Attenuation of PAMP-triggered immunity in maize requires down-regulation of the key β-1,6-glucan synthesis genes KRE5 and KRE6 in biotrophic hyphae of Colletotrichum graminicola

The galactose metabolism genes UGE1 and UGM1 are novel virulence factors of the maize anthracnose fungus Colletotrichum graminicola

Fungal cell walls represent the frontline contact with the host and play a prime role in pathogenesis. While the roles of the cell wall polymers like chitin and branched β-glucan are well understood in vegetative and pathogenic development, that of the most prominent galactose-containing polymers galactosaminogalactan and fungal-type galactomannan is unknown in plant pathogenic fungi. Mining the genome of the maize pathogen Colletotrichum graminicola identified the single-copy key galactose metabolism genes UGE1 and UGM1, encoding a UDP-glucose-4-epimerase and UDP-galactopyranose mutase, respectively. UGE1 is thought to be required for biosynthesis of both polymers, whereas UGM1 is specifically required for fungal-type galactomannan formation. Promoter:eGFP fusion strains revealed that both genes are expressed in vegetative and in pathogenic hyphae at all stages of pathogenesis. Targeted deletion of UGE1 and UGM1, and fluorescence-labeling of galactosaminogalactan and fungal-type galactomannan confirmed that Δuge1 mutants were unable to synthesize either of these polymers, and Δugm1 mutants did not exhibit fungal-type galactomannan. Appressoria of Δuge1, but not of Δugm1 mutants, were defective in adhesion, highlighting a function of galactosaminogalactan in the establishment of these infection cells on hydrophobic surfaces. Both Δuge1 and Δugm1 mutants showed cell wall defects in older vegetative hyphae and severely reduced appressorial penetration competence. On intact leaves of Zea mays, both mutants showed strongly reduced disease symptom severity, indicating that UGE1 and UGM1 represent novel virulence factors of C. graminicola.

Deciphering antifungal and antibiofilm mechanisms of isobavachalcone against Cryptococcus neoformans through RNA-seq and functional analyses

Cryptococcus neoformans has been designated as critical fungal pathogens by the World Health Organization, mainly due to limited treatment options and the prevalence of antifungal resistance. Consequently, the utilization of novel antifungal agents is crucial for the effective treatment of C. neoformans infections. This study exposed that the minimum inhibitory concentration (MIC) of isobavachalcone (IBC) against C. neoformans H99 was 8 µg/mL, and IBC dispersed 48-h mature biofilms by affecting cell viability at 16 µg/mL. The antifungal efficacy of IBC was further validated through microscopic observations using specific dyes and in vitro assays, which confirmed the disruption of cell wall/membrane integrity. RNA-Seq analysis was employed to decipher the effect of IBC on the C. neoformans H99 transcriptomic profiles. Real-time quantitative reverse transcription PCR (RT-qPCR) analysis was performed to validate the transcriptomic data and identify the differentially expressed genes. The results showed that IBC exhibited various mechanisms to impede the growth, biofilm formation, and virulence of C. neoformans H99 by modulating multiple dysregulated pathways related to cell wall/membrane, drug resistance, apoptosis, and mitochondrial homeostasis. The transcriptomic findings were corroborated by the antioxidant analyses, antifungal drug sensitivity, molecular docking, capsule, and melanin assays. In vivo antifungal activity analysis demonstrated that IBC extended the lifespan of C. neoformans-infected Caenorhabditis elegans. Overall, the current study unveiled that IBC targeted multiple pathways simultaneously to inhibit growth significantly, biofilm formation, and virulence, as well as to disperse mature biofilms of C. neoformans H99 and induce cell death.

Temporally-coordinated bivalent histone modifications of BCG1 enable fungal invasion and immune evasion

Bivalent histone modifications, including functionally opposite H3K4me3 and H3K27me3 marks simultaneously on the same nucleosome, control various cellular processes by fine-tuning the gene expression in eukaryotes. However, the role of bivalent histone modifications in fungal virulence remains elusive. By mapping the genome-wide landscape of H3K4me3 and H3K27me3 dynamic modifications in Fusarium graminearum (Fg) during invasion, we identify the infection-related bivalent chromatin-marked genes (BCGs). BCG1 gene, which encodes a secreted Fusarium-specific xylanase containing a G/Q-rich motif, displays the highest increase of bivalent modification during Fg infection. We report that the G/Q-rich motif of BCG1 is a stimulator of its xylanase activity and is essential for the full virulence of Fg. Intriguingly, this G/Q-rich motif is recognized by pattern-recognition receptors to trigger plant immunity. We discover that Fg employs H3K4me3 modification to induce BCG1 expression required for host cell wall degradation. After breaching the cell wall barrier, this active chromatin state is reset to bivalency by co-modifying with H3K27me3, which enables epigenetic silencing of BCG1 to escape from host immune surveillance. Collectively, our study highlights how fungal pathogens deploy bivalent epigenetic modification to achieve temporally-coordinated activation and suppression of a critical fungal gene, thereby facilitating successful infection and host immune evasion.

