MoSec61β, the beta subunit of Sec61, is involved in fungal development and pathogenicity, plant immunity, and ER-phagy in Magnaporthe oryzae

Role of BcSfb3, the subunit of COPII vesicles, in fungal development and pathogenicity, ER-phagy and autophagy in the gray mold fungus Botrytis cinerea

Cytoplasmic coat protein complex II (COPII) plays a multifunctional role in the transport of newly synthesized proteins, autophagosome formation, and endoplasmic reticulum (ER)-ER-phagy. However, the molecular mechanisms of the COPII subunit in ER-phagy in plant pathogens remain unknown. Here, we identified the subunit of COPII vesicles (BcSfb3) and explored the importance of BcSfb3 in Botrytis cinerea. BcSfb3 deletion affected vegetative growth, conidiation, conidial morphology, and plasma membrane integrity. We confirmed that the increase in infectious hyphal growth was delayed in the ΔBcSfb3 mutant, reducing its pathogenicity in the host plant. Furthermore, the ΔBcSfb3 mutant was sensitive to ER stress, which caused massive ER expansion and induced the formation of ER whorls that were taken up into the vacuole. Further examination demonstrated that BcSfb3 deletion caused ER stress initiated by unfolded protein response, and which led to the promotion of ER-phagy and autophagy that participate in sclerotia formation. In conclusion, these results demonstrate that BcSfb3 plays an important role in fungal development, pathogenesis, ER-phagy and autophagy in B. cinerea.

CfErp3 regulates growth, conidiation, inducing ipomeamarone and the pathogenicity of Ceratocystis fimbriata

The Erp3 protein, which is an important member of the p24 family, is primarily responsible for the transport of cargo from the ER to the Golgi apparatus in Saccharomyces cerevisiae. However, the function of Erp3 in plant pathogenic fungi has not been reported. In this study, we characterized the ERP3 gene in Ceratocystis fimbriata, which causes the devastating disease sweetpotato black rot. The ΔCferp3 mutants exhibited slow growth, reduced conidia production, attenuated virulence, and reduced ability to induce host to produce toxins. Further analysis revealed that CfErp3 was localized in the ER and vesicles and regulated endocytosis, cell wall integrity, and osmotic stress responses, modulated ROS levels, and the production of ipomeamarone during pathogen-host interactions. These results indicate that CfErp3 regulates C. fimbriata growth and pathogenicity as well as the production of ipomeamarone in sweetpotato by controlling endocytosis, oxidative homeostasis, and responses to cell wall and osmotic stresses.

Autophagy and cell wall integrity pathways coordinately regulate the development and pathogenicity through MoAtg4 phosphorylation in Magnaporthe oryzae

Autophagy and Cell wall integrity (CWI) signaling are critical stress-responsive processes during fungal infection of host plants. In the rice blast fungus Magnaporthe oryzae, autophagy-related (ATG) proteins phosphorylate CWI kinases to regulate virulence; however, how autophagy interplays with CWI signaling to coordinate such regulation remains unknown. Here, we have identified the phosphorylation of ATG protein MoAtg4 as an important process in the coordination between autophagy and CWI in M. oryzae. The ATG kinase MoAtg1 phosphorylates MoAtg4 to inhibit the deconjugation and recycling of the key ATG protein MoAtg8. At the same time, MoMkk1, a core kinase of CWI, also phosphorylates MoAtg4 to attenuate the C-terminal cleavage of MoAtg8. Significantly, these two phosphorylation events maintain proper autophagy levels to coordinate the development and pathogenicity of the rice blast fungus.

Recent Advances in Effector Research of Magnaporthe oryzae

Recalcitrant rice blast disease is caused by Magnaporthe oryzae, which has a significant negative economic reverberation on crop productivity. In order to induce the disease onto the host, M. oryzae positively generates many types of small secreted proteins, here named as effectors, to manipulate the host cell for the purpose of stimulating pathogenic infection. In M. oryzae, by engaging with specific receptors on the cell surface, effectors activate signaling channels which control an array of cellular activities, such as proliferation, differentiation and apoptosis. The most recent research on effector identification, classification, function, secretion, and control mechanism has been compiled in this review. In addition, the article also discusses directions and challenges for future research into an effector in M. oryzae.

