Ectopic expression of MgSM1, a Cerato-platanin family protein from Magnaporthe grisea, confers broad-spectrum disease resistance in Arabidopsis

Recent Advances in Effector Research of Magnaporthe oryzae

Recalcitrant rice blast disease is caused by Magnaporthe oryzae, which has a significant negative economic reverberation on crop productivity. In order to induce the disease onto the host, M. oryzae positively generates many types of small secreted proteins, here named as effectors, to manipulate the host cell for the purpose of stimulating pathogenic infection. In M. oryzae, by engaging with specific receptors on the cell surface, effectors activate signaling channels which control an array of cellular activities, such as proliferation, differentiation and apoptosis. The most recent research on effector identification, classification, function, secretion, and control mechanism has been compiled in this review. In addition, the article also discusses directions and challenges for future research into an effector in M. oryzae.

Analysis of post-translational modification dynamics unveiled novel insights into Rice responses to MSP1

Magnaporthe oryzae snodprot1 homologous protein (MSP1) is known to function as a pathogen-associated molecular pattern (PAMP) and trigger PAMP-triggered immunity (PTI) in rice including induction of programmed cell death and expression of defense-related genes. The involvement of several post-translational modifications (PTMs) in the regulation of plant immune response, especially PTI, is well established, however, the information on the regulatory roles of these PTMs in response to MSP1-induced signaling is currently elusive. Here, we report the phosphoproteome, ubiquitinome, and acetylproteome to investigate the MSP1-induced PTMs alterations in MSP1 overexpressed and wild-type rice. Our analysis identified a total of 4666 PTMs-modified sites in rice leaves including 4292 phosphosites, 189 ubiquitin sites, and 185 acetylation sites. Among these, the PTM status of 437 phosphorylated, 53 ubiquitinated, and 68 acetylated peptides was significantly changed by MSP1. Functional annotation of MSP1 modulated peptides by MapMan analysis revealed that these were majorly associated with cellular immune responses including signaling, transcription factors, DNA and RNA regulation, and protein metabolism, among others. Taken together, our study provides novel insights into post-translational mediated regulation of rice proteins in response to M. oryzae secreted PAMP which help in understanding the molecular mechanism of MSP1-induced signaling in rice in greater detail. SIGNIFICANCE: The research investigates the effect of overexpression of MSP1 protein in rice leaves on the phosphoproteome, acetylome, and ubiquitinome. The study found that MSP1 is involved in rice protein phosphorylation, particularly in signaling pathways, and identified a key component, PTAC16, in MSP1-induced signaling. The analysis also revealed MSP1's role in protein degradation and modification by inducing ubiquitination of the target rice proteins. The research identified potential kinases involved in the phosphorylation of rice proteins, including casein kinase II, 14-3-3 domain binding motif, β-adrenergic receptor kinase, ERK1,2 kinase substrate motif, and casein kinase I motifs. Overall, the findings provide insights into the molecular mechanisms underlying of MSP1 induced signaling in rice which may have implications for improving crop yield and quality.

Expression of EPL1 from Trichoderma atroviride in Arabidopsis Confers Resistance to Bacterial and Fungal Pathogens

During plant interaction with beneficial microorganisms, fungi secrete a battery of elicitors that trigger plant defenses against pathogenic microorganisms. Among the elicitor molecules secreted by Trichoderma are cerato-platanin proteins, such as EPL1, from Trichoderma atroviride. In this study, Arabidopsis thaliana plants that express the TaEPL1 gene were challenged with phytopathogens to evaluate whether expression of EPL1 confers increased resistance to the bacterial pathogen Pseudomonas syringae and the necrotrophic fungus Botrytis cinerea. Infection assays showed that Arabidopsis EPL1-2, EPL1-3, EPL1-4 expressing lines were more resistant to both pathogens in comparison to WT plants. After Pseudomonas syringae infection, there were reduced disease symptoms (e.g., small chlorotic spots) and low bacterial titers in the three 35S::TaEPL1 expression lines. Similarly; 35S::TaEPL1 expression lines were more resistant to Botrytis cinerea infection, showing smaller lesion size in comparison to WT. Interestingly, an increase in ROS levels was detected in 35S::TaEPL1 expression lines when compared to WT. A higher expression of SA- and JA-response genes occurred in the 35S::TaEPL1 lines, which could explain the resistance of these EPL1 expression lines to both pathogens. We propose that EPL1 is an excellent elicitor, which can be used to generate crops with improved resistance to broad-spectrum diseases.

Fusarium oxysporum f. sp. niveum Pumilio 1 Regulates Virulence on Watermelon through Interacting with the ARP2/3 Complex and Binding to an A-Rich Motif in the 3' UTR of Diverse Transcripts

Fusarium oxysporum f. sp. niveum (Fon), a soilborne phytopathogenic fungus, causes watermelon Fusarium wilt, resulting in serious yield losses worldwide. However, the underlying molecular mechanism of Fon virulence is largely unknown. The present study investigated the biological functions of six FonPUFs, encoding RNA binding Pumilio proteins, and especially explored the molecular mechanism of FonPUF1 in Fon virulence. A series of phenotypic analyses indicated that FonPUFs have distinct but diverse functions in vegetative growth, asexual reproduction, macroconidia morphology, spore germination, cell wall, or abiotic stress response of Fon. Notably, the deletion of FonPUF1 attenuates Fon virulence by impairing the invasive growth and colonization ability inside the watermelon plants. FonPUF1 possesses RNA binding activity, and its biochemical activity and virulence function depend on the RNA recognition motif or Pumilio domains. FonPUF1 associates with the actin-related protein 2/3 (ARP2/3) complex by interacting with FonARC18, which is also required for Fon virulence and plays an important role in regulating mitochondrial functions, such as ATP generation and reactive oxygen species production. Transcriptomic profiling of ΔFonPUF1 identified a set of putative FonPUF1-dependent virulence-related genes in Fon, possessing a novel A-rich binding motif in the 3' untranslated region (UTR), indicating that FonPUF1 participates in additional mechanisms critical for Fon virulence. These findings highlight the functions and molecular mechanism of FonPUFs in Fon virulence. IMPORTANCE Fusarium oxysporum is a devastating plant-pathogenic fungus that causes vascular wilt disease in many economically important crops, including watermelon, worldwide. F. oxysporum f. sp. nievum (Fon) causes serious yield loss in watermelon production. However, the molecular mechanism of Fusarium wilt development by Fon remains largely unknown. Here, we demonstrate that six putative Pumilio proteins-encoding genes (FonPUFs) differentially operate diverse basic biological processes, including stress response, and that FonPUF1 is required for Fon virulence. Notably, FonPUF1 possesses RNA binding activity and associates with the actin-related protein 2/3 complex to control mitochondrial functions. Furthermore, FonPUF1 coordinates the expression of a set of putative virulence-related genes in Fon by binding to a novel A-rich motif present in the 3' UTR of a diverse set of target mRNAs. Our study disentangles the previously unexplored molecular mechanism involved in regulating Fon virulence, providing a possibility for the development of novel strategies for disease management.

