of Colletotrichum fructicola

Camellia oleifera is a woody edible-oil plant in China, and anthracnose occurs wherever it is grown, causing serious losses each year. We previously identified that the histone acetyltransferase CfGcn5 orchestrates growth, development, and pathogenicity in Colletotrichum fructicola, the major causal agent of anthracnose on C. oleifera. To elucidate the underlying mechanism, we conducted a transcriptome analysis and found that CfGcn5 is mainly involved in ribosomes, catalytic and metabolic processes, primary metabolism, and autophagy. In addition, we provided evidence showing that CfGcn5 serves as an autophagy repressor to mediate the expression of many autophagy-related genes (ATG) and undergoes degradation during autophagy. Moreover, we found that the CfATG8 and CfATG9 gene-deletion mutants had defects in mitosis and autophagy, resulting in their decreased appressoria formation rates and lower turgor pressure. These combined effects caused the failure of their appressoria functions and caused defects on their pathogenicity, revealing the importance of autophagy in pathogenicity. Taken together, our study illustrates that the autophagy repressor CfGcn5 undergoes degradation in order to regulate autophagy-dependent pathogenicity in C. fructicola.

Oxidative Stress and Autophagy Are Important Processes in Post Ripeness and Brown Film Formation in Mycelium of Lentinula edodes

Lentinula edodes (Berk.) Pegler, the shiitake mushroom, is one of the most important mushrooms in the global mushroom industry. Although mycelium post ripeness and brown film (BF) formation are crucial for fruiting body initiation, the underlying molecular mechanisms of BF formation are largely unknown. In this study, proteomic quantification (relative and absolute) and metabolomic profiling of L. edodes were performed using isobaric tags and gas chromatography-mass spectroscopy, respectively. A total of 2,474 proteins were identified, which included 239 differentially expressed proteins. Notably, several proteins associated with autophagy were upregulated, including RPD3, TOR1, VAC8, VPS1, and VPS27. Transmission electron microscopy also indicated that autophagy occurred in post ripeness and BF formation. In time-dependent analysis of the metabolome, metabolites associated with oxidative stress and autophagy changed significantly, including mannitol, trehalose, myo-inositol, glucose, leucine, valine, glutamine, and 4-aminobutyric acid. Thus, oxidative stress and autophagy were important processes in post ripeness and BF formation in L. edodes, and new insights were gained into molecular mechanisms at proteome and metabolome levels.

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.

Downregulation of RAB7 and Caveolin-1 increases MMP-2 activity in renal tubular epithelial cells under hypoxic conditions

Tubulointerstitial fibrosis leads to tubular basement membrane thickening and accumulation of interstitial extracellular matrix (ECM). Matrix metallopeptidase-2 (MMP-2) is involved in the breakdown of ECM. Chronic hypoxia often occurs in the kidney tissues of patients with chronic kidney disease. Our previous study reported the effect of autophagy and endocytosis on MMP-2 activity in hypoxia-treated HK-2 cells. In this study, the relationship between the Ras-related protein Rab-7a (RAB7) and MMP-2 activity was further investigated. RAB7 overexpression decreased MMP-2 activity. In contrast, the results for RAB7 knockdown displayed the opposite pattern. Short hairpin RNA technology was used to knockdown Caveolin-1 (Cav-1) or Beclin-1 (Bec-1) in HK-2 cells. The two genes displayed differential effects on MMP-2 activity. Cav-1 and RAB7 interference increased MMP-2 activity. This study suggested that autophagy and endocytosis, RAB7, Cav-1, and Bec-1 may serve as potential mediators for altered MMP-2 activity.

Labour-saving construction of a target protein interaction network by selective culture and high-throughput sequencing

Yeast two-hybrid (Y2H) is a classic method of screening for protein-protein interactions. However, the operation process is labor intensive and time consuming, and there is a high possibility of false positives and false negatives. Combined with wet lab operation and bioinformatics analysis, we developed a novel method of Y2H library screening using Chrysanthemum morifolium CmMPK3 as an example. The protocol can not only greatly simplify the steps of traditional Y2H library screening but also identify as many interacting proteins as possible. Furthermore, this protocol is applicable to any species, even if no genomic information is available yet.

