Non-canonical fungal G-protein coupled receptors promote Fusarium head blight on wheat

The nematode-trapping fungus Arthrobotrys oligospora detects prey pheromones via G protein-coupled receptors

The ability to sense prey-derived cues is essential for predatory lifestyles. Under low-nutrient conditions, Arthrobotrys oligospora and other nematode-trapping fungi develop dedicated structures for nematode capture when exposed to nematode-derived cues, including a conserved family of pheromones, the ascarosides. A. oligospora senses ascarosides via conserved MAPK and cAMP-PKA pathways; however, the upstream receptors remain unknown. Here, using genomic, transcriptomic and functional analyses, we identified two families of G protein-coupled receptors (GPCRs) involved in sensing distinct nematode-derived cues. GPCRs homologous to yeast glucose receptors are required for ascaroside sensing, whereas Pth11-like GPCRs contribute to ascaroside-independent nematode sensing. Both GPCR classes activate conserved cAMP-PKA signalling to trigger trap development. This work demonstrates that predatory fungi use multiple GPCRs to sense several distinct nematode-derived cues for prey recognition and to enable a switch to a predatory lifestyle. Identification of these receptors reveals the molecular mechanisms of cross-kingdom communication via conserved pheromones also sensed by plants and animals.

Identification and functional characterisation of a locus for target site integration in Fusarium graminearum

BACKGROUND

Fusarium Head Blight (FHB) is a destructive floral disease of different cereal crops. The Ascomycete fungus Fusarium graminearum (Fg) is one of the main causal agents of FHB in wheat and barley. The role(s) in virulence of Fg genes include genetic studies that involve the transformation of the fungus with different expression cassettes. We have observed in several studies where Fg genes functions were characterised that integration of expression cassettes occurred randomly. Random insertion of a cassette may disrupt gene expression and/or protein functions and hence the overall conclusion of the study. Target site integration (TSI) is an approach that consists of identifying a chromosomal region where the cassette can be inserted. The identification of a suitable locus for TSI in Fg would avert the potential risks of ectopic integration.

RESULTS

Here, we identified a highly conserved intergenic region on chromosome 1 suitable for TSI. We named this intergenic region TSI locus 1. We developed an efficient cloning vector system based on the Golden Gate method to clone different expression cassettes for use in combination with TSI locus 1. We present evidence that integrations in the TSI locus 1 affects neither fungal virulence nor fungal growth under different stress conditions. Integrations at the TSI locus 1 resulted in the expression of different gene fusions. In addition, the activities of Fg native promoters were not altered by integration into the TSI locus 1. We have developed a bespoke bioinformatic pipeline to analyse the existence of ectopic integrations, cassette truncations and tandem insertions of the cassette that may occurred during the transformation process. Finally, we established a protocol to study protein secretion in wheat coleoptiles using confocal microscopy and the TSI locus 1.

CONCLUSION

The TSI locus 1 can be used in Fg and potentially other cereal infecting Fusarium species for diverse studies including promoter activity analysis, protein secretion, protein localisation studies and gene complementation. The bespoke bioinformatic pipeline developed in this work together with PCR amplification of the insert could be an alternative to Southern blotting, the gold standard technique used to identify ectopic integrations, cassette truncations and tandem insertions in fungal transformation.

Antifungal Effect of Metabolites from Bacterial Symbionts of Entomopathogenic Nematodes on Fusarium Head Blight of Wheat

Fungal diseases such as Fusarium head blight (FHB) are significant biotic stressors, negatively affecting wheat production and quality. This study explored the antifungal activity of the metabolites produced by the bacterial symbionts of entomopathogenic nematodes (EPNs) against FHB-causing Fusarium sp. Fusarium graminearum. To achieve this, the symbiotic bacteria of nine EPN isolates from the EPN collection at the Agricultural Research Council-Small Grains (ARC-SG) were isolated from the cadavers of Galleria mellonella (Lepidoptera: Pyralidae) larvae after infection with EPNs. Broth cultures (crude) and their supernatants (filtered and autoclaved) of each bacterial isolate were used as bacterial metabolite treatments to test their inhibitory effect on the mycelial growth and spore germination of F. graminearum. Mycelial growth inhibition rates varied among both bacterial isolates and treatments. Crude metabolite treatments proved to be more effective than filtered and autoclaved metabolite treatments, with an overall inhibition rate of 75.25% compared to 23.93% and 13.32%, respectively. From the crude metabolite treatments, the Xenorhabdus khoisanae SGI 197 bacterial isolate from Steinernema beitlechemi SGI 197 had the highest mean inhibition rate of 96.25%, followed by Photorhabdus luminescens SGI 170 bacteria isolated from Heterorhabditis bacteriophora SGI 170 with a 95.79% mean inhibition rate. The filtered metabolite treatments of all bacterial isolates were tested for their inhibitory activity against Fusarium graminearum spore germination. Mean spore germination inhibition rates from Xenorhabdus spp. bacterial isolates were higher (83.91 to 96.29%) than those from Photorhabdus spp. (6.05 to 14.74%). The results obtained from this study suggest that EPN symbiotic bacterial metabolites have potential use as biological control agents of FHB. Although field efficacy against FHB was not studied, the significant inhibition of mycelial growth and spore germination suggest that the application of these metabolites at the flowering stage may provide protection to plants against infection with or spread of F. graminearum. These metabolites have the potential to be employed as part of integrated pest management (IPM) to inhibit/delay conidia germination until the anthesis (flowering stage) of wheat seedlings has passed.

