BcIEB1, a Botrytis cinerea secreted protein, elicits a defense response in plants

Integrated Transcriptome and Proteome Analysis Reveals That Cell Wall Activity Affects Phelipanche aegyptiaca Parasitism

Phelipanche aegyptiaca can infect many crops, causing large agricultural production losses. It is important to study the parasitism mechanism of P. aegyptiaca to control its harm. In this experiment, the P. aegyptiaca HY13M and TE9M from Tacheng Prefecture and Hami City in Xinjiang, respectively, were used to analyze the parasitical mechanism of P. aegyptiaca by means of transcriptome and proteome analyses. The parasitic capacity of TE9M was significantly stronger than that of HY13M in Citrullus lanatus. The results showed that the DEGs and DEPs were prominently enriched in the cell wall metabolism pathways, including "cell wall organization or biogenesis", "cell wall organization", and "cell wall". Moreover, the functions of the pectinesterase enzyme gene (TR138070_c0_g), which is involved in the cell wall metabolism of P. aegyptiaca in its parasitism, were studied by means HIGS. The number and weight of P. aegyptiaca were significantly reduced when TR138070_c0_g1, which encodes a cell-wall-degrading protease, was silenced, indicating that it positively regulates P. aegyptiaca parasitism. Thus, these results suggest that the cell wall metabolism pathway is involved in P. aegyptiaca differentiation of the parasitic ability and that the TR138070_c0_g1 gene plays an important role in P. aegyptiaca's parasitism.

Characterization of two SGNH family cell death-inducing proteins from the horticulturally important fungal pathogen Botrytis cinerea based on the optimized prokaryotic expression system

Botrytis cinerea is one of the most destructive phytopathogenic fungi, causing significant losses to horticultural crops. As a necrotrophic fungus, B. cinerea obtains nutrients by killing host cells. Secreted cell death-inducing proteins (CDIPs) play a crucial role in necrotrophic infection; however, only a limited number have been reported. For high-throughput CDIP screening, we optimized the prokaryotic expression system and compared its efficiency with other commonly used protein expression systems. The optimized prokaryotic expression system showed superior effectiveness and efficiency and was selected for subsequent CDIP screening. The screening system verified fifty-five candidate proteins and identified two novel SGNH family CDIPs: BcRAE and BcFAT. BcRAE and BcFAT exhibited high expression levels throughout the infection process. Site-directed mutagenesis targeting conserved Ser residues abolished the cell death-inducing activity of both BcRAE and BcFAT. Moreover, the transient expression of BcRAE and BcFAT in plants enhanced plant resistance against B. cinerea without inducing cell death, independent of their enzymatic activities. Our results suggest a high-efficiency screening system for high-throughput CDIP screening and provide new targets for further study of B. cinerea-plant interactions.

The Botrytis cinerea transglycosylase BcCrh4 is a cell death-inducing protein with cell death-promoting and -suppressing domains

Botrytis cinerea is a necrotrophic fungal plant pathogen that causes grey mould and rot diseases in many crops. Here, we show that the B. cinerea BcCrh4 transglycosylase is secreted during plant infection and induces plant cell death and pattern-triggered immunity (PTI), fulfilling the characteristics of a cell death-inducing protein (CDIP). The CDIP activity of BcCrh4 is independent of the transglycosylase enzymatic activity, it takes place in the apoplast and does not involve the receptor-like kinases BAK1 and SOBIR1. During saprophytic growth, BcCrh4 is localized in the endoplasmic reticulum and in vacuoles, but during plant infection, it accumulates in infection cushions (ICs) and is then secreted to the apoplast. Two domains within the BcCrh4 protein determine the CDIP activities: a 20aa domain at the N' end activates intense cell death and PTI, while a stretch of 52aa in the middle of the protein induces a weaker response and suppresses the activity of the 20aa N' domain. Deletion of bccrh4 affected fungal development and IC formation in particular, resulting in reduced virulence. Collectively, our findings demonstrate that BcCrh4 is required for fungal development and pathogenicity, and hint at a dual mechanism that balances the virulence activity of this, and potentially other CDIPs.