RAB7 activity is required for the regulation of mitophagy in oocyte meiosis and oocyte quality control during ovarian aging

There is increasing evidence that mitophagy, a specialized form of autophagy to degrade and clear long-lived or damaged mitochondria, is impaired in aging and age-related disease. Previous study has demonstrated the obesity-exposed oocytes accumulate and transmit damaged mitochondria due to an inability to activate mitophagy. However, it remains unknown whether mitophagy functions in oocyte and what's the regulatory mechanism in oocyte aging. In the study, when fully grown oocytes were treated with CCCP, an uncoupling agent to induce mitophagy, we found the activation of the PRKN-mediated mitophagy pathway accompanied the blockage of meiosis at metaphase I stage. Our result then demonstrated its association with the decreased activity of RAB7 and all the observed defects in CCCP treated oocytes could be effectively rescued by microinjection of mRNA encoding active RAB7^Q67L or treatment with the RAB7 activator ML098. Further study indicated PRKN protein level as a rate-limiting factor to facilitate degradation of RAB7 and its GEF (guanine nucleotide exchange factor) complex CCZ1-MON1 through the ubiquitin-proteasome system. In GV oocytes collected during ovarian aging, we found the age-related increase of PINK1 and PRKN proteins and a significant decrease of RAB7 which resulted in defects of mitophagosome formation and the accumulation of damaged mitochondria. The age-related retardation of female fertility was improved after in vivo treatment of ML098. Thus, RAB7 activity is required to maintain the balance between mitophagy and chromosome stability and RAB7 activator is a good candidate to ameliorate age-related deterioration of oocyte quality.Abbreviations: ATG9: autophagy related 9A; ATP: adenosine triphosphate; CALCOCO2/NDP52: calcium binding and coiled-coil domain 2; CCCP: carbonyl cyanide 3-chlorophenylhydrazone; CCZ1: CCZ1 vacuolar protein trafficking and biogenesis associated; GAPDH: glyceraldehyde-3-phosphate dehydrogenase; GAPs: GTPase-activating proteins; GEF: guanine nucleotide exchange factor; GV: germinal vesicle; GVBD: germinal vesicle breakdown; LAMP1: lysosomal-associated membrane protein 1; MI: metaphase I stage of meiosis; MII: metaphase II stage of meiosis; Mito: MitoTracker; mtDNA: mitochondrial DNA; MON1: MON1 homolog, secretory trafficking associated; OPTN: optineurin; PINK1: PTEN induced putative kinase 1; PRKN: parkin RBR E3 ubiquitin protein ligase; RAB7: RAB7, member RAS oncogene family; ROS: reactive oxygen species; TEM: transmission electron microscopy; TOMM20/TOM20: translocase of outer mitochondrial membrane 20; TUBB: tubulin, beta; UB: ubiquitin.

The Small GTPases in Fungal Signaling Conservation and Function

Monomeric GTPases, which belong to the Ras superfamily, are small proteins involved in many biological processes. They are fine-tuned regulated by guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). Several families have been identified in organisms from different kingdoms. Overall, the most studied families are Ras, Rho, Rab, Ran, Arf, and Miro. Recently, a new family named Big Ras GTPases was reported. As a general rule, the proteins of all families have five characteristic motifs (G1–G5), and some specific features for each family have been described. Here, we present an exhaustive analysis of these small GTPase families in fungi, using 56 different genomes belonging to different phyla. For this purpose, we used distinct approaches such as phylogenetics and sequences analysis. The main functions described for monomeric GTPases in fungi include morphogenesis, secondary metabolism, vesicle trafficking, and virulence, which are discussed here. Their participation during fungus–plant interactions is reviewed as well.

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.