Identification of Essential Genes for the Establishment of Spray-Induced Gene Silencing-Based Disease Control in Fusarium graminearum

As resistance to chemical fungicides continues to increase inFusarium graminearum, there is a growing need to develop novel disease control strategies. To discover essential genes that could serve as new disease control targets, we selected essential gene candidates that had failed to be deleted in previous studies. Thirteen genes were confirmed to be essential, either by constructing conditional promoter replacement mutants or by employing a clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR-associated protein 9 (Cas9)-mediated editing strategy. We synthesized double-stranded RNAs (dsRNAs) targeting these essential genes and analyzed their protective effects in plants using a spray-induced gene silencing (SIGS) method. When dsRNAs targeting Fg10360, Fg13150, and Fg06123 were applied to detached barley leaves prior to fungal inoculation, disease lesions were greatly reduced. Our findings provide evidence of the potential of essential genes identified by a SIGS method to be effective targets for the control of fungal diseases.

Wheat adaptation to environmental stresses under climate change: Molecular basis and genetic improvement

Wheat (Triticum aestivum) is a staple food for about 40% of the world's population. As the global population has grown and living standards improved, high yield and improved nutritional quality have become the main targets for wheat breeding. However, wheat production has been compromised by global warming through the more frequent occurrence of extreme temperature events, which have increased water scarcity, aggravated soil salinization, caused plants to be more vulnerable to diseases, and directly reduced plant fertility and suppressed yield. One promising option to address these challenges is the genetic improvement of wheat for enhanced resistance to environmental stress. Several decades of progress in genomics and genetic engineering has tremendously advanced our understanding of the molecular and genetic mechanisms underlying abiotic and biotic stress responses in wheat. These advances have heralded what might be considered a "golden age" of functional genomics for the genetic improvement of wheat. Here, we summarize the current knowledge on the molecular and genetic basis of wheat resistance to abiotic and biotic stresses, including the QTLs/genes involved, their functional and regulatory mechanisms, and strategies for genetic modification of wheat for improved stress resistance. In addition, we also provide perspectives on some key challenges that need to be addressed.

Genetic and Transcriptional Regulatory Mechanisms of Lipase Activity in the Plant Pathogenic Fungus Fusarium graminearum

Lipases, which catalyze the hydrolysis of long-chain triglycerides, diglycerides, and monoglycerides into free fatty acids and glycerol, participate in various biological pathways in fungi. In this study, we examined the biological functions and regulatory mechanisms of fungal lipases via two approaches. First, we performed a systemic functional characterization of 86 putative lipase-encoding genes in the plant-pathogenic fungus Fusarium graminearum. The phenotypes were assayed for vegetative growth, asexual and sexual reproduction, stress responses, pathogenicity, mycotoxin production, and lipase activity. Most mutants were normal in the assessed phenotypes, implying overlapping roles for lipases in F. graminearum. In particular, FgLip1 and Fgl1 were revealed as core extracellular lipases in F. graminearum. Second, we examined the lipase activity of previously constructed transcription factor (TF) mutants of F. graminearum and identified three TFs and one histone acetyltransferase that significantly affect lipase activity. The relative transcript levels of FgLIP1 and FGL1 were markedly reduced or enhanced in these TF mutants. Among them, Gzzc258 was identified as a key lipase regulator that is also involved in the induction of lipase activity during sexual reproduction. To our knowledge, this study is the first comprehensive functional analysis of fungal lipases and provides significant insights into the genetic and regulatory mechanisms underlying lipases in fungi. IMPORTANCE Fusarium graminearum is an economically important plant-pathogenic fungus that causes Fusarium head blight (FHB) on wheat and barley. Here, we constructed a gene knockout mutant library of 86 putative lipase-encoding genes and established a comprehensive phenotypic database of the mutants. Among them, we found that FgLip1 and Fgl1 act as core extracellular lipases in this pathogen. Moreover, several putative transcription factors (TFs) that regulate the lipase activities in F. graminearum were identified. The disruption mutants of F. graminearum-lipase regulatory TFs all showed defects in sexual reproduction, which implies a strong relationship between sexual development and lipase activity in this fungus. These findings provide valuable insights into the genetic mechanisms regulating lipase activity as well as its importance to the developmental stages of this plant-pathogenic fungus.