The Botrytis cinerea effector BcXYG1 suppresses immunity in Fragaria vesca by targeting FvBPL4 and FvACD11

Botrytis cinerea is one of the most destructive pathogens in strawberry cultivation. Successful infection by B. cinerea requires releasing a large number of effectors that interfere with the plant's immune system. One of the effectors required by B. cinerea for optimal virulence is the secreted protein BcXYG1, which is thought to associate with proteins near the plasma membrane of the host plant to induce necrosis. However, the host proteins that associate with BcXYG1 at the plasma membrane are currently unknown. We found that BcXYG1 binds to FvBPL4 and FvACD11 at the plasma membrane. Both FvBPL4 and FvACD11 are negative regulators of plant immunity in strawberry. Our results demonstrate that degradation of FvBPL4 by BcXYG1 promotes disease resistance while stabilization of FvACD11 by BcXYG1 suppresses the immune response. These findings suggest that BcXYG1 suppresses plant immunity and promotes B. cinerea infection by regulating FvBPL4 and FvACD11 protein levels.

Botrytis cinerea tolerates phytoalexins produced by Solanaceae and Fabaceae plants through an efflux transporter BcatrB and metabolizing enzymes

Botrytis cinerea, a plant pathogenic fungus with a wide host range, has reduced sensitivity to fungicides as well as phytoalexins, threatening cultivation of economically important fruits and vegetable crops worldwide. B. cinerea tolerates a wide array of phytoalexins, through efflux and/or enzymatic detoxification. Previously, we provided evidence that a distinctive set of genes were induced in B. cinerea when treated with different phytoalexins such as rishitin (produced by tomato and potato), capsidiol (tobacco and bell pepper) and resveratrol (grape and blueberry). In this study, we focused on the functional analyses of B. cinerea genes implicated in rishitin tolerance. LC/MS profiling revealed that B. cinerea can metabolize/detoxify rishitin into at least 4 oxidized forms. Heterologous expression of Bcin08g04910 and Bcin16g01490, two B. cinerea oxidoreductases upregulated by rishitin, in a plant symbiotic fungus Epichloë festucae revealed that these rishitin-induced enzymes are involved in the oxidation of rishitin. Expression of BcatrB, encoding an exporter of structurally unrelated phytoalexins and fungicides, was significantly upregulated by rishitin but not by capsidiol and was thus expected to be involved in the rishitin tolerance. Conidia of BcatrB KO (ΔbcatrB) showed enhanced sensitivity to rishitin, but not to capsidiol, despite their structural similarity. ΔbcatrB showed reduced virulence on tomato, but maintained full virulence on bell pepper, indicating that B. cinerea activates BcatrB by recognizing appropriate phytoalexins to utilize it in tolerance. Surveying 26 plant species across 13 families revealed that the BcatrB promoter is mainly activated during the infection of B. cinerea in plants belonging to the Solanaceae, Fabaceae and Brassicaceae. The BcatrB promoter was also activated by in vitro treatments of phytoalexins produced by members of these plant families, namely rishitin (Solanaceae), medicarpin and glyceollin (Fabaceae), as well as camalexin and brassinin (Brassicaceae). Consistently, ΔbcatrB showed reduced virulence on red clover, which produces medicarpin. These results suggest that B. cinerea distinguishes phytoalexins and induces differential expression of appropriate genes during the infection. Likewise, BcatrB plays a critical role in the strategy employed by B. cinerea to bypass the plant innate immune responses in a wide variety of important crops belonging to the Solanaceae, Brassicaceae and Fabaceae.

Botrytis cinerea BcCDI1 protein triggers both plant cell death and immune response

Cell death-inducing proteins (CDIPs) play important roles in the infection of Botrytis cinerea, a broad host-range necrotrophic phytopathogen. Here, we show that the secreted protein BcCDI1 (Cell Death Inducing 1) can cause necrosis in tobacco leaves and at the same time elicit plant defense. The transcription of Bccdi1 was induced at the infection stage. Deletion or overexpression of Bccdi1 resulted in no notable change in disease lesion on bean, tobacco, and Arabidopsis leaves, indicating that Bccdi1 has no effect on the final outcome of B. cinerea infection. Furthermore, the plant receptor-like kinases BAK1 and SOBIR1 are required to transduce the cell death-promoting signal induced by BcCDI1. These findings suggest that BcCDI1 is possibly recognized by plant receptors and then induces plant cell death.