In Planta Study Localizes an Effector Candidate from Austropuccinia psidii Strain MF-1 to the Nucleus and Demonstrates In Vitro Cuticular Wax-Dependent Differential Expression

Austropuccinia psidii is a biotrophic fungus that causes myrtle rust. First described in Brazil, it has since spread to become a globally important pathogen that infects more than 480 myrtaceous species. One of the most important commercial crops affected by A. psidii is eucalypt, a widely grown forestry tree. The A. psidii-Eucalyptus spp. interaction is poorly understood, but pathogenesis is likely driven by pathogen-secreted effector molecules. Here, we identified and characterized a total of 255 virulence effector candidates using a genome assembly of A. psidii strain MF-1, which was recovered from Eucalyptus grandis in Brazil. We show that the expression of seven effector candidate genes is modulated by cell wax from leaves sourced from resistant and susceptible hosts. Two effector candidates with different subcellular localization predictions, and with specific gene expression profiles, were transiently expressed with GFP-fusions in Nicotiana benthamiana leaves. Interestingly, we observed the accumulation of an effector candidate, Ap28303, which was upregulated under cell wax from rust susceptible E. grandis and described as a peptidase inhibitor I9 domain-containing protein in the nucleus. This was in accordance with in silico analyses. Few studies have characterized nuclear effectors. Our findings open new perspectives on the study of A. psidii-Eucalyptus interactions by providing a potential entry point to understand how the pathogen manipulates its hosts in modulating physiology, structure, or function with effector proteins.

Cell Wall Carbohydrate Dynamics during the Differentiation of Infection Structures by the Apple Scab Fungus, Venturia inaequalis

Scab, caused by the biotrophic fungal pathogen Venturia inaequalis, is the most economically important disease of apples. During infection, V. inaequalis colonizes the subcuticular host environment, where it develops specialized infection structures called runner hyphae and stromata. These structures are thought to be involved in nutrient acquisition and effector (virulence factor) delivery, but also give rise to conidia that further the infection cycle. Despite their importance, very little is known about how these structures are differentiated. Likewise, nothing is known about how these structures are protected from host defenses or recognition by the host immune system. To better understand these processes, we first performed a glycosidic linkage analysis of sporulating tubular hyphae from V. inaequalis developed in culture. This analysis revealed that the V. inaequalis cell wall is mostly composed of glucans (44%) and mannans (37%), whereas chitin represents a much smaller proportion (4%). Next, we used transcriptomics and confocal laser scanning microscopy to provide insights into the cell wall carbohydrate composition of runner hyphae and stromata. These analyses revealed that, during subcuticular host colonization, genes of V. inaequalis putatively associated with the biosynthesis of immunogenic carbohydrates, such as chitin and β-1,6-glucan, are downregulated relative to growth in culture, while on the surface of runner hyphae and stromata, chitin is deacetylated to the less-immunogenic carbohydrate chitosan. These changes are anticipated to enable the subcuticular differentiation of runner hyphae and stromata by V. inaequalis, as well as to protect these structures from host defenses and recognition by the host immune system. IMPORTANCE Plant-pathogenic fungi are a major threat to food security. Among these are subcuticular pathogens, which often cause latent asymptomatic infections, making them difficult to control. A key feature of these pathogens is their ability to differentiate specialized subcuticular infection structures that, to date, remain largely understudied. This is typified by Venturia inaequalis, which causes scab, the most economically important disease of apples. In this study, we show that, during subcuticular host colonization, V. inaequalis downregulates genes associated with the biosynthesis of two immunogenic cell wall carbohydrates, chitin and β-1,6-glucan, and coats its subcuticular infection structures with a less-immunogenic carbohydrate, chitosan. These changes are anticipated to enable host colonization by V. inaequalis and provide a foundation for understanding subcuticular host colonization by other plant-pathogenic fungi. Such an understanding is important, as it may inform the development of novel control strategies against subcuticular plant-pathogenic fungi.

Overexpression of a beta-1,6-glucanase gene GluM in transgenic rice confers high resistance to rice blast, sheath blight and false smut

BACKGROUND

Frequent fungal diseases tend to lead to severe losses in rice production. As a main component of the fungal cell wall, glucan plays an important role in the growth and development of fungi. Glucanase can inhibit the growth of fungi by breaking glycosidic bonds, and may be a promising target for developing rice varieties with broad-spectrum disease resistance.

RESULTS

We transferred a codon-optimized β-1,6-glucanase gene (GluM) from myxobacteria into the japonica rice variety Zhonghua11 (ZH11), and obtained a large number of individual transgenic plants with GluM overexpression. Based on molecular analysis, three single-copy homozygous lines with GluM overexpression were selected for assessment of fungal disease resistance at the T₃ generation. Compared with that of the recipient cultivar ZH11, the area of rice blast lesion in transgenic rice was reduced by 82.71%; that of sheath blight lesion was decreased by 35.76%-43.67%; the sheath blight resistance in the field was enhanced by an average of 0.75 grade over 3 years; and the incidence of diseased panicles due to rice false smut was decreased by 65.79%. More importantly, there was no obvious loss of yield (without a significant effect on agronomic traits). Furthermore, plants overexpressing a β-1,6-glucanase gene showed higher disease resistance than rice plants overexpressing a β-1,3-glucanase gene derived from tobacco.

CONCLUSION

The β-1,6-glucanase gene GluM can confer broad-spectrum disease resistance to rice, providing an environmentally friendly alternative way to effectively manage fungal pathogens in rice production. © 2023 Society of Chemical Industry.

The right microbe-associated molecular patterns for effective recognition by plants

Plants are constantly exposed to diverse microbes and thus develop a sophisticated perceive system to distinguish non-self from self and identify non-self as friends or foes. Plants can detect microbes in apoplast via recognition of microbe-associated molecular patterns (MAMPs) by pattern recognition receptors (PRRs) on the cell surface to activate appropriate signaling in response to microbes. MAMPs are highly conserved but essential molecules of microbes and often buried in microbes’ complex structure. Mature MAMPs are released from microbes by invasion-induced hydrolytic enzymes in apoplast and accumulate in proximity of plasma membrane-localized PRRs to be perceived as ligands to activate downstream signaling. In response, microbes developed strategies to counteract these processing. Here, we review how the form, the concentration, and the size of mature MAMPs affect the PRR-mediated immune signaling. In particular, we describe some potential applications and explore potential open questions in the fields.