TMT-based quantitative membrane proteomics identified PRRs potentially involved in the perception of MSP1 in rice leaves

Pathogen-associated molecular patterns (PAMPs) play a key role in triggering PAMPs triggered immunity (PTI) in plants. In the case of the rice-Magnaporthe oryzae pathosystem, fewer PAMPs and their pattern recognition receptors (PRRs) have been characterized. Recently, a M. oryzae snodprot1 homolog protein (MSP1) has been identified that functions as PAMP and triggering the PTI responses in rice. However, the molecular mechanism underlying MSP1-induced PTI is currently elusive. Therefore, we generated MSP1 overexpressed transgenic lines of rice, and a tandem mass tag (TMT)-based quantitative membrane proteomic analysis was employed to decipher the potential MSP1-induced signaling in rice using total cytosolic as well as membrane protein fractions. This approach led to the identification of 8033 proteins of which 1826 were differentially modulated in response to overexpression of MSP1 and/or exogenous jasmonic acid treatment. Of these, 20 plasma membrane-localized receptor-like kinases (RLKs) showed increased abundance in MSP1 overexpression lines. Moreover, activation of proteins related to the protein degradation and modification, calcium signaling, redox, and MAPK signaling was observed in transgenic lines expressing MSP1 in the apoplast. Taken together, our results identified potential PRR candidates involved in MSP1 recognition and suggested the overview mechanism of the MSP1-induced PTI signaling in rice leaves. SIGNIFICANCE: In plants, recognition of pathogen pathogen-derived molecules, such as PAMPs, by plant plant-derived PRRs has an essential role for in the activation of PTI against pathogen invasion. Typically, PAMPs are recognized by plasma membrane (PM) localized PRRs, however, identifying the PM-localized PRR proteins is challenging due to their low abundance. In this study, we performed an integrated membrane protein enrichment by microsomal membrane extraction (MME) method and subsequent TMT-labeling-based quantitative proteomic analysis using MSP1 overexpressed rice. Based on these results, we successfully identified various intracellular and membrane membrane-localized proteins that participated in the MSP1-induced immune response and characterized the potential PM-localized PRR candidates in rice.

Comparative Proteomic Analysis of Rhizoctonia solani Isolates Identifies the Differentially Expressed Proteins with Roles in Virulence

Sheath blight of rice is a destructive disease that could be calamitous to rice cultivation. The significant objective of this study is to contemplate the proteomic analysis of the high virulent and less virulent isolate of Rhizoctonia solani using a quantitative LC-MS/MS-based proteomic approach to identify the differentially expressed proteins promoting higher virulence. Across several rice-growing regions in Odisha, Eastern India, 58 Rhizoctonia isolates were obtained. All the isolates varied in their pathogenicity. The isolate RS15 was found to be the most virulent and RS22 was identified as the least virulent. The PCR amplification confirmed that the RS15 and RS22 belonged to the Rhizoctonia subgroup of AG1-IA with a specific primer. The proteomic information generated has been deposited in the PRIDE database with PXD023430. The virulent isolate consisted of 48 differentially abundant proteins, out of which 27 proteins had higher abundance, while 21 proteins had lower abundance. The analyzed proteins acquired functionality in fungal development, sporulation, morphology, pathogenicity, detoxification, antifungal activity, essential metabolism and transcriptional activities, protein biosynthesis, glycolysis, phosphorylation and catalytic activities in fungi. A Quantitative Real-Time PCR (qRT-PCR) was used to validate changes in differentially expressed proteins at the mRNA level for selected genes. The abundances of proteins and transcripts were positively correlated. This study provides the role of the proteome in the pathogenicity of R. solani AG1-IA in rice and underpins the mechanism behind the pathogen’s virulence in causing sheath blight disease.

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.

Computational Structural Genomics Unravels Common Folds and Novel Families in the Secretome of Fungal Phytopathogen Magnaporthe oryzae

Structural biology has the potential to illuminate the evolution of pathogen effectors and their commonalities that cannot be readily detected at the primary sequence level. Recent breakthroughs in protein structure modeling have demonstrated the feasibility to predict the protein folds without depending on homologous templates. These advances enabled a genome-wide computational structural biology approach to help understand proteins based on their predicted folds. In this study, we employed structure prediction methods on the secretome of the destructive fungal pathogen Magnaporthe oryzae. Out of 1,854 secreted proteins, we predicted the folds of 1,295 proteins (70%). We showed that template-free modeling by TrRosetta captured 514 folds missed by homology modeling, including many known effectors and virulence factors, and that TrRosetta generally produced higher quality models for secreted proteins. Along with sensitive homology search, we employed structure-based clustering, defining not only homologous groups with divergent members but also sequence-unrelated structurally analogous groups. We demonstrate that this approach can reveal new putative members of structurally similar MAX effectors and novel analogous effector families present in M. oryzae and possibly in other phytopathogens. We also investigated the evolution of expanded putative ADP-ribose transferases with predicted structures. We suggest that the loss of catalytic activities of the enzymes might have led them to new evolutionary trajectories to be specialized as protein binders. Collectively, we propose that computational structural genomics approaches can be an integral part of studying effector biology and provide valuable resources that were inaccessible before the advent of machine learning-based structure prediction.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

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.

Cell wall-associated effectors of plant-colonizing fungi

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

Expansin-related proteins: biology, microbe-plant interactions and associated plant-defense responses

Expansins, cerato-platanins and swollenins (which we will henceforth refer to as expansin-related proteins) are a group of microbial proteins involved in microbe-plant interactions. Although they share very low sequence similarity, some of their composing domains are near-identical at the structural level. Expansin-related proteins have their target in the plant cell wall, in which they act through a non-enzymatic, but still uncharacterized, mechanism. In most cases, mutagenesis of expansin-related genes affects plant colonization or plant pathogenesis of different bacterial and fungal species, and thus, in many cases they are considered virulence factors. Additionally, plant treatment with expansin-related proteins activate several plant defenses resulting in the priming and protection towards subsequent pathogen encounters. Plant-defence responses induced by these proteins are reminiscent of pattern-triggered immunity or hypersensitive response in some cases. Plant immunity to expansin-related proteins could be caused by the following: (i) protein detection by specific host-cell receptors, (ii) alterations to the cell-wall-barrier properties sensed by the host, (iii) displacement of cell-wall polysaccharides detected by the host. Expansin-related proteins may also target polysaccharides on the wall of the microbes that produced them under certain physiological instances. Here, we review biochemical, evolutionary and biological aspects of these relatively understudied proteins and different immune responses they induce in plant hosts.

