Cell death of Nicotiana benthamiana is induced by secreted protein NIS1 of Colletotrichum orbiculare and is suppressed by a homologue of CgDN3

Understanding and manipulating the recognition of necrosis-inducing secreted protein 1 (NIS1) by BRI1-associated receptor kinase 1 (BAK1)

Necrosis-inducing secreted protein 1 (NIS1) is a core effector of Ascomycota and Basidiomycota fungi. They inhibit the immune responses of host plants mainly through interaction with the multi-functional coreceptor BRI1-associated receptor kinase 1 (BAK1). However, the structural mechanism of the NIS1 family and how they are recognized by BAK1 are unknown. Herein, we report the first crystal structure of the NIS1 family protein, the Magnaporthe oryzae NIS1 (MoNIS1), analyze the recognition mechanism of NIS1s by BAK1, and explore regulation of the NIS1-BAK1 interaction by a chemical compound. MoNIS1 exists as a β barrel formed by eight β strands, a folding mode that has not been reported. Hydrogen/deuterium exchange mass spectrometry (HDX-MS) assay suggested that β4-β5 loop and β5 strand of MoNIS1 participate in OsBAK1 interaction, which was supported by further single-point mutational assays. For OsBAK1, HDX-MS assay suggested four regions involved in MoNIS1 interaction. Additionally, we identified a compound that blocks MoNIS1-OsBAK1 interaction in vitro and inhibits the virulence of M. oryzae on rice. Collectively, we determined the first structure of NIS1 family effectors, presented the recognition mechanism of NIS1 by BAK1, and showed that blocking NIS1-BAK1 interaction could be a new target for fungicide development.

Fusarium sacchari FsNis1 induces plant immunity

Pokkah Boeng disease (PBD), caused by Fusarium sacchari, severely affects sugarcane yield and quality. Necrosis-inducing secreted protein 1 (Nis1) is a fungal secreted effector that induces necrotic lesions in plants. It interacts with host receptor-like kinases and inhibits their kinase activity. FsNis1 contains the Nis1 structure and triggered a pathogen-associated molecular pattern-triggered immune response in Nicotiana benthamiana, as reflected by causing reactive oxygen species production, callose accumulation, and the upregulated expression of defense response genes. Knockout of this gene in F. sacchari revealed a significant reduction in its pathogenicity, whereas the pathogenicity of the complementary mutant recovered to the wild-type levels, making this gene an important virulence factor for F. sacchari. In addition, the signal peptide of FsNis1 was required for the induction of cell death and PTI response in N. benthamiana. Thus, FsNis1 may not only be a key virulence factor for F. sacchari but may also induce defense responses in plants. These findings provide new insights into the function of Nis1 in host-pathogen interactions.

Genomic insights into the evolution and adaptation of secondary metabolite gene clusters in fungicolous species Cladobotryum mycophilum ATHUM6906

Mycophilic or fungicolous fungi can be found wherever fungi exist since they are able to colonize other fungi, which occupy a diverse range of habitats. Some fungicolous species cause important diseases on Basidiomycetes, and thus, they are the main reason for the destruction of mushroom cultivations. Nonetheless, despite their ecological significance, their genomic data remain limited. Cladobotryum mycophilum is one of the most aggressive species of the genus, destroying the economically important Agaricus bisporus cultivations. The 40.7 Mb whole genome of the Greek isolate ATHUM6906 is assembled in 16 fragments, including the mitochondrial genome and 2 small circular mitochondrial plasmids, in this study. This genome includes a comprehensive set of 12,282 protein coding, 56 rRNA, and 273 tRNA genes. Transposable elements, CAZymes, and pathogenicity related genes were also examined. The genome of C. mycophilum contained a diverse arsenal of genes involved in secondary metabolism, forming 106 biosynthetic gene clusters, which renders this genome as one of the most BGC abundant among fungicolous species. Comparative analyses were performed for genomes of species of the family Hypocreaceae. Some BGCs identified in C. mycophilum genome exhibited similarities to clusters found in the family Hypocreaceae, suggesting vertical heritage. In contrast, certain BGCs showed a scattered distribution among Hypocreaceae species or were solely found in Cladobotryum genomes. This work provides evidence of extensive BGC losses, horizontal gene transfer events, and formation of novel BGCs during evolution, potentially driven by neutral or even positive selection pressures. These events may increase Cladobotryum fitness under various environmental conditions and potentially during host-fungus interaction.

Guanosine-specific single-stranded ribonuclease effectors of a phytopathogenic fungus potentiate host immune responses

Plants activate immunity upon recognition of pathogen-associated molecular patterns. Although phytopathogens have evolved a set of effector proteins to counteract plant immunity, some effectors are perceived by hosts and induce immune responses. Here, we show that two secreted ribonuclease effectors, SRN1 and SRN2, encoded in a phytopathogenic fungus, Colletotrichum orbiculare, induce cell death in a signal peptide- and catalytic residue-dependent manner, when transiently expressed in Nicotiana benthamiana. The pervasive presence of SRN genes across Colletotrichum species suggested the conserved roles. Using a transient gene expression system in cucumber (Cucumis sativus), an original host of C. orbiculare, we show that SRN1 and SRN2 potentiate host pattern-triggered immunity responses. Consistent with this, C. orbiculare SRN1 and SRN2 deletion mutants exhibited increased virulence on the host. In vitro analysis revealed that SRN1 specifically cleaves single-stranded RNAs at guanosine, leaving a 3'-end phosphate. Importantly, the potentiation of C. sativus responses by SRN1 and SRN2, present in the apoplast, depends on ribonuclease catalytic residues. We propose that the pathogen-derived apoplastic guanosine-specific single-stranded endoribonucleases lead to immunity potentiation in plants.

A perspective on varied fungal virulence factors causing infection in host plants

Pathogenic fungi and their spores are ubiquitously present and invade the tissues of higher living plants causing pathogenesis and inevitably death or retarded growth. A group of fungi kills its hosts and consume the dead tissues (necrotrophs), while others feed on living tissue (biotrophs) or combination of two (hemibiotrophs). A number of virulent factors is used by fungal pathogens to inhabit new hosts and cause illness. Fungal pathogens develop specialized structures for complete invasion into plant organs to regulate pathogenic growth. Virulence factors like effectors, mycotoxins, cell wall degrading enzymes and organic acids have varied roles depending on the infection strategy and assist the pathogens to possess control on living tissues of the plants. Infection strategies employed by fungi generally masks the plant defense mechanism, however necrotrophs are best known to harm plant tissues with their poisonous secretion. Interestingly, the effector chemicals released by Biotrophs reduce plant cell growth and regulate plant metabolism in their advantage causing no direct death. All these virulence tools cause huge loss to the agricultural product of pre- harvest crops and post-harvest yields causing low output leading to huge economic losses. This review focusses on comprehensive study of range of virulence factors of the pathogenic fungi responsible for their invasion inside the healthy tissues of plants. The compiled information would influence researchers to design antidote against all virulence factors of fungi relevant to their area of research which could pave way for protection against plant pathogenesis.