Overview of fungal terpene synthases and their regulation

Terpenes and terpenoids are a group of isoprene-derived molecules that constitute the largest group of natural products and secondary metabolites produced by living things, with more than 25,000 compounds reported. These compounds are synthesized by enzymes called terpene synthases, which include several families of cyclases and enzymes. These are responsible for adding functional groups to cyclized structures. Fungal terpenoids are of great interest for their pharmacological properties; therefore, understanding the mechanisms that regulate their synthesis (regulation of the mevalonate pathway, regulation of gene expression, and availability of cofactors) is essential to direct their production. For this reason, this review addresses the detailed study of the biosynthesis of fungal terpenoids and their regulation by various physiological and environmental factors.

Selective Quantification of Chemotropic Responses of Fusarium graminearum

Chemotropism refers to the directional growth of a living organism toward a chemical stimulus. Molecular mechanisms underlying chemotropism of fungal pathogens have recently been enabled by advancements in biological chemotropic assays, with a particular focus on the roles of G-protein-coupled receptors and their plant-derived ligands in chemotropism. Here we describe in detail an assay that enables quantification of chemotropic responses of Fusarium graminearum, with variations recently reported for Fusarium oxysporum and Trichoderma atroviride.

Fungal CFEM effectors negatively regulate a maize wall-associated kinase by interacting with its alternatively spliced variant to dampen resistance

The fungus Fusarium graminearum causes a devastating disease Gibberella stalk rot of maize. Our knowledge of molecular interactions between F. graminearum effectors and maize immunity factors is lacking. Here, we show that a group of cysteine-rich common in fungal extracellular membrane (CFEM) domain proteins of F. graminearum are required for full virulence in maize stalk infection and that they interact with two secreted maize proteins, ZmLRR5 and ZmWAK17ET. ZmWAK17ET is an alternative splicing isoform of a wall-associated kinase ZmWAK17. Both ZmLRR5 and ZmWAK17ET interact with the extracellular domain of ZmWAK17. Transgenic maize overexpressing ZmWAK17 shows increased resistance to F. graminearum, while ZmWAK17 mutants exhibit enhanced susceptibility to F. graminearum. Transient expression of ZmWAK17 in Nicotiana benthamiana triggers hypersensitive cell death, whereas co-expression of CFEMs with ZmWAK17ET or ZmLRR5 suppresses the ZmWAK17-triggered cell death. Our results show that ZmWAK17 mediates stalk rot resistance and that F. graminearum delivers apoplastic CFEMs to compromise ZmWAK17-mediated resistance.

Upstream Regulation of Development and Secondary Metabolism in Aspergillus Species

In filamentous fungal Aspergillus species, growth, development, and secondary metabolism are genetically programmed biological processes, which require precise coordination of diverse signaling elements, transcription factors (TFs), upstream and downstream regulators, and biosynthetic genes. For the last few decades, regulatory roles of these controllers in asexual/sexual development and primary/secondary metabolism of Aspergillus species have been extensively studied. Among a wide spectrum of regulators, a handful of global regulators govern upstream regulation of development and metabolism by directly and/or indirectly affecting the expression of various genes including TFs. In this review, with the model fungus Aspergillus nidulans as the central figure, we summarize the most well-studied main upstream regulators and their regulatory roles. Specifically, we present key functions of heterotrimeric G proteins and G protein-coupled receptors in signal transduction), the velvet family proteins governing development and metabolism, LaeA as a global regulator of secondary metabolism, and NsdD, a key GATA-type TF, affecting development and secondary metabolism and provide a snapshot of overall upstream regulatory processes underlying growth, development, and metabolism in Aspergillus fungi.

Insights into intracellular signaling network in Fusarium species

Fusarium is a large genus of filamentous fungi including numerous important plant pathogens. In addition to causing huge economic losses of crops, some Fusarium species produce a wide range of mycotoxins in cereal crops that affect human and animal health. The intracellular signaling in Fusarium plays an important role in growth, sexual and asexual developments, pathogenesis, and mycotoxin biosynthesis. In this review, we highlight the recent advances and provide insight into signal sensing and transduction in Fusarium species. G protein-coupled receptors and other conserved membrane receptors mediate recognition of environmental cues and activate complex intracellular signaling. Once activated, the cAMP-PKA and three well-conserved MAP kinase pathways activate downstream transcriptional regulatory networks. The functions of individual signaling pathways have been well characterized in a variety of Fusarium species, showing the conserved components with diverged functions. Furthermore, these signaling pathways crosstalk and coordinately regulate various fungal development and infection-related morphogenesis.