The Small Ras Superfamily GTPase Rho4 of the Maize Anthracnose Fungus Colletotrichum graminicola Is Required for β-1,3-glucan Synthesis, Cell Wall Integrity, and Full Virulence

Small Ras superfamily GTPases are highly conserved regulatory factors of fungal cell wall biosynthesis and morphogenesis. Previous experiments have shown that the Rho4-like protein of the maize anthracnose fungus Colletotrichum graminicola, formerly erroneously annotated as a Rho1 protein, physically interacts with the β-1,3-glucan synthase Gls1 (Lange et al., 2014; Curr. Genet. 60:343–350). Here, we show that Rho4 is required for β-1,3-glucan synthesis. Accordingly, Δrho4 strains formed distorted vegetative hyphae with swellings, and exhibited strongly reduced rates of hyphal growth and defects in asexual sporulation. Moreover, on host cuticles, conidia of Δrho4 strains formed long hyphae with hyphopodia, rather than short germ tubes with appressoria. Hyphopodia of Δrho4 strains exhibited penetration defects and often germinated laterally, indicative of cell wall weaknesses. In planta differentiated infection hyphae of Δrho4 strains were fringy, and anthracnose disease symptoms caused by these strains on intact and wounded maize leaf segments were significantly weaker than those caused by the WT strain. A retarded disease symptom development was confirmed by qPCR analyses. Collectively, we identified the Ras GTPase Rho4 as a new virulence factor of C. graminicola.

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

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

Fungal Pathogenesis-Related Cell Wall Biogenesis, with Emphasis on the Maize Anthracnose Fungus Colletotrichum graminicola

The genus Colletotrichum harbors many plant pathogenic species, several of which cause significant yield losses in the field and post harvest. Typically, in order to infect their host plants, spores germinate, differentiate a pressurized infection cell, and display a hemibiotrophic lifestyle after plant invasion. Several factors required for virulence or pathogenicity have been identified in different Colletotrichum species, and adaptation of cell wall biogenesis to distinct stages of pathogenesis has been identified as a major pre-requisite for the establishment of a compatible parasitic fungus-plant interaction. Here, we highlight aspects of fungal cell wall biogenesis during plant infection, with emphasis on the maize leaf anthracnose and stalk rot fungus, Colletotrichum graminicola.

Microbial interaction mediated programmed cell death in plants

Food demand of growing population can only be met by finding solutions for sustaining the crop yield. The understanding of basic mechanisms employed by microorganisms for the establishment of parasitic relationship with plants is a complex phenomenon. Symbionts and biotrophs are dependent on living hosts for completing their life cycle, whereas necrotrophs utilize dead cells for their growth and establishment. Hemibiotrophs as compared to other microbes associate themselves with plants in two phase's, viz. early bio-phase and later necro-phase. Plants and microbes interact with each other using receptors present on host cell surface and elicitors (PAMPs and effectors) produced by microbes. Plant-microbe interaction either leads to compatible or incompatible reaction. In response to various biotic and abiotic stress factors, plant undergoes programmed cell death which restricts the growth of biotrophs or hemibiotrophs while necrotrophs as an opportunist starts growing on dead tissue for their own benefit. PCD regulation is an outcome of plant-microbe crosstalk which entirely depends on various biochemical events like generation of reactive oxygen species, nitric oxide, ionic efflux/influx, CLPs, biosynthesis of phytohormones, phytoalexins, polyamines and certain pathogenesis-related proteins. This phenomenon mostly occurs in resistant and non-host plants during invasion of pathogenic microbes. The compatible or incompatible host-pathogen interaction depends upon the presence or absence of host plant resistance and pathogenic race. In addition to host-pathogen interaction, the defense induction by beneficial microbes must also be explored and used to the best of its potential. This review highlights the mechanism of microbe- or symbiont-mediated PCD along with defense induction in plants towards symbionts, biotrophs, necrotrophs and hemibiotrophs. Here we have also discussed the possible use of beneficial microbes in inducing systemic resistance in plants against pathogenic microbes.

Secreted Glycoside Hydrolase Proteins as Effectors and Invasion Patterns of Plant-Associated Fungi and Oomycetes

During host colonization, plant-associated microbes, including fungi and oomycetes, deliver a collection of glycoside hydrolases (GHs) to their cell surfaces and surrounding extracellular environments. The number and type of GHs secreted by each organism is typically associated with their lifestyle or mode of nutrient acquisition. Secreted GHs of plant-associated fungi and oomycetes serve a number of different functions, with many of them acting as virulence factors (effectors) to promote microbial host colonization. Specific functions involve, for example, nutrient acquisition, the detoxification of antimicrobial compounds, the manipulation of plant microbiota, and the suppression or prevention of plant immune responses. In contrast, secreted GHs of plant-associated fungi and oomycetes can also activate the plant immune system, either by acting as microbe-associated molecular patterns (MAMPs), or through the release of damage-associated molecular patterns (DAMPs) as a consequence of their enzymatic activity. In this review, we highlight the critical roles that secreted GHs from plant-associated fungi and oomycetes play in plant-microbe interactions, provide an overview of existing knowledge gaps and summarize future directions.

