A Cdc42 homolog in Colletotrichum gloeosporioides regulates morphological development and is required for ROS-mediated plant infection

Transcription Factor CgSte12 Regulates Pathogenicity by Affecting Appressorium Structural Development in the Anthracnose-Causing Fungus Colletotrichum gloeosporioides

Colletotrichum gloeosporioides is the causal agent of poplar anthracnose, which induces major economic losses and adversely affects the ecosystem services of poplar forests. The appressorium serves as a penetration structure for many pathogenic fungi, including C. gloeosporioides. The production of mucilage and the formation of penetration pegs are critically important for the appressorium-mediated penetration of host tissues. We previously found that CgPmk1 is a key protein involved in appressorium formation, penetration, and pathogenicity. Although CgSte12, which is a transcription factor that functions downstream of CgPmk1, regulates the formation of penetration pegs, its role in C. gloeosporioides appressorium development and pathogenicity has not been elucidated. Here, we developed C. gloeosporioides CgSTE12 mutants and characterized the molecular and cellular functions of CgSTE12. The results showed that mycelial growth and morphology were not affected in the CgSTE12 knockout mutants, which produced normal melanized appressoria. However, these mutants had less mucilage secreted around the appressoria, impaired appressorial cone formation, and the inability to form penetration pores and pegs, which ultimately led to a significant loss of pathogenicity. Our comparative transcriptome analysis revealed that CgSte12 controls the expression of genes involved in appressorium development and function, including genes encoding cutinases, NADPH oxidase, spermine biosynthesis-related proteins, ceramide biosynthesis-related proteins, fatty acid metabolism-related proteins, and glycerophospholipid metabolism-related proteins. Overall, our findings indicate that CgSte12 is a critical regulator of appressorium development and affects C. gloeosporioides pathogenicity by modulating the structural integrity of appressoria.

Gcc1 homologs regulate growth, oxidative stress, conidiation and appressorium formation in Colletotrichum siamense and Colletotrichum graminicola

The Zn2Cys6 transcription factor is a fungal-specific zinc finger protein, which plays an important role in regulating growth, development and pathogenicity of pathogenic fungi. In this study, we characterized two Zn2Cys6 transcription factors, CsGcc1 and CgrGcc1 in Colletotrichum siamense and C. graminicola, respectively, which are homologous to Gcc1 in Magnaporthe oryzae. Both CsGcc1 and CgrGcc1 contain a typical GAL4 DNA-binding domain. Deletion of CsGCC1 or CgrGCC1 decreased the growth rate and lowered the tolerance to H2O2. In addition, disrupting CsGCC1 reduced conidial yield and lowered the germination rate and appressorium formation rate of C. siamense. Cellophane assays showed that deletion of CsGCC1 also weakened the penetration ability of appressoria. In C. graminicola, CgrGcc1 did not affect the production and germination of oval conidia, but its deletion significantly decreased the yield of the falcate conidium, and led to abnormal appressorium formation. In terms of pathogenicity, CsGcc1 slightly reduced the virulence of C. siamense, while deleting CgrGcc1 did not affect virulence of C. graminicola. In conclusion, the Zn2Cys6 transcription factors CsGcc1 and CgrGcc1 are involved in the regulation of vegetative growth, oxidative stress, conidial/falcate conidial production and appressorium formation in C. siamense and C. graminicola.

The velvet proteins CsVosA and CsVelB coordinate growth, cell wall integrity, sporulation, conidial viability and pathogenicity in the rubber anthracnose fungus Colletotrichum siamense

Colletotrichum siamense, a member of Colletotrichum gloeosporioides complex species, is the primary pathogen causing rubber anthracnose, which leads to significant economic loss in natural rubber production. Velvet family proteins are fungal-specific proteins and play an essential role in regulating development and secondary metabolism. In this study, we characterized two velvet proteins CsVosA and CsVelB in C. siamense as the orthologs of VosA and VelB in Aspergillus nidulans. CsVosA is located in the nucleus, and CsVelB displays a localization in both the nucleus and the cytoplasm. Deleting CsvosA or CsvelB results in a slow growth rate, and the CsvelB-knockout mutants also exhibit low mycelial density. CsVosA and CsVelB are involved in regulating chitin metabolism and distribution, leading to the variation in the cell wall integrity of C. siamense. Furthermore, disruption of CsvosA or CsvelB can decrease conidial production and viability, and the ΔCsvosA and ΔCsvelB mutants also lose the ability to produce fruiting bodies. Pathogenicity assays show that deleting CsvosA or CsvelB can lower the virulence, and the two velvet genes are essential for the full virulence of C. siamense. Based on the results of the yeast two-hybrid analysis and bimolecular fluorescence complementation assays, CsVosA can interact with CsVelB and form the complex CsVosA-CsVelB in the conidia of C. siamense, which may play essential roles in maintaining the cell wall integrity and conidial viability. In addition, CsVelB is also involved in regulating melanin production of C. siamense. In conclusion, CsVosA and CsVelB regulate vegetative growth, cell wall integrity, asexual/sexual sporulation, conidial viability and virulence in C. siamense.

