The thioredoxin MoTrx2 protein mediates reactive oxygen species (ROS) balance and controls pathogenicity as a target of the transcription factor MoAP1 in Magnaporthe oryzae

Organic fertilizer mitigated the oxidative stress of tomato induced by nanoplastics through affecting rhizosphere soil microorganisms and bacteriophage functions

Nanoplastics (NPs), which are widely present in agricultural soils, are difficult to remove and are potentially harmful to plant growth and development. However, few studies have focused on how to mitigation the oxidative stress in plants induced by soil NPs exposure. Therefore, in this study, the effects of organic and chemical fertilizers on the oxidative stress of tomato under exposure to polystyrene nanoplastics (PS-NPs) in soil were investigated. Compared with chemical fertilizer under exposure to PS-NPs, the organic fertilizer reduced the reactive oxygen species (ROS) content by 25.63 % and the H2O2 content by 34.58 % in tomato stems, whereas no significant effects were observed with respect to the amount of PS-NP internalized in tomato. Additionally, organic fertilizer increased the accumulation of the phytohormones salicylic acid (SA) and abscisic acid (ABA) by 76.53 % and 22.54 %, respectively, and these factors are key for reducing the ROS and H2O2 contents in stems. In the rhizosphere microbiome of organic fertilizer group under exposure to PS-NPs, enrichment in Actinomycetes and an increased abundance of terpenoids and polyketides metabolism were the main factors affecting the accumulation of ABA and SA. Moreover, bacteriophage activity in the rhizosphere indirectly contributed to the increase in this function. These changes ultimately resulted in a reduction in oxidative stress in tomato stems and protected tomato growth. The results of this study will provide a better understanding of the interaction between plants and nanoplastics in soil and provide a new reference for alleviating the oxidative stress caused by nanoplastics in plants.

Revealing the size effect mechanisms of micro(nano)plastics on nitrogen removal performance of constructed wetland

Micro(nano)plastics (MPs) in aquatic environments can disrupt wastewater treatment, particularly nitrogen removal in constructed wetlands (CWs). However, their broader effects on microbial and plant nitrogen metabolism remain unclear. This study investigated the effects of different-sized MPs (4 mm, 100 µm, and 100 nm) on nitrogen transformation in CWs. Results revealed that 4 mm- and 100 µm-MPs did not significantly affect total nitrogen (TN) removal, although 100 µm-MPs significantly increased leaf antioxidant enzyme activities and reduced plant uptake of nitrogen by 12.95 % (p < 0.05). In contrast, 100 nm-MPs decreased the TN removal efficiency by 7.97 % via inhibiting both nitrification and denitrification, since 100 nm-MPs penetrated cell membranes, disrupted reactive oxygen species balance, and reduced bacterial viability, thus suppressing microbial nitrogen degradation by 8.07 % (p < 0.05). Additionally, 100 nm-MPs significantly inhibited plant growth and reduced plant nitrogen uptake by 16.05 % (p < 0.05). Furthermore, 100 µm-MPs increased the abundance of nitrifiers but reduced denitrifiers and functional genes, whereas 100 nm-MPs reduced the abundance of both nitrifiers and denitrifiers along with their functional genes (p < 0.05). These findings highlight the need to improve waste management to mitigate the adverse effects of MPs on nitrogen removal.

The dual-specificity kinase MoLKH1-mediated cell cycle, autophagy, and suppression of plant immunity is critical for development and pathogenicity of Magnaporthe oryzae

Cell cycle progression, autophagic cell death during appressorium development, and ROS degradation at the infection site are important for the development of rice blast disease. However, the association of cell cycle, autophagy and ROS detoxification remains largely unknown in M. oryzae. Here, we identify the dual-specificity kinase MoLKH1, which serves as an important cell cycle regulator required for appressorium formation by regulating cytokinesis and cytoskeleton in M. oryzae. MoLKH1 is transcriptionally activated by H2O2 and required for H2O2-induced autophagic cell death and suppression of ROS-activated plant defense during plant invasion of M. oryzae. In addition, the Molkh1 mutant also showed several phenotypic defects, including delayed growth, abnormal conidiation, damaged cell wall integrity, impaired glycogen and lipid transport, reduced secretion of extracellular enzymes and effectors, and attenuated virulence of M. oryzae. Nuclear localization of MoLKH1 requires the nuclear localization sequence, Lammer motif, as well as the kinase active site and ATP-binding site in this protein. Site-directed mutagenesis showed that each of them plays crucial roles in fungal growth and pathogenicity of M. oryzae. In conclusion, our results demonstrate that MoLKH1-mediated cell cycle, autophagy, and suppression of plant immunity play crucial roles in development and pathogenicity of M. oryzae.

Phospho-code of a conserved transcriptional factor underpins fungal virulence

BACKGROUND

Cell wall integrity (CWI) is crucial for fungal growth, pathogenesis, and adaptation to extracellular environments. Calcofluor white (CFW) is a cell wall perturbant that inhibits fungal growth, yet little is known about how phytopathogenic fungi respond to the CFW-induced stress.

RESULTS

In this study, we unveiled a significant discovery that CFW triggered the translocation of the transcription factor CgCrzA from the cytoplasm to the nucleus in Colletotrichum gloeosporioides. This translocation was regulated by an interacting protein, CgMkk1, a mitogen-activated protein kinase involved in the CWI pathway. Further analysis revealed that CgMkk1 facilitated nuclear translocation by phosphorylating CgCrzA at the Ser280 residue. Using chromatin immunoprecipitation sequencing, we identified two downstream targets of CgCrzA, namely CgCHS5 and CgCHS6, which are critical for growth, cell wall integrity, and pathogenicity as chitin synthase genes.

CONCLUSIONS

These findings provide a novel insight into the regulatory mechanism of CgMkk1-CgCrzA-CgChs5/6, which enables response of the cell wall inhibitor CFW and facilitates infectious growth for C. gloeosporioides.

MoRgs3 functions in intracellular reactive oxygen species perception-integrated cAMP signaling to promote appressorium formation in Magnaporthe oryzae

Regulator of G-protein signaling (RGS) proteins exhibit GTPase-accelerating protein activities to govern G-protein function. In the rice blast fungus Magnaporthe oryzae, there is a family of at least eight RGS and RGS-like proteins (MoRgs1 to MoRgs8), each exhibiting distinct or shared functions in the growth, appressorium formation, and pathogenicity. MoRgs3 recently emerged as one of the crucial regulators that senses intracellular oxidation during appressorium formation. To explore this unique regulatory mechanism of MoRgs3, we identified the nucleoside diphosphate kinase MoNdk1 that interacts with MoRgs3. MoNdk1 phosphorylates MoRgs3 under induced intracellular reactive oxygen species levels, and MoRgs3 phosphorylation is required for appressorium formation and pathogenicity. In addition, we showed that MoRgs3 phosphorylation determines its interaction with MoCrn1, a coronin-like actin-binding protein homolog, which regulates MoRgs3 internalization. Finally, we provided evidence demonstrating that MoRgs3 functions in MoMagA-mediated cAMP signaling to regulate normal appressorium induction. By revealing a novel signal perception mechanism, our studies highlighted the complexity of regulation during the appressorium function and pathogenicity of the blast fungus.