Involvement of the Cell Wall-Integrity Pathway in Signal Recognition, Cell-Wall Biosynthesis, and Virulence in Magnaporthe oryzae

The fungal cell wall is the first layer exposed to the external environment. The cell wall has key roles in regulating cell functions, such as cellular stability, permeability, and protection against stress. Understanding the structure of the cell wall and the mechanism of its biogenesis is important for the study of fungi. Highly conserved in fungi, including Magnaporthe oryzae, the cell wall-integrity (CWI) pathway is the primary signaling cascade regulating cell-wall structure and function. The CWI pathway has been demonstrated to correlate with pathogenicity in many phytopathogenic fungi. In the synthesis of the cell wall, the CWI pathway cooperates with multiple signaling pathways to regulate cell morphogenesis and secondary metabolism. Many questions have arisen regarding the cooperation of different signaling pathways with the CWI pathway in regulating cell-wall synthesis and pathogenicity. In this review, we summarized the latest advances in the M. oryzae CWI pathway and cell-wall structure. We discussed the CWI pathway components and their involvement in different aspects, such as virulence factors, the possibility of the pathway as a target for antifungal therapies, and crosstalk with other signaling pathways. This information will aid in better understanding the universal functions of the CWI pathway in regulating cell-wall synthesis and pathogenicity in M. oryzae. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

MoCbp7, a Novel Calcineurin B Subunit-Binding Protein, Is Involved in the Calcium Signaling Pathway and Regulates Fungal Development, Virulence, and ER Homeostasis in Magnaporthe oryzae

Calcineurin, a key regulator of the calcium signaling pathway, is involved in calcium signal transduction and calcium ion homeostasis. Magnaporthe oryzae is a devastating filamentous phytopathogenic fungus in rice, yet little is known about the function of the calcium signaling system. Here, we identified a novel calcineurin regulatory-subunit-binding protein, MoCbp7, which is highly conserved in filamentous fungi and was found to localize in the cytoplasm. Phenotypic analysis of the MoCBP7 gene deletion mutant (ΔMocbp7) showed that MoCbp7 influenced the growth, conidiation, appressorium formation, invasive growth, and virulence of M. oryzae. Some calcium-signaling-related genes, such as YVC1, VCX1, and RCN1, are expressed in a calcineurin/MoCbp7-dependent manner. Furthermore, MoCbp7 synergizes with calcineurin to regulate endoplasmic reticulum homeostasis. Our research indicated that M. oryzae may have evolved a new calcium signaling regulatory network to adapt to its environment compared to the fungal model organism Saccharomyces cerevisiae.

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 ER Protein Translocation Channel Subunit Sbh1 Controls Virulence of Cryptococcus neoformans