A VASt-domain protein regulates autophagy, membrane tension, and sterol homeostasis in rice blast fungus

Sterols are a class of lipids critical for fundamental biological processes and membrane dynamics. These molecules are synthesized in the endoplasmic reticulum (ER) and are transported bi-directionally between the ER and plasma membrane (PM). However, the trafficking mechanism of sterols and their relationship with macroautophagy/autophagy are still poorly understood in the rice blast fungus Magnaporthe oryzae. Here, we identified the VAD1 Analog of StAR-related lipid transfer (VASt) domain-containing protein MoVast1 via co-immunoprecipitation in M. oryzae. Loss of MoVAST1 resulted in conidial defects, impaired appressorium development, and reduced pathogenicity. The MoTor (target of rapamycin in M. oryzae) activity is inhibited because MoVast1 deletion leads to high levels of sterol accumulation in the PM. Site-directed mutagenesis showed that the 902 T site is essential for localization and function of MoVast1. Through filipin or Flipper-TR staining, autophagic flux detection, MoAtg8 lipidation, and drug sensitivity assays, we uncovered that MoVast1 acts as a novel autophagy inhibition factor that monitors tension in the PM by regulating the sterol content, which in turn modulates the activity of MoTor. Lipidomics and transcriptomics analyses further confirmed that MoVast1 is an important regulator of lipid metabolism and the autophagy pathway. Our results revealed and characterized a novel sterol transfer protein important for M. oryzae pathogenicity.Abbreviations: AmB: amphotericin B; ATMT: Agrobacterium tumefaciens-mediated transformation; CM: complete medium; dpi: days post-inoculation; ER: endoplasmic reticulum; Flipper-TR: fluorescent lipid tension reporter; GO: Gene ontology; hpi: hours post-inoculation; IH: invasive hyphae; KEGG: kyoto encyclopedia of genes and genomes; MoTor: target of rapamycin in Magnaporthe oryzae; PalmC: palmitoylcarnitine; PM: plasma membrane; SD-N: synthetic defined medium without amino acids and ammonium sulfate; TOR: target of rapamycin; VASt: VAD1 Analog of StAR-related lipid transfer; YFP, yellow fluorescent protein.

Salt-induced expression of intracellular vesicle trafficking genes, CaRab-GTP, and their association with Na+ accumulation in leaves of chickpea (Cicer arietinum L.)

BACKGROUND

Chickpea is an important legume and is moderately tolerant to salinity stress during the growing season. However, the level and mechanisms for salinity tolerance can vary among accessions and cultivars. A large family of CaRab-GTP genes, previously identified in chickpea, is homologous to intracellular vesicle trafficking superfamily genes that play essential roles in response to salinity stress in plants.

RESULTS

To determine which of the gene family members are involved in the chickpea salt response, plants from six selected chickpea accessions (Genesis 836, Hattrick, ICC12726, Rupali, Slasher and Yubileiny) were exposed to salinity stress and expression profiles resolved for the major CaRab-GTP gene clades after 5, 9 and 15 days of salt exposure. Gene clade expression profiles (using degenerate primers targeting all members of each clade) were tested for their relationship to salinity tolerance measures, namely plant biomass and Na⁺ accumulation. Transcripts representing 11 out of the 13 CaRab clades could be detected by RT-PCR, but only six (CaRabA2, -B, -C, -D, -E and -H) could be quantified using qRT-PCR due to low expression levels or poor amplification efficiency of the degenerate primers for clades containing several gene members. Expression profiles of three gene clades, CaRabB, -D and -E, were very similar across all six chickpea accessions, showing a strongly coordinated network. Salt-induced enhancement of CaRabA2 expression at 15 days showed a very strong positive correlation (R² = 0.905) with Na⁺ accumulation in leaves. However, salinity tolerance estimated as relative plant biomass production compared to controls, did not correlate with Na⁺ accumulation in leaves, nor with expression profiles of any of the investigated CaRab-GTP genes.

CONCLUSION

A coordinated network of CaRab-GTP genes, which are likely involved in intracellular trafficking, are important for the salinity stress response of chickpea plants.