IMPORTANCE

We report that MoRgs3 becomes phosphorylated in an oxidative intracellular environment during the appressorium formation stage. We found that this phosphorylation is carried out by MoNdk1, a nucleoside diphosphate kinase. In addition, this phosphorylation leads to a higher binding affinity between MoRgs3 and MoCrn1, a coronin-like actin-binding protein that was implicated in the endocytic transport of several other RGS proteins of Magnaporthe oryzae. We further found that the internalization of MoRgs3 is indispensable for its GTPase-activating protein function toward the Gα subunit MoMagA. Importantly, we characterized how such cellular regulatory events coincide with cAMP signaling-regulated appressorium formation and pathogenicity in the blast fungus. Our studies uncovered a novel intracellular reactive oxygen species signal-transducing mechanism in a model pathogenic fungus with important basic and applied implications.

Unraveling the Role of the Liriodendron Thioredoxin (TRX) Gene Family in an Abiotic Stress Response

Thioredoxin (TRX) is a small protein with REDOX activity that plays a crucial role in a plant's growth, development, and stress resistance. The TRX family has been extensively studied in Arabidopsis, rice, and wheat, and so it is likely that its members have similar biological functions in Liriodendron that have not been reported in Liriodendron. In this study, we performed the genome-wide identification of the TRX gene family based on the Liriodendron chinense genome, leading to a total of 42 LcTRX gene members. A phylogenetic analysis categorized these 42 LcTRX proteins into 13 subfamilies. We further characterized their chromosome distributions, gene structures, conserved protein motifs, and cis-elements in the promoter regions. In addition, based on the publicly available transcriptome data for Liriodendron hybrid and following RT-qPCR experiments, we explored the expression patterns of LhTRXs to different abiotic stressors, i.e., drought, cold, and heat stress. Notably, we found that several LhTRXs, especially LhTRX-h3, were significantly upregulated in response to abiotic stress. In addition, the subcellular localization assay showed that LhTRX-h3 was mainly distributed in the cytoplasm. Subsequently, we obtained LhTRX-h3 overexpression (OE) and knockout (KO) callus lines in Liriodendron hybrid. Compared to the wild type (WT) and LhTRX-h3-KO callus proliferation of LhTRX-h3-OE lines was significantly enhanced with reduced reactive oxygen species (ROS) accumulation under drought stress. Our findings that LhTRX-h3 is sufficient to improve drought tolerance. and underscore the significance of the TRX gene family in environmental stress responses in Liriodendron.

Sirt5-mediated lysine desuccinylation regulates oxidative stress adaptation in Magnaporthe oryzae during host intracellular infection

Plant pathogenic fungi elaborate numerous detoxification strategies to suppress host reactive oxygen species (ROS), but their coordination is not well-understood. Here, we show that Sirt5-mediated protein desuccinylation in Magnaporthe oryzae is central to host ROS detoxification. SIRT5 encodes a desuccinylase important for virulence via adaptation to host oxidative stress. Quantitative proteomics analysis identified a large number of succinylated proteins targeted by Sirt5, most of which were mitochondrial proteins involved in oxidative phosphorylation, TCA cycle, and fatty acid oxidation. Deletion of SIRT5 resulted in hypersuccinylation of detoxification-related enzymes, and significant reduction in NADPH : NADP+ and GSH : GSSG ratios, disrupting redox balance and impeding invasive growth. Sirt5 desuccinylated thioredoxin Trx2 and glutathione peroxidase Hyr1 to activate their enzyme activity, likely by affecting proper folding. Altogether, this work demonstrates the importance of Sirt5-mediated desuccinylation in controlling fungal process required for detoxifying host ROS during M. oryzae infection.

Response mechanism of non-biodegradable polyethylene terephthalate microplastics and biodegradable polylactic acid microplastics to nitrogen removal in activated sludge system

The issue of microplastics (MPs) has gained more attention among researchers and the public; however, there is still a lot to be studied about its impact on biological wastewater treatment. In this study, the effects of non-biodegradable polyethylene terephthalate (PET) and biodegradable polylactic acid (PLA) on wastewater treatment by sequencing batch reactor (SBR) were compared. The results showed that PET and PLA reduced the removal efficiency of NH4+-N by 1.7 % and 21.2 %, respectively. Structural equation functional model (SEM) analysis was used to infer the potential mechanism of PLA affecting ammonia oxidation. PLA primarily inhibits the activity of ammonia monooxygenase (AMO), while promoting an increase in reactive oxygen species (ROS) and antioxidant enzyme activity. Accordingly, the toxic effect of PLA further reduced the abundance of ammonia-oxidizing bacteria. This study showed that biodegradable MPs have a greater potential impact on wastewater treatment than non-biodegradable MPs, which warrants further investigation.

MAPK signaling pathway orchestrates and fine-tunes the pathogenicity of Colletotrichum falcatum

Colletotrichum falcatum is the causal organism of red rot, the most devastating disease of sugarcane. Mitogen-activated protein kinase (MAPK) signaling pathway plays pivotal role in coordinating the process of pathogenesis. We identified eighteen proteins implicated in MAPK signaling pathway in C. falcatum, through nanoLCMS/MS based proteomics approach. Twelve of these proteins were the part of core MAPK signaling pathway, whereas remaining proteins were indirectly implicated in MAPK signaling. Majority of these proteins had enhanced abundance in C. falcatum samples cultured with host sugarcane stalks. To validate the findings, core MAPK pathway genes (MAPKKK-NSY1, MAPK 17-MAPK17, MAPKKK 5-MAPKKK5, MAPK-HOG1B, MAPKKK-MCK1/STE11, MAPK-MST50/STE50, MAPKK-SEK1, MAPKK-MEK1/MST7/STE7, MAPKK-MKK2/STE7, MAPKKK-MST11/STE11, MAPK 5-MPK5, and MAPK-MPK-C) were analyzed by qPCR to confirm the real-time expression in C. falcatum samples cultured with host sugarcane stalks. The results of qPCR-based expression of genes were largely in agreement with the findings of proteomics. String association networks of MAPKK- MEK1/MST7/STE7, and MAPK- MPK-C revealed strong association with plenty of assorted proteins implicated in the process of pathogenesis/virulence. This is the novel and first large scale study of MAPK proteins in C. falcatum, responsible for red rot epidemics of sugarcane various countries. KEY MESSAGE: Our findings demonstrate the pivotal role of MAPK proteins in orchestrating the pathogenicity of Colletotrichum falcatum, responsible devastating red rot disease of sugarcane. SIGNIFICANCE: Our findings are novel and the first large scale study demonstrating the pivotal role of MAPK proteins in C. falcatum, responsible devastating red rot disease of sugarcane. The study will be useful for future researchers in terms of manipulating the fungal pathogenicity through genome editing.