Candidate Effectors of Plasmodiophora brassicae Pathotype 5X During Infection of Two Brassica napus Genotypes

Clubroot, caused by Plasmodiophora brassicae, is one of the most important diseases of canola (Brassica napus) in Canada. Disease management relies heavily on planting clubroot resistant (CR) cultivars, but in recent years, new resistance-breaking pathotypes of P. brassicae have emerged. Current efforts against the disease are concentrated in developing host resistance using traditional genetic breeding, omics and molecular biology. However, because of its obligate biotrophic nature, limited resources have been dedicated to investigating molecular mechanisms of pathogenic infection. We previously performed a transcriptomic study with the cultivar resistance-breaking pathotype 5X on two B. napus hosts presenting contrasting resistance/susceptibility, where we evaluated the mechanisms of host response. Since cultivar-pathotype interactions are very specific, and pathotype 5X is one of the most relevant resistance-breaking pathotypes in Canada, in this study, we analyze the expression of genes encoding putative secreted proteins from this pathotype, predicted using a bioinformatics pipeline, protein modeling and orthologous comparisons with effectors from other pathosystems. While host responses were found to differ markedly in our previous study, many common effectors are found in the pathogen while infecting both hosts, and the gene response among biological pathogen replicates seems more consistent in the effectors associated with the susceptible interaction, especially at 21 days after inoculation. The predicted effectors indicate the predominance of proteins with interacting domains (e.g., ankyrin), and genes bearing kinase and NUDIX domains, but also proteins with protective action against reactive oxygen species from the host. Many of these genes confirm previous predictions from other clubroot studies. A benzoic acid/SA methyltransferase (BSMT), which methylates SA to render it inactive, showed high levels of expression in the interactions with both hosts. Interestingly, our data indicate that E3 ubiquitin proteasome elements are also potentially involved in pathogenesis. Finally, a gene with similarity to indole-3-acetaldehyde dehydrogenase is a promising candidate effector because of its involvement in indole acetic acid synthesis, since auxin is one of the major players in clubroot development.

Defeated by the nines: nine extracellular strategies to avoid microbe-associated molecular patterns recognition in plants

Recognition of microbe-associated molecular patterns (MAMPs) by cell-surface receptors is pivotal in host-microbe interactions. Both pathogens and symbionts establish plant-microbe interactions using fascinating intricate extracellular strategies to avoid recognition. Here we distinguish nine different extracellular strategies to avoid recognition by the host, acting at three different levels. To avoid the accumulation of MAMP precursors (Level 1), microbes take advantage of polymorphisms in both MAMP proteins and glycans, or downregulate MAMP production. To reduce hydrolytic MAMP release (Level 2), microbes shield MAMP precursors with proteins or glycans and inhibit or degrade host-derived hydrolases. And to prevent MAMP perception directly (Level 3), microbes degrade or sequester MAMPs before they are perceived. We discuss examples of these nine strategies and envisage three additional extracellular strategies to avoid MAMP perception in plants.

Cell wall-associated effectors of plant-colonizing fungi

Plant-colonizing fungi secrete a cocktail of effector proteins during colonization. After secretion, some of these effectors are delivered into plant cells to directly dampen the plant immune system or redirect host processes benefitting fungal growth. Other effectors function in the apoplastic space either as released proteins modulating the activity of plant enzymes associated with plant defense or as proteins bound to the fungal cell wall. For such fungal cell wall-bound effectors, we know particularly little about their molecular function. In this review, we describe effectors that are associated with the fungal cell wall and discuss how they contribute to colonization.

Unraveling the sugar code: the role of microbial extracellular glycans in plant-microbe interactions

To defend against microbial invaders but also to establish symbiotic programs, plants need to detect the presence of microbes through the perception of molecular signatures characteristic of a whole class of microbes. Among these molecular signatures, extracellular glycans represent a structurally complex and diverse group of biomolecules that has a pivotal role in the molecular dialog between plants and microbes. Secreted glycans and glycoconjugates such as symbiotic lipochitooligosaccharides or immunosuppressive cyclic β-glucans act as microbial messengers that prepare the ground for host colonization. On the other hand, microbial cell surface glycans are important indicators of microbial presence. They are conserved structures normally exposed and thus accessible for plant hydrolytic enzymes and cell surface receptor proteins. While the immunogenic potential of bacterial cell surface glycoconjugates such as lipopolysaccharides and peptidoglycan has been intensively studied in the past years, perception of cell surface glycans from filamentous microbes such as fungi or oomycetes is still largely unexplored. To date, only few studies have focused on the role of fungal-derived cell surface glycans other than chitin, highlighting a knowledge gap that needs to be addressed. The objective of this review is to give an overview on the biological functions and perception of microbial extracellular glycans, primarily focusing on their recognition and their contribution to plant-microbe interactions.

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.

Molecular Characterization of the Purine Degradation Pathway Genes ALA1 and URE1 of the Maize Anthracnose Fungus Colletotrichum graminicola Identified Urease as a Novel Target for Plant Disease Control

Fungal pathogenicity is governed by environmental factors, with nitrogen playing a key role in triggering pathogenic development. Spores germinating on the plant cuticle are exposed to a nitrogen-free environment, and reprograming of nitrogen metabolism is required for bridging the time needed to gain access to the nitrogen sources of the host. Although degradation of endogenous purine bases efficiently generates ammonium and may allow the fungus to bridge the preinvasion nitrogen gap, the roles of the purine degradation pathway and of the key genes encoding allantoicase and urease are largely unknown in plant pathogenic fungi. To investigate the roles of the allantoicase and urease genes ALA1 and URE1 of the maize anthracnose fungus Colletotrichum graminicola in pathogenic development, we generated ALA1:eGFP and URE1:eGFP fusion strains as well as allantoicase- and urease-deficient mutants. Virulence assays, live cell, and differential interference contrast imaging, chemical complementation and employment of a urease inhibitor showed that the purine degradation genes ALA1 and URE1 are required for bridging nitrogen deficiency at early phases of the infection process and for full virulence. Application of the urease inhibitor acetohydroxamic acid did not only protect maize from C. graminicola infection, but also interfered with the infection process of the wheat powdery mildew fungus Blumeria graminis f. sp. tritici, the maize and broad bean rusts Puccinia sorghi and Uromyces viciae-fabae, and the potato late blight pathogen Phytophthora infestans. Our data strongly suggest that inhibition of the purine degradation pathway might represent a novel approach to control plant pathogenic fungi and oomycetes.