Magnaporthe oryzae effector MoSPAB1 directly activates rice Bsr-d1 expression to facilitate pathogenesis

Fungal pathogens typically use secreted effector proteins to suppress host immune activators to facilitate invasion. However, there is rarely evidence supporting the idea that fungal secretory proteins contribute to pathogenesis by transactivating host genes that suppress defense. We previously found that pathogen Magnaporthe oryzae induces rice Bsr-d1 to facilitate infection and hypothesized that a fungal effector mediates this induction. Here, we report that MoSPAB1 secreted by M. oryzae directly binds to the Bsr-d1 promoter to induce its expression, facilitating pathogenesis. Amino acids 103-123 of MoSPAB1 are required for its binding to the Bsr-d1 promoter. Both MoSPAB1 and rice MYBS1 compete for binding to the Bsr-d1 promoter to regulate Bsr-d1 expression. Furthermore, MoSPAB1 homologues are highly conserved among fungi. In particular, Colletotrichum fructicola CfSPAB1 and Colletotrichum sublineola CsSPAB1 activate kiwifruit AcBsr-d1 and sorghum SbBsr-d1 respectively, to facilitate pathogenesis. Taken together, our findings reveal a conserved module that may be widely utilized by fungi to enhance pathogenesis.

Effector secretion and stability in the maize anthracnose pathogen Colletotrichum graminicola requires N-linked protein glycosylation and the ER chaperone pathway

N-linked protein glycosylation is a conserved and essential modification mediating protein processing and quality control in the endoplasmic reticulum (ER), but how this contributes to the infection cycle of phytopathogenic fungi is largely unknown. In this study, we discovered that inhibition of protein N-glycosylation severely affected vegetative growth, hyphal tip development, conidial germination, appressorium formation, and, ultimately, the ability of the maize (Zea mays) anthracnose pathogen Colletotrichum graminicola to infect its host. Quantitative proteomics analysis showed that N-glycosylation can coordinate protein O-glycosylation, glycosylphosphatidylinositol anchor modification, and endoplasmic reticulum quality control (ERQC) by directly targeting the proteins from the corresponding pathway in the ER. We performed a functional study of the N-glycosylation pathway-related protein CgALG3 and of the ERQC pathway-related protein CgCNX1, which demonstrated that N-glycosylation of ER chaperone proteins is essential for effector stability, secretion, and pathogenicity of C. graminicola. Our study provides concrete evidence for the regulation of effector protein stability and secretion by N-glycosylation.

Prediction of Effector Proteins from Trichoderma longibrachiatum Through Transcriptome Sequencing

Trichoderma longibrachiatum SMF2 is an important biocontrol strain isolated by our group that can promote plant growth and induce plant disease resistance. To further study its biocontrol mechanism, the effector proteins secreted by T. longibrachiatum SMF2 were analyzed through bioinformatics and transcriptome sequencing. Overall, 478 secretory proteins produced by T. longibrachiatum were identified, of which 272 were upregulated after treatment with plants. Functional annotation showed that 36 secretory proteins were homologous with different groups of effectors from pathogenic microorganisms. Moreover, the quantitative PCR results of six putative effector proteins were consistent with those of transcriptome sequencing. Taken together, these findings indicate that the secretory proteins secreted by T. longibrachiatum SMF2 may act as effectors to facilitate its own growth and colonization or to induce plant immunity response.

Selective deployment of virulence effectors correlates with host specificity in a fungal plant pathogen

The hemibiotrophic fungal plant pathogen Colletotrichum orbiculare is predicted to secrete hundreds of effector proteins when the pathogen infects cucurbit crops, such as cucumber and melon, and tobacco (Nicotiana benthamiana), a distantly related Solanaceae species. Here, we report the identification of sets of C. orbiculare effector genes that are differentially required for fungal virulence to two phylogenetically distant host species. Through targeted gene knockout screening of C. orbiculare 'core' effector candidates defined based on in planta gene expression, we identified: four host-specific virulence effectors (named effector proteins for cucurbit infection, or EPCs) that are required for full virulence of C. orbiculare to cucurbit hosts, but not to the Solanaceae host N. benthamiana; and five host-nonspecific virulence effectors, which collectively contribute to fungal virulence to both hosts. During host infection, only a small subset of genes, including the host-specific EPC effector genes, showed preferential expression on one of the hosts, while gene expression profiles of the majority of other genes, including the five host-nonspecific effector genes, were common to both hosts. This work suggests that C. orbiculare adopts a host-specific effector deployment strategy, in addition to general host-blind virulence mechanisms, for adaptation to cucurbit hosts.

Zymoseptoria tritici white-collar complex integrates light, temperature and plant cues to initiate dimorphism and pathogenesis

Transitioning from spores to hyphae is pivotal to host invasion by the plant pathogenic fungus Zymoseptoria tritici. This dimorphic switch can be initiated by high temperature in vitro (~27 °C); however, such a condition may induce cellular heat stress, questioning its relevance to field infections. Here, we study the regulation of the dimorphic switch by temperature and other factors. Climate data from wheat-growing areas indicate that the pathogen sporadically experiences high temperatures such as 27 °C during summer months. However, using a fluorescent dimorphic switch reporter (FDR1) in four wild-type strains, we show that dimorphic switching already initiates at 15-18 °C, and is enhanced by wheat leaf surface compounds. Transcriptomics reveals 1261 genes that are up- or down-regulated in hyphae of all strains. These pan-strain core dimorphism genes (PCDGs) encode known effectors, dimorphism and transcription factors, and light-responsive proteins (velvet factors, opsins, putative blue light receptors). An FDR1-based genetic screen reveals a crucial role for the white-collar complex (WCC) in dimorphism and virulence, mediated by control of PCDG expression. Thus, WCC integrates light with biotic and abiotic cues to orchestrate Z. tritici infection.

Recent progress on harm, pathogen classification, control and pathogenic molecular mechanism of anthracnose of oil-tea

Oil tea (Camellia oleifera), mainly used to produce high-quality edible oil, is an important cash crop in China. Anthracnose of oil tea is a considerable factor that limits the yield of tea oil. In order to effectively control the anthracnose of oil tea, researchers have worked hard for many years, and great progress has been made in the research of oil tea anthracnose. For instance, researchers isolated a variety of Colletotrichum spp. from oil tea and found that Colletotrichum fructicola was the most popular pathogen in oil tea. At the same time, a variety of control methods have been explored, such as cultivating resistant varieties, pesticides, and biological control, etc. Furthermore, the research on the molecular pathogenesis of Colletotrichum spp. has also made good progress, such as the elaboration of the transcription factors and effector functions of Colletotrichum spp. The authors summarized the research status of the harm, pathogen types, control, and pathogenic molecular mechanism of oil tea anthracnose in order to provide theoretical support and new technical means for the green prevention and control of oil tea anthracnose.

Proteinaceous Effector Discovery and Characterization in Plant Pathogenic Colletotrichum Fungi

Anthracnose caused by plant pathogenic Colletotrichum fungi results in large economic losses in field crop production worldwide. To aid the establishment of plant host infection, Colletotrichum pathogens secrete numerous effector proteins either in apoplastic space or inside of host cells for effective colonization. Understanding these effector repertoires is critical for developing new strategies for resistance breeding and disease management. With the advance of genomics and bioinformatics tools, a large repertoire of putative effectors has been identified in Colletotrichum genomes, and the biological functions and molecular mechanisms of some studied effectors have been summarized. Here, we review recent advances in genomic identification, understanding of evolutional characteristics, transcriptional profiling, and functional characterization of Colletotrichum effectors. We also offer a perspective on future research.