Partitioning the structural features that underlie expansin-like and elicitor activities of cerato-platanin

Cerato-platanin family (CPF) proteins are produced by fungi and elicit defences when applied to plants, behaving as PAMPs/MAMPs. CPF proteins share structural similarity to plant and bacterial expansins, and have been demonstrated, in some cases, to possess expansin-like loosening activity on cellulose. This is the case of cerato-platanin (CP), the founder of the CPF, which shows both eliciting and cellulose-loosening activities, raising the question as to whether the expansin-like activity may be responsible for defence activation. To pinpoint structural and thermodynamic features underlying eliciting and expansin-like activity of CP, we carried out site-directed mutagenesis targeting separately net charge (N84D mutation), conformational stability (V63A mutation), or conserved position previously shown to affect expansin-like activity in CP (D77A mutation), and characterized wild-type protein and its variants. Removing or adding negative charges on the protein surface led to reducing or increasing, respectively, the expansin-like activity. The activity was instead not affected by mutations affecting protein fold and stability. In contrast, all the mutants showed reduced capacity to elicit defences in plants. We conclude that the expansin-like activity of CP depends on net charge and ability to (weakly) bind cellulose, whereas the eliciting activity on plants does not depend on the cellulose-loosening activity.

Characterization of cmcp Gene as a Pathogenicity Factor of Ceratocystis manginecans

Ceratocystis manginecans causes mango wilt with significant economic losses. In the infection court, cerato-platanin (CP) family proteins (CPPs) are believed to involve in pathogenesis but has not been determined in C. manginecans. To confirm this function, a CP protein (CmCP) of C. manginecans was characterized in this study. A protoplast of C. manginecans was prepared by treating its mycelia with driselase and lysing enzymes. The cmcp gene was edited using CRISPR/Cas-U6-1 expression vectors in 60% PEG and 50 μg/mL hygromycin B in the medium, resulting in mutants with cmcp deletion (Δcmcp). A complemented mutant (Δcmcp-C) was obtained by transforming cmcp to Δcmcp. Both Δcmcp and Δcmcp-C were characterized by comparing them with a wild-type strain on morphology, mycelial growth, conidial production and pathogenicity. Additionally, cmcp was transformed and expressed in Pichia pastoris, and the derived recombinant protein CmCP caused a severe necrosis on Nicotiana tabacum leaves. CmCP-treated plant leaves showed symptoms of hypersensitive response including electrolyte leakage, reactive oxygen species generation and overexpression of defense-related genes PR-1, PAD3, ERF1, HSR203J, and HIN1. All those results suggested that cmcp gene was required for the growth development of C. manginecans and functioned as a major pathogenicity factor in mango infection.

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.

A Cerato-Platanin Family Protein FocCP1 Is Essential for the Penetration and Virulence of Fusarium oxysporum f. sp. cubense Tropical Race 4

Fusarium oxysporum f. sp. cubense tropical race 4 (Foc TR4) is well-known as the causal agent of Fusarium wilt of banana and is one of the most destructive phytopathogens for banana plants. The molecular mechanisms underlying Foc TR4 virulence remain elusive. Here, we demonstrate that a cerato-platanin (CP) protein, FocCP1, functions as a virulence factor that is required by Foc TR4 for penetration and full virulence. The FocCP1 gene was expressed in every condition studied, showing a high transcript level in planta at the early stage of infection. Infiltration of the recombinant FocCP1 protein induced significant cell death and upregulated defence-related gene expression. FocCP1 knock-out strains showed a significant decrease in aerial growth rather than aqueous growth, which is reminiscent of hydrophobins. Furthermore, deletion of FocCP1 significantly reduced virulence and dramatically reduced infective growth in banana roots, likely resulting from a defective penetration ability. Taken together, the results of this study provide novel insight into the function of the recently identified FocCP1 as a virulence factor in Foc TR4.

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.

Recent Advances in Mechanisms of Plant Defense to Sclerotinia sclerotiorum

Sclerotinia sclerotiorum (Lib.) de Bary is an unusual pathogen which has the broad host range, diverse infection modes, and potential double feeding lifestyles of both biotroph and necrotroph. It is capable of infecting over 400 plant species found worldwide and more than 60 names have agriculturally been used to refer to diseases caused by this pathogen. Plant defense to S. sclerotiorum is a complex biological process and exhibits a typical quantitative disease resistance (QDR) response. Recent studies using Arabidopsis thaliana and crop plants have obtained new advances in mechanisms used by plants to cope with S. sclerotiorum infection. In this review, we focused on our current understanding on plant defense mechanisms against this pathogen, and set up a model for the defense process including three stages: recognition of this pathogen, signal transduction and defense response. We also have a particular interest in defense signaling mediated by diverse signaling molecules. We highlight the current challenges and unanswered questions in both the defense process and defense signaling. Essentially, we discussed candidate resistance genes newly mapped by using high-throughput experiments in important crops, and classified these potential gene targets into different stages of the defense process, which will broaden our understanding of the genetic architecture underlying quantitative resistance to S. sclerotiorum. We proposed that more powerful mapping population(s) will be required for accurate and reliable QDR gene identification.

Transcriptomic Analysis of Oryza sativa Leaves Reveals Key Changes in Response to Magnaporthe oryzae MSP1

Rice blast disease, caused by Magnaporthe oryzae, results in an extensive loss of rice productivity. Previously, we identified a novel M. oryzae secreted protein, termed MSP1 which causes cell death and pathogen-associated molecular pattern (PAMP)-triggered immune (PTI) responses in rice. Here, we report the transcriptome profile of MSP1-induced response in rice, which led to the identification of 21,619 genes, among which 4,386 showed significant changes (P < 0.05 and fold change > 2 or < 1/2) in response to exogenous MSP1 treatment. Functional annotation of differentially regulated genes showed that the suppressed genes were deeply associated with photosynthesis, secondary metabolism, lipid synthesis, and protein synthesis, while the induced genes were involved in lipid degradation, protein degradation, and signaling. Moreover, expression of genes encoding receptor-like kinases, MAPKs, WRKYs, hormone signaling proteins and pathogenesis-related (PR) proteins were also induced by MSP1. Mapping these differentially expressed genes onto various pathways revealed critical information about the MSP1-triggered responses, providing new insights into the molecular mechanism and components of MSP1-triggered PTI responses in rice.