The ADP-ribosylation factor-like small GTPase FgArl1 participates in growth, pathogenicity and DON production in Fusarium graminearum

Fusarium graminearum is the main pathogen of Fusarium head blight (FHB) in wheat and related species, which causes serious production decreases and economic losses and produces toxins such as deoxynivalenol (DON), which endangers the health of humans and livestock. Vesicle transport is a basic physiological process required for cell survival in eukaryotes. Many regulators of vesicle transport are reported to be involved in the pathogenicity of fungi. In yeast and mammalian cells, the ADP-ribosylation factor-like small GTPase Arl1 and its orthologs are involved in regulating vesicular trafficking, cytoskeletal reorganization and other significant biological processes. However, the role of Arl1 in F. graminearum is not well understood. In this study, we characterized the Arl1-homologous protein FgArl1 in F. graminearum and showed that FgArl1 is located in the trans-Golgi apparatus. The deletion of FgARL1 resulted in a significant decrease in vegetative growth and pathogenicity. Further analyses of the ΔFgarl1 mutant revealed defects in the production of DON. Taken together, these results indicate that FgArl1 is important in the development and pathogenicity of F. graminearum.

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.

FgVps9, a Rab5 GEF, Is Critical for DON Biosynthesis and Pathogenicity in Fusarium graminearum

Rab GTPases play an important role in vesicle-mediated membrane trafficking in eukaryotes. Previous studies have demonstrated that deletion of RAB5/VPS21 reduces endocytosis and virulence of fungal phytopathogens in their host plants. However, Rab5 GTPase cycle regulators have not been characterized in Fusarium graminearum, the causal agent of Fusarium head blight (FHB) or head scab disease in cereal crops. In this study, we have identified and characterized a Rab5 guanine nucleotide exchange factor (GEF), the Vps9 homolog FgVps9, in F. graminearum. Yeast two hybrid (Y2H) assays have shown that FgVps9 specifically interacts with the guanosine diphosphate (GDP)-bound (inactive) forms of FgRab51 and FgRab52, the Rab5 isoforms in F. graminearum. Deletion of FgVPS9 shows impaired fungal growth and conidiation. Pathogenicity assays indicate that deletion of FgVPS9 can significantly decrease the virulence of F. graminearum in wheat. Cytological analyses have indicated that FgVps9 colocalizes with FgRab51 and FgRab52 on early endosomes and regulates endocytosis and autophagy processes. Gene expression and cytological examination have shown that FgVps9 and FgRab51 or FgRab52 function in concert to control deoxynivalenol (DON) biosynthesis by regulating the expression of trichothecene biosynthesis-related genes and toxisome biogenesis. Taken together, FgVps9 functions as a GEF for FgRab51 and FgRab52 to regulate endocytosis, which, as a basic cellular function, has significant impact on the vegetative growth, asexual development, autophagy, DON production, and plant infection in F. graminearum.

FgHtf1 Regulates Global Gene Expression towards Aerial Mycelium and Conidiophore Formation in the Cereal Fungal Pathogen Fusarium graminearum

The homeobox gene family of transcription factors (HTF) controls many developmental pathways and physiological processes in eukaryotes. We previously showed that a conserved HTF in the plant-pathogenic fungus Fusarium graminearum, Htf1 (FgHtf1), regulates conidium morphology in that organism. This study investigated the mechanism of FgHtf1-mediated regulation and identified putative FgHtf1 target genes by a chromatin immunoprecipitation assay combined with parallel DNA sequencing (ChIP-seq) and RNA sequencing. A total of 186 potential binding peaks, including 142 genes directly regulated by FgHtf1, were identified. Subsequent motif prediction analysis identified two DNA-binding motifs, TAAT and CTTGT. Among the FgHtf1 target genes were FgHTF1 itself and several important conidiation-related genes (e.g., FgCON7), the chitin synthase pathway genes, and the aurofusarin biosynthetic pathway genes. In addition, FgHtf1 may regulate the cAMP-protein kinase A (PKA)-Msn2/4 and Ca²⁺-calcineurin-Crz1 pathways. Taken together, these results suggest that, in addition to autoregulation, FgHtf1 also controls global gene expression and promotes a shift to aerial growth and conidiation in F. graminearum by activation of FgCON7 or other conidiation-related genes.IMPORTANCE The homeobox gene family of transcription factors is known to be involved in the development and conidiation of filamentous fungi. However, the regulatory mechanisms and downstream targets of homeobox genes remain unclear. FgHtf1 is a homeobox transcription factor that is required for phialide development and conidiogenesis in the plant pathogen F. graminearum In this study, we identified FgHtf1-controlled target genes and binding motifs. We found that, besides autoregulation, FgHtf1 also controls global gene expression and promotes conidiation in F. graminearum by activation of genes necessary for aerial growth, FgCON7, and other conidiation-related genes.