Effector Sntf2 Interacted with Chloroplast-Related Protein Mdycf39 Promoting the Colonization of Colletotrichum gloeosporioides in Apple Leaf

Glomerella leaf spot of apple, caused by Colletotrichumgloeosporioides, is a devastating disease that leads to severe defoliation and fruit spots. The Colletotrichum species secretes a series of effectors to manipulate the host’s immune response, facilitating its colonization in plants. However, the mechanism by which the effector of C. gloeosporioides inhibits the defenses of the host remains unclear. In this study, we reported a novel effector Sntf2 of C. gloeosporioides. The transient expression of SNTF2 inhibits BAX-induced cell death in tobacco plants. Sntf2 suppresses plant defense responses by reducing callose deposition and H₂O₂ accumulation. SNTF2 is upregulated during infection, and its deletion reduces virulence to the plant. Sntf2 is localized to the chloroplasts and interacts with Mdycf39 (a chloroplast PSII assembly factor) in apple leaves. The Mdycf39 overexpression line increases susceptibility to C. gloeosporioides, whereas the Mdycf39 transgenic silent line does not grow normally with pale white leaves, indicating that Sntf2 disturbs plant defense responses and growth by targeting Mdycf39.

A novel effector, CsSp1, from Bipolaris sorokiniana, is essential for colonization in wheat and is also involved in triggering host immunity

The hemibiotrophic pathogen Bipolaris sorokiniana causes root rot, leaf blotching, and black embryos in wheat and barley worldwide, resulting in significant yield and quality reductions. However, the mechanism underlying the host-pathogen interactions between B. sorokiniana and wheat or barley remains unknown. The B. sorokiniana genome encodes a large number of uncharacterized putative effector proteins. In this study, we identified a putative secreted protein, CsSp1, with a classic N-terminal signal peptide, that is induced during early infection. A split-marker approach was used to knock out CsSP1 in the Lankao 9-3 strain. Compared with the wild type, the deletion mutant ∆Cssp1 displayed less radial growth on potato dextrose agar plates and produced fewer spores, and complementary transformation completely restored the phenotype of the deletion mutant to that of the wild type. The pathogenicity of the deletion mutant in wheat was attenuated even though appressoria still penetrated the host. Additionally, the infectious hyphae in the deletion mutant became swollen and exhibited reduced growth in plant cells. The signal peptide of CsSp1 was functionally verified through a yeast YTK12 secretion system. Transient expression of CsSp1 in Nicotiana benthamiana inhibited lesion formation caused by Phytophthora capsici. Moreover, CsSp1 localized in the nucleus and cytoplasm of plant cells. In B. sorokiniana-infected wheat leaves, the salicylic acid-regulated genes TaPAL, TaPR1, and TaPR2 were down-regulated in the ∆Cssp1 strain compared with the wild-type strain under the same conditions. Therefore, CsSp1 is a virulence effector and is involved in triggering host immunity.

Two NIS1-like proteins from apple canker pathogen (Valsa mali) play distinct roles in plant recognition and pathogen virulence

Conserved effectors produced by phytopathogens play critical roles in plant-microbe interactions. NIS1-like proteins represent a newly identified family of effectors distributed in multiple fungal species. However, their biological functions in a majority of pathogenic fungi remain largely elusive and require further investigation. In this study, we characterized two NIS1-like proteins VmNIS1 and VmNIS2 from Valsa mali, the causal agent of apple Valsa canker. Both of these two proteins were predicted to be secreted. Using agroinfiltration, we found that VmNIS1 induced intense cell death, whereas VmNIS2 suppressed INF1 elicitin-triggered cell death in Nicotiana benthamiana. Treatment of N. benthamiana with VmNIS1 recombinant protein produced by Escherichia coli activated a series of immune responses and enhanced plant disease resistance against Phytophthora capsici. In contrast, VmNIS2 suppressed plant immune responses and promoted P. capsici infection when transiently expressed in N. benthamiana. Both VmNIS1 and VmNIS2 were shown to be highly induced at late stage of V. mali infection. By individually knocking out of these two genes in V. mali, however, only VmNIS2 was shown to be required for pathogen virulence as well as tolerance to oxidative stress. Notably, we further showed that C-terminal extension of VmNIS1 was essential for plant recognition and VmNIS2 may escape plant detection via sequence truncation. Our data collectively indicate that VmNIS1 and VmNIS2 play distinct roles in plant recognition and pathogen virulence, which provided new insights into the function of NIS1-like proteins in plant-microbe interactions.

Fungal effector SIB1 of Colletotrichum orbiculare has unique structural features and can suppress plant immunity in Nicotiana benthamiana

Fungal plant pathogens secrete virulence-related proteins, called effectors, to establish host infection; however, the details are not fully understood yet. Functional screening of effector candidates using Agrobacterium-mediated transient expression assay in Nicotiana benthamiana identified two virulence-related effectors, named SIB1 and SIB2 (Suppression of Immunity in N. benthamiana), of an anthracnose fungus Colletotrichum orbiculare, which infects both cucurbits and N. benthamiana. The Agrobacterium-mediated transient expression of SIB1 or SIB2 increased the susceptibility of N. benthamiana to C. orbiculare, which suggested these effectors can suppress immune responses in N. benthamiana. The presence of SIB1 and SIB2 homologs was found to be limited to the genus Colletotrichum. SIB1 suppressed both (i) the generation of reactive oxygen species triggered by two different pathogen-associated molecular patterns, chitin and flg22, and (ii) the cell death response triggered by the Phytophthora infestans INF1 elicitin in N. benthamiana. We determined the NMR-based structure of SIB1 to obtain its structural insights. The three-dimensional structure of SIB1 comprises five β-strands, each containing three disulfide bonds. The overall conformation was found to be a cylindrical shape, such as the well-known antiparallel β-barrel structure. However, the β-strands were found to display a unique topology, one pair of these β-strands formed a parallel β-sheet. These results suggest that the effector SIB1 present in Colletotrichum fungi has unique structural features and can suppress pathogen-associated molecular pattern-triggered immunity in N. benthamiana.

Extracellular Vesicles from Fusarium graminearum Contain Protein Effectors Expressed during Infection of Corn

Fusarium graminearum (Fgr) is a devastating filamentous fungal pathogen that causes diseases in cereals, while producing mycotoxins that are toxic for humans and animals, and render grains unusable. Low efficiency in managing Fgr poses a constant need for identifying novel control mechanisms. Evidence that fungal extracellular vesicles (EVs) from pathogenic yeast have a role in human disease led us to question whether this is also true for fungal plant pathogens. We separated EVs from Fgr and performed a proteomic analysis to determine if EVs carry proteins with potential roles in pathogenesis. We revealed that protein effectors, which are crucial for fungal virulence, were detected in EV preparations and some of them did not contain predicted secretion signals. Furthermore, a transcriptomic analysis of corn (Zea mays) plants infected by Fgr revealed that the genes of some of the effectors were highly expressed in vivo, suggesting that the Fgr EVs are a mechanism for the unconventional secretion of effectors and virulence factors. Our results expand the knowledge on fungal EVs in plant pathogenesis and cross-kingdom communication, and may contribute to the discovery of new antifungals.