Corn Stunt Disease: An Ideal Insect-Microbial-Plant Pathosystem for Comprehensive Studies of Vector-Borne Plant Diseases of Corn

Over 700 plant diseases identified as vector-borne negatively impact plant health and food security globally. The pest control of vector-borne diseases in agricultural settings is in urgent need of more effective tools. Ongoing research in genetics, molecular biology, physiology, and vector behavior has begun to unravel new insights into the transmission of phytopathogens by their insect vectors. However, the intricate mechanisms involved in phytopathogen transmission for certain pathosystems warrant further investigation. In this review, we propose the corn stunt pathosystem (Zea mays-Spiroplasma kunkelii-Dalbulus maidis) as an ideal model for dissecting the molecular determinants and mechanisms underpinning the persistent transmission of a mollicute by its specialist insect vector to an economically important monocotyledonous crop. Corn stunt is the most important disease of corn in the Americas and the Caribbean, where it causes the severe stunting of corn plants and can result in up to 100% yield loss. A comprehensive study of the corn stunt disease system will pave the way for the discovery of novel molecular targets for genetic pest control targeting either the insect vector or the phytopathogen.

GPI7-mediated glycosylphosphatidylinositol anchoring regulates appressorial penetration and immune evasion during infection of Magnaporthe oryzae

Glycosylphosphatidylinositol (GPI) anchoring plays key roles in many biological processes by targeting proteins to the cell wall; however, its roles are largely unknown in plant pathogenic fungi. Here, we reveal the roles of the GPI anchoring in Magnaporthe oryzae during plant infection. The GPI-anchored proteins were found to highly accumulate in appressoria and invasive hyphae. Disruption of GPI7, a GPI anchor-pathway gene, led to a significant reduction in virulence. The Δgpi7 mutant showed significant defects in penetration and invasive growth. This mutant also displayed defects of the cell wall architecture, suggesting GPI7 is required for cell wall biogenesis. Removal of GPI-anchored proteins in the wild-type strain by hydrofluoric acid (HF) pyridine treatment exposed both the chitin and β-1,3-glucans to the host immune system. Exposure of the chitin and β-1,3-glucans was also observed in the Δgpi7 mutant, indicating GPI-anchored proteins are required for immune evasion. The GPI anchoring can regulate subcellular localization of the Gel proteins in the cell wall for appressorial penetration and abundance of which for invasive growth. Our results indicate the GPI anchoring facilitates the penetration of M. oryzae into host cells by affecting the cell wall integrity and the evasion of host immune recognition.

The Pattern and Function of DNA Methylation in Fungal Plant Pathogens

To successfully infect plants and trigger disease, fungal plant pathogens use various strategies that are dependent on characteristics of their biology and genomes. Although pathogenic fungi are different from animals and plants in the genomic heritability, sequence feature, and epigenetic modification, an increasing number of phytopathogenic fungi have been demonstrated to share DNA methyltransferases (MTases) responsible for DNA methylation with animals and plants. Fungal plant pathogens predominantly possess four types of DNA MTase homologs, including DIM-2, DNMT1, DNMT5, and RID. Numerous studies have indicated that DNA methylation in phytopathogenic fungi mainly distributes in transposable elements (TEs), gene promoter regions, and the repetitive DNA sequences. As an important and heritable epigenetic modification, DNA methylation is associated with silencing of gene expression and transposon, and it is responsible for a wide range of biological phenomena in fungi. This review highlights the relevant reports and insights into the important roles of DNA methylation in the modulation of development, pathogenicity, and secondary metabolism of fungal plant pathogens. Recent evidences prove that there are massive links between DNA and histone methylation in fungi, and they commonly regulate fungal development and mycotoxin biosynthesis.

Two genes in a pathogenicity gene cluster encoding secreted proteins are required for appressorial penetration and infection of the maize anthracnose fungus Colletotrichum graminicola

To avoid pathogen-associated molecular pattern recognition, the hemibiotrophic maize pathogen Colletotrichum graminicola secretes proteins mediating the establishment of biotrophy. Targeted deletion of 26 individual candidate genes and seven gene clusters comprising 32 genes of C. graminicola identified a pathogenicity cluster (CLU5) of five co-linear genes, all of which, with the exception of CLU5b, encode secreted proteins. Targeted deletion of all genes of CLU5 revealed that CLU5a and CLU5d are required for full appressorial penetration competence, with virulence deficiencies independent of the host genotype and organ inoculated. Cytorrhysis experiments and microscopy showed that Δclu5a mutants form pressurized appressoria, but they are hampered in forming penetration pores and fail to differentiate a penetration peg. Whereas Δclu5d mutants elicited WT-like papillae, albeit at increased frequencies, papillae induced by Δclu5a mutants were much smaller than those elicited by the WT. Synteny of CLU5 is not only conserved in Colletotrichum spp. but also in additional species of Sordariomycetes including insect pathogens and saprophytes suggesting importance of CLU5 for fungal biology. Since CLU5a and CLU5d also occur in non-pathogenic fungi and since they are expressed prior to plant invasion and even in vegetative hyphae, the encoded proteins probably do not act primarily as effectors.