CfPDIP1, a novel secreted protein of Colletotrichum falcatum, elicits defense responses in sugarcane and triggers hypersensitive response in tobacco

Colletotrichum falcatum, a hemibiotrophic fungal pathogen, causes one of the major devastating diseases of sugarcane-red rot. C. falcatum secretes a plethora of molecular signatures that might play a crucial role during its interaction with sugarcane. Here, we report the purification and characterization of a novel secreted protein of C. falcatum that elicits defense responses in sugarcane and triggers hypersensitive response (HR) in tobacco. The novel protein purified from the culture filtrate of C. falcatum was identified by MALDI TOF/TOF MS and designated as C. falcatum plant defense-inducing protein 1 (CfPDIP1). Temporal transcriptional profiling showed that the level of CfPDIP1 expression was greater in incompatible interaction than the compatible interaction until 120 h post-inoculation (hpi). EffectorP, an in silico tool, has predicted CfPDIP1 as a potential effector. Functional characterization of full length and two other domain deletional variants (CfPDIP1ΔN1-21 and CfPDIP1ΔN1-45) of recombinant CfPDIP1 proteins has indicated that CfPDIP1ΔN1-21 variant elicited rapid alkalinization and induced a relatively higher production of hydrogen peroxide (H2O2) in sugarcane suspension culture. However, in Nicotiana tabacum, all the three forms of recombinant CfPDIP1 proteins triggered HR along with the induction of H2O2 production and callose deposition. Further characterization using detached leaf bioassay in sugarcane revealed that foliar priming with CfPDIP1∆1-21 has suppressed the extent of lesion development, even though the co-infiltration of CfPDIP1∆1-21 with C. falcatum on unprimed leaves increased the extent of lesion development than control. Besides, the foliar priming has induced systemic expression of major defense-related genes with the concomitant reduction of pathogen biomass and thereby suppression of red rot severity in sugarcane. Comprehensively, the results have suggested that the novel protein, CfPDIP1, has the potential to trigger a multitude of defense responses in sugarcane and tobacco upon priming and might play a potential role during plant-pathogen interactions.

A proteomic insight into the MSP1 and flg22 induced signaling in Oryza sativa leaves

Previously, we reported a novel Magnaporthe oryzae- secreted protein MSP1, which triggers cell death and pathogen-associated molecular pattern (PAMP)-triggered immune (PTI) responses in rice. To investigate the MSP1 induced defense response in rice at the protein level, we employed a label-free quantitative proteomic approach, in parallel with flg22 treatment, which is a well-known elicitor. Exogenous application of MSP1 to rice leaves induced an oxidative burst, MAPK3/6 activation, and activation of pathogenesis-related genes (DUF26, PBZ, and PR-10). MaxQuant based label free proteome analysis led to the identification of 4167 protein groups of which 433 showed significant differences in response to MSP1 and/or flg22 treatment. Functional annotation of the differential proteins showed that majority of the proteins related to primary, secondary, and lipid metabolism were decreased, while proteins associated mainly with the stress response, post-translational modification and signaling were increased in abundance. Moreover, several peroxidases and receptor kinases were induced by both the elicitors, highlighting their involvement in MSP1 and flg22 induced signaling in rice. Taken together, the results reported here contribute to our understanding of MSP1 and flg22 triggered immune responses at the proteome level, thereby increasing our overall understanding of PTI signaling in rice. BIOLOGICAL SIGNIFICANCE: MSP1 is a M. oryzae secreted protein, which triggers defense responses in rice. Previous reports have shown that MSP1 is required for the pathogenicity of rice blast fungus, however, the exact mechanism of its action and its downstream targets in rice are currently unknown. Identification of the downstream targets is required in order to understand the MSP1 induced signaling in rice. Moreover, key proteins identified could also serve as potential candidates for the generation of disease resistance crops by modulating stress signaling pathways. Therefore, here we employed, for the first time, a label-free quantitative proteomic approach to investigate the MSP1 induced signaling in rice together with flg22. Functional annotation of the differential proteins showed that majority of the proteins related to primary, secondary, and lipid metabolism were decreased, while proteins related to the defense response, signaling and ROS detoxification were majorly increased. Thus, as an elicitor, recombinant MSP1 proteins could be utilized to inducing broad pathogen resistance in crops by priming the local immune responses.

Comprehensive analysis of Verticillium nonalfalfae in silico secretome uncovers putative effector proteins expressed during hop invasion

The vascular plant pathogen Verticillium nonalfalfae causes Verticillium wilt in several important crops. VnaSSP4.2 was recently discovered as a V. nonalfalfae virulence effector protein in the xylem sap of infected hop. Here, we expanded our search for candidate secreted effector proteins (CSEPs) in the V. nonalfalfae predicted secretome using a bioinformatic pipeline built on V. nonalfalfae genome data, RNA-Seq and proteomic studies of the interaction with hop. The secretome, rich in carbohydrate active enzymes, proteases, redox proteins and proteins involved in secondary metabolism, cellular processing and signaling, includes 263 CSEPs. Several homologs of known fungal effectors (LysM, NLPs, Hce2, Cerato-platanins, Cyanovirin-N lectins, hydrophobins and CFEM domain containing proteins) and avirulence determinants in the PHI database (Avr-Pita1 and MgSM1) were found. The majority of CSEPs were non-annotated and were narrowed down to 44 top priority candidates based on their likelihood of being effectors. These were examined by spatio-temporal gene expression profiling of infected hop. Among the highest in planta expressed CSEPs, five deletion mutants were tested in pathogenicity assays. A deletion mutant of VnaUn.279, a lethal pathotype specific gene with sequence similarity to SAM-dependent methyltransferase (LaeA), had lower infectivity and showed highly reduced virulence, but no changes in morphology, fungal growth or conidiation were observed. Several putative secreted effector proteins that probably contribute to V. nonalfalfae colonization of hop were identified in this study. Among them, LaeA gene homolog was found to act as a potential novel virulence effector of V. nonalfalfae. The combined results will serve for future characterization of V. nonalfalfae effectors, which will advance our understanding of Verticillium wilt disease.

SsSm1, a Cerato-platanin family protein, is involved in the hyphal development and pathogenic process of Sclerotinia sclerotiorum

The filamentous fungus Sclerotinia sclerotiorum is an important plant pathogen with a worldwide distribution. It can infect a wide variety of plants, causing serious disease in many types of crops, such as rapeseed, sunflower and soybean. Sclerotinia stem rot caused by this fungus affects main crops and has led to great economic loss. Elicitors are a group of compounds that inspire the host plant to produce an immune response against invading pathogens. This study describes a protein that has high homology with the Trichoderma elicitor Sm1 and was found in the genome of S. sclerotiorum. We named this protein SsSm1. To determine whether this protein has an elicitor function like its homology protein, we constructed a heterologous expression vector for SsSm1 and expressed it in Escherichia coli. The protein of heterologous expression led to the formation of lesions in tobacco that closely resemble hypersensitive response lesions. Transient expression of the encoding gene of SsSm1 in tobacco leaves also caused hypersensitive response. Then, RNA silencing was used to identify the function of SsSm1. The hyphal growth and pathogenicity of silenced transformants were shown to be obviously lagging and branched abnormally. Transformants produced less infection cushions and deformed sclerotiorum. In addition, SsSm1 silencing caused weak tolerance to NaCl, sorbitol and SDS, and the sensitivity of mutants to carbendazim was also significantly decreased. Based on the above results, we speculate that this protein may be related to the development of hyphae, infection cushions and sclerotiorum, but the specific molecular mechanism needs to be studied further.

