Sorting nexin (MoVps17) is required for fungal development and plant infection by regulating endosome dynamics in the rice blast fungus

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.

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.

Alleviating vacuolar transport improves cellulase production in Trichoderma reesei

Increasing cellulase production in cellulolytic fungus Trichoderma reesei is of interest for biofuels and biorefineries. Previous studies indicated that secreted protein was occasionally accumulated in vacuoles; this phenomenon has also been reported in T. reesei. Therefore, alleviating vacuolar transport seems to be a promising strategy for improving cellulase production in T. reesei. Herein, we found that knockout of vps10, vps13, and vps21, among 11 vacuolar protein sorting factors, improved cellulase production in T. reesei. The filter paper activity in Δvps10, Δvps13, and Δvps21 increased by 1.28-, 2.45-, and 2.11-fold than that of the parent strain. Moreover, the β-glucosidase activity in Δvps13 and Δvps21 increased by 3.22- and 3.56-fold after 6 days of fermentation. Furthermore, we also found that the vacuolar trafficking towards vacuoles was partially impaired in three knockout mutants, and disruption of vps13 alleviated the autophagy process. These results indicated that alleviated transport and degradation towards vacuole in Δvps10, Δvps13, and Δvps21 might improve cellulase production. Of note, the expression of cellulase genes in Δvps13 and Δvps21 was dramatically increased in the late induction phase compared to the parent. These results suggested that Vps13 and Vps21 might influence the cellulase production at transcription level. And further transcriptome analysis indicated that increased cellulase gene expression might be attributed to the differential expression of sugar transporters. Our study unravels the effect of alleviating vacuolar transport through knockout vps10, vps13, and vps21 for efficient cellulase secretion, providing new clues for higher cellulase production in T. reesei. KEY POINTS: • Disruption of vps10, vps13 or vps21 improves cellulase production • Vacuolar transport is impaired in three vps KO mutants • Deletion of vps13 or vps21 increases the transcript of cellulase genes in late stage.

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.

Comparative Proteomic Analysis of Rhizoctonia solani Isolates Identifies the Differentially Expressed Proteins with Roles in Virulence

Sheath blight of rice is a destructive disease that could be calamitous to rice cultivation. The significant objective of this study is to contemplate the proteomic analysis of the high virulent and less virulent isolate of Rhizoctonia solani using a quantitative LC-MS/MS-based proteomic approach to identify the differentially expressed proteins promoting higher virulence. Across several rice-growing regions in Odisha, Eastern India, 58 Rhizoctonia isolates were obtained. All the isolates varied in their pathogenicity. The isolate RS15 was found to be the most virulent and RS22 was identified as the least virulent. The PCR amplification confirmed that the RS15 and RS22 belonged to the Rhizoctonia subgroup of AG1-IA with a specific primer. The proteomic information generated has been deposited in the PRIDE database with PXD023430. The virulent isolate consisted of 48 differentially abundant proteins, out of which 27 proteins had higher abundance, while 21 proteins had lower abundance. The analyzed proteins acquired functionality in fungal development, sporulation, morphology, pathogenicity, detoxification, antifungal activity, essential metabolism and transcriptional activities, protein biosynthesis, glycolysis, phosphorylation and catalytic activities in fungi. A Quantitative Real-Time PCR (qRT-PCR) was used to validate changes in differentially expressed proteins at the mRNA level for selected genes. The abundances of proteins and transcripts were positively correlated. This study provides the role of the proteome in the pathogenicity of R. solani AG1-IA in rice and underpins the mechanism behind the pathogen’s virulence in causing sheath blight disease.

The retromer CSC subcomplex is recruited by MoYpt7 and sequentially sorted by MoVps17 for effective conidiation and pathogenicity of the rice blast fungus

In eukaryotic cells, Rab GTPases and the retromer complex are important regulators of intracellular protein transport. However, the mechanistic relationship between Rab GTPases and the retromer complex in relation to filamentous fungal development and pathogenesis is unknown. In this study, we used Magnaporthe oryzae, an important pathogen of rice and other cereals, as a model filamentous fungus to dissect this knowledge gap. Our data demonstrate that the core retromer subunit MoVps35 interacts with the Rab GTPase MoYpt7 and they colocalize to the endosome. Without MoYpt7, MoVps35 is mislocalized in the cytoplasm, indicating that MoYpt7 plays an important role in the recruitment of MoVps35. We further demonstrate that the expression of an inactive MoYpt7-DN (GDP-bound form) mutant in M. oryzae mimicks the phenotype defects of retromer cargo-sorting complex (CSC) null mutants and blocks the proper localization of MoVps35. In addition, our data establish that MoVps17, a member of the sorting nexin family, is situated at the endosome independent of retromer CSC but regulates the sorting function of MoVps35 after its recruitment to the endosomal membrane by MoYpt7. Taken together, these results provide insight into the precise mechanism of retromer CSC recruitment to the endosome by MoYpt7 and subsequent sorting by MoVps17 for efficient conidiation and pathogenicity of M. oryzae.

MoMCP1, a Cytochrome P450 Gene, Is Required for Alleviating Manganese Toxin Revealed by Transcriptomics Analysis in Magnaporthe oryzae

Manganese, as an essential trace element, participates in many physiological reactions by regulating Mn associated enzymes. Magnaporthe oryzae is a serious pathogen and causes destructive losses for rice production. We identified a cytochrome P450 gene, MoMCP1, involving the alleviation of manganese toxin and pathogenicity. To identify the underlying mechanisms, transcriptomics were performed. The results indicated that many pathogenicity related genes were regulated, especially hydrophobin related genes in ∆Momcp1. Furthermore, the Mn²⁺ toxicity decreased the expressions of genes involved in the oxidative phosphorylation and energy production, and increased the reactive oxygen species (ROS) levels, which might impair the functions of mitochondrion and vacuole, compromising the pathogenicity and development in ∆Momcp1. Additionally, our results provided further information about Mn associated the gene network for Mn metabolism in cells.

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.

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

Fusarium graminearum is a fungal pathogen that causes Fusarium head blight (FHB) in wheat and barley. Autophagy is a highly conserved vacuolar degradation pathway essential for cellular homeostasis in which Atg9 serves as a multispanning membrane protein important for generating membranes for the formation of phagophore assembly site. However, the mechanism of autophagy or autophagosome formation in phytopathogens awaits further clarifications. In this study, we identified and characterized the Atg9 homolog (FgAtg9) in F. graminearum by live cell imaging, biochemical and genetic analyses. We find that GFP-FgAtg9 localizes to late endosomes and trans-Golgi network under both nutrient-rich and nitrogen starvation conditions and also show its dynamic actin-dependent trafficking in the cell. Further targeted gene deletion of FgATG9 demonstrates that it is important for growth, aerial hyphae development, and pathogenicity in F. graminearum. Furthermore, the deletion mutant (ΔFgatg9) shows severe defects in autophagy and lipid metabolism in response to carbon starvation. Interestingly, small GTPase FgRab7 is found to be required for the dynamic trafficking of FgAtg9, and co-immunoprecipitation (Co-IP) assays show that FgAtg9 associates with FgRab7 in vivo. Finally, heterologous complementation assay shows that Atg9 is functionally conserved in F. graminearum and Magnaporthe oryzae. Taken together, we conclude that FgAtg9 is essential for autophagy-dependent development and pathogenicity of F. graminearum, which may be regulated by the small GTPase FgRab7.