Phytohormones (Auxin, Gibberellin) and ACC Deaminase In Vitro Synthesized by the Mycoparasitic Trichoderma DEMTkZ3A0 Strain and Changes in the Level of Auxin and Plant Resistance Markers in Wheat Seedlings Inoculated with this Strain Conidia

Both hormonal balance and plant growth may be shaped by microorganisms synthesizing phytohormones, regulating its synthesis in the plant and inducing plant resistance by releasing elicitors from cell walls (CW) by degrading enzymes (CWDE). It was shown that the Trichoderma DEMTkZ3A0 strain, isolated from a healthy rye rhizosphere, colonized the rhizoplane of wheat seedlings and root border cells (RBC) and caused approximately 40% increase of stem weight. The strain inhibited (in over 90%) the growth of polyphagous Fusarium spp. (F. culmorum, F. oxysporum, F. graminearum) phytopathogens through a mechanism of mycoparasitism. Chitinolytic and glucanolytic activity, strongly stimulated by CW of F. culmorum in the DEMTkZ3A0 liquid culture, is most likely responsible for the lysis of hyphae and macroconidia of phytopathogenic Fusarium spp. as well as the release of plant resistance elicitors. In DEMTkZ3A0 inoculated plants, an increase in the activity of the six tested plant resistance markers and a decrease in the concentration of indoleacetic acid (IAA) auxin were noted. IAA and gibberellic acid (GA) but also the 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase (ACCD) enzyme regulating ethylene production by plant were synthesized by DEMTkZ3A0 in the liquid culture. IAA synthesis was dependent on tryptophan and negatively correlated with temperature, whereas GA synthesis was positively correlated with the biomass and temperature.

The MAP Kinase CfPMK1 Is a Key Regulator of Pathogenesis, Development, and Stress Tolerance of Colletotrichum fructicola

The Ascomycetes fungus Colletotrichum fructicola causes severe diseases on a wide range of crops, fruits, and vegetables. Its pathogenic mechanisms, however, remain poorly understood. Mitogen-activated protein kinases (MAPKs) are conserved regulators of fungal development and pathogenesis. In this study, a Fus3/Kss1-related MAPK from C. fructicola was functionally characterized via gene deletion. On potato dextrose agar (PDA) and oatmeal agar media, the CfPMK1 gene deletion mutants (ΔCfPMK1) were slightly reduced in radial growth rate, severely limited in aerial hyphal differentiation and hyphal melanization, and formed deformed perithecia that were smaller in size and more compactly organized relative to wild type. When artificially inoculated on plants, conidia of these mutants failed to differentiate appressoria or penetrate cuticle, and their pathogenicity defect could not be rescued by wounding plant tissue prior to inoculation. On PDA, ΔCfPMK1 mutants were hypersensitive to osmotic stresses, but were more tolerant to membrane and cell wall stresses. Genetic complementation rescued all phenotypic changes associated with CfPMK1 gene deletion. Based on GFP fusion expression, CfPMK1 protein accumulation was detected at all life stages, and the accumulation level was higher in nascent appressoria relative to conidia. Overall, this study identified CfPMK1 as a key regulator of appressorium and sexual development, pathogenesis, and stress tolerance in C. fructicola.

A novel outer membrane β-1,6-glucanase is deployed in the predation of fungi by myxobacteria

Myxobacterial predation on bacteria has been investigated for several decades. However, their predation on fungi has received less attention. Here, we show that a novel outer membrane β-1,6-glucanase GluM from Corallococcus sp. strain EGB is essential for initial sensing and efficient decomposition of fungi during predation. GluM belongs to an unstudied family of outer membrane β-barrel proteins with potent specific activity up to 24,000 U/mg, whose homologs extensively exist in myxobacteria. GluM was able to digest fungal cell walls efficiently and restrict Magnaporthe oryzae infection of rice plants. Genetic complementation with gluM restored the fungal predation ability of Myxococcus xanthus CL1001, which was abolished by the disruption of gluM homolog oar. The inability to prey on fungi with cell walls that lack β-1,6-glucans indicates that β-1,6-glucans are targeted by GluM. Our results demonstrate that GluM confers myxobacteria with the ability to feed on fungi, and provide new insights for understanding predator-prey interactions. Considering the attack mode of GluM, we suggest that β-1,6-glucan is a promising target for the development of novel broad-spectrum antifungal agents.

FGB1 and WSC3 are in planta-induced β-glucan-binding fungal lectins with different functions

In the root endophyte Serendipita indica, several lectin-like members of the expanded multigene family of WSC proteins are transcriptionally induced in planta and are potentially involved in β-glucan remodeling at the fungal cell wall. Using biochemical and cytological approaches we show that one of these lectins, SiWSC3 with three WSC domains, is an integral fungal cell wall component that binds to long-chain β1-3-glucan but has no affinity for shorter β1-3- or β1-6-linked glucose oligomers. Comparative analysis with the previously identified β-glucan-binding lectin SiFGB1 demonstrated that whereas SiWSC3 does not require β1-6-linked glucose for efficient binding to branched β1-3-glucan, SiFGB1 does. In contrast to SiFGB1, the multivalent SiWSC3 lectin can efficiently agglutinate fungal cells and is additionally induced during fungus-fungus confrontation, suggesting different functions for these two β-glucan-binding lectins. Our results highlight the importance of the β-glucan cell wall component in plant-fungus interactions and the potential of β-glucan-binding lectins as specific detection tools for fungi in vivo.

Cell wall glucans of fungi. A review

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Glucans are the most abundant compounds in the fungal cell walls.

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The most common type of glucose bonding is 1 → 3, both alpha and beta.

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Microfibrillar glucans with chitin provide rigidity to the fungal wall.

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Fungal beta glucans act as PAMPS during infection of animals and plants.

Glucans are the most abundant polysaccharides in the cell walls of fungi, and their structures are highly variable. Accordingly, their glucose moieties may be joined through either or both alpha (α) or beta (β) linkages, they are either lineal or branched, and amorphous or microfibrillar. Alpha 1,3 glucans sensu strictu (pseudonigerans) are the most abundant alpha glucans present in the cell walls of fungi, being restricted to dikarya. They exist in the form of structural microfibrils that provide resistance to the cell wall. The structure of beta glucans is more complex. They are linear or branched, and contain mostly β 1,3 and β 1,6 linkages, existing in the form of microfibrils. Together with chitin they constitute the most important structural components of fungal cell walls. They are the most abundant components of the cell walls in members of all fungal phyla, with the exception of Microsporidia, where they are absent.