Deletion of all three MAP kinase genes results in severe defects in stress responses and pathogenesis in Fusarium graminearum

Mitogen-activated protein kinase (MAPK) cascades are activated by external stimuli and convert signals to cellular changes. Individual MAPKs have been characterized in a number of plant pathogenic fungi for their roles in pathogenesis and responses to biotic or abiotic stresses. However, mutants deleted of all the MAPK genes have not been reported in filamentous fungi. To determine the MAPK-less effects in a fungal pathogen, in this study we generated and characterized mutants deleted of all three MAPK genes in the wheat scab fungus Fusarium graminearum. The Gpmk1 mgv1 Fghog1 triple mutants had severe growth defects and was non-pathogenic. It was defective in infection cushion formation and DON production. Conidiation was reduced in the triple mutant, which often produced elongated conidia with more septa than the wild-type conidia. The triple mutant was blocked in sexual reproduction due to the loss of female fertility. Lack of any MAPKs resulted in an increased sensitivity to various abiotic stress including cell wall, osmotic, oxidative stresses, and phytoalexins, which are likely related to the defects of the triple mutant in environmental adaptation and plant infection. The triple mutant also had increased sensitivity to the biocontrol bacterium Bacillus velezensis and fungus Clonostachys rosea. In co-incubation assays with B. velezensis, the Gpmk1 mgv1 Fghog1 mutant had more severe growth limitation than the wild type and was defective in conidium germination and germ tube growth. In confrontation assays, the triple mutant was defective in defending against mycoparasitic activities of C. rosea and the latter could grow over the mutant but not wild-type F. graminearum. RNA-seq and metabolomics analyses showed that the MAPK triple mutant was altered in the expression of many ATP-binding cassette (ABC) and major facilitator superfamily (MFS) transporter genes and the accumulation of metabolites related to arachidonic acid, linoleic acid, and alpha-linolenic acid metabolisms. Overall, as the first study on mutants deleted of all three MAPKs in fungal pathogens, our results showed that although MAPKs are not essential for growth and asexual reproduction, the Gpmk1 mgv1 Fghog1 triple mutant was blocked in plant infection and sexual reproductions. It also had severe defects in responses to various abiotic stresses and bacterial- or fungal-fungal interactions.

Prey sensing and response in a nematode-trapping fungus is governed by the MAPK pheromone response pathway

Detection of surrounding organisms in the environment plays a major role in the evolution of interspecies interactions, such as predator-prey relationships. Nematode-trapping fungi (NTF) are predators that develop specialized trap structures to capture, kill, and consume nematodes when food sources are limited. Despite the identification of various factors that induce trap morphogenesis, the mechanisms underlying the differentiation process have remained largely unclear. Here, we demonstrate that the highly conserved pheromone-response MAPK pathway is essential for sensing ascarosides, a conserved molecular signature of nemaotdes, and is required for the predatory lifestyle switch in the NTF Arthrobotrys oligospora. Gene deletion of STE7 (MAPKK) and FUS3 (MAPK) abolished nematode-induced trap morphogenesis and conidiation and impaired the growth of hyphae. The conserved transcription factor Ste12 acting downstream of the pheromone-response pathway also plays a vital role in the predation of A. oligospora. Transcriptional profiling of a ste12 mutant identified a small subset of genes with diverse functions that are Ste12 dependent and could trigger trap differentiation. Our work has revealed that A. oligospora perceives and interprets the ascarosides produced by nematodes via the conserved pheromone signaling pathway in fungi, providing molecular insights into the mechanisms of communication between a fungal predator and its nematode prey.

The vesicular trafficking system component MIN7 is required for minimizing Fusarium graminearum infection

Plants have developed intricate defense mechanisms, referred to as innate immunity, to defend themselves against a wide range of pathogens. Plants often respond rapidly to pathogen attack by the synthesis and delivery to the primary infection sites of various antimicrobial compounds, proteins, and small RNA in membrane vesicles. Much of the evidence regarding the importance of vesicular trafficking in plant-pathogen interactions comes from studies involving model plants whereas this process is relatively understudied in crop plants. Here we assessed whether the vesicular trafficking system components previously implicated in immunity in Arabidopsis play a role in the interaction with Fusarium graminearum, a fungal pathogen well-known for its ability to cause Fusarium head blight disease in wheat. Among the analysed vesicular trafficking mutants, two independent T-DNA insertion mutants in the AtMin7 gene displayed a markedly enhanced susceptibility to F. graminearum. Earlier studies identified this gene, encoding an ARF-GEF protein, as a target for the HopM1 effector of the bacterial pathogen Pseudomonas syringae pv. tomato, which destabilizes MIN7 leading to its degradation and weakening host defenses. To test whether this key vesicular trafficking component may also contribute to defense in crop plants, we identified the candidate TaMin7 genes in wheat and knocked-down their expression through virus-induced gene silencing. Wheat plants in which TaMin7 genes were silenced displayed significantly more Fusarium head blight disease. This suggests that disruption of MIN7 function in both model and crop plants compromises the trafficking of innate immunity signals or products resulting in hypersusceptibility to various pathogens.