Thioredoxin VdTrx1, an unconventional secreted protein, is a virulence factor in Verticillium dahliae

Understanding how plant pathogenic fungi adapt to their hosts is of critical importance to securing optimal crop productivity. In response to pathogenic attack, plants produce reactive oxygen species (ROS) as part of a multipronged defense response. Pathogens, in turn, have evolved ROS scavenging mechanisms to undermine host defense. Thioredoxins (Trx) are highly conserved oxidoreductase enzymes with a dithiol-disulfide active site, and function as antioxidants to protect cells against free radicals, such as ROS. However, the roles of thioredoxins in Verticillium dahliae, an important vascular pathogen, are not clear. Through proteomics analyses, we identified a putative thioredoxin (VdTrx1) lacking a signal peptide. VdTrx1 was present in the exoproteome of V. dahliae cultured in the presence of host tissues, a finding that suggested that it plays a role in host-pathogen interactions. We constructed a VdTrx1 deletion mutant ΔVdTrx1 that exhibited significantly higher sensitivity to ROS stress, H2O2, and tert-butyl hydroperoxide (t-BOOH). In vivo assays by live-cell imaging and in vitro assays by western blotting revealed that while VdTrx1 lacking the signal peptide can be localized within V. dahliae cells, VdTrx1 can also be secreted unconventionally depending on VdVps36, a member of the ESCRT-II protein complex. The ΔVdTrx1 strain was unable to scavenge host-generated extracellular ROS fully during host invasion. Deletion of VdTrx1 resulted in higher intracellular ROS levels of V. dahliae mycelium, displayed impaired conidial production, and showed significantly reduced virulence on Gossypium hirsutum, and model plants, Arabidopsis thaliana and Nicotiana benthamiana. Thus, we conclude that VdTrx1 acts as a virulence factor in V. dahliae.

MoMih1 is indispensable for asexual development, cell wall integrity, and pathogenicity of Magnaporthe oryzae

Asexual spore serves as essential inoculum of rice blast during the disease cycle, and differentiation of young conidium from conidiophore is intimately regulated by cell cycle. Mih1 encodes a dual-specificity phosphatase that involved in the G2/M transition of the mitotic cell cycle by regulating the Cdk1 activity in eukaryotes. Till now, the roles of Mih1 homologue, however, remain unclear in Magnaporthe oryzae. We here functionally characterized the Mih1 homologue MoMih1 in M. oryzae. MoMih1 is localized to both the cytoplasm and nucleus and can physically interact with the CDK protein MoCdc28 in vivo. Loss of MoMih1 led to delayed nucleus division and a high level of Tyr15 phosphorylation of MoCdc28. The MoMih1 mutants showed retarded mycelial growth with a defective polar growth, less fungal biomass, and shorter distance between diaphragms, compared with the KU80. Asexual reproduction altered in MoMih1 mutants, with both abnormal conidial morphogenesis and decreased conidiation. The MoMih1 mutants severely attenuated the virulence to host plants due to the impaired ability of penetration and biotrophic growth. The incapability of scavenging of host-derived reactive oxygen species, which was possibly ascribed to the severely decreased extracellular enzymes activities, were partially associated with deficiency of pathogenicity. Besides, the MoMih1 mutants displayed also improper localization of retromer protein MoVps26 and polarisome component MoSpa2, and defects of cell wall integrity (CWI), melanin pigmentation, chitin synthesis, and hydrophobicity. In conclusion, our results demonstrate that MoMih1 plays pleiotropic roles during fungal development and plant infection of M. oryzae.

Two Magnaporthe appressoria-specific (MAS) proteins, MoMas3 and MoMas5, are required for suppressing host innate immunity and promoting biotrophic growth in rice cells

In the devastating rice blast fungus Magnaporthe oryzae, six Magnaporthe appressoria-specific (MAS) proteins are encoded by MoGAS1, MoGAS2 and MoMAS3-MoMAS6. MoGAS1 and MoGAS2 were previously characterized as M. oryzae virulence factors; however, the roles of the other four genes are unknown. Here, we found that, although the loss of any MAS gene did not affect appressorial formation or vegetative growth, ∆Momas3 and ∆Momas5 mutant strains (but not the others) were reduced in virulence on susceptible CO-39 rice seedlings. Focusing on ∆Momas3 and ∆Momas5 mutant strains, we found that they could penetrate host leaf surfaces and fill the first infected rice cell but did not spread readily to neighbouring cells, suggesting they were impaired for biotrophic growth. Live-cell imaging of fluorescently labelled MoMas3 and MoMas5 proteins showed that during biotrophy, MoMas3 localized to the apoplastic compartment formed between fungal invasive hyphae and the plant-derived extra-invasive hyphal membrane while MoMas5 localized to the appressoria and the penetration peg. The loss of either MoMAS3 or MoMAS5 resulted in the accumulation of reactive oxygen species (ROS) in infected rice cells, resulting in the triggering of plant defences that inhibited mutant growth in planta. ∆Momas3 and ∆Momas5 biotrophic growth could be remediated by inhibiting host NADPH oxidases and suppressing ROS accumulation. Thus, MoMas3 and MoMas5 are novel virulence factors involved in suppressing host plant innate immunity to promote biotrophic growth.

Distinct and essential roles of bZIP transcription factors in the stress response and pathogenesis in Alternaria alternata

The ability to cope with environmental abiotic stress and biotic stress is crucial for the survival of plants and microorganisms, which enable them to occupy multiple niches in the environment. Previous studies have shown that transcription factors play crucial roles in regulating various biological processes including multiple stress tolerance and response in eukaryotes. This work identified multiple critical transcription factor genes, metabolic pathways and gene ontology (GO) terms related to abiotic stress response were broadly activated by analyzing the transcriptome of phytopathogenic fungus Alternaria alternata under metal ions stresses, oxidative stress, salt stresses, and host-pathogen interaction. We investigated the biological functions and regulatory roles of the bZIP transcriptional factor (TF) genes in the phytopathogenic fungus A.alternata by analyzing targeted gene disrupted mutants. Morphological analysis provides evidence that the bZIP transcription factors (Gcn4, MeaB, Atf1, the ER stress regulator Hac1, and the all development altered-1 gene Ada1) are required for morphogenesis as the colony morphology of these gene deletion mutants was significantly different from that of the wild-type. In addition, bZIPs are involved in the resistance to multiple stresses such as oxidative stress (Ada1, Yap1, MetR) and virulence (Hac1, MetR, Yap1, Ada1) at varying degrees. Transcriptome data demonstrated that the inactivation of bZIPs (Hac1, Atf1, Ada1 and Yap1) significantly affected many genes in multiple critical metabolism pathways and gene ontology (GO) terms. Moreover,the ΔHac1 mutants displayed reduced aerial hypha and are hypersensitivity to endoplasmic reticulum disruptors such as tunicamycin and dithiothreitol. Transcriptome analysis showed that inactivation of Hac1 significantly affected the proteasome process and its downstream unfolded protein binding, indicating that Hac1 participates in the endoplasmic reticulum stress response through the conserved unfolded protein response. Taken together, our findings reveal that bZIP transcription factors function as key regulators of fungal morphogenesis, abiotic stress response and pathogenesis, and expand our understanding of how microbial pathogens utilize these genes to deal with environmental stresses and achieve successful infection in the host plant.