A cerato-platanin protein SsCP1 targets plant PR1 and contributes to virulence of Sclerotinia sclerotiorum

Cerato-platanin proteins (CPs), which are secreted by filamentous fungi, are phytotoxic to host plants, but their functions have not been well defined to date. Here we characterized a CP (SsCP1) from the necrotrophic phytopathogen Sclerotinia sclerotiorum. Sscp1 transcripts accumulated during plant infection, and deletion of Sscp1 significantly reduced virulence. SsCP1 could induce significant cell death when expressed in Nicotiana benthamiana. Using yeast two-hybrid, GST pull-down, co-immunoprecipitation and bimolecular florescence complementation, we found that SsCP1 interacts with PR1 in the apoplast to facilitate infection by S. sclerotiorum. Overexpressing PR1 enhanced resistance to the wild-type strain, but not to the Sscp1 knockout strain of S. sclerotiorum. Sscp1-expressing transgenic plants showed increased concentrations of salicylic acid (SA) and higher levels of resistance to several plant pathogens (namely Botrytis cinerea, Alternaria brassicicola and Golovinomyces orontii). Our results suggest that SsCP1 is important for virulence of S. sclerotiorum and that it can be recognized by plants to trigger plant defense responses. Our results also suggest that the SA signaling pathway is involved in CP-mediated plant defense .

Function, Discovery, and Exploitation of Plant Pattern Recognition Receptors for Broad-Spectrum Disease Resistance

Plants are constantly exposed to would-be pathogens and pests, and thus have a sophisticated immune system to ward off these threats, which otherwise can have devastating ecological and economic consequences on ecosystems and agriculture. Plants employ receptor kinases (RKs) and receptor-like proteins (RLPs) as pattern recognition receptors (PRRs) to monitor their apoplastic environment and detect non-self and damaged-self patterns as signs of potential danger. Plant PRRs contribute to both basal and non-host resistances, and treatment with pathogen-/microbe-associated molecular patterns (PAMPs/MAMPs) or damage-associated molecular patterns (DAMPs) recognized by plant PRRs induces both local and systemic immunity. Here, we comprehensively review known PAMPs/DAMPs recognized by plants as well as the plant PRRs described to date. In particular, we describe the different methods that can be used to identify PAMPs/DAMPs and PRRs. Finally, we emphasize the emerging biotechnological potential use of PRRs to improve broad-spectrum, and potentially durable, disease resistance in crops.

A new proteinaceous pathogen-associated molecular pattern (PAMP) identified in Ascomycete fungi induces cell death in Solanaceae

Pathogen-associated molecular patterns (PAMPs) are detected by plant pattern recognition receptors (PRRs), which gives rise to PAMP-triggered immunity (PTI). We characterized a novel fungal PAMP, Cell Death Inducing 1 (RcCDI1), identified in the Rhynchosporium commune transcriptome sampled at an early stage of barley (Hordeum vulgare) infection. The ability of RcCDI1 and its homologues from different fungal species to induce cell death in Nicotiana benthamiana was tested following agroinfiltration or infiltration of recombinant proteins produced by Pichia pastoris. Virus-induced gene silencing (VIGS) and transient expression of Phytophthora infestans effectors PiAVR3a and PexRD2 were used to assess the involvement of known components of PTI in N. benthamiana responses to RcCDI1. RcCDI1 was highly upregulated early during barley colonization with R. commune. RcCDI1 and its homologues from different fungal species, including Zymoseptoria tritici, Magnaporthe oryzae and Neurospora crassa, exhibited PAMP activity, inducing cell death in Solanaceae but not in other families of dicots or monocots. RcCDI1-triggered cell death was shown to require N. benthamiana Brassinosteroid insensitive 1-Associated Kinase 1 (NbBAK1), N. benthamiana suppressor of BIR1-1 (NbSOBIR1) and N. benthamiana SGT1 (NbSGT1), but was not suppressed by PiAVR3a or PexRD2. We report the identification of a novel Ascomycete PAMP, RcCDI1, recognized by Solanaceae but not by monocots, which activates cell death through a pathway that is distinct from that triggered by the oomycete PAMP INF1.

Overexpression of MoSM1, encoding for an immunity-inducing protein from Magnaporthe oryzae, in rice confers broad-spectrum resistance against fungal and bacterial diseases

Potential of MoSM1, encoding for a cerato-platanin protein from Magnaporthe oryzae, in improvement of rice disease resistance was examined. Transient expression of MoSM1 in rice leaves initiated hypersensitive response and upregulated expression of defense genes. When transiently expressed in tobacco leaves, MoSM1 targeted to plasma membrane. The MoSM1-overexpressing (MoSM1-OE) transgenic rice lines showed an improved resistance, as revealed by the reduced disease severity and decreased in planta pathogen growth, against 2 strains belonging to two different races of M. oryzae, causing blast disease, and against 2 strains of Xanthomonas oryzae pv. oryzae, causing bacterial leaf blight disease. However, no alteration in resistance to sheath blight disease was observed in MoSM1-OE lines. The MoSM1-OE plants contained elevated levels of salicylic acid (SA) and jasmonic acid (JA) and constitutively activated the expression of SA and JA signaling-related regulatory and defense genes. Furthermore, the MoSM1-OE plants had no effect on drought and salt stress tolerance and on grain yield. We conclude that MoSM1 confers a broad-spectrum resistance against different pathogens through modulating SA- and JA-mediated signaling pathways without any penalty on abiotic stress tolerance and grain yield, providing a promising potential for application of MoSM1 in improvement of disease resistance in crops.