Taking into consideration the importance of glucans in the structure and physiology of the fungi, in the present review we describe the following aspects of these polysaccharides: i) types and distribution of fungal glucans, ii) their structure, iii) their roles, iv) the mechanism of synthesis of the most important ones, and v) the phylogentic relationships of the enzymes involved in their synthesis.

The core effector Cce1 is required for early infection of maize by Ustilago maydis

The biotrophic pathogen Ustilago maydis, the causative agent of corn smut disease, infects one of the most important crops worldwide - Zea mays. To successfully colonize its host, U. maydis secretes proteins, known as effectors, that suppress plant defense responses and facilitate the establishment of biotrophy. In this work, we describe the U. maydis effector protein Cce1. Cce1 is essential for virulence and is upregulated during infection. Through microscopic analysis and in vitro assays, we show that Cce1 is secreted from hyphae during filamentous growth of the fungus. Strikingly, Δcce1 mutants are blocked at early stages of infection and induce callose deposition as a plant defense response. Cce1 is highly conserved among smut fungi and the Ustilago bromivora ortholog complemented the virulence defect of the SG200Δcce1 deletion strain. These data indicate that Cce1 is a core effector with apoplastic localization that is essential for U. maydis to infect its host.

Colletotrichum higginsianum as a Model for Understanding Host⁻Pathogen Interactions: A Review

Colletotrichum higginsianum is a hemibiotrophic ascomycetous fungus that causes economically important anthracnose diseases on numerous monocot and dicot crops worldwide. As a model pathosystem, the Colletotrichum⁻Arabidopsis interaction has the significant advantage that both organisms can be manipulated genetically. The goal of this review is to provide an overview of the system and to point out recent significant studies that update our understanding of the pathogenesis of C. higginsianum and resistance mechanisms of Arabidopsis against this hemibiotrophic fungus. The genome sequence of C. higginsianum has provided insights into how genome structure and pathogen genetic variability has been shaped by transposable elements, and allows systematic approaches to longstanding areas of investigation, including infection structure differentiation and fungal⁻plant interactions. The Arabidopsis-Colletotrichum pathosystem provides an integrated system, with extensive information on the host plant and availability of genomes for both partners, to illustrate many of the important concepts governing fungal⁻plant interactions, and to serve as an excellent starting point for broad perspectives into issues in plant pathology.

Tools of the crook- infection strategies of fungal plant pathogens

Fungi represent an ecologically diverse group of microorganisms that includes plant pathogenic species able to cause considerable yield loses in crop production systems worldwide. In order to establish compatible interactions with their hosts, pathogenic fungi rely on the secretion of molecules of diverse nature during host colonization to modulate host physiology, manipulate other environmental factors or provide self-defence. These molecules, collectively known as effectors, are typically small secreted cysteine-rich proteins, but may also comprise secondary metabolites and sRNAs. Here, we discuss the most common strategies that fungal plant pathogens employ to subvert their host plants in order to successfully complete their life cycle and secure the release of abundant viable progeny.

The Fungal Cell Wall: Structure, Biosynthesis, and Function

The molecular composition of the cell wall is critical for the biology and ecology of each fungal species. Fungal walls are composed of matrix components that are embedded and linked to scaffolds of fibrous load-bearing polysaccharides. Most of the major cell wall components of fungal pathogens are not represented in humans, other mammals, or plants, and therefore the immune systems of animals and plants have evolved to recognize many of the conserved elements of fungal walls. For similar reasons the enzymes that assemble fungal cell wall components are excellent targets for antifungal chemotherapies and fungicides. However, for fungal pathogens, the cell wall is often disguised since key signature molecules for immune recognition are sometimes masked by immunologically inert molecules. Cell wall damage leads to the activation of sophisticated fail-safe mechanisms that shore up and repair walls to avoid catastrophic breaching of the integrity of the surface. The frontiers of research on fungal cell walls are moving from a descriptive phase defining the underlying genes and component parts of fungal walls to more dynamic analyses of how the various components are assembled, cross-linked, and modified in response to environmental signals. This review therefore discusses recent advances in research investigating the composition, synthesis, and regulation of cell walls and how the cell wall is targeted by immune recognition systems and the design of antifungal diagnostics and therapeutics.

The Role of the Fungal Cell Wall in the Infection of Plants

The polysaccharide-rich wall, which envelopes the fungal cell, is pivotal to the maintenance of cellular integrity and for the protection of the cell from external aggressors - such as environmental fluxes and during host infection. This review considers the commonalities in the composition of the wall across the fungal kingdom, addresses how little is known about the assembly of the polysaccharide matrix, and considers changes in the wall of plant-pathogenic fungi during on and in planta growth, following the elucidation of infection structures requiring cell wall alterations. It highlights what is known about the phytopathogenic fungal wall and what needs to be discovered.

Genetic dissection of the maize (Zea mays L.) MAMP response

Loci associated with variation in maize responses to two microbe-associated molecular patterns (MAMPs) were identified. MAMP responses were correlated. No relationship between MAMP responses and quantitative disease resistance was identified. Microbe-associated molecular patterns (MAMPs) are highly conserved molecules commonly found in microbes which can be recognized by plant pattern recognition receptors. Recognition triggers a suite of responses including production of reactive oxygen species (ROS) and nitric oxide (NO) and expression changes of defense-related genes. In this study, we used two well-studied MAMPs (flg22 and chitooctaose) to challenge different maize lines to determine whether there was variation in the level of responses to these MAMPs, to dissect the genetic basis underlying that variation and to understand the relationship between MAMP response and quantitative disease resistance (QDR). Naturally occurring quantitative variation in ROS, NO production, and defense genes expression levels triggered by MAMPs was observed. A major quantitative traits locus (QTL) associated with variation in the ROS production response to both flg22 and chitooctaose was identified on chromosome 2 in a recombinant inbred line (RIL) population derived from the maize inbred lines B73 and CML228. Minor QTL associated with variation in the flg22 ROS response was identified on chromosomes 1 and 4. Comparison of these results with data previously obtained for variation in QDR and the defense response in the same RIL population did not provide any evidence for a common genetic basis controlling variation in these traits.