The bZIP Transcription Factor LtAP1 Modulates Oxidative Stress Tolerance and Virulence in the Peach Gummosis Fungus Lasiodiplodia theobromae

Lasiodiplodia theobromae is one of the primary causal agents in peach gummosis disease, leading to enormous losses in peach production. In our previous study, a redox-related gene, LtAP1, from the fungus was significantly upregulated in peach shoots throughout infection. Here, we characterized LtAP1, a basic leucine zipper transcription factor, during peach gummosis progression using the CRISPR-Cas9 system and homologous recombination. The results showed that LtAP1-deletion mutant had slower vegetative growth and increased sensitivity to several oxidative and nitrosative stress agents. LtAP1 was highly induced by exogenous oxidants treatment in the L. theobromae wild-type strain. In a pathogenicity test, the deletion mutant showed decreased virulence (reduced size of necrotic lesions, less gum release, and decreased pathogen biomass) on infected peach shoots compared to the wild-type strain. The mutant showed severely reduced transcription levels of genes related to glutaredoxin and thioredoxin in L. theobroame under oxidative stress or during infection, indicating an attenuated capacity for reactive oxygen species (ROS) detoxification. When shoots were treated with an NADPH oxidase inhibitor, the pathogenicity of the mutant was partially restored. Moreover, ROS production and plant defense response were strongly activated in peach shoots infected by the mutant. These results highlight the crucial role of LtAP1 in the oxidative stress response, and further that it acts as an important virulence factor through modulating the fungal ROS-detoxification system and the plant defense response.

A double-edged sword: reactive oxygen species (ROS) during the rice blast fungus and host interaction

Magnaporthe oryzae is a hemibiotrophic fungus that also needs host nutrients for propagation during infection. During its interaction with rice, reactive oxygen species (ROS) mediate important signaling reactions impacting both the pathogen and the host. In M. oryzae, the accumulation of ROS is important for the formation and maturation of the infectious structure appressorium. On the other hand, upon M. oryzae infection, rice generates further ROS to restrict invasive hyphae (IH) spreading. Despite ROS receptors remaining to be identified, M. oryzae recruits several strategies to respond and suppress ROS accumulation through the secretion of various effector molecules. These findings suggest that the balance between the generation and scavenging of ROS is sophisticatedly controlled during M. oryzae-rice interaction. In this review, we discuss advances to understand the regulation mechanisms for the generation, accumulation, and transduction of ROS.

Regulation of biotic interactions and responses to abiotic stresses by MAP kinase pathways in plant pathogenic fungi

Like other eukaryotes, fungi use MAP kinase (MAPK) pathways to mediate cellular changes responding to external stimuli. In the past two decades, three well-conserved MAP kinase pathways have been characterized in various plant pathogenic fungi for regulating responses and adaptations to a variety of biotic and abiotic stresses encountered during plant infection or survival in nature. The invasive growth (IG) pathway is homologous to the yeast pheromone response and filamentation pathways. In plant pathogens, the IG pathway often is essential for pathogenesis by regulating infection-related morphogenesis, such as appressorium formation, penetration, and invasive growth. The cell wall integrity (CWI) pathway also is important for plant infection although the infection processes it regulates vary among fungal pathogens. Besides its universal function in cell wall integrity, it often plays a minor role in responses to oxidative and cell wall stresses. Both the IG and CWI pathways are involved in regulating known virulence factors as well as effector genes during plant infection and mediating defenses against mycoviruses, bacteria, and other fungi. In contrast, the high osmolarity growth (HOG) pathway is dispensable for virulence in some fungi although it is essential for plant infection in others. It regulates osmoregulation in hyphae and is dispensable for appressorium turgor generation. The HOG pathway also plays a major role for responding to oxidative, heat, and other environmental stresses and is overstimulated by phenylpyrrole fungicides. Moreover, these three MAPK pathways crosstalk and coordinately regulate responses to various biotic and abiotic stresses. The IG and CWI pathways, particularly the latter, also are involved in responding to abiotic stresses to various degrees in different fungal pathogens, and the HOG pathway also plays a role in interactions with other microbes or fungi. Furthermore, some infection processes or stress responses are co-regulated by MAPK pathways with cAMP or Ca2+/CaM signaling. Overall, functions of individual MAP kinase pathways in pathogenesis and stress responses have been well characterized in a number of fungal pathogens, showing the conserved genetic elements with diverged functions, likely by rewiring transcriptional regulatory networks. In the near future, applications of genomics and proteomics approaches will likely lead to better understanding of crosstalk among the MAPKs and with other signaling pathways as well as roles of MAPKs in defense against other microbes (biotic interactions).

Insights into enhanced biodegradation of sulfadimethoxine by catalyst: Transcriptomic responses and free radical interactions

The occurrence of sulfonamides in the environment is a severe global threat to public health due to the increasing prevalence of antibiotic selection pressure that may lead to the development of antibiotic resistance. We report an enhanced biodegradation of sulfadimethoxine (SDM) by Phanerochaete chrysosporium (Pc) with lignocellulosic biomass (Lb) using Fe₃O₄-ZSM-5 as a catalyst (Pc/Fe₃O₄-ZSM-5/Lb). SDM was completely degraded within 4 days at pH 7.0 in the Pc/Fe₃O₄-ZSM-5/Lb system. Transcriptomic, metabolites and free radical analyses were performed to explore the detailed molecular mechanisms of SDM degradation. A total of 246 genes of Pc in the Pc/Fe₃O₄-ZSM-5/Lb system exhibited significant upregulation compared to that in Pc alone. Upregulated genes encoding cellulases, cytochrome P450, cellobiose quinone oxidoreductase, and cellobiose dehydrogenase were involved in SDM degradation in the Pc/Fe₃O₄-ZSM-5/Lb system. In addition, genes encoding glutathione S-transferase and cytochrome P450 genes related to oxidative stress and detoxification were all significantly upregulated (P < 0.01). Electron paramagnetic resonance revealed the generation of OH suggesting a free radical pathway could be catalyzed by Fe₃O₄-ZSM-5 and the enzymes. These findings of catalyst-assisted SDM biodegradation will be valuable for remediation of antibiotics from contaminated wastewater.

Surviving the odds: From perception to survival of fungal phytopathogens under host-generated oxidative burst

Fungal phytopathogens pose a serious threat to global crop production. Only a handful of strategies are available to combat these fungal infections, and the increasing incidence of fungicide resistance is making the situation worse. Hence, the molecular understanding of plant-fungus interactions remains a primary focus of plant pathology. One of the hallmarks of host-pathogen interactions is the overproduction of reactive oxygen species (ROS) as a plant defense mechanism, collectively termed the oxidative burst. In general, high accumulation of ROS restricts the growth of pathogenic organisms by causing localized cell death around the site of infection. To survive the oxidative burst and achieve successful host colonization, fungal phytopathogens employ intricate mechanisms for ROS perception, ROS neutralization, and protection from ROS-mediated damage. Together, these countermeasures maintain the physiological redox homeostasis that is essential for cell viability. In addition to intracellular antioxidant systems, phytopathogenic fungi also deploy interesting effector-mediated mechanisms for extracellular ROS modulation. This aspect of plant-pathogen interactions is significantly under-studied and provides enormous scope for future research. These adaptive responses, broadly categorized into "escape" and "exploitation" mechanisms, are poorly understood. In this review, we discuss the oxidative stress response of filamentous fungi, their perception signaling, and recent insights that provide a comprehensive understanding of the distinct survival mechanisms of fungal pathogens in response to the host-generated oxidative burst.