Killing softly: a roadmap of Botrytis cinerea pathogenicity

Botrytis cinerea, a widespread plant pathogen with a necrotrophic lifestyle, causes gray mold disease in many crops. Massive secretion of enzymes and toxins was long considered to be the main driver of infection, but recent studies have uncovered a rich toolbox for B. cinerea pathogenicity. The emerging picture is of a multilayered infection process governed by the exchange of factors that collectively contribute to disease development. No plant shows complete resistance against B. cinerea, but pattern-triggered plant immune responses have the potential to significantly reduce disease progression, opening new possibilities for producing B. cinerea-tolerant plants. We examine current B. cinerea infection models, highlight knowledge gaps, and suggest directions for future studies.

Botrytis hypersensitive response inducing protein 1 triggers noncanonical PTI to induce plant cell death

According to their lifestyle, plant pathogens are divided into biotrophic and necrotrophic organisms. Biotrophic pathogens exclusively nourish living host cells, whereas necrotrophic pathogens rapidly kill host cells and nourish cell walls and cell contents. To this end, the necrotrophic fungus Botrytis cinerea secretes large amounts of phytotoxic proteins and cell wall-degrading enzymes. However, the precise role of these proteins during infection is unknown. Here, we report on the identification and characterization of the previously unknown toxic protein hypersensitive response-inducing protein 1 (Hip1), which induces plant cell death. We found the adoption of a structurally conserved folded Alternaria alternata Alt a 1 protein structure to be a prerequisite for Hip1 to exert its necrosis-inducing activity in a host-specific manner. Localization and the induction of typical plant defense responses by Hip1 indicate recognition as a pathogen-associated molecular pattern at the plant plasma membrane. In contrast to other secreted toxic Botrytis proteins, the activity of Hip1 does not depend on the presence of the receptor-associated kinases BRI1-associated kinase 1 and suppressor of BIR1-1. Our results demonstrate that recognition of Hip1, even in the absence of obvious enzymatic or pore-forming activity, induces strong plant defense reactions eventually leading to plant cell death. Botrytis hip1 overexpression strains generated by CRISPR/Cas9 displayed enhanced infection, indicating the virulence-promoting potential of Hip1. Taken together, Hip1 induces a noncanonical defense response which might be a common feature of structurally conserved fungal proteins from the Alt a 1 family.

Analysis of plant cell death-inducing proteins of the necrotrophic fungal pathogens Botrytis squamosa and Botrytis elliptica

Fungal plant pathogens secrete proteins that manipulate the host in order to facilitate colonization. Necrotrophs have evolved specialized proteins that actively induce plant cell death by co-opting the programmed cell death machinery of the host. Besides the broad host range pathogen Botrytis cinerea, most other species within the genus Botrytis are restricted to a single host species or a group of closely related hosts. Here, we focused on Botrytis squamosa and B. elliptica, host specific pathogens of onion (Allium cepa) and lily (Lilium spp.), respectively. Despite their occurrence on different hosts, the two fungal species are each other's closest relatives. Therefore, we hypothesize that they share a considerable number of proteins to induce cell death on their respective hosts. In this study, we first confirmed the host-specificity of B. squamosa and B. elliptica. Then we sequenced and assembled high quality genomes. The alignment of these two genomes revealed a high level of synteny with few balanced structural chromosomal arrangements. To assess the cell death-inducing capacity of their secreted proteins, we produced culture filtrates of B. squamosa and B. elliptica that induced cell death responses upon infiltration in host leaves. Protein composition of the culture filtrate was analysed by mass spectrometry, and we identified orthologous proteins that were present in both samples. Subsequently, the expression of the corresponding genes during host infection was compared. RNAseq analysis showed that the majority of the orthogroups of the two sister species display similar expression patterns during infection of their respective host. The analysis of cell death-inducing proteins of B. squamosa and B. elliptica provides insights in the mechanisms used by these two Botrytis species to infect their respective hosts.

Pathogenic strategies and immune mechanisms to necrotrophs: Differences and similarities to biotrophs and hemibiotrophs

Pathogenesis in plant diseases is complex comprising diverse pathogen virulence and plant immune mechanisms. These pathogens cause damaging plant diseases by deploying specialized and generic virulence strategies that are countered by intricate resistance mechanisms. The significant challenges that necrotrophs pose to crop production are predicted to increase with climate change. Immunity to biotrophs and hemibiotrophs is dominated by intracellular receptors that recognize specific effectors and activate resistance. These mechanisms play only minor roles in resistance to necrotrophs. Pathogen- or host-derived conserved pattern molecules trigger immune responses that broadly contribute to plant immunity. However, certain pathogen or host-derived immune elicitors are enriched by the virulence activities of necrotrophs. Different plant hormones modulate systemic resistance and cell death that have differential impacts on resistance to pathogens of different lifestyles. Knowledge of mechanisms that contribute to resistance to necrotrophs has expanded. Besides toxins and cell wall degrading enzymes that dominate the pathogenesis of necrotrophs, other effectors with subtle contributions are being identified.