Multiple Colletotrichum species commonly exhibit focal effector accumulation in a biotrophic interface at the primary invasion sites in their host plants

Fungal plant pathogens deploy a suite of secreted proteins, called effectors, to facilitate successful infection. Several fungal pathogens have been reported to secrete and accumulate their effector proteins in the host-pathogen interfacial spaces. Previously, we reported that the strain 104-T of the cucurbit anthracnose pathogen Colletotrichum orbiculare secretes and accumulates mCherry-tagged effectors along with the formation of ring-shaped fluorescence signals beneath the appressoria. However, it was unclear whether these effector accumulation patterns occur in other C. orbiculare isolates and other species belonging to the Colletotrichum genus. Here, we investigated the effector localization during host infection of C. orbiculare MAFF306589, C. trifolii MAFF305078, which infects alfalfa, and C. higginsianum MAFF305635, which infects Brassicaceae plants. We generated effector-reporter lines of each species, which constitutively expressed mCherry-tagged CoDN3 effector (CoDN3:mCherry). Immunoblotting analysis of the liquid culture fluids of the generated lines detected CoDN3:mCherry, which confirmed secretion of CoDN3:mCherry by fungal cells. Via inoculation assays in the corresponding host plants, we detected ring-shaped CoDN3:mCherry fluorescence around the appressorial invasion sites in all tested reporter lines. These results suggest that pathogens in the Colletotrichum genus have evolutionarily conserved the trait of effector secretion in the infection stage irrespective of differences in their hosts.

The Conserved Colletotrichum spp. Effector Candidate CEC3 Induces Nuclear Expansion and Cell Death in Plants

Plant pathogens secrete proteins, known as effectors, that promote infection by manipulating host cells. Members of the phytopathogenic fungal genus Colletotrichum collectively have a broad host range and generally adopt a hemibiotrophic lifestyle that includes an initial biotrophic phase and a later necrotrophic phase. We hypothesized that Colletotrichum fungi use a set of conserved effectors during infection to support the two phases of their hemibiotrophic lifestyle. This study aimed to examine this hypothesis by identifying and characterizing conserved effectors among Colletotrichum fungi. Comparative genomic analyses using genomes of ascomycete fungi with different lifestyles identified seven effector candidates that are conserved across the genus Colletotrichum. Transient expression assays showed that one of these putative conserved effectors, CEC3, induces nuclear expansion and cell death in Nicotiana benthamiana, suggesting that CEC3 is involved in promoting host cell death during infection. Nuclear expansion and cell death induction were commonly observed in CEC3 homologs from four different Colletotrichum species that vary in host specificity. Thus, CEC3 proteins could represent a novel class of core effectors with functional conservation in the genus Colletotrichum.

Remote homology clustering identifies lowly conserved families of effector proteins in plant-pathogenic fungi

Plant diseases caused by fungal pathogens are typically initiated by molecular interactions between 'effector' molecules released by a pathogen and receptor molecules on or within the plant host cell. In many cases these effector-receptor interactions directly determine host resistance or susceptibility. The search for fungal effector proteins is a developing area in fungal-plant pathology, with more than 165 distinct confirmed fungal effector proteins in the public domain. For a small number of these, novel effectors can be rapidly discovered across multiple fungal species through the identification of known effector homologues. However, many have no detectable homology by standard sequence-based search methods. This study employs a novel comparison method (RemEff) that is capable of identifying protein families with greater sensitivity than traditional homology-inference methods, leveraging a growing pool of confirmed fungal effector data to enable the prediction of novel fungal effector candidates by protein family association. Resources relating to the RemEff method and data used in this study are available from https://figshare.com/projects/Effector_protein_remote_homology/87965.

Signaling Pathways and Downstream Effectors of Host Innate Immunity in Plants

Phytopathogens, such as biotrophs, hemibiotrophs and necrotrophs, pose serious stress on the development of their host plants, compromising their yields. Plants are in constant interaction with such phytopathogens and hence are vulnerable to their attack. In order to counter these attacks, plants need to develop immunity against them. Consequently, plants have developed strategies of recognizing and countering pathogenesis through pattern-triggered immunity (PTI) and effector-triggered immunity (ETI). Pathogen perception and surveillance is mediated through receptor proteins that trigger signal transduction, initiated in the cytoplasm or at the plasma membrane (PM) surfaces. Plant hosts possess microbe-associated molecular patterns (P/MAMPs), which trigger a complex set of mechanisms through the pattern recognition receptors (PRRs) and resistance (R) genes. These interactions lead to the stimulation of cytoplasmic kinases by many phosphorylating proteins that may also be transcription factors. Furthermore, phytohormones, such as salicylic acid, jasmonic acid and ethylene, are also effective in triggering defense responses. Closure of stomata, limiting the transfer of nutrients through apoplast and symplastic movements, production of antimicrobial compounds, programmed cell death (PCD) are some of the primary defense-related mechanisms. The current article highlights the molecular processes involved in plant innate immunity (PII) and discusses the most recent and plausible scientific interventions that could be useful in augmenting PII.

Comparative transient expression analyses on two conserved effectors of Colletotrichum orbiculare reveal their distinct cell death-inducing activities between Nicotiana benthamiana and melon

Colletotrichum orbiculare infects cucurbits, such as cucumber and melon (Cucumis melo), as well as the model Solanaceae plant Nicotiana benthamiana, by secreting an arsenal of effectors that suppress the immunity of these distinct plants. Two conserved effectors of C. orbiculare, called NLP1 and NIS1, induce cell death responses in N. benthamiana, but it is unclear whether they exhibit the same activity in Cucurbitaceae plants. In this study, we established a new Agrobacterium-mediated transient expression system to investigate the cell death-inducing activity of NLP1 and NIS1 in melon. NLP1 strongly induced cell death in melon but, in contrast to the effects seen in N. benthamiana, mutations either in the heptapeptide motif or in the putative glycosylinositol phosphorylceramide-binding site did not cancel its cell death-inducing activity in melon. Furthermore, NLP1 lacking the signal peptide caused cell death in melon but not in N. benthamiana. Study of the transient expression of NIS1 also revealed that, unlike in N. benthamiana, NIS1 did not induce cell death in melon. In contrast, NIS1 suppressed flg22-induced reactive oxygen species generation in melon, as seen in N. benthamiana. These findings indicate distinct cell death-inducing activities of NLP1 and NIS1 in these two plant species that C. orbiculare infects.

Effectors of Plant Necrotrophic Fungi

Plant diseases caused by necrotrophic fungal pathogens result in large economic losses in field crop production worldwide. Effectors are important players of plant-pathogen interaction and deployed by pathogens to facilitate plant colonization and nutrient acquisition. Compared to biotrophic and hemibiotrophic fungal pathogens, effector biology is poorly understood for necrotrophic fungal pathogens. Recent bioinformatics advances have accelerated the prediction and discovery of effectors from necrotrophic fungi, and their functional context is currently being clarified. In this review we examine effectors utilized by necrotrophic fungi and hemibiotrophic fungi in the latter stages of disease development, including plant cell death manipulation. We define "effectors" as secreted proteins and other molecules that affect plant physiology in ways that contribute to disease establishment and progression. Studying and understanding the mechanisms of necrotrophic effectors is critical for identifying avenues of genetic intervention that could lead to improved resistance to these pathogens in plants.