Comparative genomics reveals a core gene toolbox for lifestyle transitions in Hypocreales fungi

Fungi have evolved diverse lifestyles and adopted pivotal new roles in both natural ecosystems and human environments. However, the molecular mechanisms underlying their adaptation to new lifestyles are obscure. Here, we hypothesize that genes shared across all species with the same lifestyle, but absent in genera with alternative lifestyles, are crucial to that lifestyle. By analysing dozens of species within four genera in a fungal order, with each genus following a different lifestyle, we find that genus-specific genes are typically few in number. Notably, not all genus-specific genes appear to derive from de novo birth, with most instead reflecting recurrent loss across the fungi. Importantly, however, a subset of these genus-specific genes are shared by fungi with the same lifestyle in quite different evolutionary orders, thus supporting the view that some genus-specific genes are necessary for specific lifestyles. These lifestyle-specific genes are enriched for key functional classes and often exhibit specialized expression patterns. Genus-specific selection also contributes to lifestyle transitions, and is especially associated with intensity of pathogenesis. Our study, therefore, suggests that fungal adaptation to new lifestyles often requires just a small number of core genes, with gene turnover and positive selection playing complementary roles.

Breakpoint: Cell Wall and Glycoproteins and their Crucial Role in the Phytopathogenic Fungi Infection

The cell wall that surrounds fungal cells is essential for their survival, provides protection against physical and chemical stresses, and plays relevant roles during infection. In general, the fungal cell wall is composed of an outer layer of glycoprotein and an inner skeletal layer of β-glucans or α- glucans and chitin. Chitin synthase genes have been shown to be important for septum formation, cell division and virulence. In the same way, chitin can act as a potent elicitor to activate defense response in several plant species; however, the fungi can convert chitin to chitosan during plant infection to evade plant defense mechanisms. Moreover, α-1,3-Glucan, a non-degradable polysaccharide in plants, represents a key feature in fungal cell walls formed in plants and plays a protective role for this fungus against plant lytic enzymes. A similar case is with β-1,3- and β-1,6-glucan which are essential for infection, structure rigidity and pathogenicity during fungal infection. Cell wall glycoproteins are also vital to fungi. They have been associated with conidial separation, the increase of chitin in conidial cell walls, germination, appressorium formation, as well as osmotic and cell wall stress and virulence; however, the specific roles of glycoproteins in filamentous fungi remain unknown. Fungi that can respond to environmental stimuli distinguish these signals and relay them through intracellular signaling pathways to change the cell wall composition. They play a crucial role in appressorium formation and penetration, and release cell wall degrading enzymes, which determine the outcome of the interaction with the host. In this review, we highlight the interaction of phypatophogen cell wall and signaling pathways with its host and their contribution to fungal pathogenesis.

Heterotrimeric G-Protein Signaling Is Required for Cellulose Degradation in Neurospora crassa

Filamentous fungi are critical for the recycling of plant litter in the biosphere by degrading lignocellulosic biomass into simpler compounds for metabolism. Both saprophytic and pathogenic fungi utilize plant cell wall-degrading enzymes to liberate carbon for metabolism. Several studies have demonstrated a role for cellulase enzymes during infection of economically relevant crops by fungal pathogens. Especially in developing countries, severe plant disease means loss of entire crops, sometimes leading to starvation. In this study, we demonstrate that G-protein signaling is a key component of cellulase production. Therefore, understanding the role of G-protein signaling in the regulation of the unique metabolism of cellulose by these organisms can inform innovations in strain engineering of industrially relevant species for biofuel production and in combatting food shortages caused by plant pathogens.