Appressoria-Small but Incredibly Powerful Structures in Plant-Pathogen Interactions

Plant-pathogenic fungi are responsible for many of the most severe crop diseases in the world and remain very challenging to control. Improving current protection strategies or designating new measures based on an overall understanding of molecular host-pathogen interaction mechanisms could be helpful for disease management. The attachment and penetration of the plant surface are the most important events among diverse plant-fungi interactions. Fungi evolved as small but incredibly powerful infection structure appressoria to facilitate attachment and penetration. Appressoria are indispensable for many diseases, such as rusts, powdery mildews, and blast diseases, as well as devastating oomycete diseases. Investigation into the formation of plant-pathogen appressoria contributes to improving the understanding of the molecular mechanisms of plant-pathogen interactions. Fungal host attachment is a vital step of fungal pathogenesis. Here, we review recent advances in the molecular mechanisms regulating the formation of appressoria. Additionally, some biocontrol agents were revealed to act on appressorium. The regulation of fungal adhesion during the infective process by acting on appressoria formation is expected to prevent the occurrence of crop disease caused by some pathogenic fungi.

Investigation of bioactive compounds from Bacillus sp. against protein homologs CDC42 of Colletotrichum gloeosporioides causing anthracnose disease in cassava by using molecular docking and dynamics studies

Manihot esculenta, commonly called cassava, is an economically valuable crop and important staple food, grown in tropical and subtropical regions of the world. Demand for cassava in the food and fuel industry is growing worldwide. However, anthracnose disease caused by Colletotrichum gloeosporioides severely affects cassava yield and production. The bioactive molecules from Bacillus are widely used to control fungal diseases in several plants. Therefore, in this study, bioactive compounds (erucamide, behenic acid, palmitic acid, phenylacetic acid, and β-sitosterol) from Bacillus megaterium were assessed against CDC42, a key protein for virulence, from C. gloeosporioides. Structure of the CDC42 protein was generated through the comparative homology modeling method. The binding site of the ligands and the stability of the complex were analyzed through docking and molecular dynamics simulation studies, respectively. Furthermore, a protein interaction network was envisaged through the STRING database, followed by enrichment analysis in the WebGestalt tool. From the enrichment analysis, it is apparent that bioactive from B. megaterium chiefly targets the MAP kinase pathway that is essential for filamentous growth and virulence. Further exploration through experimental studies could be advantageous for cassava improvement as well as to combat against C. gloeosporioides pathogen.

SsCox17, a copper chaperone, is required for pathogenic process and oxidative stress tolerance of Sclerotinia sclerotiorum

Stem rot, caused by Sclerotinia sclerotiorum has emerged as one of the major fungal pathogens of oilseed Brassica across the world. The pathogenic development is exquisitely dependent on reactive oxygen species (ROS) modulation. Cox17 is a crucial factor that shuttles copper ions from the cytosol to the mitochondria for the cytochrome c oxidase (CCO) assembly. Currently, no data is available regarding the impact of Cox17 in fungal pathogenesis. The present research was carried out to functionally characterize the role of Cox17 in S. sclerotiorum pathogenesis. SsCox17 transcripts showed high expression levels during inoculation on rapeseed. Intramitochondrial copper content and CCO activity were decreased in SsCox17 gene-silenced strains. The SsCox17 gene expression was up-regulated in the hyphae under oxidative stress and a deficiency response to oxidative stress was detected in SsCox17 gene-silenced strains. Compared to the S. sclerotiorum wild-type strain, there was a concomitant reduction in the virulence of SsCox17 gene-silenced strains. The SsCox17 overexpression strain was further found to increase copper content, CCO activity, tolerance to oxidative stress and virulence. We also observed a certain correlation of appressoria formation and SsCox17. These results provide evidence that SsCox17 is positively associated with fungal virulence and oxidative detoxification.