CgEnd3 Regulates Endocytosis, Appressorium Formation, and Virulence in the Poplar Anthracnose Fungus Colletotrichum gloeosporioides

The hemibiotrophic ascomycete fungus Colletotrichum gloeosporioides is the causal agent of anthracnose on numerous plants, and it causes considerable economic losses worldwide. Endocytosis is an essential cellular process in eukaryotic cells, but its roles in C. gloeosporioides remain unknown. In our study, we identified an endocytosis-related protein, CgEnd3, and knocked it out via polyethylene glycol (PEG)-mediated protoplast transformation. The lack of CgEnd3 resulted in severe defects in endocytosis. C. gloeosporioides infects its host through a specialized structure called appressorium, and ΔCgEnd3 showed deficient appressorium formation, melanization, turgor pressure accumulation, penetration ability of appressorium, cellophane membrane penetration, and pathogenicity. CgEnd3 also affected oxidant adaptation and the expression of core effectors during the early stage of infection. CgEnd3 contains one EF hand domain and four calcium ion-binding sites, and it is involved in calcium signaling. A lack of CgEnd3 changed the responses to cell-wall integrity agents and fungicide fludioxonil. However, CgEnd3 regulated appressorium formation and endocytosis in a calcium signaling-independent manner. Taken together, these results demonstrate that CgEnd3 plays pleiotropic roles in endocytosis, calcium signaling, cell-wall integrity, appressorium formation, penetration, and pathogenicity in C. gloeosporioides, and it suggests that CgEnd3 or endocytosis-related genes function as promising antifungal targets.

Size-exclusion chromatography allows the isolation of EVs from the filamentous fungal plant pathogen Fusarium oxysporum f. sp. vasinfectum (Fov)

Extracellular vesicles (EVs) are nano-sized compartments involved in cell communication and macromolecule transport that are well characterized in mammalian organisms. Fungal EVs transport virulence-related cargo and modulate the host immune response, but most work has been focused on human yeast pathogens. Additionally, the study of EVs from filamentous fungi has been hindered by the lack of protein markers and efficient isolation methods. In this study we performed the isolation and proteomic characterization of EVs from the filamentous cotton pathogen Fusarium oxysporum f. sp. vasinfectum (Fov). EVs were recovered from two different growth media, Czapek Dox and Saboraud's dextrose broth, and purified by size-exclusion chromatography. Our results show that the EV proteome changes depending on the growth medium but EV production remains constant. EVs contained proteins involved in polyketide synthesis, cell wall modifications, proteases and potential effectors. These results support a role in modulation of host-pathogen interactions for Fov EVs.

Proteinaceous effector discovery and characterization in filamentous plant pathogens

The complicated interplay of plant-pathogen interactions occurs on multiple levels as pathogens evolve to constantly evade the immune responses of their hosts. Many economically important crops fall victim to filamentous pathogens that produce small proteins called effectors to manipulate the host and aid infection/colonization. Understanding the effector repertoires of pathogens is facilitating an increased understanding of the molecular mechanisms underlying virulence as well as guiding the development of disease control strategies. The purpose of this review is to give a chronological perspective on the evolution of the methodologies used in effector discovery from physical isolation and in silico predictions, to functional characterization of the effectors of filamentous plant pathogens and identification of their host targets.

UhAVR1, an HR-Triggering Avirulence Effector of Ustilago hordei, Is Secreted via the ER-Golgi Pathway, Localizes to the Cytosol of Barley Cells during in Planta-Expression, and Contributes to Virulence Early in Infection

The basidiomycete Ustilago hordei causes covered smut disease of barley and oats. Virulence effectors promoting infection and supporting pathogen lifestyle have been described for this fungus. Genetically, six avirulence genes are known and one codes for UhAVR1, the only proven avirulence effector identified in smuts to date that triggers complete immunity in barley cultivars carrying resistance gene Ruh1. A prerequisite for resistance breeding is understanding the host targets and molecular function of UhAVR1. Analysis of this effector upon natural infection of barley coleoptiles using teliospores showed that UhAVR1 is expressed during the early stages of fungal infection where it leads to HR triggering in resistant cultivars or performs its virulence function in susceptible cultivars. Fungal secretion of UhAVR1 is directed by its signal peptide and occurs via the BrefeldinA-sensitive ER-Golgi pathway in cell culture away from its host. Transient in planta expression of UhAVR1 in barley and a nonhost, Nicotiana benthamiana, supports a cytosolic localization. Delivery of UhAVR1 via foxtail mosaic virus or Pseudomonas species in both barley and N. benthamiana reveals a role in suppressing components common to both plant systems of Effector- and Pattern-Triggered Immunity, including necrosis triggered by Agrobacterium-delivered cell death inducers.

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.

Target the core: durable plant resistance against filamentous plant pathogens through effector recognition

Plant pathogens colonize their host through the secretion of effector proteins that modulate plant metabolism and immune responses to their benefit. Plants evolve towards effector recognition, leading to host immunity. Typically, pathogen effectors are targets for recognition through plant receptors that are encoded by resistance genes. Resistance gene mediated crop immunity puts a tremendous pressure on pathogens to adapt and alter their effector repertoire to overcome recognition. We argue that the type of effector that is recognized by the host may have considerable implications on the durability of resistance against filamentous plant pathogens. Effector genes that are conserved among pathogens and reside in core genome regions are most likely to hold indispensable virulence functions. Consequently, the cost for the pathogen to overcome recognition by the host is higher than for diversified, host-specific effectors with a quantitative impact on virulence. Consequently, resistance genes that directly target conserved effector proteins without the interception of other effector proteins are potentially excellent resistance resources. © 2019 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

The Effector AGLIP1 in Rhizoctonia solani AG1 IA Triggers Cell Death in Plants and Promotes Disease Development Through Inhibiting PAMP-Triggered Immunity in Arabidopsis thaliana

Rhizoctonia solani, one of the most detrimental necrotrophic pathogens, causes rice sheath blight and poses a severe threat to production. Focus on the function of effectors secreted by necrotrophic pathogens during infection has grown rapidly in recent years. However, little is known about the virulence and mechanisms of these proteins. In this study, we performed functional studies on putative effectors in R. solani and revealed that AGLIP1 out of 13 putative effectors induced cell death in Nicotiana benthamiana. AGLIP1 was also demonstrated to trigger cell death in rice protoplasts. The predicted lipase active sites and signal peptide (SP) of this protein were required for the cell death-inducing ability. AGLIP1 was greatly induced during R. solani infection in rice sheath. The AGLIP1's virulence function was further demonstrated by transgenic technology. The pathogenesis-related genes induced by pathogen-associated molecular pattern and bacteria were remarkably inhibited in AGLIP1-expressing transgenic Arabidopsis lines. Ectopic expression of AGLIP1 strongly facilitated disease progression in Arabidopsis caused by the type III secretion system-defective mutant from Pseudomonas syringae pv. tomato DC3000. Collectively, these results indicate that AGLIP1 is a possible effector that plays a significant role in pathogen virulence through inhibiting basal defenses and promoting disease development in plants.

Ca2+-dependent interaction between calmodulin and CoDN3, an effector of Colletotrichum orbiculare

Nuclear magnetic resonance (NMR) data directly indicated a Ca²⁺-dependent interaction between calmodulin (CaM) and CoDN3, a small effector of the plant pathogenic fungus Colletotrichum orbiculare, which is the causal agent of cucumber anthracnose. The overall conformation of CoDN3 is intrinsically disordered, and the CaM-binding site spans residues 34-53 of its C-terminal region. Experiments employing a chemically synthesized peptide corresponding to the CaM-binding site indicated that the CaM-binding region of CoDN3 in the Ca²⁺-bound CaM complex takes an α-helical conformation. Cell death suppression assay using a CoDN3 mutant lacking the CaM-binding ability suggested that the wild type CaM-binding site is necessary for full CoDN3 function in vivo.