The filamentous fungus Neurospora crassa decomposes lignocellulosic biomass to generate soluble sugars as carbon sources. In this study, we investigated a role for heterotrimeric G-protein signaling in cellulose degradation. Loss of the Gα subunit genes gna-1 and gna-3, the Gβ subunit genes gnb-1 and cpc-2, the Gγ gene gng-1, or the gene for downstream effector adenylyl cyclase (cr-1) resulted in loss of detectable cellulase activity. This defect was also observed in strains expressing a constitutively active version of gna-3 (gna-3^Q208L). We found that GNA-1 levels are greatly reduced in Δgna-3, Δgnb-1, and Δgng-1 strains, likely contributing to cellulase defects in these genetic backgrounds. The observation that gna-3^Q208L Δgnb-1 strains exhibit cellulase activity, despite greatly reduced levels of GNA-1 protein, is consistent with positive control of cellulase production by GNA-3 that is manifested in the absence of gnb-1. Expression patterns for five cellulase genes showed that Δgna-1, Δgnb-1, and Δgna-3 mutants produce less cellulase mRNA than the wild type, consistent with transcriptional regulation. Δcpc-2 mutants had wild-type levels of cellulase transcripts, suggesting posttranscriptional control. In contrast, results for Δcr-1 mutants support both transcriptional and posttranscriptional control of cellulase activity by cAMP signaling. Cellulase activity defects in Δgna-3 mutants were fully remediated by cAMP supplementation, consistent with GNA-3 operating upstream of cAMP signaling. In contrast, cAMP addition only partially corrected cellulase activity defects in Δgna-1 and Δgnb-1 mutants, suggesting participation of GNA-1 and GNB-1 in additional cAMP-independent pathways that control cellulase activity.

Epigenetic regulation of gene expression improves Fusarium head blight resistance in durum wheat

Eight advanced durum-breeding lines were treated with 5-methyl-azacytidine to test the feasibility of generating sources of Fusarium head blight (FHB) resistance. Of the 800 treated seeds, 415 germinated and were advanced up to four (M4) generations by selfing. Thirty-two of the resulting 415 M4 lines were selected following preliminary screening and were further tested for FHB resistance for three years at two field locations, and in the greenhouse. Five of the 32 M4 lines showed less than 30% disease severity, as compared to the parental lines and susceptible checks. Fusarium-damaged kernels and deoxynivalenol analyses supported the findings of the field and greenhouse disease assessments. Two of the most resistant M4 lines were crossed to a susceptible parent, advanced to third generation (BC₁:F₃) and were tested for stability and inheritance of the resistance. About, one third of the BC₁:F₃ lines showed FHB resistance similar to their M4 parents. The overall methylation levels (%) were compared using FASTmC method, which did not show a significant difference between M4 and parental lines. However, transcriptome analysis of one M4 line revealed significant number of differentially expressed genes related to biosynthesis of secondary metabolites, MAPK signaling, photosynthesis, starch and sucrose metabolism, plant hormone signal transduction and plant-pathogen interaction pathways, which may have helped in improved FHB resistance.

Aspergillus fumigatus G-Protein Coupled Receptors GprM and GprJ Are Important for the Regulation of the Cell Wall Integrity Pathway, Secondary Metabolite Production, and Virulence

A. fumigatus is the main etiological agent of invasive pulmonary aspergillosis, a life-threatening fungal disease that occurs in severely immunocompromised humans. Withstanding the host environment is essential for A. fumigatus virulence, and sensing of extracellular cues occurs primarily through G-protein coupled receptors (GPCRs) that activate signal transduction pathways, which, in turn, regulate fungal development, metabolism, virulence, and mycotoxin biosynthesis. The A. fumigatus genome encodes 15 putative classical GPCRs, with only three having been functionally characterized to date. In this work, we show that the two GPCRs GprM and GprJ regulate the phosphorylation of the mitogen-activated protein kinase MpkA and thus control the regulation of the cell wall integrity pathway. GprM and GprJ are also involved in the regulation of the production of the secondary metabolites fumagillin, pyripyropene, fumigaclavine C, fumiquinazoline, melanin, and fumitremorgin, and this regulation partially occurs through the activation of MpkA. Furthermore, GprM and GprJ are important for virulence in the insect model Galleria mellonella. This work therefore functionally characterizes two GPCRs and shows how they regulate several intracellular pathways that have been shown to be crucial for A. fumigatus virulence.