Auxilin-like protein MoSwa2 promotes effector secretion and virulence as a clathrin uncoating factor in the rice blast fungus Magnaporthe oryzae

Plant pathogens exploit the extracellular matrix (ECM) to inhibit host immunity during their interactions with the host. The formation of ECM involves a series of continuous steps of vesicular transport events. To understand how such vesicle trafficking impacts ECM and virulence in the rice blast fungus Magnaporthe oryzae, we characterised MoSwa2, a previously identified actin-regulating kinase MoArk1 interacting protein, as an orthologue of the auxilin-like clathrin uncoating factor Swa2 of the budding yeast Saccharomyces cerevisiae. We found that MoSwa2 functions as an uncoating factor of the coat protein complex II (COPII) via an interaction with the COPII subunit MoSec24-2. Loss of MoSwa2 led to a deficiency in the secretion of extracellular proteins, resulting in both restricted growth of invasive hyphae and reduced inhibition of host immunity. Additionally, extracellular fluid (ECF) proteome analysis revealed that MoSwa2-regulated extracellular proteins include many redox proteins such as the berberine bridge enzyme-like (BBE-like) protein MoSef1. We further found that MoSef1 functions as an apoplastic virulent factor that inhibits the host immune response. Our studies revealed a novel function of a COPII uncoating factor in vesicular transport that is critical in the suppression of host immunity and pathogenicity of M. oryzae.

Balancing of the mitotic exit network and cell wall integrity signaling governs the development and pathogenicity in Magnaporthe oryzae

The fungal cell wall plays an essential role in maintaining cell morphology, transmitting external signals, controlling cell growth, and even virulence. Relaxation and irreversible stretching of the cell wall are the prerequisites of cell division and development, but they also inevitably cause cell wall stress. Both Mitotic Exit Network (MEN) and Cell Wall Integrity (CWI) are signaling pathways that govern cell division and cell stress response, respectively, how these pathways cross talk to govern and coordinate cellular growth, development, and pathogenicity remains not fully understood. We have identified MoSep1, MoDbf2, and MoMob1 as the conserved components of MEN from the rice blast fungus Magnaporthe oryzae. We have found that blocking cell division results in abnormal CWI signaling. In addition, we discovered that MoSep1 targets MoMkk1, a conserved key MAP kinase of the CWI pathway, through protein phosphorylation that promotes CWI signaling. Moreover, we provided evidence demonstrating that MoSep1-dependent MoMkk1 phosphorylation is essential for balancing cell division with CWI that maintains the dynamic stability required for virulence of the blast fungus.

The cell wall is a relatively rigid structure for supporting the cell shape and against extracellular stresses. However, it also maintains plasticity to cope with cell division, growth, and differentiation. In the rice blast pathogenic fungus Magnaporthe oryzae, such differentiation corresponds directly to its virulence. Thus, how to balance the “strong for shaping” with the “malleable for growth and virulence” poses as an important question of both basic- and applied science of significance. We here report that the protein kinase MoSep1 links the Mitotic Exit Network (MEN) to the Cell Wall Integrity (CWI) signaling through the phosphorylation of the CWI MAP kinase kinase MoMkk1. We found that the MoSep1-dependent phosphorylation of MoMkk1 relieves the cell wall stress caused by cell division and that the MEN-CWI-mediated balance of rigid and remodeling of the cell wall is important in the growth, development, and virulence of the blast fungus. Our study provides a new evidence on how the blast fungus adapts to self-generated stress for growth and virulence and it sheds new light on the crosstalk between MEN and CWI signaling.

A self-balancing circuit centered on MoOsm1 kinase governs adaptive responses to host-derived ROS in Magnaporthe oryzae

The production of reactive oxygen species (ROS) is a ubiquitous defense response in plants. Adapted pathogens evolved mechanisms to counteract the deleterious effects of host-derived ROS and promote infection. How plant pathogens regulate this elaborate response against ROS burst remains unclear. Using the rice blast fungus Magnaporthe oryzae, we uncovered a self-balancing circuit controlling response to ROS in planta and virulence. During infection, ROS induces phosphorylation of the high osmolarity glycerol pathway kinase MoOsm1 and its nuclear translocation. There, MoOsm1 phosphorylates transcription factor MoAtf1 and dissociates MoAtf1-MoTup1 complex. This releases MoTup1-mediated transcriptional repression on oxidoreduction-pathway genes and activates the transcription of MoPtp1/2 protein phosphatases. In turn, MoPtp1/2 dephosphorylate MoOsm1, restoring the circuit to its initial state. Balanced interactions among proteins centered on MoOsm1 provide a means to counter host-derived ROS. Our findings thereby reveal new insights into how M. oryzae utilizes a phosphor-regulatory circuitry to face plant immunity during infection.

Nanoplastics Disturb Nitrogen Removal in Constructed Wetlands: Responses of Microbes and Macrophytes

Nanosized plastics (nanoplastics) releasing into the wastewater may pose a potential threat to biological nitrogen removal. Constructed wetland (CW), a wastewater treatment or shore buffer system, is an important sink of nanoplastics, while it is unclear how nitrogen removal in CWs occurs in response to nanoplastics. Here, we investigated the effects of polystyrene (PS) nanoplastics (0, 10, and 1000 μg/L) on nitrogen removal for 180 days in CWs. The results revealed that total nitrogen removal efficiency decreased by 29.5-40.6%. We found that PS penetrated the cell membrane and destroyed both membrane integrity and reactive oxygen species balance. Furthermore, PS inhibited microbial activity in vivo, including enzyme (ammonia monooxygenase, nitrate reductase, and nitrite reductase) activities and electron transport system activity (ETSA). These adverse effects, accompanied by a decline in the relative abundance of nitrifiers (e.g., Nitrosomonas and Nitrospira) and denitrifiers (e.g., Thauera and Zoogloea), directly accounted for the strong deterioration observed in nitrogen removal. The decline in leaf and root activities decreased nitrogen uptake by plants, which is an important factor of deterioration in nitrogen removal. Overall, our results imply that the presence of nanoplastics in the aquatic environment is a hidden danger to the global nitrogen cycle and should receive more attention.

Lasiodiplodia theobromae-induced alteration in ROS metabolism and its relation to gummosis development in Prunus persica

Peach gummosis caused by Lasiodiplodia theobromae is one of the most detrimental diseases to peaches in southern China. Reactive oxygen species (ROS) play major roles in plant-pathogen interactions, however, their roles in the pathogenesis of peach gummosis, especially shoot disease in perennials, are largely unknown. In this study, the effects of L. theobromae on ROS production-scavenging systems and on signalling transduction during L. theobromae-induced gummosis in current-year peach shoots were investigated. The infection by L. theobromae led to a ROS burst and activated the plant antioxidant enzyme-dependent scavenging system. With disease progression, the capacity of the plant antioxidant machinery declined, and allowed for ROS accumulation and eventual malondialdehyde production. As for the fungus L. theobromae, the transcripts of genes related to ROS production were significantly repressed, and concomitantly the expression of genes related to antioxidant systems and oxidative stress resistance was markedly upregulated, perhaps to alleviate oxidative stress for successful colonisation. Moreover, genes involved in phytohormones biosynthesis and pathogenesis-related proteins were all markedly promoted, which could contribute to the restriction of disease development in peach shoots. Overall, the results showed that the ROS production-scavenging system in P. persica might affect disease development during peach-L. theobromae interaction. Our findings lay the foundations for future in-depth investigations of the molecular mechanisms and regulatory networks underlying L. theobromae-mediated shoot diseases in terms of ROS production-scavenging systems.