Establishment of a selection marker recycling system for sequential transformation of the plant-pathogenic fungus Colletotrichum orbiculare

Genome sequencing of pathogenic fungi has revealed the presence of various effectors that aid pathogen invasion by the manipulation of plant immunity. Effectors are often individually dispensable because of duplication and functional redundancy as a result of the arms race between host plants and pathogens. To study effectors that have functional redundancy, multiple gene disruption is often required. However, the number of selection markers that can be used for gene targeting is limited. Here, we established a marker recycling system that allows the use of the same selection marker in successive transformations in the model fungal pathogen Colletotrichum orbiculare, a causal agent of anthracnose disease in plants belonging to the Cucurbitaceae. We identified two C. orbiculare homologues of yeast URA3/pyrG, designated as URA3A and URA3B, which can be used as selection markers on medium with no uridine. The gene can then be removed from the genome via homologous recombination when the fungus is grown in the presence of 5-fluoroorotic acid (5-FOA), a chemical that is converted into a toxin by URA3 activity. The ura3a/b double mutants showed auxotrophy for uridine and insensitivity to 5-FOA. Using the ura3a/b mutants, transformation with the URA3B marker and its removal were successfully applied to disrupt the virulence-related gene, PKS1. The pks1 mutants showed a reduction in virulence, demonstrating that the method can be used to study virulence-related genes in C. orbiculare. The establishment of a URA3-based marker recycling system in plant-pathogenic fungi enables the genetic analysis of multiple genes that have redundant functions, including effector genes.

Conserved fungal effector suppresses PAMP-triggered immunity by targeting plant immune kinases

Plant pathogens have optimized their own effector sets to adapt to their hosts. However, certain effectors, regarded as core effectors, are conserved among various pathogens, and may therefore play an important and common role in pathogen virulence. We report here that the widely distributed fungal effector NIS1 targets host immune components that transmit signaling from pattern recognition receptors (PRRs) in plants. NIS1 from two Colletotrichum spp. suppressed the hypersensitive response and oxidative burst, both of which are induced by pathogen-derived molecules, in Nicotiana benthamianaMagnaporthe oryzae NIS1 also suppressed the two defense responses, although this pathogen likely acquired the NIS1 gene via horizontal transfer from Basidiomycota. Interestingly, the root endophyte Colletotrichum tofieldiae also possesses a NIS1 homolog that can suppress the oxidative burst in N. benthamiana We show that NIS1 of multiple pathogens commonly interacts with the PRR-associated kinases BAK1 and BIK1, thereby inhibiting their kinase activities and the BIK1-NADPH oxidase interaction. Furthermore, mutations in the NIS1-targeting proteins, i.e., BAK1 and BIK1, in Arabidopsis thaliana also resulted in reduced immunity to Colletotrichum fungi. Finally, M. oryzae lacking NIS1 displayed significantly reduced virulence on rice and barley, its hosts. Our study therefore reveals that a broad range of filamentous fungi maintain and utilize the core effector NIS1 to establish infection in their host plants and perhaps also beneficial interactions, by targeting conserved and central PRR-associated kinases that are also known to be targeted by bacterial effectors.

Transcriptomic analysis reveals candidate genes regulating development and host interactions of Colletotrichum fructicola

Background

Colletotrichum is a fungal genus in Ascomycota that contain many plant pathogens. Among all Colletotrichum genomes that have been sequenced, C. fructicola contains the largest number of candidate virulence factors, such as plant cell wall degrading enzymes, secondary metabolite (SM) biosynthetic enzymes, secreted proteinases, and small secreted proteins. Systematic analysis of the expressional patterns of these factors would be an important step toward identifying key virulence determinants.

Results

In this study, we obtained and compared the global transcriptome profiles of four types of infection-related structures: conidia, appressoria, infected apple leaves, and cellophane infectious hyphae (bulbous hyphae spreading inside cellophane) of C. fructicola. We also compared the expression changes of candidate virulence factors among these structures in a systematic manner. A total of 3189 genes were differentially expressed in at least one pairwise comparison. Genes showing in planta-specific expressional upregulations were enriched with small secreted proteins (SSPs), cytochrome P450s, carbohydrate-active enzymes (CAZYs) and secondary metabolite (SM) synthetases, and included homologs of several known candidate effectors and one SM gene cluster specific to the Colletotrichum genus. In conidia, tens of genes functioning in triacylglycerol biosynthesis showed coordinately expressional upregulation, supporting the viewpoint that C. fructicola builds up lipid droplets as energy reserves. Several phosphate starvation responsive genes were coordinately up-regulated during early plant colonization, indicating a phosphate-limited in planta environment immediately faced by biotrophic infectious hyphae.

Conclusion

This study systematically analyzes the expression patterns of candidate virulence genes, and reveals biological activities related to the development of several infection-related structures of C. fructicola. Our findings lay a foundation for further dissecting infection mechanisms in Colletotrichum and identifying disease control targets.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4934-0) contains supplementary material, which is available to authorized users.

Inappropriate Expression of an NLP Effector in Colletotrichum orbiculare Impairs Infection on Cucurbitaceae Cultivars via Plant Recognition of the C-Terminal Region

The hemibiotrophic pathogen Colletotrichum orbiculare preferentially expresses a necrosis and ethylene-inducing peptide 1 (Nep1)-like protein named NLP1 during the switch to necrotrophy. Here, we report that the constitutive expression of NLP1 in C. orbiculare blocks pathogen infection in multiple Cucurbitaceae cultivars via their enhanced defense responses. NLP1 has a cytotoxic activity that induces cell death in Nicotiana benthamiana. However, C. orbiculare transgenic lines constitutively expressing a mutant NLP1 lacking the cytotoxic activity still failed to infect cucumber, indicating no clear relationship between cytotoxic activity and the NLP1-dependent enhanced defense. NLP1 also possesses the microbe-associated molecular pattern (MAMP) sequence called nlp24, recognized by Arabidopsis thaliana at its central region, similar to NLPs of other pathogens. Surprisingly, inappropriate expression of a mutant NLP1 lacking the MAMP signature is also effective for blocking pathogen infection, uncoupling the infection block from the corresponding MAMP. Notably, the deletion analyses of NLP1 suggested that the C-terminal region of NLP1 is critical to enhance defense in cucumber. The expression of mCherry fused with the C-terminal 32 amino acids of NLP1 was enough to trigger the defense of cucurbits, revealing that the C-terminal region of the NLP1 protein is recognized by cucurbits and, then, terminates C. orbiculare infection.

Subcellular Localization Screening of Colletotrichum higginsianum Effector Candidates Identifies Fungal Proteins Targeted to Plant Peroxisomes, Golgi Bodies, and Microtubules

The genome of the hemibiotrophic anthracnose fungus, Colletotrichum higginsianum, encodes a large inventory of putative secreted effector proteins that are sequentially expressed at different stages of plant infection, namely appressorium-mediated penetration, biotrophy and necrotrophy. However, the destinations to which these proteins are addressed inside plant cells are unknown. In the present study, we selected 61 putative effector genes that are highly induced in appressoria and/or biotrophic hyphae. We then used Agrobacterium-mediated transformation to transiently express them as N-terminal fusions with fluorescent proteins in cells of Nicotiana benthamiana for imaging by confocal microscopy. Plant compartments labeled by the fusion proteins in N. benthamiana were validated by co-localization with specific organelle markers, by transient expression of the proteins in the true host plant, Arabidopsis thaliana, and by transmission electron microscopy-immunogold labeling. Among those proteins for which specific subcellular localizations could be verified, nine were imported into plant nuclei, three were imported into the matrix of peroxisomes, three decorated cortical microtubule arrays and one labeled Golgi stacks. Two peroxisome-targeted proteins harbored canonical C-terminal tripeptide signals for peroxisome import via the PTS1 (peroxisomal targeting signal 1) pathway, and we showed that these signals are essential for their peroxisome localization. Our findings provide valuable information about which host processes are potentially manipulated by this pathogen, and also reveal plant peroxisomes, microtubules, and Golgi as novel targets for fungal effectors.