G-protein coupled receptors (GPCRs) are extracellular signaling receptors that sense environmental cues. Fungi sense their environment primarily through GPCR-mediated signaling pathways, which, in turn, regulate fungal development, metabolism, virulence, and mycotoxin biosynthesis. Aspergillus fumigatus is an important human pathogen that causes aspergillosis, a heterogeneous group of diseases that present a wide range of clinical manifestations. Here, we investigate in detail the role of the GPCRs GprM and GprJ in growth and gene expression. GprM and GprJ are important for melanin production and the regulation of the cell wall integrity (CWI) pathway. Overexpression of gprM and gprJ causes a 20 and 50% reduction in growth rate compared to the wild-type (WT) strain and increases sensitivity to cell wall-damaging agents. Phosphorylation of the CWI protein kinase MpkA is increased in the ΔgprM and ΔgprJ strains and decreased in the overexpression mutants compared to the WT strain. Furthermore, differences in cell wall polysaccharide concentrations and organization were observed in these strains. Transcriptome sequencing suggests that GprM and GprJ negatively regulate genes encoding secondary metabolites (SMs). Mass spectrometry analysis confirmed that the production of fumagillin, pyripyropene, fumigaclavine C, fumiquinazoline, and fumitremorgin is reduced in the ΔgprM and ΔgprJ strains, at least partially through the activation of MpkA. Overexpression of grpM also resulted in the regulation of many transcription factors, with AsgA predicted to function downstream of GprM and MpkA signaling. Finally, we show that the ΔgprM and ΔgprJ mutants are reduced in virulence in the Galleria mellonella insect model of invasive aspergillosis.

Identification of a key G-protein coupled receptor in mediating appressorium formation and fungal virulence against insects

Fungal G-protein coupled receptors (GPCRs) play essential roles in sensing environmental cues including host signals. The study of GPCR in mediating fungus-insect interactions is still limited. Here we report the evolution of GPCR genes encoded in the entomopathogenic Metarhizium species and found the expansion of Pth11-like GPCRs in the generalist species with a wide host range. By deletion of ten candidate genes MrGpr1-MrGpr10 selected from the six obtained subfamilies in the generalist M. robertsii, we found that each of them played a varied level of roles in mediating appressorium formation. In particular, deletion of MrGpr8 resulted in the failure of appressorium formation on different substrates and the loss of virulence during topical infection of insects but not during injection assays when compared with the wild-type (WT) strain. Further analysis revealed that disruption of MrGpr8 substantially impaired the nucleus translocation of the mitogen-activated protein kinase (MAPK) Mero-Fus3 but not the MAPK Mero-Slt2 during appressorium formation. We also found that the defect of AMrGpr8 could not be rescued with the addition of cyclic AMP for appressorium formation. Relative to the WT, differential expression of the selected genes have also been detected in AMrGpr8. The results of this study may benefit the understanding of fungus-interactions mediated by GPCRs.

Characterization of the Role of a Non-GPCR Membrane-Bound CFEM Protein in the Pathogenicity and Germination of Botrytis cinerea

The necrotrophic fungus Botrytis cinerea, is considered a major cause of postharvest losses in a wide range of crops. The common fungal extracellular membrane protein (CFEM), containing a conserved eight-cysteine pattern, was found exclusively in fungi. Previous studies in phytopathogenic fungi have demonstrated the role of membrane-bound and secreted CFEM-containing proteins in different aspects of fungal virulence. However, non-G protein-coupled receptor (non-GPCR) membrane CFEM proteins have not been studied yet in phytopathogenic fungi. In the present study, we have identified a non-GPCR membrane-bound CFEM-containing protein, Bcin07g03260, in the B. cinerea genome, and generated deletion mutants, ΔCFEM-Bcin07g03260, to study its potential role in physiology and virulence. Three independent ΔCFEM-Bcin07g03260 mutants showed significantly reduced progression of a necrotic lesion on tomato (Solanum lycopersicum) leaves. Further analysis of the mutants revealed significant reduction (approximately 20–30%) in conidial germination and consequent germ tube elongation compared with the WT. Our data complements a previous study of secreted ΔCFEM1 mutants of B. cinerea that showed reduced progression of necrotic lesions on leaves, without effect on germination. Considering various functions identified for CFEM proteins in fungal virulence, our work illustrates a potential new role for a non-GPCR membrane CFEM in pathogenic fungi to control virulence in the fungus B. cinerea.

Apoplastic Cell Death-Inducing Proteins of Filamentous Plant Pathogens: Roles in Plant-Pathogen Interactions

Filamentous pathogens, such as phytopathogenic oomycetes and fungi, secrete a remarkable diversity of apoplastic effector proteins to facilitate infection, many of which are able to induce cell death in plants. Over the past decades, over 177 apoplastic cell death-inducing proteins (CDIPs) have been identified in filamentous oomycetes and fungi. An emerging number of studies have demonstrated the role of many apoplastic CDIPs as essential virulence factors. At the same time, apoplastic CDIPs have been documented to be recognized by plant cells as pathogen-associated molecular patterns (PAMPs). The recent findings of extracellular recognition of apoplastic CDIPs by plant leucine-rich repeat-receptor-like proteins (LRR-RLPs) have greatly advanced our understanding of how plants detect them and mount a defense response. This review summarizes the latest advances in identifying apoplastic CDIPs of plant pathogenic oomycetes and fungi, and our current understanding of the dual roles of apoplastic CDIPs in plant-filamentous pathogen interactions.