The fungal pathogen Cryptococcus neoformans is distinguished by a cell-wall-anchored polysaccharide capsule that is critical for virulence. Biogenesis of both cell wall and capsule relies on the secretory pathway. Protein secretion begins with polypeptide translocation across the endoplasmic reticulum (ER) membrane through a highly conserved channel formed by three proteins: Sec61, Sbh1, and Sss1. Sbh1, the most divergent, contains multiple phosphorylation sites, which may allow it to regulate entry into the secretory pathway in a species- and protein-specific manner. Absence of SBH1 causes a cell-wall defect in both Saccharomyces cerevisiae and C. neoformans, although other phenotypes differ. Notably, proteomic analysis showed that when cryptococci are grown in conditions that mimic aspects of the mammalian host environment (tissue culture medium, 37°C, 5% CO₂), a set of secretory and transmembrane proteins is upregulated in wild-type, but not in Δsbh1 mutant cells. The Sbh1-dependent proteins show specific features of their ER targeting sequences that likely cause them to transit less efficiently into the secretory pathway. Many also act in cell-wall biogenesis, while several are known virulence factors. Consistent with these observations, the C. neoformans Δsbh1 mutant is avirulent in a mouse infection model. We conclude that, in the context of conditions encountered during infection, Sbh1 controls the entry of virulence factors into the secretory pathway of C. neoformans, and thereby regulates fungal pathogenicity. IMPORTANCE Cryptococcus neoformans is a yeast that causes almost 200,000 deaths worldwide each year, mainly of immunocompromised individuals. The surface structures of this pathogen, a protective cell wall surrounded by a polysaccharide capsule, are made and maintained by proteins that are synthesized inside the cell and travel outwards through the secretory pathway. A protein called Sbh1 is part of the machinery that determines which polypeptides enter this export pathway. We found that when Sbh1 is absent, both C. neoformans and the model yeast S. cerevisiae show cell-wall defects. Lack of Sbh1 also changes the pattern of secretion of both transmembrane and soluble proteins, in a manner that depends on characteristics of their sequences. Notably, multiple proteins that are normally upregulated in conditions similar to those encountered during infection, including several needed for cryptococcal virulence, are no longer increased. Sbh1 thereby regulates the ability of this important pathogen to cause disease.

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.

The COPII subunit MoSec24B is involved in development, pathogenicity and autophagy in the rice blast fungus

The endoplasmic reticulum (ER) acts as the starting point of the secretory pathway, where approximately one-third of the proteins are correctly folded and modified, loaded into vesicles, and transported to the Golgi for further processing and modification. In this process, COPII vesicles are responsible for transporting cargo proteins from the ER to the Golgi. Here, we identified the inner shell subunit of COPII vesicles (MoSec24B) and explored the importance of MoSec24B in the rice blast fungus. The targeted disruption of MoSec24B led to decreased growth, reduced conidiation, restricted glycogen and lipids utilization, sensitivity to the cell wall and hypertonic stress, the failure of septin-mediated repolarization of appressorium, impaired appressorium turgor pressure, and decreased ability to infect, which resulted in reduced pathogenicity to the host plant. Furthermore, MoSec24B functions in the three mitogen-activated protein kinase (MAPK) signaling pathways by acting with MoMst50. Deletion of MoSec24B caused reduced lipidation of MoAtg8, accelerated degradation of exogenously introduced GFP-MoAtg8, and increased lipidation of MoAtg8 upon treatment with a late inhibitor of autophagy (BafA1), suggesting that MoSec24B regulates the fusion of late autophagosomes with vacuoles. Together, these results suggest that MoSec24B exerts a significant role in fungal development, the pathogenesis of filamentous fungi and autophagy.

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.

Magnaporthe oryzae Transcription Factor MoBZIP3 Regulates Appressorium Turgor Pressure Formation during Pathogenesis

The devastating fungus Magnaporthe oryzae (M. oryzae) forms a specialized infection structure known as appressorium, which generates enormous turgor, to penetrate the plant cells. However, how M. oryzae regulates the appressorium turgor formation, is not well understood. In this study, we identified MoBZIP3, a bZIP transcription factor that functioned in pathogenesis in M. oryzae. We found that the pathogenicity of the MoBZIP3 knockout strain (Δmobzip3) was significantly reduced, and the defect was restored after re-expression of MoBZIP3, indicating that MoBZIP3 is required for M. oryzae virulence. Further analysis showed that MoBZIP3 functions in utilization of glycogen and lipid droplets for generation of glycerol in appressorium. MoBZIP3 localized in the nucleus and could bind directly to the promoters of the glycerol synthesis-related genes, MoPTH2, MoTGL1 and MoPEX6, and regulate their expression which is critical for glycerol synthesis in the appressorium turgor pressure generation. Furthermore, the critical turgor sensor gene MoSln1 was also down regulated and its subcellular localization was aberrant in Δmobzip3, which leads to a disordered actin assembly in the Δmobzip3 appressorium. Taken together, these results revealed new regulatory functions of the bZIP transcription factor MoBZIP3, in regulating M. oryzae appressorium turgor formation and infection.