The Verticillium dahliae SnodProt1-Like Protein VdCP1 Contributes to Virulence and Triggers the Plant Immune System

During pathogenic infection, hundreds of proteins that play vital roles in the Verticillium dahliae-host interaction are secreted. In this study, an integrated proteomic analysis of secreted V. dahliae proteins was performed, and a conserved secretory protein, designated VdCP1, was identified as a member of the SnodProt1 phytotoxin family. An expression analysis of the vdcp1 gene revealed that the transcript is present in every condition studied and displays elevated expression throughout the infection process. To investigate the natural role of VdCP1 in V. dahliae, two vdcp1 knockout mutants and their complementation strains were generated. Bioassays of these mutants revealed no obvious phenotypic differences from the wild-type (WT) in terms of mycelial growth, conidial production or mycelial/spore morphology. However, compared with the WT, the vdcp1 knockout mutants displayed attenuated pathogenicity in cotton plants. Furthermore, treating plants with purified recombinant VdCP1 protein expressed in Pichia pastoris induced the accumulation of reactive oxygen species (ROS), expression of several defense-related genes, leakage of ion electrolytes, enhancement of defense-related enzyme activity and production of salicylic acid. Moreover, VdCP1 conferred resistance to Botrytis cinerea and Pseudomonas syringae pv. tabaci in tobacco and to V. dahliae in cotton. Further research revealed that VdCP1 possesses chitin-binding properties and that the growth of vdcp1 knockout mutants was more affected by treatments with chitinase, indicating that VdCP1 could protect V. dahliae cell wall from enzymatic degradation, which suggests an effector role of VdCP1 in infecting hosts.

Magnaporthe oryzae-Secreted Protein MSP1 Induces Cell Death and Elicits Defense Responses in Rice

The Magnaporthe oryzae snodprot1 homolog (MSP1), secreted by M. oryzae, is a cerato-platanin family protein. msp1-knockout mutants have reduced virulence on barley leaves, indicating that MSP1 is required for the pathogenicity of rice blast fungus. To investigate the functional roles of MSP1 and its downstream signaling in rice, recombinant MSP1 was produced in Escherichia coli and was assayed for its functionality. Application of MSP1 triggered cell death and elicited defense responses in rice. MSP1 also induced H2O2 production and autophagic cell death in both suspension-cultured cells and rice leaves. One or more protein kinases triggered cell death, jasmonic acid and abscisic acid enhanced cell death, while salicylic acid suppressed it. We demonstrated that the secretion of MSP1 into the apoplast is a prerequisite for triggering cell death and activating defense-related gene expression. Furthermore, pretreatment of rice with a sublethal MSP1 concentration potentiated resistance to the pathogen. Taken together, our results showed that MSP1 induces a high degree of cell death in plants, which might be essential for its virulence. Moreover, rice can recognize MSP1, resulting in the induction of pathogen-associated molecular pattern-triggered immunity.

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.

Changes in the Proteome of Xylem Sap in Brassica oleracea in Response to Fusarium oxysporum Stress

Fusarium oxysporum f.sp. conlutinans (Foc) is a serious root-invading and xylem-colonizing fungus that causes yellowing in Brassica oleracea. To comprehensively understand the interaction between F. oxysporum and B. oleracea, composition of the xylem sap proteome of the non-infected and Foc-infected plants was investigated in both resistant and susceptible cultivars using liquid chromatography-tandem mass spectrometry (LC-MS/MS) after in-solution digestion of xylem sap proteins. Whole genome sequencing of Foc was carried out and generated a predicted Foc protein database. The predicted Foc protein database was then combined with the public B. oleracea and B. rapa protein databases downloaded from Uniprot and used for protein identification. About 200 plant proteins were identified in the xylem sap of susceptible and resistant plants. Comparison between the non-infected and Foc-infected samples revealed that Foc infection causes changes to the protein composition in B. oleracea xylem sap where repressed proteins accounted for a greater proportion than those of induced in both the susceptible and resistant reactions. The analysis on the proteins with concentration change > = 2-fold indicated a large portion of up- and down-regulated proteins were those acting on carbohydrates. Proteins with leucine-rich repeats and legume lectin domains were mainly induced in both resistant and susceptible system, so was the case of thaumatins. Twenty-five Foc proteins were identified in the infected xylem sap and 10 of them were cysteine-containing secreted small proteins that are good candidates for virulence and/or avirulence effectors. The findings of differential response of protein contents in the xylem sap between the non-infected and Foc-infected samples as well as the Foc candidate effectors secreted in xylem provide valuable insights into B. oleracea-Foc interactions.

Effector discovery in the fungal wheat pathogen Zymoseptoria tritici

Fungal plant pathogens, such as Zymoseptoria tritici (formerly known as Mycosphaerella graminicola), secrete repertoires of effectors to facilitate infection or trigger host defence mechanisms. The discovery and functional characterization of effectors provides valuable knowledge that can contribute to the design of new and effective disease management strategies. Here, we combined bioinformatics approaches with expression profiling during pathogenesis to identify candidate effectors of Z. tritici. In addition, a genetic approach was conducted to map quantitative trait loci (QTLs) carrying putative effectors, enabling the validation of both complementary strategies for effector discovery. In planta expression profiling revealed that candidate effectors were up-regulated in successive waves corresponding to consecutive stages of pathogenesis, contrary to candidates identified by QTL mapping that were, overall, expressed at low levels. Functional analyses of two top candidate effectors (SSP15 and SSP18) showed their dispensability for Z. tritici pathogenesis. These analyses reveal that generally adopted criteria, such as protein size, cysteine residues and expression during pathogenesis, may preclude an unbiased effector discovery. Indeed, genetic mapping of genomic regions involved in specificity render alternative effector candidates that do not match the aforementioned criteria, but should nevertheless be considered as promising new leads for effectors that are crucial for the Z. tritici-wheat pathosystem.

Beyond plant defense: insights on the potential of salicylic and methylsalicylic acid to contain growth of the phytopathogen Botrytis cinerea

Using Botrytis cinerea we confirmed in the present work several previous studies showing that salicylic acid, a main plant hormone, inhibits fungal growth in vitro. Such an inhibitory effect was also observed for the two salicylic acid derivatives, methylsalicylic and acetylsalicylic acid. In marked contrast, 5-sulfosalicylic acid was totally inactive. Comparative proteomics from treated vs. control mycelia showed that both the intracellular and extracellular proteomes were affected in the presence of salicylic acid or methylsalicylic acid. These data suggest several mechanisms that could potentially account for the observed fungal growth inhibition, notably pH regulation, metal homeostasis, mitochondrial respiration, ROS accumulation and cell wall remodeling. The present observations support a role played by the phytohormone SA and derivatives in directly containing the pathogen. Data are available via ProteomeXchange with identifier PXD002873.