The deubiquitinating enzyme MoUbp8 is required for infection-related development, pathogenicity, and carbon catabolite repression in Magnaporthe oryzae

Deubiquitination is an essential regulatory step in the Ub-dependent pathway. Deubiquitinating enzymes (DUBs) mediate the removal of ubiquitin moieties from substrate proteins, which are involved in many regulatory mechanisms. As a component of the DUB module (Ubp8/Sgf11/Sus1/Sgf73) in the SAGA (Spt-Ada-Gcn5-acetyltransferase) complex, Ubp8 plays a crucial role in both Saccharomyces cerevisiae and humans. In S. cerevisiae, Ubp8-mediated deubiquitination regulates transcriptional activation processes. To investigate the contributions of Ubp8 to physiological and pathological development of filamentous fungi, we generated the deletion mutant of ortholog MoUBP8 (MGG-03527) in Magnaporthe oryzae (syn. Pyricularia oryzae). The ΔMoubp8 strain showed reduced sporulation, pathogenicity, and resistance to distinct stresses. Even though the conidia of the ΔMoubp8 mutant were delayed in appressorium formation, the normal and abnormal (none-septum or one-septum) conidia could finally form appressoria. Reduced melanin in the ΔMoubp8 mutant is highly responsible for the attenuated pathogenicity since the appressoria of the ΔMoubp8 mutant was much more fragile than those of the wild type, due to the defective turgidity. The weakened ability to detoxify or scavenge host-derived reactive oxygen species (ROS) further restricted the invasion of the pathogen. We also showed that carbon derepression, on the one hand, rendered the ΔMoubp8 strain highly sensitive to allyl alcohol, on the other hand, it enhances the resistance of the MoUBP8 defective strain to deoxyglucose. Overall, we suggest that MoUbp8 is not only required for sporulation, melanin formation, appressoria development, and pathogenicity but also involved in carbon catabolite repression of M. oryzae.

Magnaporthe oryzae Auxiliary Activity Protein MoAa91 Functions as Chitin-Binding Protein To Induce Appressorium Formation on Artificial Inductive Surfaces and Suppress Plant Immunity

The appressoria that are generated by the rice blast fungus Magnaporthe oryzae in response to surface cues are important for successful colonization. Previous work showed that regulators of G-protein signaling (RGS) and RGS-like proteins play critical roles in appressorium formation. However, the mechanisms by which these proteins orchestrate surface recognition for appressorium induction remain unclear. Here, we performed comparative transcriptomic studies of ΔMorgs mutant and wild-type strains and found that M. oryzae Aa91 (MoAa91), a homolog of the auxiliary activity family 9 protein (Aa9), was required for surface recognition of M. oryzae We found that MoAA91 was regulated by the MoMsn2 transcription factor and that its disruption resulted in defects in both appressorium formation on the artificial inductive surface and full virulence of the pathogen. We further showed that MoAa91 was secreted into the apoplast space and was capable of competing with the immune receptor chitin elicitor-binding protein precursor (CEBiP) for chitin binding, thereby suppressing chitin-induced plant immune responses. In summary, we have found that MoAa91 is a novel signaling molecule regulated by RGS and RGS-like proteins and that MoAa91 not only governs appressorium development and virulence but also functions as an effector to suppress host immunity.IMPORTANCE The rice blast fungus Magnaporthe oryzae generates infection structure appressoria in response to surface cues largely due to functions of signaling molecules, including G-proteins, regulators of G-protein signaling (RGS), mitogen-activated protein (MAP) kinase pathways, cAMP signaling, and TOR signaling pathways. M. oryzae encodes eight RGS and RGS-like proteins (MoRgs1 to MoRgs8), and MoRgs1, MoRgs3, MoRgs4, and MoRgs7 were found to be particularly important in appressorium development. To explore the mechanisms by which these proteins regulate appressorium development, we have performed a comparative in planta transcriptomic study and identified an auxiliary activity family 9 protein (Aa9) homolog that we named MoAa91. We showed that MoAa91 was secreted from appressoria and that the recombinant MoAa91 could compete with a chitin elicitor-binding protein precursor (CEBiP) for chitin binding, thereby suppressing chitin-induced plant immunity. By identifying MoAa91 as a novel signaling molecule functioning in appressorium development and an effector in suppressing host immunity, our studies revealed a novel mechanism by which RGS and RGS-like proteins regulate pathogen-host interactions.

MicroRNA-like milR236, regulated by transcription factor MoMsn2, targets histone acetyltransferase MoHat1 to play a role in appressorium formation and virulence of the rice blast fungus Magnaporthe oryzae

MicroRNAs (miRNAs) play important roles in various cellular growth and developmental processes through post-transcriptional gene regulation via mRNA cleavage and degradation and the inhibition of protein translation. To explore if miRNAs play a role in appressoria formation and virulence that are also governed by the regulators of G-protein signaling (RGS) proteins in the rice blast fungus Magnaporthe oryzae, we have compared small RNA (sRNA) production between several ΔMorgs mutant and the wild-type strains. We have identified sRNA236 as a microRNA-like milR236 that targets the encoding sequence of MoHat1, a histone acetyltransferase type B catalytic subunit involved in appressorium function and virulence. We have also found that milR236 overexpression induces delayed appressorium formation and virulence attenuation, similar to those displayed by the ΔMohat1 mutant strain. Moreover, we have shown that the transcription factor MoMsn2 binds to the promoter sequence of milR236 to further suppress MoHAT1 transcription and MoHat1-regulated appressorium formation and virulence. In summary, by identifying a novel regulatory role of sRNA in the blast fungus, our studies reveal a new paradigm in the multifaceted regulatory pathways that govern the appressorium formation and virulence of M. oryzae.