The Fungal Cell Wall: Structure, Biosynthesis, and Function

The molecular composition of the cell wall is critical for the biology and ecology of each fungal species. Fungal walls are composed of matrix components that are embedded and linked to scaffolds of fibrous load-bearing polysaccharides. Most of the major cell wall components of fungal pathogens are not represented in humans, other mammals, or plants, and therefore the immune systems of animals and plants have evolved to recognize many of the conserved elements of fungal walls. For similar reasons the enzymes that assemble fungal cell wall components are excellent targets for antifungal chemotherapies and fungicides. However, for fungal pathogens, the cell wall is often disguised since key signature molecules for immune recognition are sometimes masked by immunologically inert molecules. Cell wall damage leads to the activation of sophisticated fail-safe mechanisms that shore up and repair walls to avoid catastrophic breaching of the integrity of the surface. The frontiers of research on fungal cell walls are moving from a descriptive phase defining the underlying genes and component parts of fungal walls to more dynamic analyses of how the various components are assembled, cross-linked, and modified in response to environmental signals. This review therefore discusses recent advances in research investigating the composition, synthesis, and regulation of cell walls and how the cell wall is targeted by immune recognition systems and the design of antifungal diagnostics and therapeutics.

Regulation of proteinaceous effector expression in phytopathogenic fungi

Effectors are molecules used by microbial pathogens to facilitate infection via effector-triggered susceptibility or tissue necrosis in their host. Much research has been focussed on the identification and elucidating the function of fungal effectors during plant pathogenesis. By comparison, knowledge of how phytopathogenic fungi regulate the expression of effector genes has been lagging. Several recent studies have illustrated the role of various transcription factors, chromosome-based control, effector epistasis, and mobilisation of endosomes within the fungal hyphae in regulating effector expression and virulence on the host plant. Improved knowledge of effector regulation is likely to assist in improving novel crop protection strategies.

Fructans As DAMPs or MAMPs: Evolutionary Prospects, Cross-Tolerance, and Multistress Resistance Potential

This perspective paper proposes that endogenous apoplastic fructans in fructan accumulating plants, released after stress-mediated cellular leakage, or increased by exogenous application, can act as damage-associated molecular patterns (DAMPs), priming plant innate immunity through ancient receptors and defense pathways that most probably evolved to react on microbial fructans acting as microbe-associated molecular patterns (MAMPs). The proposed model is placed in an evolutionary perspective. How this type of DAMP signaling may contribute to cross-tolerance and multistress resistance effects in plants is discussed. Besides apoplastic ATP, NAD and fructans, apoplastic polyamines, secondary metabolites, and melatonin may be considered potential players in DAMP-mediated stress signaling. It is proposed that mixtures of DAMP priming formulations hold great promise as natural and sustainable alternatives for toxic agrochemicals.

Convergent evolution of filamentous microbes towards evasion of glycan-triggered immunity

896 I. 896 II. 896 III. 897 IV. 898 V. 899 VI. 899 900 References 900 SUMMARY: All filamentous microbes produce and release a wide range of glycans, which are essential determinants of microbe-microbe and microbe-host interactions. Major cell wall constituents, such as chitin and β-glucans, are elicitors of host immune responses. The widespread capacity for glycan perception in plants has driven the evolution of various strategies that help filamentous microbes to evade detection. Common strategies include structural and chemical modifications of cell wall components as well as the secretion of effector proteins that suppress chitin- and β-glucan-triggered immune responses. Thus, the necessity to avoid glycan-triggered immunity represents a driving force in the convergent evolution of filamentous microbes towards its suppression.

The Glycosylphosphatidylinositol Anchor Biosynthesis Genes GPI12, GAA1, and GPI8 Are Essential for Cell-Wall Integrity and Pathogenicity of the Maize Anthracnose Fungus Colletotrichum graminicola

Glycosylphosphatidylinositol (GPI) anchoring of proteins is one of the most common posttranslational modifications of proteins in eukaryotic cells and is important for associating proteins with the cell surface. In fungi, GPI-anchored proteins play essential roles in cross-linking of β-glucan cell-wall polymers and cell-wall rigidity. GPI-anchor synthesis is successively performed at the cytoplasmic and the luminal face of the ER membrane and involves approximately 25 proteins. While mutagenesis of auxiliary genes of this pathway suggested roles of GPI-anchored proteins in hyphal growth and virulence, essential genes of this pathway have not been characterized. Taking advantage of RNA interference (RNAi) we analyzed the function of the three essential genes GPI12, GAA1 and GPI8, encoding a cytoplasmic N-acetylglucosaminylphosphatidylinositol deacetylase, a metallo-peptide-synthetase and a cystein protease, the latter two representing catalytic components of the GPI transamidase complex. RNAi strains showed drastic cell-wall defects, resulting in exploding infection cells on the plant surface and severe distortion of in planta-differentiated infection hyphae, including formation of intrahyphal hyphae. Reduction of transcript abundance of the genes analyzed resulted in nonpathogenicity. We show here for the first time that the GPI synthesis genes GPI12, GAA1, and GPI8 are indispensable for vegetative development and pathogenicity of the causal agent of maize anthracnose, Colletotrichum graminicola.