Functional evaluation of a homologue of plant rapid alkalinisation factor (RALF) peptides in Fusarium graminearum

The cereal infecting fungus Fusarium graminearum is predicted to possess a single homologue of plant RALF (rapid alkalinisation factor) peptides. Fusarium mutant strains lacking FgRALF were generated and found to exhibit wildtype virulence on wheat and Arabidopsis floral tissue. Arabidopsis lines constitutively overexpressing FgRALF exhibited no obvious change in susceptibility to F. graminearum leaf infection. In contrast transient virus-mediated over-expression (VOX) of FgRALF in wheat prior to F. graminearum infection, slightly increased the rate of fungal colonisation of floral tissue. Ten putative Feronia (FER) receptors of RALF peptide were identified bioinformatically in hexaploid wheat (Triticum aestivum). Transient silencing of two wheat FER homoeologous genes prior to F. graminearum inoculation did not alter the subsequent interaction outcome. Collectively, our VOX results show that the fungal RALF peptide may be a minor contributor in F. graminearum virulence but results from fungal gene deletion experiments indicate potential functional redundancy within the F. graminearum genome. We demonstrate that virus-mediated over-expression is a useful tool to provide novel information about gene/protein function when results from gene deletion/disruption experimentation were uninformative.

PHI-base: the pathogen-host interactions database

The pathogen–host interactions database (PHI-base) is available at www.phi-base.org. PHI-base contains expertly curated molecular and biological information on genes proven to affect the outcome of pathogen–host interactions reported in peer reviewed research articles. PHI-base also curates literature describing specific gene alterations that did not affect the disease interaction phenotype, in order to provide complete datasets for comparative purposes. Viruses are not included, due to their extensive coverage in other databases. In this article, we describe the increased data content of PHI-base, plus new database features and further integration with complementary databases. The release of PHI-base version 4.8 (September 2019) contains 3454 manually curated references, and provides information on 6780 genes from 268 pathogens, tested on 210 hosts in 13,801 interactions. Prokaryotic and eukaryotic pathogens are represented in almost equal numbers. Host species consist of approximately 60% plants (split 50:50 between cereal and non-cereal plants), and 40% other species of medical and/or environmental importance. The information available on pathogen effectors has risen by more than a third, and the entries for pathogens that infect crop species of global importance has dramatically increased in this release. We also briefly describe the future direction of the PHI-base project, and some existing problems with the PHI-base curation process.

Fusarium head blight in wheat: contemporary status and molecular approaches

Fusarium head blight (FHB) disease that occurs in wheat is caused by Fusarium graminearum and is a major risk to wheat yield. Although several research efforts focusing on FHB have been conducted in the past several decades, conditions have become more critical due to the increase in its virulent forms. In such a scenario, conferring complete resistance in plants seems to be difficult for handling this issue. The phenotyping for FHB and finding a solution for it at the genetic level comprises a long-term process as FHB infection is largely affected by environmental conditions. Modern molecular strategies have played a crucial role in revealing the host-pathogen interaction in FHB. The integration of molecular biology-based methods such as genome-wide association studies and marker-based genomic selection has provided potential cultivars for breeding programs. In this review, we aim at outlining the contemporary status of the studies conducted on FHB in wheat. The influence of FHB in wheat on animals and human health is also discussed. In addition, a summary of the advancement in the molecular technologies for identifying and developing the FHB-resistant wheat genetic resources is provided. It also suggests the future measures that are required to reduce the world's vulnerability to FHB which was one of the main goals of the US Wheat and Barley Scab Initiative.

GPCR-mediated glucose sensing system regulates light-dependent fungal development and mycotoxin production

Microorganisms sense environmental fluctuations in nutrients and light, coordinating their growth and development accordingly. Despite their critical roles in fungi, only a few G-protein coupled receptors (GPCRs) have been characterized. The Aspergillus nidulans genome encodes 86 putative GPCRs. Here, we characterise a carbon starvation-induced GPCR-mediated glucose sensing mechanism in A. nidulans. This includes two class V (gprH and gprI) and one class VII (gprM) GPCRs, which in response to glucose promote cAMP signalling, germination and hyphal growth, while negatively regulating sexual development in a light-dependent manner. We demonstrate that GprH regulates sexual development via influencing VeA activity, a key light-dependent regulator of fungal morphogenesis and secondary metabolism. We show that GprH and GprM are light-independent negative regulators of sterigmatocystin biosynthesis. Additionally, we reveal the epistatic interactions between the three GPCRs in regulating sexual development and sterigmatocystin production. In conclusion, GprH, GprM and GprI constitute a novel carbon starvation-induced glucose sensing mechanism that functions upstream of cAMP-PKA signalling to regulate fungal development and mycotoxin production.