Plant Pathogenic Fungi

Fungi are among the dominant causal agents of plant diseases. To colonize plants and cause disease, pathogenic fungi use diverse strategies. Some fungi kill their hosts and feed on dead material (necrotrophs), while others colonize the living tissue (biotrophs). For successful invasion of plant organs, pathogenic development is tightly regulated and specialized infection structures are formed. To further colonize hosts and establish disease, fungal pathogens deploy a plethora of virulence factors. Depending on the infection strategy, virulence factors perform different functions. While basically all pathogens interfere with primary plant defense, necrotrophs secrete toxins to kill plant tissue. In contrast, biotrophs utilize effector molecules to suppress plant cell death and manipulate plant metabolism in favor of the pathogen. This article provides an overview of plant pathogenic fungal species and the strategies they use to cause disease.

Colletotrichum orbiculare WHI2, a Yeast Stress-Response Regulator Homolog, Controls the Biotrophic Stage of Hemibiotrophic Infection Through TOR Signaling

The hemibiotrophic fungus Colletotrichum orbiculare first establishes a biotrophic infection stage in cucumber (Cucumber sativus) epidermal cells and subsequently transitions to a necrotrophic stage. Here, we found that C. orbiculare established hemibiotrophic infection via C. orbiculare WHI2, a yeast stress regulator homolog, and TOR (target of rapamycin) signaling. Plant defense responses such as callose deposition, H2O2, and antimicrobial proteins were strongly induced by the C. orbiculare whi2Δ mutant, resulting in defective pathogenesis. Expression analysis of biotrophy-specific genes evaluated by the promoter VENUS fusion gene indicated weaker VENUS signal intensity in the whi2Δ mutant, thereby suggesting that C. orbiculare WHI2 plays a key role in regulating biotrophic infection of C. orbiculare. The involvement of CoWHI2 in biotrophic infection was further explored with a DNA microarray. In the Cowhi2Δ mutant, TOR-dependent ribosomal protein-related genes were strikingly upregulated compared with the wild type. Moreover, callose deposition in the host plant after inoculation with the Cowhi2Δ mutant treated with rapamycin, which inhibits TOR activity, was reduced, and the mutant remained biotrophic in contrast to the untreated mutant. Thus, regulation of TOR by Whi2 is apparently crucial to the biotrophic stage of hemibiotrophic infection in C. orbiculare.

Correspondence between symptom development of Colletotrichum graminicola and fungal biomass, quantified by a newly developed qPCR assay, depends on the maize variety

BACKGROUND

Penetration attempts of the hemibiotroph Colletotrichum graminicola may activate PAMP-triggered immunity (PTI) on different cultivars of Zea mays to different extent. However, in most events, this does not prevent the establishment of a compatible pathogenic interaction. In this study, we investigate the extent to which the host variety influences PTI. Furthermore, we assess whether visual disease symptoms occurring on different maize varieties reliably reflect fungal biomass development in planta as determined by qPCR and GFP tracing.

RESULTS

Employing a set of four maize varieties, which were selected from a panel of 27 varieties, for in-depth assessment of pathogenesis of the wild type strain of C. graminicola, revealed considerable differences in susceptibility as evidenced by symptom severity that decreased from variety Golden Jubilee to Mikado to Farmtop to B73. However, a newly developed qPCR assay and microscopical observation of a GFP-labelled strain showed that disease symptoms are in some instances inconsistent when compared with other indicators of susceptibility. Of the four varieties assessed, either Golden Jubilee, Mikado and B73, or Golden Jubilee, Farmtop and B73 showed a direct correlation between symptom and fungal biomass development. In a pairwise comparison, however, Mikado and Farmtop showed an inverse correlation for these features.

CONCLUSIONS

The genotype of maize contributes to the severity of symptoms resulting from an infection with C. graminicola. Partially, this may be attributed to the extent of PTI activated in different varieties, as reflected by papilla formation. Furthermore, when evaluating the susceptibility of a variety, it should be considered that symptom severity must not have to reflect the extent of fungal growth in the infected tissue.

Genus-Wide Comparative Genome Analyses of Colletotrichum Species Reveal Specific Gene Family Losses and Gains during Adaptation to Specific Infection Lifestyles

Members from Colletotrichum genus adopt a diverse range of lifestyles during infection of plants and represent a group of agriculturally devastating pathogens. In this study, we present the draft genome of Colletotrichum incanum from the spaethianum clade of Colletotrichum and the comparative analyses with five other Colletotrichum species from distinct lineages. We show that the C. incanum strain, originally isolated from Japanese daikon radish, is able to infect both eudicot plants, such as certain ecotypes of the eudicot Arabidopsis, and monocot plants, such as lily. Being closely related to Colletotrichum species both in the graminicola clade, whose members are restricted strictly to monocot hosts, and to the destructivum clade, whose members are mostly associated with dicot infections, C. incanum provides an interesting model system for comparative genomics to study how fungal pathogens adapt to monocot and dicot hosts. Genus-wide comparative genome analyses reveal that Colletotrichum species have tailored profiles of their carbohydrate-degrading enzymes according to their infection lifestyles. In addition, we show evidence that positive selection acting on secreted and nuclear localized proteins that are highly conserved may be important in adaptation to specific hosts or ecological niches.

Identification of Multiple Phytotoxins Produced by Fusarium virguliforme Including a Phytotoxic Effector (FvNIS1) Associated With Sudden Death Syndrome Foliar Symptoms

Sudden death syndrome (SDS) of soybean is caused by a soilborne pathogen, Fusarium virguliforme. Phytotoxins produced by F. virguliforme are translocated from infected roots to leaves, in which they cause SDS foliar symptoms. In this study, additional putative phytotoxins of F. virguliforme were identified, including three secondary metabolites and 11 effectors. While citrinin, fusaric acid, and radicicol induced foliar chlorosis and wilting, Soybean mosaic virus (SMV)-mediated overexpression of F. virguliforme necrosis-inducing secreted protein 1 (FvNIS1) induced SDS foliar symptoms that mimicked the development of foliar symptoms in the field. The expression level of fvnis1 remained steady over time, although foliar symptoms were delayed compared with the expression levels. SMV::FvNIS1 also displayed genotype-specific toxicity to which 75 of 80 soybean cultivars were susceptible. Genome-wide association mapping further identified three single nucleotide polymorphisms at two loci, where three leucine-rich repeat receptor-like protein kinase (LRR-RLK) genes were found. Culture filtrates of fvnis1 knockout mutants displayed a mild reduction in phytotoxicity, indicating that FvNIS1 is one of the phytotoxins responsible for SDS foliar symptoms and may contribute to the quantitative susceptibility of soybean by interacting with the LRR-RLK genes.