Multiple knockout mutants reveal a high redundancy of phytotoxic compounds contributing to necrotrophic pathogenesis of Botrytis cinerea

Botrytis cinerea is a major plant pathogen infecting more than 1400 plant species. During invasion, the fungus rapidly kills host cells, which is believed to be supported by induction of programmed plant cell death. To comprehensively evaluate the contributions of most of the currently known plant cell death inducing proteins (CDIPs) and metabolites for necrotrophic infection, an optimized CRISPR/Cas9 protocol was established which allowed to perform serial marker-free mutagenesis to generate multiple deletion mutants lacking up to 12 CDIPs. Whole genome sequencing of a 6x and 12x deletion mutant revealed a low number of off-target mutations which were unrelated to Cas9-mediated cleavage. Secretome analyses confirmed the loss of secreted proteins encoded by the deleted genes. Infection tests with the mutants revealed a successive decrease in virulence with increasing numbers of mutated genes, and varying effects of the knockouts on different host plants. Comparative analysis of mutants confirmed significant roles of two polygalacturonases (PG1, PG2) and the phytotoxic metabolites botrydial and botcinins for infection, but revealed no or only weak effects of deletion of the other CDIPs. Nicotiana benthamiana plants with mutated or silenced coreceptors of pattern recognition receptors, SOBIR1 and BAK1, showed similar susceptibility as control plants to infection by B. cinerea wild type and a 12x deletion mutant. These results raise doubts about a major role of manipulation of these plant defence regulators for B. cinerea infection. Despite the loss of most of the known phytotoxic compounds, the on planta secretomes of the multiple mutants retained substantial phytotoxic activity, proving that further, as yet unknown CDIPs contribute to necrosis and virulence. Our study has addressed for the first time systematically the functional redundancy of fungal virulence factors, and demonstrates that B. cinerea releases a highly redundant cocktail of proteins to achieve necrotrophic infection of a wide variety of host plants.

Botrytis cinerea BcSSP2 protein is a late infection phase, cytotoxic effector

Botrytis cinerea is a broad-host-range necrotrophic phytopathogen responsible for serious diseases in leading crops. To facilitate infection, B. cinerea secretes a large number of effectors that induce plant cell death. In screening secretome data of B. cinerea during infection stage, we identified a phytotoxic protein (BcSSP2) that can also induce immune resistance in plants. BcSSP2 is a small, cysteine-rich protein without any known domains. Transient expression of BcSSP2 in leaves caused chlorosis that intensifies with time and eventually leads to death. Point mutations in eight of 10 cysteine residues abolished phytotoxicity, but residual toxic activity remained after heating treatment, suggesting contribution of unknown epitopes to protein phytotoxicity. The expression of bcssp2 was low during the first 36 h after inoculation and increased sharply upon transition to late infection stage. Deletion of bcssp2 did not cause statistically significant changes in lesions size on bean and tobacco leaves. Further analyses indicated that the phytotoxicity of BcSSP2 is negatively regulated by the receptor-like kinases BAK1 and SOBIR1. Collectively, our findings show that BcSSP2 is an effector protein that toxifies the host cells, but is also recognized by the plant immune system.

Plant immunity inducers: from discovery to agricultural application

While conventional chemical fungicides directly eliminate pathogens, plant immunity inducers activate or prime plant immunity. In recent years, considerable progress has been made in understanding the mechanisms of immune regulation in plants. The development and application of plant immunity inducers based on the principles of plant immunity represent a new field in plant protection research. In this review, we describe the mechanisms of plant immunity inducers in terms of plant immune system activation, summarize the various classes of reported plant immunity inducers (proteins, oligosaccharides, chemicals, and lipids), and review methods for the identification or synthesis of plant immunity inducers. The current situation, new strategies, and future prospects in the development and application of plant immunity inducers are also discussed.