Early Secretory Pathway-Associated Proteins SsEmp24 and SsErv25 Are Involved in Morphogenesis and Pathogenicity in a Filamentous Phytopathogenic Fungus

Proper protein secretion is critical for fungal development and pathogenesis. However, the potential roles of proteins involved in the early secretory pathway are largely undescribed in filamentous fungi. p24 proteins are cargo receptors that cycle between the endoplasmic reticulum (ER) and Golgi apparatus in the early secretory pathway and recruit cargo proteins to nascent vesicles. This study characterized the function of two p24 family proteins (SsEmp24 and SsErv25) in a phytopathogenic fungus, Sclerotinia sclerotiorum. Both SsEmp24 and SsErv25 were upregulated during the early stages of S. sclerotiorum infection. ΔSsEmp24 mutant and ΔSsErv25 mutant displayed abnormal vegetative growth and sclerotium formation, were defective in infection cushion formation, and showed lower virulence on host plants. ΔSsEmp24 mutant had a more severe abnormal phenotype than ΔSsErv25 mutant, implying that SsEmp24 could play a central role in the early secretory pathway. Similar to their Saccharomyces cerevisiae counterparts, SsEmp24 interacted with SsErv25 and predominantly colocalized in the ER or nuclear envelope. The absence of SsEmp24 or SsErv25 led to defective in protein secretion in S. sclerotiorum, including the pathogenicity-related extracellular hydrolytic enzymes and effectors. It is proposed that SsEmp24 and SsErv25, components in the early secretory pathway, are involved in modulating morphogenesis and pathogenicity in S. sclerotiorum by mediating protein secretion. IMPORTANCE Understanding the reproduction and pathogenesis mechanism of phytopathogens could provide new opinions to effectively control fungal diseases. Although it has been known that effectors and extracellular hydrolytic enzymes secreted by phytopathogenic fungi play important roles in fungus-host interactions, the secretion system for the delivery of virulence factors to the host is still largely undescribed. Although the role of the early secretory pathway-associated p24 proteins in S. cerevisiae has been well characterized, the function of these proteins in filamentous fungi was scarcely known prior to this study. The present research provides evidence that p24 proteins participate in the reproduction and pathogenesis of phytopathogenic fungi through the mediation of protein secretion. This research advances our understanding of p24 proteins in filamentous phytopathogenic fungi. In addition, the candidate cargos of the two p24 proteins, SsEmp24 and SsErv25, were screened out by comparative proteomics, which could aid the identification of novel development and virulence-associated factors in phytopathogenic fungi.

Vacuolar Protein-Sorting Receptor MoVps13 Regulates Conidiation and Pathogenicity in Rice Blast Fungus Magnaporthe oryzae

Magnaporthe oryzae (synonym Pyricularia oryzae) is a filamentous fungal pathogen that causes major yield losses in cultivated rice worldwide. However, the mechanisms of infection of M. oryzae are not well characterized. The VPS13 proteins play vital roles in various biological processes in many eukaryotic organisms, including in the organization of actin cytoskeleton, vesicle trafficking, mitochondrial fusion, and phagocytosis. Nevertheless, the function of the Vps13 protein in plant pathogenic fungi has not been explored. Here, we analysed the biological functions of the Vps13 protein in the development and pathogenicity of M. oryzae. Deletion mutants of MoVps13 significantly reduced the conidiation and decreased the rate of fungal infection on hosts. Moreover, the loss of MoVps13 resulted in defective cell wall integrity (CWI) and plasma membrane (PM) homeostasis when treated with chemicals for inducing cell wall stress (200 mg/mL Congo Red or 0.005% SDS) and sphingolipid synthesis inhibitors (2 μM myriocin or 2 μM amphotericin B). This indicated that MoVps13 is also involved in cell wall synthesis and sphingolipid synthesis. Through immunoblotting, autophagic flux detection, co-localization, and chemical drug sensitivity assays, we confirmed the involvement of Movps13 in ER-phagy and the response to ER stress. Additionally, we generated the C-terminal structure of MoVps13 with high accuracy using the alphaflod2 database. Our experimental evidence indicates that MoVps13 is an important virulence factor that regulates the pathogenicity of M. oryzae by controlling CWI, lipid metabolism and the ER-phagy pathway. These results have expanded our knowledge about pathogenic fungi and will help exploration for novel therapeutic strategies against the rice blast fungus.