Unveiling a Novel Role of Cdc42 in Pyruvate Metabolism Pathway to Mediate Insecticidal Activity of Beauveria bassiana

The small GTPase Cdc42 acts as a molecular switch essential for cell cycles and polar growth in model yeast, but has not been explored in Beaurveria bassiana, an insect-pathogenic fungus serving as a main source of fungal formulations against arthropod pests. Here, we show the indispensability of Cdc42 for fungal insecticidal activity. Deletion of cdc42 in B. bassiana resulted in a great loss of virulence to Galleria mellonella, a model insect, via normal cuticle infection as well as defects in conidial germination, radial growth, aerial conidiation, and conidial tolerance to heat and UVB irradiation. The deleted mutant’s hyphae formed fewer or more septa and produced unicellular blastospores with disturbed cell cycles under submerged-culture conditions. Transcriptomic analysis revealed differential expression of 746 genes and dysregulation of pyruvate metabolism and related pathways, which were validated by marked changes in intracellular pyruvate content, ATP content, related enzyme activities, and in extracellular beauvericin content and Pr1 protease activity vital for fungal virulence. These findings uncover a novel role for Cdc42 in the pathways of pyruvate metabolism and the pyruvate-involved tricarboxylic acid cycle (TCA cycle) and a linkage of the novel role with its indispensability for the biological control potential of B. bassiana against arthropod pests.

Protein Phosphatase CgPpz1 Regulates Potassium Uptake, Stress Responses, and Plant Infection in Colletotrichum gloeosporioides

Protein phosphatases play important roles in the regulation of various cellular processes in eukaryotes. The ascomycete Colletotrichum gloeosporioides is a causal agent of anthracnose disease on some important crops and trees. In this study, CgPPZ1, a protein phosphate gene and a homolog of yeast PPZ1, was identified in C. gloeosporioides. Targeted gene deletion showed that CgPpz1 was important for vegetative growth and asexual development, conidial germination, and plant infection. Cytological examinations revealed that CgPpz1 was localized to the cytoplasm. The ΔCgppz1 mutant was hypersensitive to osmotic stresses, cell wall stressors, and oxidative stressors. Taken together, our results indicated that CgPpz1 plays an important role in the fungal development and virulence of C. gloeosporioides and the multiple stress responses generated.

Mucin Msb2 cooperates with the transmembrane protein Sho1 in various plant surface signal sensing and pathogenic processes in the poplar anthracnose fungus Colletotrichum gloeosporioides

Colletotrichum gloeosporioides is a hemibiotrophic ascomycete fungus that causes anthracnose on numerous plants worldwide and forms a specialized infection structure known as an appressorium in response to various plant surface signals. However, the associated mechanism of host surface signal recognition remains unclear. In the present study, three putative sensors, namely the mucin Msb2, the membrane sensor protein Sho1, and the G-protein-coupled receptor Pth11, were identified and characterized. The results showed that CgMsb2 plays a major role in the recognition of various host surface signals; deletion of CgMsb2 resulted in significant defects in appressorium formation, appressorium penetration, cellophane membrane penetration, and pathogenicity. CgSho1 plays a minor role and together with CgMsb2 cooperatively regulates host signal recognition, cellophane membrane penetration, and pathogenicity; deletion of CgSho1 resulted in an expansion defect of infection hyphae. Deletion of CgPth11 in wildtype, ΔCgMsb2, and ΔCgSho1 strains only resulted in a slight defect in appressorium formation at the early stage, and CgPth11 was dispensable for penetration and pathogenicity. However, exogenous cAMP failed to restore the defect of appressorium formation in ΔCgPth11 at the early stage. CgMsb2 contributed to the phosphorylation of the mitogen-activated protein kinase CgMk1, which is essential for infection-associated functions, while CgSho1 was unable to activate CgMk1 alone but rather cooperated with CgMsb2 to activate CgMk1. These data suggest that CgMsb2 contributes to the activation of CgMk1 and has overlapping functions with CgSho1 in plant surface sensing, appressorium formation, and pathogenicity.

The Role of Plant Hormones in the Interaction of Colletotrichum Species with Their Host Plants

Colletotrichum is a plant pathogenic fungus which is able to infect virtually every economically important plant species. Up to now no common infection mechanism has been identified comparing different plant and Colletotrichum species. Plant hormones play a crucial role in plant-pathogen interactions regardless whether they are symbiotic or pathogenic. In this review we analyze the role of ethylene, abscisic acid, jasmonic acid, auxin and salicylic acid during Colletotrichum infections. Different Colletotrichum strains are capable of auxin production and this might contribute to virulence. In this review the role of different plant hormones in plant—Colletotrichum interactions will be discussed and thereby auxin biosynthetic pathways in Colletotrichum spp. will be proposed.