Clathrin Is Important for Virulence Factors Delivery in the Necrotrophic Fungus Botrytis cinerea

Fungi are the most prevalent plant pathogens, causing annually important damages. To infect and colonize their hosts, they secrete effectors including hydrolytic enzymes able to kill and macerate plant tissues. These secreted proteins are transported from the Endoplasmic Reticulum and the Golgi apparatus to the extracellular space through intracellular vesicles. In pathogenic fungi, intracellular vesicles were described but their biogenesis and their role in virulence remain unclear. In this study, we report the essential role of clathrin heavy chain (CHC) in the pathogenicity of Botrytis cinerea, the agent of gray mold disease. To investigate the importance of this protein involved in coat vesicles formation in eukaryotic cells, a T-DNA insertional mutant reduced in the expression of the CHC-encoding gene, and a mutant expressing a dominant-negative form of CHC were studied. Both mutants were strongly affected in pathogenicity. Characterization of the mutants revealed altered infection cushions and an important defect in protein secretion. This study demonstrates the essential role of clathrin in the infectious process of a plant pathogenic fungus and more particularly its role in virulence factors delivery.

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.

The infection cushion of Botrytis cinerea: a fungal 'weapon' of plant-biomass destruction

The necrotrophic plant-pathogen fungus Botrytis cinerea produces multicellular appressoria dedicated to plant penetration, named infection cushions (IC). A microarray analysis was performed to identify genes upregulated in mature IC. The expression data were validated by RT-qPCR analysis performed in vitro and in planta, proteomic analysis of the IC secretome and biochemical assays. 1231 upregulated genes and 79 up-accumulated proteins were identified. The data support the secretion of effectors by IC: phytotoxins, ROS, proteases, cutinases, plant cell wall-degrading enzymes and plant cell death-inducing proteins. Parallel upregulation of sugar transport and sugar catabolism-encoding genes would indicate a role of IC in nutrition. The data also reveal a substantial remodelling of the IC cell wall and suggest a role for melanin and chitosan in IC function. Lastly, mutagenesis of two upregulated genes in IC identified secreted fasciclin-like proteins as actors in the pathogenesis of B. cinerea. These results support the role of IC in plant penetration and also introduce other unexpected functions for this fungal organ, in colonization, necrotrophy and nutrition of the pathogen.

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.

New tools for high-throughput expression of fungal secretory proteins in Saccharomyces cerevisiae and Pichia pastoris

Heterologous protein expression in yeast, mostly in Saccharomyces cerevisiae and Pichia pastoris, is a well-established and widely used technique. It typically requires the construction of an expression vector in Escherichia coli containing the foreign gene and its subsequent transformation into yeast. Although simple, this procedure has important limitations for the expression of large numbers of proteins, that is, for the generation of protein libraries. We describe here the development of a novel system for the easy and fast expression of heterologous proteins both in S. cerevisiae and in P. pastoris, under the control of the GAL1 and AOX1 promoters respectively. Expression in S. cerevisiae requires only the transformation of yeast cells with an unpurified PCR product carrying the gene to be expressed, and the expression of the same gene in P. pastoris requires only the isolation of the plasmid generated in S. cerevisiae and its transformation into this second yeast, thus making this system suitable for high-throughput projects. The system has been tested by the extracellular expression of 30 secretory fungal proteins.

The Novel Cerato-Platanin-Like Protein FocCP1 from Fusarium oxysporum Triggers an Immune Response in Plants

Panama disease, or Fusarium wilt, the most serious disease in banana cultivation, is caused by Fusarium oxysporum f. sp. cubense (FOC) and has led to great economic losses worldwide. One effective way to combat this disease is by enhancing host plant resistance. The cerato-platanin protein (CPP) family is a group of small secreted cysteine-rich proteins in filamentous fungi. CPPs as elicitors can trigger the immune system resulting in defense responses in plants. In this study, we characterized a novel cerato-platanin-like protein in the secretome of Fusarium oxysporum f. sp. cubense race 4 (FOC4), named FocCP1. In tobacco, the purified recombinant FocCP1 protein caused accumulation of reactive oxygen species (ROS), formation of necrotic reaction, deposition of callose, expression of defense-related genes, and accumulation of salicylic acid (SA) and jasmonic acid (JA) in tobacco. These results indicated that FocCP1 triggered a hypersensitive response (HR) and systemic acquired resistance (SAR) in tobacco. Furthermore, FocCP1 enhanced resistance tobacco mosaic virus (TMV) disease and Pseudomonas syringae pv. tabaci 6605 (Pst. 6605) infection in tobacco and improved banana seedling resistance to FOC4. All results provide the possibility of further research on immune mechanisms of plant and pathogen interactions, and lay a foundation for a new biological strategy of banana wilt control in the future.