Elicitin-like proteins Oli-D1 and Oli-D2 from Pythium oligandrum trigger hypersensitive response in Nicotiana benthamiana and induce resistance against Botrytis cinerea in tomato

The biocontrol agent Pythium oligandrum and its elicitin-like proteins oligandrins have been shown to induce disease resistance in a range of plants. In the present study, the ability of two oligandrins, Oli-D1 and Oli-D2, to induce an immune response and the possible molecular mechanism regulating the defence responses in Nicotiana benthamiana and tomato were investigated. Infiltration of recombinant Oli-D1 and Oli-D2 proteins induced a typical immune response in N. benthamiana including the induction of a hypersensitive response (HR), accumulation of reactive oxygen species and production of autofluorescence. Agrobacterium-mediated transient expression assays revealed that full-length Oli-D1 and Oli-D2 were required for full HR-inducing activity in N. benthamiana, and virus-induced gene silencing-mediated knockdown of some of the signalling regulatory genes demonstrated that NbSGT1 and NbNPR1 were required for Oli-D1 and Oli-D2 to induce HR in N. benthamiana. Subcellular localization analyses indicated that both Oli-D1 and Oli-D2 were targeted to the plasma membrane of N. benthamiana. When infiltrated or transiently expressed in leaves, Oli-D1 and Oli-D2 induced resistance against Botrytis cinerea in tomato and activated the expression of a set of genes involved in the jasmonic acid/ethylene (JA/ET)-mediated signalling pathway. Our results demonstrate that Oli-D1 and Oli-D2 are effective elicitors capable of inducing immune responses in plants, probably through the JA/ET-mediated signalling pathway, and that both Oli-D1 and Oli-D2 have potential for the development of bioactive formulae for crop disease control in practice.

A paralog of the proteinaceous elicitor SM1 is involved in colonization of maize roots by Trichoderma virens

The biocontrol agent, Trichoderma virens, has the ability to protect plants from pathogens by eliciting plant defense responses, involvement in mycoparasitism, or secreting antagonistic secondary metabolites. SM1, an elicitor of induced systemic resistance (ISR), was found to have three paralogs within the T. virens genome. The paralog sm2 is highly expressed in the presence of plant roots. Gene deletion mutants of sm2 were generated and the mutants were found to overproduce SM1. The ability to elicit ISR in maize against Colletotrichum graminicola was not compromised for the mutants compared to that of wild type isolate. However, the deletion strains had a significantly lowered ability to colonize maize roots. This appears to be the first report on the involvement of an effector-like protein in colonization of roots by Trichoderma.

Sm2, a paralog of the Trichoderma cerato-platanin elicitor Sm1, is also highly important for plant protection conferred by the fungal-root interaction of Trichoderma with maize

Background

The proteins Sm1 and Sm2 from the biocontrol fungus Trichoderma virens belong to the cerato-platanin protein family. Members of this family are small, secreted proteins that are abundantly produced by filamentous fungi with all types of life-styles. Some species of the fungal genus Trichoderma are considered as biocontrol fungi because they are mycoparasites and are also able to directly interact with plants, thereby stimulating plant defense responses. It was previously shown that the cerato-platanin protein Sm1 from T. virens - and to a lesser extent its homologue Epl1 from Trichoderma atroviride - induce plant defense responses. The plant protection potential of other members of the cerato-platanin protein family in Trichoderma, however, has not yet been investigated.

Results

In order to analyze the function of the cerato-platanin protein Sm2, sm1 and sm2 knockout strains were generated and characterized. The effect of the lack of Sm1 and Sm2 in T. virens on inducing systemic resistance in maize seedlings, challenged with the plant pathogen Cochliobolus heterostrophus, was tested. These plant experiments were also performed with T. atroviride epl1 and epl2 knockout strains. In our plant-pathogen system T. virens was a more effective plant protectant than T. atroviride and the results with both Trichoderma species showed concordantly that the level of plant protection was more strongly reduced in plants treated with the sm2/epl2 knockout strains than with sm1/epl1 knockout strains.

Conclusions

Although the cerato-platanin genes sm1/epl1 are more abundantly expressed than sm2/epl2 during fungal growth, Sm2/Epl2 are, interestingly, more important than Sm1/Epl1 for the promotion of plant protection conferred by Trichoderma in the maize-C. heterostrophus pathosystem.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-014-0333-0) contains supplementary material, which is available to authorized users.

Molecular effects of resistance elicitors from biological origin and their potential for crop protection

Plants contain a sophisticated innate immune network to prevent pathogenic microbes from gaining access to nutrients and from colonizing internal structures. The first layer of inducible response is governed by the plant following the perception of microbe- or modified plant-derived molecules. As the perception of these molecules results in a plant response that can provide efficient resistance toward non-adapted pathogens they can also be described as "defense elicitors." In compatible plant/microbe interactions, adapted microorganisms have means to avoid or disable this resistance response and promote virulence. However, this requires a detailed spatial and temporal response from the invading pathogens. In agricultural practice, treating plants with isolated defense elicitors in the absence of pathogens can promote plant resistance by uncoupling defense activation from the effects of pathogen virulence determinants. The plant responses to plant, bacterial, oomycete, or fungal-derived elicitors are not, in all cases, universal and need elucidating prior to the application in agriculture. This review provides an overview of currently known elicitors of biological rather than synthetic origin and places their activity into a molecular context.

Cerato-platanins: elicitors and effectors

Cerato-platanins are an interesting group of small, secreted, cysteine-rich proteins that have been implicated in virulence of certain plant pathogenic fungi. The relatively recent discovery of these proteins in plant beneficial fungi like Trichoderma spp., and their positive role in induction of defense in plants against invading pathogens has raised the question as to whether these proteins are effectors or elicitor molecules. Here we present a comprehensive review on the occurrence of these conserved proteins across the fungal kingdom, their structure-function relationships, and their physiological roles in plant pathogenic and symbiotic fungi. We also discuss the usefulness of these proteins in evolving strategies for crop protection through a transgenic approach or direct application as elicitors.

A novel protein elicitor (SsCut) from Sclerotinia sclerotiorum induces multiple defense responses in plants

In this study, we report the cloning of the SsCut gene encoding cutinase from Sclerotinia sclerotiorum. We isolated a 609-bp cDNA encoding a polypeptide of 202 amino acids with a molecular weight of 20.4 kDa. Heterologous expression of SsCut in Escherichia coli (His-SsCut) caused the formation of lesions in tobacco that closely resembled hypersensitive response lesions. Mutational analysis identified the C-terminal-half peptide and the same amino acids indispensable for both enzyme and elicitor activity. His-SsCut was caused cell death in Arabidopsis, soybean (Glycine max), oilseed rape (Brassica napus), rice (Oryza sativa), maize (Zea mays), and wheat (Triticum aestivum), indicating that both dicot and monocot species are responsive to the elicitor. Furthermore, the elicitation of tobacco was effective in the induction of the activities of hydrogen peroxide, phenylalanine ammonia-lyase, peroxides, and polyphenol oxidase. His-SsCut-treated plants exhibited enhanced resistance as indicated by a significant reduction in the number and size of S. sclerotiorum, Phytophthora sojae, and P. nicotianae lesions on leaves relative to controls. Real-time PCR results indicated that the expression of defense-related genes and genes involved in signal transduction were induced by His-SsCut. Our results demonstrate that SsCut is an elicitor that triggers defense responses in plants and will help to clarify its relationship to downstream signaling pathways that induce defense responses.