System-Wide Characterization of MoArf GTPase Family Proteins and Adaptor Protein MoGga1 Involved in the Development and Pathogenicity of Magnaporthe oryzae

ADP ribosylation factor (Arf) small GTPase family members are involved in vesicle trafficking and organelle maintenance in organisms ranging from Saccharomyces cerevisiae to humans. A previous study identified Magnaporthe oryzae Arf6 (MoArf6) as one of the Arf proteins that regulates growth and conidiation in the rice blast fungus M. oryzae, but the remaining family proteins remain unknown. Here, we identified six additional Arf proteins, including MoArf1, MoArl1, MoArl3, MoArl8, MoCin4, and MoSar1, as well as their sole adaptor protein, MoGga1, and determined their shared and specific functions. We showed that the majority of these proteins exhibit positive regulatory functions, most notably, in growth. Importantly, MoArl1, MoCin4, and MoGga1 are involved in pathogenicity through the regulation of host penetration and invasive hyphal growth. MoArl1 and MoCin4 also regulate normal vesicle trafficking, and MoCin4 further controls the formation of the biotrophic interfacial complex (BIC). Moreover, we showed that Golgi-cytoplasm cycling of MoArl1 is required for its function. Finally, we demonstrated that interactions between MoArf1 and MoArl1 with MoGga1 are important for Golgi localization and pathogenicity. Collectively, our findings revealed the shared and specific functions of Arf family members in M. oryzae and shed light on how these proteins function through conserved mechanisms to govern growth, transport, and virulence of the blast fungus.IMPORTANCEMagnaporthe oryzae is the causal agent of rice blast, representing the most devastating diseases of rice worldwide, which results in losses of amounts of rice that could feed more than 60 million people each year. Arf (ADP ribosylation factor) small GTPase family proteins are involved in vesicle trafficking and organelle maintenance in eukaryotic cells. To investigate the function of Arf family proteins in M. oryzae, we systematically characterized all seven Arf proteins and found that they have shared and specific functions in governing the growth, development, and pathogenicity of the blast fungus. We have also identified the pathogenicity-related protein MoGga1 as the common adaptor of MoArf1 and MoArl1. Our findings are important because they provide the first comprehensive characterization of the Arf GTPase family proteins and their adaptor protein MoGga1 functioning in a plant-pathogenic fungus, which could help to reveal new fungicide targets to control this devastating disease.

The Secreted Protein MoHrip1 Is Necessary for the Virulence of Magnaporthe oryzae

Secreted effectors from Magnaporthe oryzae play critical roles in the interaction with rice to facilitate fungal infection and disease development. M. oryzae-secreted protein MoHrip1 can improve plant defense as an elicitor in vitro, however, its biological function in fungal infection is not clear. In this study, we found that the expression of mohrip1 was significantly induced in the stages of fungal penetration and colonization. Although dispensable for the growth and conidiation, MoHrip1 was necessary for the full virulence of M. oryzae. Deletion of mohrip1 remarkably compromised fungal virulence on rice seedlings and even on rice leaves with wounds. Rice sheath inoculation assay further demonstrated the defects of mohrip1-deleted mutants on penetration and proliferation in rice cells. Additionally, compared with WT and complementation strain, the inoculation of mohrip1-deleted mutants induced a higher expression of specific defense related genes and a higher production of specific defensive compounds in rice leaves. These data collectively indicated that MoHrip1 is necessary for fungal penetration and invasive expansion, and further full virulence of rice blast fungus.

Every Coin Has Two Sides: Reactive Oxygen Species during Rice⁻Magnaporthe oryzae Interaction

Reactive oxygen species (ROS) are involved in many important processes, including the growth, development, and responses to the environments, in rice (Oryza sativa) and Magnaporthe oryzae. Although ROS are known to be critical components in rice⁻M. oryzae interactions, their regulations and pathways have not yet been completely revealed. Recent studies have provided fascinating insights into the intricate physiological redox balance in rice⁻M. oryzae interactions. In M. oryzae, ROS accumulation is required for the appressorium formation and penetration. However, once inside the rice cells, M. oryzae must scavenge the host-derived ROS to spread invasive hyphae. On the other side, ROS play key roles in rice against M. oryzae. It has been known that, upon perception of M. oryzae, rice plants modulate their activities of ROS generating and scavenging enzymes, mainly on NADPH oxidase OsRbohB, by different signaling pathways to accumulate ROS against rice blast. By contrast, the M. oryzae virulent strains are capable of suppressing ROS accumulation and attenuating rice blast resistance by the secretion of effectors, such as AvrPii and AvrPiz-t. These results suggest that ROS generation and scavenging of ROS are tightly controlled by different pathways in both M. oryzae and rice during rice blast. In this review, the most recent advances in the understanding of the regulatory mechanisms of ROS accumulation and signaling during rice⁻M. oryzae interaction are summarized.

MoPpe1 partners with MoSap1 to mediate TOR and cell wall integrity signalling in growth and pathogenicity of the rice blast fungus Magnaporthe oryzae

In the rice blast fungus Magnaporthe oryzae, the cell wall integrity (CWI) signalling pathway governs cell wall changes in response to external cues and normal CWI signalling is critical for appressorium function and pathogenicity. We previously characterized the mitogen-activated protein kinase (MAPK) kinase MoMkk1 as an integral component of the CWI pathway. Using the affinity purification approach, we have identified MoMkk1-interacting MoPpe1 as a homologue of Saccharomyces cerevisiae serine/threonine protein phosphatase Sit4/Ppe1. We found that MoPpe1 is required for vegetative growth, conidiation and full virulence. In addition, we found that MoPpe1 interacts with MoSap1, a protein with functions similar to MoPpe1. Intriguingly, we found that MoPpe1-MoSap1 interaction is related to CWI and target of rapamycin (TOR) pathways. We presented evidence suggesting that MoPpe1 and MoSap1 function as an adaptor complex linking CWI and TOR signalling and that the activation of the TOR pathway leads to suppression of CWI signalling, resulting in defects in appressorium function and pathogenicity. Taken together, our studies not only reveal important functions of MoMkk1-MoPpe1-MoSap1 interactions in growth and pathogenicity of the blast fungus, but also highlight the complexity of regulatory networks involving conserved yet novel regulatory mechanisms of CWI and TOR signalling.

Thioredoxin and Glutaredoxin Systems Required for Oxidative Stress Resistance, Fungicide Sensitivity, and Virulence of Alternaria alternata

This study determined the function of thioredoxin and glutaredoxin systems in the phytopathogenic fungus Alternaria alternata via analyzing mutants obtained from the targeted deletion of genes encoding thioredoxin peroxidase (Tsa1), thioredoxin reductase (Trr1), and glutathione reductase (Glr1). Trr1 and Glr1, but not Tsa1, are required for growth and conidiation. The reduced growth and conidiation seen in the Trr1 or Glr1 deletion mutant can be restored by glutathione. Deletion mutants showing growth inhibition by oxidants are defective for H₂O₂ detoxification and induce smaller lesions on citrus leaves. Trr1 and Glr1, but not Tsa1, also contribute to NaCl resistance. Glr1 is required for sorbitol resistance and is responsible for resistance to mancozeb and boscalid but not chlorothalonil fungicides, a novel phenotype that has not been reported in fungi. Trr1 is required for resistance to boscalid and chlorothalonil fungicides but confers susceptibility to mancozeb. The Tsa1 deletion mutant displays wild-type sensitivity to the tested fungicides. The expression of Tsa1 and Trr1 is regulated by the oxidative stress responsive regulators Yap1, Hog1, and Skn7. The expression of Tsa1, but not Trr1, is also regulated indirectly by the NADPH oxidase. The results indicate that the capability to resist oxidative stress is required for virulence of A. alternataIMPORTANCE The thioredoxin and glutaredoxin systems are important thiol antioxidant systems in cells, and knowledge of these two systems in the plant-pathogenic fungus A. alternata is useful for finding new strategies to reduce the virulence of this pathogen. In this study, we demonstrated that thiol antioxidant system-related genes (Tsa1, Trr1, and Glr1) are required for H₂O₂ detoxification and virulence in A. alternata Moreover, deletion of Trr1 results in hypersensitivity to the fungicides chlorothalonil and boscalid, and Glr1 deletion mutants are highly sensitive to mancozeb, which is the fungicide mostly used in citrus fields. Therefore, our findings demonstrate that the ability to detoxify reactive oxygen species (ROS) plays a critical role in pathogenesis on citrus and provide novel insights into the physiological functions of thiol-containing systems in fungicide sensitivity for A. alternata.