The dual role of oxalic acid on the resistance of tomato against Botrytis cinerea

In order to define the role of oxalic acid (OA) in the invasion of Botrytis cinerea in tomato plants, the OA induction of resistance related to oxalate oxidase (O×O) and germin was examined. In greenhouse experiments, OA at 3 mmol/L significantly induced resistance in tomato plants against B. cinerea strains B05.10 and T4, reducing lesion size of 37.55% and 24.91% by compared with distilled water control, respectively, while 20 mmol/L OA increasing by 36.14% and 41.48%. OA contents were 98 and 46 µg/mL when tomato plants were infected by B. cinerea strains B05.10 and T4, respectively. To define the molecular-genetic mechanisms, we compared the gene expression under four different conditions: 3 mmol/L OA-treated plants, 20 mmol/L OA-treated plants, B. cinerea strain B05.10-infected plants (B05.10 Inf plants) and B. cinerea strain T4-infected plants (T4 Inf plants). In 3 mmol/L OA-treated plants, the expressions of O×O and Germin peaked at 48 h after spraying, with approximate threefold and 18-fold increase compared with the control expression, respectively. In T4 Inf plants, the expression (mRNA accumulation) of O×O and Germin reached the highest levels at 24 h after inoculation, with 3- and 13-times that immediately after inoculation, respectively. In total, these findings suggest that elevated levels of OA correlated with increased fungal invasion and lower OA induced resistance in tomato plants by increasing expressions of O×O and Germin.

Verticillium dahliae PevD1, an Alt a 1-like protein, targets cotton PR5-like protein and promotes fungal infection

Alt a 1 family proteins (AA1s) have only been observed in the Dothideomycetes and Sordariomycetes classes of fungi, and their biological functions have remained poorly understood. Verticillium dahliae, a soil-borne pathogen that causes plant wilt disease, secretes hundreds of proteins during the process of pathogenic infection, including the AA1 member PevD1. In this study, we found that the pevd1 transcript was present in all of the hosts studied (cotton, Arabidopsis, tomato, and tobacco) and showed elevated expression throughout the infection process. Furthermore, pevd1 knockout mutants displayed attenuated pathogenicity compared with the wild-type (WT) strain and complemented strains in hosts. A partner protein of PevD1, pathogenesis-related protein 5 (PR5)-like protein GhPR5, was isolated from cotton (Gossypium hirsutum) plants by co-purification assays, and the PevD1-GhPR5 interaction was determined to be localized in the C-terminus (PevD1b, amino acids residues 113-155) by pull-down and yeast two-hybrid techniques. Re-introduction of the pevd1b gene into a pevd1 knockout mutant resulted in restoration of the virulence phenotype to WT levels. In addition, PevD1b, which is similar to PevD1, decreased the antifungal activity of GhPR5 in vitro. Our findings reveal an infection strategy in which V. dahliae secretes PevD1 to inhibit GhPR5 antifungal activity in order to overcome the host defence system.

A 25-Residue Peptide From Botrytis cinerea Xylanase BcXyn11A Elicits Plant Defenses

Plants activate defense responses against a possible pathogen once pattern-recognition receptors (PRRs) perceive the presence of pathogen-associated molecular patterns (PAMPs). Glycosyl hydrolase family 11 (GH11) endoxylanases from Trichoderma, Fusarium and Botrytis species have been described as being able to induce the defense response in plants, in a way that is independent of its enzymatic activity. However, until now, it has not been possible to establish with certainty which regions of these enzymes are recognized by plants as PAMPs. We show here for the first time that a short 25-residue peptide (named Xyn25) from the Botrytis cinerea xylanase BcXyn11A can reproduce by itself all the effects observed for the treatment of plants with the whole BcXyn11A protein. These include necrosis on leaves, seedling growth inhibition, induction of a ROS burst, electrolyte leakage, cytoplasm shrinkage, autofluorescence, cell death, and induction of defense genes. Two highly conserved four-amino acid regions within Xyn25 were shown to be necessary for the elicitation activity by substituting them with tracts of four alanine residues.