Crosstalk between Endo/Exocytosis and Autophagy in Health and Disease

Imbalance between the main intracellular degradative, trafficking and intercellular shuttling pathways has been implicated in disease pathogenesis. Autophagy controls degradation of cellular components, while vesicular trafficking permits transport of material in and out of the cell. Emerging evidence has uncovered the extensive interconnectivity between these pathways, which is crucial to maintain organismal homeostasis. Thus, therapeutic intervention and drug development strategies targeting these processes, particularly in neurodegeneration, should account for this broad crosstalk, to maximize effectiveness. Here, recent findings underlining the highly dynamic nature of the crosstalk between autophagy, endosomal transport, and secretion is reviewed. Synergy of autophagy and endosomes for degradation, as well as, competition of autophagy and secretion are discussed. Perturbation of this crosstalk triggers pathology especially neurodegeneration.

FolVps9, a Guanine Nucleotide Exchange Factor for FolVps21, Is Essential for Fungal Development and Pathogenicity in Fusarium oxysporum f. sp. lycopersici

The soil-borne, asexual fungus Fusarium oxysporum f.sp. lycopersici (Fol) is the causal agent of tomato wilt disease. Autophagy plays a crucial role in the development and virulence of Fol. The Fol endosomal system is highly dynamic and has been shown to be associated with conidiogenesis and pathogenicity. Rab GTPases and the regulators are highly conserved in regulating autophagy and endocytosis in most eukaryotes. Identification and characterization of additional Rab regulators in fungal pathogens should facilitate the understanding of the autophagy and endocytosis in different filamentous fungi. Here, we have identified and characterized the yeast VPS9 homolog FolVPS9 in Fol. Targeted gene deletion showed that FolVPS9 is important for growth, conidiation and virulence in Fol. Cytological examination revealed that FolVps9 co-localized with FolVps21 (a marker of early endosome) and played a critical role in endocytosis and autophagosome degradation. Pull-down assays showed that FolVps9 interacted with FolVps21, which was also important for development and plant infection in Fol. Yeast two-hybrid, bimolecular fluorescence complementation and co-immunoprecipitation assays revealed that FolVps9 specifically interacts with the GDP-bound form of FolVps21. Furthermore, a constitutively active form of FolVps21 (Q72L) was able to rescue defects of ΔFolvps9 and ΔFolvps21 mutants. In summary, our study provides solid evidence that FolVps9 acts as a FolVps21 guanine nucleotide exchange factor (GEFs) to modulate endocytosis and autophagy, thereby controlling vegetative growth, asexual development and pathogenicity in Fol.

Si vis pacem para bellum: A prospective in silico analysis of miRNA-based plant defenses against fungal infections

Fungal pathogens are an important threat for plant crops, being responsible for important reductions of production yields and a consequent economic impact. Among the molecular mediators of fungal infections of plant crops, non-coding RNAs (ncRNAs) have been described as relevant players either in the plant immune responses and mechanism of defense or in the colonization of plant tissues by fungi. Acting as a mechanism of defense, some plant small ncRNAs such as miRNAs and tasiRNAs can be secreted by cells and directed to target the transcriptome of pathogenic fungi, triggering an RNAi-like interference mechanism able to silence the expression of specific fungal genes. The detailed knowledge of this mechanism of defense against fungal pathogens could open new possibilities for the protection of human important crops. To infer putative functional relationships mediated by ncRNA communication, we performed a prospective analysis to determine potential plant miRNAs able to target the genome of fungal pathogens, which resulted in the description of enriched specific plant miRNA families and their putative fungal targets that could be further studied in the context of plant-fungi interactions. The expression profile of specific members of the enriched miRNAs families showed an infection-dependent behavior in laboratory models of infection. Plant miRNAs showed sequence complementarity with coding genes of their cognate fungal pathogens. Plant miRNAs could potentially target fungal genes belonging to functional families related to stress response, membrane architecture, vacuolar transport, membrane traffic, and anabolic processes. Families of specific infection-responsive miRNAs are included in the putative plant defense mechanism.