Endosomal sorting complexes required for transport-0 (ESCRT-0) are essential for fungal development, pathogenicity, autophagy and ER-phagy in Magnaporthe oryzae

Magnaporthe oryzae is an important plant pathogen that causes rice blast. Hse1 and Vps27 are components of ESCRT-0 involved in the multivesicular body (MVB) sorting pathway and biogenesis. To date, the biological functions of ESCRT-0 in M. oryzae have not been determined. In this study, we identified and characterized Hse1 and Vps27 in M. oryzae. Disruption of MoHse1 and MoVps27 caused pleiotropic defects in growth, conidiation, sexual development and pathogenicity, thereby resulting in loss of virulence in rice and barley leaves. Disruption of MoHse1 and MoVps27 triggered increased lipidation of MoAtg8 and degradation of GFP-MoAtg8, indicating that ESCRT-0 is involved in the regulation of autophagy. ESCRT-0 was determined to interact with coat protein complex II (COPII), a regulator functioning in homeostasis of the endoplasmic reticulum (ER homeostasis), and disruption of MoHse1 and MoVps27 also blocked activation of the unfolded protein response (UPR) and autophagy of the endoplasmic reticulum (ER-phagy). Overall, our results indicate that ESCRT-0 plays critical roles in regulating fungal development, virulence, autophagy and ER-phagy in M. oryzae.

PoRal2 Is Involved in Appressorium Formation and Virulence via Pmk1 MAPK Pathways in the Rice Blast Fungus Pyricularia oryzae

Pyricularia oryzae is an important plant pathogenic fungus that can severely damage rice and wheat crops, leading to significant reductions in crop productivity. To penetrate into and invade tissues of its plant host, this fungus relies on an invasive structure known as an appressorium. Appressorium formation is rigorously regulated by the cAMP-PKA and Pmk1 MAPK pathways. Here, we identified PoRal2, a homologous protein of Schizosaccharomyces pombe Ral2, and characterized its roles in fungal development and virulence in P. oryzae. PoRal2 contains N-terminal kelch repeats and C-terminal BTB domains. PoRal2 is involved in sporulation, aerial hypha and conidiophore differentiation, appressorium formation, plant penetration, and virulence. During appressorium formation, ∆Poral2 mutants generate appressoria with long germ tubes on hydrophobic surfaces. ∆Poral2 mutants exhibited a defective response to exogenous cAMP and the activated RAS2 ^G18V on a hydrophilic surface, indicating impairment in the cAMP-PKA or Pmk1 MAPK signaling pathways. Deletion of PoRAL2 leads to lowered Pmk1 phosphorylation level in the mutant. Moreover, PoRal2 is found to interact with Scd1, Smo1, and Mst50, which are involved in activation of Pmk1. In addition, the expression levels of MPG1, WISH, and PDEH in the cAMP-PKA pathway, RAS2 in both the cAMP-PKA and Pmk1 MAPK pathways, and melanin biosynthesis genes (ALB1, BUF1, and RSY1) were significantly down-regulated in the ∆Poral2. Therefore, PoRal2 is involved in fungal development and virulence by its crosstalk in the cAMP-PKA and Pmk1 MAPK signaling pathways.