Glycans as Modulators of Plant Defense Against Filamentous Pathogens

Plants and microbes utilize glycoconjugates as structural entities, energy reserves for cellular processes, and components of cellular recognition or binding events. The structural heterogeneity of carbohydrates in such systems is a result of the ability of the carbohydrate biosynthetic enzymes to reorient sugar monomers in a variety of forms, generating highly complex, linear, branched, or hierarchical structures. During the interaction between plants and their microbial pathogens, the microbial cell surface glycans, cell wall derived glycans, and glycoproteins stimulate the signaling cascades of plant immune responses, through a series of specific or broad spectrum recognition events. The microbial glycan-induced plant immune responses and the downstream modifications observed in host-plant glycan structures that combat the microbial attack have garnered immense interest among scientists in recent times. This has been enabled by technological advancements in the field of glycobiology, making it possible to study the ongoing co-evolution of the microbial and the corresponding host glycan structures, in greater detail. The new glycan analogs emerging in this evolutionary arms race brings about a fresh perspective to our understanding of plant-pathogen interactions. This review discusses the role of diverse classes of glycans and their derivatives including simple sugars, oligosaccharides, glycoproteins, and glycolipids in relation to the activation of classical Pattern-Triggered Immunity (PTI) and Effector-Triggered Immunity (ETI) defense responses in plants. While primarily encompassing the biological roles of glycans in modulating plant defense responses, this review categorizes glycans based on their structure, thereby enabling parallels to be drawn to other areas of glycobiology. Further, we examine how these molecules are currently being used to develop new bio-active molecules, potent as priming agents to stimulate plant defense response and as templates for designing environmentally friendly foliar sprays for plant protection.

BcCFEM1, a CFEM Domain-Containing Protein with Putative GPI-Anchored Site, Is Involved in Pathogenicity, Conidial Production, and Stress Tolerance in Botrytis cinerea

We experimentally isolated and characterized a CFEM protein with putative GPI-anchored site BcCFEM1 in Botrytis cinerea. BcCFEM1 contains a CFEM (common in several fungal extracellular membrane proteins) domain with the characteristic eight cysteine residues at N terminus, and a predicted GPI modification site at C terminus. BcCFEM1 was significantly up-regulated during early stage of infection on bean leaves and induced chlorosis in Nicotiana benthamiana leaves using Agrobacterium infiltration method. Targeted deletion of BcCFEM1 in B. cinerea affected virulence, conidial production and stress tolerance, but not growth rate, conidial germination, colony morphology, and sclerotial formation. However, over expression of BcCFEM1 did not make any observable phenotype change. Therefore, our data suggested that BcCFEM1 contributes to virulence, conidial production, and stress tolerance. These findings further enhance our understanding on the sophisticated pathogenicity of B. cinerea beyond necrotrophic stage, highlighting the importance of CFEM protein to B. cinerea and other broad-host-range necrotrophic pathogens.

The Botrytis cinerea elicitor protein BcIEB1 interacts with the tobacco PR5-family protein osmotin and protects the fungus against its antifungal activity

The broad-range phytopathogenic fungus Botrytis cinerea secretes hundreds of proteins during infection of its plant hosts. One of these proteins, BcIEB1, is abundantly secreted and is able to elicit plant defenses, probably as a pathogen-associated molecular pattern, although its native function in B. cinerea biology remains unknown. Pull-down experiments designed to isolate the molecular target of BcIEB1 in tobacco resulted in the identification of osmotin, a pathogenesis-related protein of family 5 that shows antifungal activity. The expression of osmotin in Escherichia coli allowed the verification of the BcIEB1-osmotin interaction with pure proteins by pull-down and far Western blot experiments, as well as the confirmation of the activity of osmotin against B. cinerea. Interestingly, B. cinerea Δbcieb1 mutants are more susceptible than the wild-type to osmotin, and the external addition of pure BcIEB1 protects the Δbcieb1 mutants, as well as Saccharomyces cerevisiae, from the antifungal action of osmotin, thus pointing at PR5 inhibition as the primary native function of BcIEB1. The question of whether osmotin is also involved in the activation of plant defenses by BcIEB1 is also addressed, and the data suggest that osmotin does not participate in the elicitation process.