CgEnd3 Regulates Endocytosis, Appressorium Formation, and Virulence in the Poplar Anthracnose Fungus Colletotrichum gloeosporioides

The hemibiotrophic ascomycete fungus Colletotrichum gloeosporioides is the causal agent of anthracnose on numerous plants, and it causes considerable economic losses worldwide. Endocytosis is an essential cellular process in eukaryotic cells, but its roles in C. gloeosporioides remain unknown. In our study, we identified an endocytosis-related protein, CgEnd3, and knocked it out via polyethylene glycol (PEG)-mediated protoplast transformation. The lack of CgEnd3 resulted in severe defects in endocytosis. C. gloeosporioides infects its host through a specialized structure called appressorium, and ΔCgEnd3 showed deficient appressorium formation, melanization, turgor pressure accumulation, penetration ability of appressorium, cellophane membrane penetration, and pathogenicity. CgEnd3 also affected oxidant adaptation and the expression of core effectors during the early stage of infection. CgEnd3 contains one EF hand domain and four calcium ion-binding sites, and it is involved in calcium signaling. A lack of CgEnd3 changed the responses to cell-wall integrity agents and fungicide fludioxonil. However, CgEnd3 regulated appressorium formation and endocytosis in a calcium signaling-independent manner. Taken together, these results demonstrate that CgEnd3 plays pleiotropic roles in endocytosis, calcium signaling, cell-wall integrity, appressorium formation, penetration, and pathogenicity in C. gloeosporioides, and it suggests that CgEnd3 or endocytosis-related genes function as promising antifungal targets.

The Cytospora chrysosperma Virulence Effector CcCAP1 Mainly Localizes to the Plant Nucleus To Suppress Plant Immune Responses

Canker disease is caused by the fungus Cytospora chrysosperma and damages a wide range of woody plants, causing major losses to crops and native plants. Plant pathogens secrete virulence-related effectors into host cells during infection to regulate plant immunity and promote colonization. However, the functions of C. chrysosperma effectors remain largely unknown. In this study, we used Agrobacterium tumefaciens-mediated transient expression system in Nicotiana benthamiana and confocal microscopy to investigate the immunoregulation roles and subcellular localization of CcCAP1, a virulence-related effector identified in C. chrysosperma CcCAP1 was significantly induced in the early stages of infection and contains cysteine-rich secretory proteins, antigen 5, and pathogenesis-related 1 proteins (CAP) superfamily domain with four cysteines. CcCAP1 suppressed the programmed cell death triggered by Bcl-2-associated X protein (BAX) and the elicitin infestin1 (INF1) in transient expression assays with Nicotiana benthamiana The CAP superfamily domain was sufficient for its cell death-inhibiting activity and three of the four cysteines in the CAP superfamily domain were indispensable for its activity. Pathogen challenge assays in N. benthamiana demonstrated that transient expression of CcCAP1 promoted Botrytis cinerea infection and restricted reactive oxygen species accumulation, callose deposition, and defense-related gene expression. In addition, expression of green fluorescent protein-labeled CcCAP1 in N. benthamiana showed that it localized to both the plant nucleus and the cytoplasm, but the nuclear localization was essential for its full immune inhibiting activity. These results suggest that this virulence-related effector of C. chrysosperma modulates plant immunity and functions mainly via its nuclear localization and the CAP domain.IMPORTANCE The data presented in this study provide a key resource for understanding the biology and molecular basis of necrotrophic pathogen responses to Nicotiana benthamiana resistance utilizing effector proteins, and CcCAP1 may be used in future studies to understand effector-triggered susceptibility processes in the Cytospora chrysosperma-poplar interaction system.

In vitro growth of Colletotrichum gloeosporioides is affected by butyl acetate, a compound produced during the co-culture of Trichoderma sp. and Bacillus subtilis

The co-culture of plant beneficial microbes to stimulate the production of antimicrobial metabolites is gaining ground. Here, the inactivated Colletotrichum gloeosporioides mycelium was used to induce the biosynthesis of antifungal compounds in the co-culture systems of Trichoderma sp. and Bacillus subtilis. The hexanic extracts obtained from the co-culture systems were tested against C. gloeosporioides. Those that inhibited the phytopathogen growth were further fractionated by column and thin-layer chromatography and analyzed by gas chromatography coupled to mass spectrometry (GC-MS). Ethyl butanoate, butyl acetate, acetic acid, 2-butoxyethanol, 3,5-di-tert-butyl-4-hydroxybenzaldehyde, 3,5-di-tert-butyl-4-hydroxybenzyl alcohol, hexadecanoic acid, and octadecanoic acid were identified. Butyl acetate was the most abundant compound, and its application affected the morphology and mycelial development of C. gloeosporioides, thereby inhibiting the radial growth, reducing spore formation, and inducing soft colonies. We conclude that co-culturing Trichoderma sp. and B. subtilis promotes the production of novel diffusible organic compounds with an antifungal effect on C. gloeosporioides.

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.