Focal effector accumulation in a biotrophic interface at the primary invasion sites of Colletotrichum orbiculare in multiple susceptible plants

We identified virulence-related effectors of a hemibiotrophic fungal pathogen Colletotrichum orbiculare, and found that a novel interface was generated by a biotrophic interaction between C. orbiculare and the host cucumber, in which the effectors secreted from the pathogen accumulated preferentially. The interface was located around the biotrophic primary hyphal neck. Here, we showed that C. orbiculare also developed this interface in a biotrophic interaction with melon, which belongs to Cucurbitaceae. Furthermore, C. orbiculare developed interface in the interaction with a susceptible plant, Nicotiana benthamiana, which is distantly related to Cucurbitaceae, suggesting that the spatial regulation strategy for effectors in C. orbiculare is not specific to cucumber; rather, it is conserved among the various plants that are susceptible to this pathogen.

Fungal effectors and plant susceptibility

Plants can be colonized by fungi that have adopted highly diverse lifestyles, ranging from symbiotic to necrotrophic. Colonization is governed in all systems by hundreds of secreted fungal effector molecules. These effectors suppress plant defense responses and modulate plant physiology to accommodate fungal invaders and provide them with nutrients. Fungal effectors either function in the interaction zone between the fungal hyphae and host or are transferred to plant cells. This review describes the effector repertoires of 84 plant-colonizing fungi. We focus on the mechanisms that allow these fungal effectors to promote virulence or compatibility, discuss common plant nodes that are targeted by effectors, and provide recent insights into effector evolution. In addition, we address the issue of effector uptake in plant cells and highlight open questions and future challenges.

Colletotrichum orbiculare Secretes Virulence Effectors to a Biotrophic Interface at the Primary Hyphal Neck via Exocytosis Coupled with SEC22-Mediated Traffic

The hemibiotrophic pathogen Colletotrichum orbiculare develops biotrophic hyphae inside cucumber (Cucumis sativus) cells via appressorial penetration; later, the pathogen switches to necrotrophy. C. orbiculare also expresses specific effectors at different stages. Here, we found that virulence-related effectors of C. orbiculare accumulate in a pathogen-host biotrophic interface. Fluorescence-tagged effectors accumulated in a ring-like region around the neck of the biotrophic primary hyphae. Fluorescence imaging of cellular components and transmission electron microscopy showed that the ring-like signals of the effectors localized at the pathogen-plant interface. Effector accumulation at the interface required induction of its expression during the early biotrophic phase, suggesting that transcriptional regulation may link to effector localization. We also investigated the route of effector secretion to the interface. An exocytosis-related component, the Rab GTPase SEC4, localized to the necks of biotrophic primary hyphae adjacent to the interface, thereby suggesting focal effector secretion. Disruption of SEC4 in C. orbiculare reduced virulence and impaired effector delivery to the ring signal interface. Disruption of the v-SNARE SEC22 also reduced effector delivery. These findings suggest that biotrophy-expressed effectors are secreted, via the endoplasmic reticulum-to-Golgi route and subsequent exocytosis, toward the interface generated between C. orbiculare and the host cell.

The Fusarium oxysporum effector Six6 contributes to virulence and suppresses I-2-mediated cell death

Plant pathogens secrete effectors to manipulate their host and facilitate colonization. Fusarium oxysporum f. sp. lycopersici is the causal agent of Fusarium wilt disease in tomato. Upon infection, F. oxysporum f. sp. lycopersici secretes numerous small proteins into the xylem sap (Six proteins). Most Six proteins are unique to F. oxysporum, but Six6 is an exception; a homolog is also present in two Colletotrichum spp. SIX6 expression was found to require living host cells and a knockout of SIX6 in F. oxysporum f. sp. lycopersici compromised virulence, classifying it as a genuine effector. Heterologous expression of SIX6 did not affect growth of Agrobacterium tumefaciens in Nicotiana benthamiana leaves or susceptibility of Arabidopsis thaliana toward Verticillium dahliae, Pseudomonas syringae, or F. oxysporum, suggesting a specific function for F. oxysporum f. sp. lycopersici Six6 in the F. oxysporum f. sp. lycopersici- tomato pathosystem. Remarkably, Six6 was found to specifically suppress I-2-mediated cell death (I2CD) upon transient expression in N. benthamiana, whereas it did not compromise the activity of other cell-death-inducing genes. Still, this I2CD suppressing activity of Six6 does not allow the fungus to overcome I-2 resistance in tomato, suggesting that I-2-mediated resistance is independent from cell death.

Evidence for a diffusible factor that induces susceptibility in the Colletotrichum-maize disease interaction

Colletotrichum graminicola, the causal agent of maize anthracnose, is a hemibiotrophic fungus that initially infects living host cells via primary hyphae surrounded by a membrane. A nonpathogenic mutant disrupted in a gene encoding a component of the signal peptidase complex, and believed to be deficient in protein processing and secretion, regained pathogenicity when it was inoculated onto maize leaf sheaths close to the wild-type fungus. Evidence is presented suggesting that the wild-type produces a diffusible factor(s) that induces the localized susceptibility of host cells at the borders of expanding colonies, causing them to become receptive to biotrophic invasion. The induced susceptibility effect is limited to a distance of approximately eight cells from the edge of the wild-type colony, is dosage dependent and is specific to C. graminicola.

Apoplastic immunity and its suppression by filamentous plant pathogens

Microbial plant pathogens have evolved a variety of strategies to enter plant hosts and cause disease. In particular, biotrophic pathogens, which parasitize living plant tissue, establish sophisticated interactions in which they modulate the plant's metabolism to their own good. The prime decision, whether or not a pathogen can accommodate itself in its host tissue, is made during the initial phase of infection. At this stage, the plant immune system recognizes conserved molecular patterns of the invading microbe, which initiate a set of basal immune responses. Induced plant defense proteins, toxic compounds and antimicrobial proteins encounter a broad arsenal of pathogen-derived virulence factors that aim to disarm host immunity. Crucial regulatory processes and protein-protein interactions take place in the apoplast, that is, intercellular spaces, plant cell walls and defined host-pathogen interfaces which are formed between the plant cytoplasm and the specialized infection structures of many biotrophic pathogens. This article aims to provide an insight into the most important principles and components of apoplastic plant immunity and its modulation by filamentous microbial pathogens.

Comparative genomic and transcriptomic analyses reveal the hemibiotrophic stage shift of Colletotrichum fungi

Hemibiotrophic fungal plant pathogens represent a group of agronomically significant disease-causing agents that grow first on living tissue and then cause host death in later, necrotrophic growth. Among these, Colletotrichum spp. are devastating pathogens of many crops. Identifying expanded classes of genes in the genomes of phytopathogenic Colletotrichum, especially those associated with specific stages of hemibiotrophy, can provide insights on how these pathogens infect a large number of hosts. The genomes of Colletotrichum orbiculare, which infects cucurbits and Nicotiana benthamiana, and C. gloeosporioides, which infects a wide range of crops, were sequenced and analyzed, focusing on features with potential roles in pathogenicity. Regulation of C. orbiculare gene expression was investigated during infection of N. benthamiana using a custom microarray. Genes expanded in both genomes compared to other fungi included sequences encoding small, secreted proteins (SSPs), secondary metabolite synthesis genes, proteases and carbohydrate-degrading enzymes. Many SSP and secondary metabolite synthesis genes were upregulated during initial stages of host colonization, whereas the necrotrophic stage of growth is characterized by upregulation of sequences encoding degradative enzymes. Hemibiotrophy in C. orbiculare is characterized by distinct stage-specific gene expression profiles of expanded classes of potential pathogenicity genes.