Phylogeny and molecular dating of the cerato-platanin-encoding genes

The cerato-platanin family consists of proteins that can induce immune responses, cause necrosis, change chemotaxis and locomotion and may be related to the growth and development of various fungi. In this work, we analyzed the phylogenetic relationships among genes encoding members of the cerato-platanin family and computed the divergence times of the genes and corresponding fungi. The results showed that cerato-platanin-encoding genes could be classified into 10 groups but did not cluster according to fungal classes or their functions. The genes transferred horizontally and showed duplication. Molecular dating and adaptive evolution analyses indicated that the cerato-platanin gene originated with the appearance of saprophytes and that the gene was under positive selection. This finding suggests that cerato-platanin-encoding genes evolved with the development of fungal parasitic characteristics.

Cerato-platanin induces resistance in Arabidopsis leaves through stomatal perception, overexpression of salicylic acid- and ethylene-signalling genes and camalexin biosynthesis

Microbe-associated molecular patterns (MAMPs) lead to the activation of the first line of plant defence. Few fungal molecules are universally qualified as MAMPs, and proteins belonging to the cerato-platanin protein (CPP) family seem to possess these features. Cerato-platanin (CP) is the name-giving protein of the CPP family and is produced by Ceratocystis platani, the causal agent of the canker stain disease of plane trees (Platanus spp.). On plane tree leaves, the biological activity of CP has been widely studied. Once applied on the leaf surface, CP acts as an elicitor of defence responses. The molecular mechanism by which CP elicits leaves is still unknown, and the protective effect of CP against virulent pathogens has not been clearly demonstrated. In the present study, we tried to address these questions in the model plant Arabidopsis thaliana. Our results suggest that stomata rapidly sense CP since they responded to the treatment with ROS signalling and stomatal closure, and that CP triggers salicylic acid (SA)- and ethylene (ET)-signalling pathways, but not the jasmonic acid (JA)-signalling pathway, as revealed by the expression pattern of 20 marker genes. Among these, EDS1, PAD4, NPR1, GRX480, WRKY70, ACS6, ERF1a/b, COI1, MYC2, PDF1.2a and the pathogenesis-related (PR) genes 1–5. CP rapidly induced MAPK phosphorylation and induced the biosynthesis of camalexin within 12 hours following treatment. The induction of localised resistance was shown by a reduced susceptibility of the leaves to the infection with Botrytis cinerea and Pseudomonas syringae pv. tomato. These results contribute to elucidate the key steps of the signalling process underlying the resistance induction in plants by CP and point out the central role played by the stomata in this process.

ALY proteins participate in multifaceted Nep1Mo-triggered responses in Nicotiana benthamiana and Arabidopsis thaliana

Previously, it was found that Nep1Mo (a Nep1-like protein from Magnaporthe oryzae) could trigger a variety of plant responses, including stomatal closure, hypersensitive cell death (HCD), and defence-related gene expression, in Nicotiana benthamiana. In this study, it was found that Nep1Mo-induced cell death could be inhibited by the virus-induced gene silencing of NbALY916 in N. benthamiana. NbALY916-silenced plants showed impaired Nep1Mo-induced stomatal closure, decreased Nep1Mo-induced production of hydrogen peroxide (H2O2) and nitric oxide (NO) in guard cells, and reduced Nep1Mo-induced resistance against Phytophthora nicotianae. It also found that the deletion of AtALY4, an orthologue of NbALY916 in Arabidopsis thaliana, impaired Nep1Mo-triggered stomatal closure, HCD, and defence-related gene expression. The compromised stomatal closure observed in the NbALY916-silenced plants and AtALY4 mutants was inhibited by the application of H2O2 and sodium nitroprusside (an NO donor), and both Nep1Mo and H2O2 stimulated guard cell NO synthesis. Conversely, NO-induced stomatal closure was found not to require H2O2 synthesis; and NO treatment did not induce H2O2 production in guard cells. Taken together, these results demonstrate that the NbAlY916/AtAlY4-H2O2-NO pathway mediates multiple Nep1Mo-triggered responses, including stomatal closure, HCD, and defence-related gene expression.

The phytotoxic activity of the cerato-platanin BcSpl1 resides in a two-peptide motif on the protein surface

Cerato-platanin family proteins are secreted and have been found in both the fungal cell wall and the extracellular medium. They elicit defence responses in a variety of plants and have been proposed to be perceived as pathogen-associated molecular patterns (PAMPs) by the plant immune system, although, in the case of the necrotroph Botrytis cinerea, the cerato-platanin BcSpl1 contributes to fungal virulence instead of plant resistance. In this study, we report that BcSpl1, which was previously found in the secretome as an abundant protein, is even more abundant in the fungal cell wall. By fusion to green fluorescent protein (GFP), we also show that BcSpl1 associates with the plant plasma membrane causing rapid morphological changes at the cellular level, such as the disorganization of chloroplasts, prior to macroscopic necrosis in the treated tissue. By a combination of serial deletion studies, synthetic peptides and chimeric proteins, we mapped the eliciting activity to a two-peptide motif in the protein surface. The expression of a chimeric protein displaying this motif in B. cinerea mutants lacking BcSpl1 undoubtedly showed that the motif is responsible for the contribution of BcSpl1 to virulence.

Cerato-platanins: a fungal protein family with intriguing properties and application potential

Cerato-platanin proteins are small, secreted proteins with four conserved cysteines that are abundantly produced by filamentous fungi with all types of lifestyles. These proteins appear to be readily recognized by other organisms and are therefore important factors in interactions of fungi with other organisms, e.g. by stimulating the induction of defence responses in plants. However, it is not known yet whether the main function of cerato-platanin proteins is associated with these fungal interactions or rather a role in fungal growth and development. Cerato-platanin proteins seem to unify several biochemical properties that are not found in this combination in other proteins. On one hand, cerato-platanins are carbohydrate-binding proteins and are able to bind to chitin and N-acetylglucosamine oligosaccharides; on the other hand, they are able to self-assemble at hydrophobic/hydrophilic interfaces and form protein layers, e.g. on the surface of aqueous solutions, thereby altering the polarity of solutions and surfaces. The latter property is reminiscent of hydrophobins, which are also small, secreted fungal proteins, but interestingly, the surface-activity-altering properties of cerato-platanins are the opposite of what can be observed for hydrophobins. The so far known biochemical properties of cerato-platanin proteins are summarized in this review, and potential biotechnological applications as well as implications of these properties for the biological functions of cerato-platanin proteins are discussed.