New findings on phosphodiesterases, MoPdeH and MoPdeL, in Magnaporthe oryzae revealed by structural analysis

The cyclic adenosine monophosphate (cAMP) signalling pathway mediates signal communication and sensing during infection-related morphogenesis in eukaryotes. Many studies have implicated cAMP as a critical mediator of appressorium development in the rice blast fungus, Magnaporthe oryzae. The cAMP phosphodiesterases, MoPdeH and MoPdeL, as key regulators of intracellular cAMP levels, play pleiotropic roles in cell wall integrity, cellular morphology, appressorium formation and infectious growth in M. oryzae. Here, we analysed the roles of domains of MoPdeH and MoPdeL separately or in chimeras. The results indicated that the HD and EAL domains of MoPdeH are indispensable for its phosphodiesterase activity and function. Replacement of the MoPdeH HD domain with the L1 and L2 domains of MoPdeL, either singly or together, resulted in decreased cAMP hydrolysis activity of MoPdeH. All of the transformants exhibited phenotypes similar to that of the ΔMopdeH mutant, but also revealed that EAL and L1 play additional roles in conidiation, and that L1 is involved in infectious growth. We further found that the intracellular cAMP level is important for surface signal recognition and hyphal autolysis. The intracellular cAMP level negatively regulates Mps1-MAPK and positively regulates Pmk1-MAPK in the rice blast fungus. Our results provide new information to better understand the cAMP signalling pathway in the development, differentiation and plant infection of the fungus.

Glycerol-3-Phosphate Shuttle Is Involved in Development and Virulence in the Rice Blast Fungus Pyricularia oryzae

The glycerol-3-phosphate (G-3-P) shuttle is an important pathway for delivery of cytosolic reducing equivalents into mitochondrial oxidative phosphorylation, and plays essential physiological roles in yeast, plants, and animals. However, its role has been unclear in filamentous and pathogenic fungi. Here, we characterize the function of the G-3-P shuttle in Pyricularia oryzae by genetic and molecular analyses. In P. oryzae, a glycerol-3-phosphate dehydrogenase 1 (PoGpd1) is involved in NO production, conidiation, and utilization of several carbon sources (pyruvate, sodium acetate, glutamate, and glutamine). A glycerol-3-phosphate dehydrogenase 2 (PoGpd2) is essential for glycerol utilization and fungal development. Deletion of PoGPD2 led to delayed aerial hyphal formation, accelerated aerial hyphal collapse, and reduced conidiation on complete medium (CM) under a light-dark cycle. Aerial mycelial surface hydrophobicity to water and Tween 20 was decreased in ΔPogpd2. Melanin synthesis genes required for cell wall construction and two transcription factor genes (COS1 and CONx2) required for conidiation and/or aerial hyphal differentiation were down-regulated in the aerial mycelia of ΔPogpd2 and ΔPogpd1. Culturing under continuous dark could complement the defects of aerial hyphal differentiation of ΔPogpd2 observed in a light-dark cycle. Two light-sensitive protein genes (PoSIR2 encoding an NAD⁺-dependent deacetylase and TRX2 encoding a thioredoxin 2) were up-regulated in ΔPogpd2 cultured on CM medium in a light-dark cycle. ΔPogpd2 showed an increased intracellular NAD⁺/NADH ratio and total NAD content, and alteration of intracellular ATP production. Culturing on minimal medium also could restore aerial hyphal differentiation of ΔPogpd2, which is deficient on CM medium in a light-dark cycle. Two glutamate synthesis genes, GDH1 and PoGLT1, which synthesize glutamate coupled with oxidation of NADH to NAD⁺, were significantly up-regulated in ΔPogpd2 in a light-dark cycle. Moreover, deletion of PoGpd1 or PoGpd2 led to reduced virulence of conidia or hyphae on rice. The glycerol-3-phosphate shuttle is involved in cellular redox, fungal development, and virulence in P. oryzae.

The Mitogen-Activated Protein Kinase CgMK1 Governs Appressorium Formation, Melanin Synthesis, and Plant Infection of Colletotrichum gloeosporioides

The fungus Colletotrichum gloeosporiodes infects plant hosts with a specialized cell called an appressorium, which is melanized and required for plant cell wall penetration. Here, we show that the mitogen-activated protein kinase CgMK1 governs appressorium formation and virulence in the poplar anthracnose fungus C. gloeosporioides. Deletion of CgMK1 impairs aerial hyphal growth and biomass accumulation, and CgMK1 is responsible for the expression of melanin biosynthesis-associated genes. CgMK1 deletion mutants are unable to form appressorium and lose the capacity to colonize either wounded or unwounded poplar leaves, leading to loss of virulence. We demonstrate that the exogenous application of cAMP fails to restore defective appressorium formation in the CgMK1 deletion mutants, suggesting that CgMK1 may function downstream or independent of a cAMP-dependent signal for appressorium formation. Moreover, CgMK1 mutants were sensitive to high osmosis, indicating that CgMK1 plays an important role in stress response. We conclude that CgMK1 plays a vital role in regulating appressorium formation, melanin biosynthesis, and virulence in C. gloeosporiodes.

The ArfGAP protein MoGlo3 regulates the development and pathogenicity of Magnaporthe oryzae

The ADP ribosylation factor (Arf) and the coat protein complex I (COPI) are involved in vesicle transport. Together with GTPase-activating proteins (ArfGAPs) and guanine exchange factors (ArfGEFs) that regulate the activity of Arf, they govern vesicle formation, COPI trafficking and the maintenance of the Golgi complex. In an ongoing effort to study the role of membrane trafficking in pathogenesis of the rice blast fungus Magnaporthe oryzae, we identified MoGlo3 as an ArfGAP protein that is homologous to Glo3p of the budding yeast Saccharomyces cerevisiae. As suspected, MoGlo3 partially complements the function of yeast Glo3p. Consistent with findings in S. cerevisiae, MoGlo3 is localized to the Golgi, and that the localization is dependent on the conserved BoCCS domain. We found that MoGlo3 is highly expressed during conidiation and early infection stages and is required for vegetative growth, conidial production and sexual development. We further found that the ΔMoglo3 mutant is defective in endocytosis, scavenging of the reactive oxygen species, and in the response to endoplasmic reticulum (ER) stress. The combined effects result in failed appressorium function and decreased pathogenicity. Moreover, we provided evidence showing that the domains including the GAP, BoCCS and GRM are all important for normal MoGlo3 functions. Our studies further illustrate the importance of normal membrane trafficking in the physiology and pathogenicity of the rice blast fungus.