Diverse Role of SNARE Protein Sec22 in Vesicle Trafficking, Membrane Fusion, and Autophagy

Protein synthesis begins at free ribosomes or ribosomes attached with the endoplasmic reticulum (ER). Newly synthesized proteins are transported to the plasma membrane for secretion through conventional or unconventional pathways. In conventional protein secretion, proteins are transported from the ER lumen to Golgi lumen and through various other compartments to be secreted at the plasma membrane, while unconventional protein secretion bypasses the Golgi apparatus. Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNARE) proteins are involved in cargo vesicle trafficking and membrane fusion. The ER localized vesicle associated SNARE (v-SNARE) protein Sec22 plays a major role during anterograde and retrograde transport by promoting efficient membrane fusion and assisting in the assembly of higher order complexes by homodimer formation. Sec22 is not only confined to ER-Golgi intermediate compartments (ERGIC) but also facilitates formation of contact sites between ER and plasma membranes. Sec22 mutation is responsible for the development of atherosclerosis and symptoms in the brain in Alzheimer's disease and aging in humans. In the fruit fly Drosophila melanogaster, Sec22 is essential for photoreceptor morphogenesis, the wingless signaling pathway, and normal ER, Golgi, and endosome morphology. In the plant Arabidopsis thaliana, it is involved in development, and in the nematode Caenorhabditis elegans, it is in involved in the RNA interference (RNAi) pathway. In filamentous fungi, it affects cell wall integrity, growth, reproduction, pathogenicity, regulation of reactive oxygen species (ROS), expression of extracellular enzymes, and transcriptional regulation of many development related genes. This review provides a detailed account of Sec22 function, summarizes its domain structure, discusses its genetic redundancy with Ykt6, discusses what is known about its localization to discrete membranes, its contributions in conventional and unconventional autophagy, and a variety of other roles across different cellular systems ranging from higher to lower eukaryotes, and highlights some of the surprises that have originated from research on Sec22.

Current opinions on autophagy in pathogenicity of fungi

The interaction between pathogens and their host plants is a ubiquitous process. Some plant fungal pathogens can form a specific infection structure, such as an appressorium, which is formed by the accumulation of a large amount of glycerin and thereby the creation of an extremely high intracellular turgor pressure, which allows the penetration peg of the appressorium to puncture the leaf cuticle of the host. Previous studies have shown that autophagy energizes the accumulation of pressure by appressoria, which induces its pathogenesis. Similar to other eukaryotic organisms, autophagy processes are highly conserved pathways that play important roles in filamentous fungal pathogenicity. This review aims to demonstrate how the autophagy process affects the pathogenicity of plant pathogens.

FgSec2A, a guanine nucleotide exchange factor of FgRab8, is important for polarized growth, pathogenicity and deoxynivalenol production in Fusarium graminearum

Sec4/Rab8 is one of the well-studied members of the Rab GTPase family, previous studies have shown that Sec4/Rab8 crucially promotes the pathogenesis of phytopathogens, but the upstream regulators of Rab8 are still unknown. Here, we have identified two Sec2 homologues FgSec2A and FgSec2B in devastating fungal pathogen Fusarium graminearum and investigated their functions and interactions with FgRab8 by live-cell imaging, genetic and functional analyses. Yeast two-hybrid assay shows that FgSec2A specifically interacts with FgRab8DN(N123I) and itself. Importantly, FgSec2A is required for growth, conidiation, DON production and virulence of F. graminearum. Live-cell imaging shows that FgSec2A and FgSec2B are both localized to the tip region of hyphae and conidia. Both N-terminal region and Sec2 domain of FgSec2A are essential for its function, but not for localization, whereas the C-terminal region is important for its polarized localization. Furthermore, constitutively active FgRab8CA(Q69L) partially rescues the defects of ΔFgsec2A. Consistently, FgSec2A is required for the polarized localization of FgRab8. Finally, FgSec2A and FgSec2B show partial functions, but FgSec2A does not interact and co-localize with FgSec2B. Taken together, these results indicate that FgSec2A acts as a FgRab8 guanine nucleotide exchange factor and is necessary for polarized growth, DON production and pathogenicity in F. graminearum.