New proteins orthologous to cerato-platanin in various Ceratocystis species and the purification and characterization of cerato-populin from Ceratocystis populicola

The secret password: Cell death-inducing proteins in filamentous phytopathogens - As versatile tools to develop disease-resistant crops

Cell death-inducing proteins (CDIPs) are some of the secreted effector proteins manifested by filamentous oomycetes and fungal pathogens to invade the plant tissue and facilitate infection. Along with their involvement in different developmental processes and virulence, CDIPs play a crucial role in plant-pathogen interactions. As the name implies, CDIPs cause necrosis and trigger localised cell death in the infected host tissues by the accumulation of higher concentrations of hydrogen peroxide (H2O2), oxidative burst, accumulation of nitric oxide (NO), and electrolyte leakage. They also stimulate the biosynthesis of defense-related phytohormones such as salicylic acid (SA), jasmonic acid (JA), abscisic acid (ABA), and ethylene (ET), as well as the expression of pathogenesis-related (PR) genes that are important in disease resistance. Altogether, the interactions result in the hypersensitive response (HR) in the host plant, which might confer systemic acquired resistance (SAR) in some cases against a vast array of related and unrelated pathogens. The CDIPs, due to their capability of inducing host resistance, are thus unique among the array of proteins secreted by filamentous plant pathogens. More interestingly, a few transgenic plant lines have also been developed expressing the CDIPs with added resistance. Thus, CDIPs have opened an interesting hot area of research. The present study critically reviews the current knowledge of major types of CDIPs identified across filamentous phytopathogens and their modes of action in the last couple of years. This review also highlights the recent breakthrough technologies in studying plant-pathogen interactions as well as crop improvement by enhancing disease resistance through CDIPs.

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.

A mechanistic model may explain the dissimilar biological efficiency of the fungal elicitors cerato-platanin and cerato-populin

Among their various functions, the members of the cerato-platanin family can stimulate plants' defense responses and induce resistance against microbial pathogens. Recent results suggest that conserved loops, also involved in chitin binding, might be a structural motif central for their eliciting activity. Here, we focus on cerato-platanin and its orthologous cerato-populin, searching for a rationale of their diverse efficiency to elicit plants' defense and to interact with oligosaccharides. A 3D model of cerato-populin has been generated by homology modeling using the NMR-derived cerato-platanin structure as template, and it has been validated by fitting with residual dipolar couplings. Loops β1-β2 and β2-β3 have been indicated as important for some CPPs members to express their biological function. When compared to cerato-platanin, in cerato-populin they present two mutations and an insertion that significantly modify their electrostatic surface. NMR relaxation experiments point to a reduced conformational plasticity of cerato-populin loops with respect to the ones of cerato-platanin. The different electrostatic surface of the loops combined with a distinct network of intra-molecular interactions are expected to be factors that, by leading to a diverse spatial organization and dissimilar collective motions, can regulate the eliciting efficacy of the two proteins and their affinity for oligosaccharides.

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 Colletotrichum gloeosporioides cerato-platanin protein, CgCP1, contributes to conidiation and plays roles in the interaction with rubber tree

Colletotrichum gloeosporioides is the causal agent of rubber tree anthracnose and leads to serious losses of natural rubber production. The pathogenesis of C. gloeosporioides on rubber tree remains unknown. Cerato-platanin proteins are small, secreted cysteine-rich proteins that contribute to virulence and function in plant–fungal interactions. A gene encoding cerato-platanin protein, CgCP1, was identified in C. gloeosporioides. In silico analysis indicated that CgCP1 belongs to a new branch of the cerato-platanin protein family. The CgCP1 knockout mutants (ΔCgCP1) and complementary strain (Res-ΔCgCP1) were generated to investigate its biological function. The results showed that the speed of growth of aerial hyphae was not significantly different among the wild-type (WT), ΔCgCP1, and Res-ΔCgCP1 strains, but conidiation of ΔCgCP1 significantly decreased in comparison with the WT. The pathogenicity test proved that the severity of symptoms caused by ΔCgCP1 was reduced significantly compared with those caused by the Res-ΔCgCP1 and WT strains. Additionally, CgCP1 induced necrosis-like cell death on tobacco leaf and accumulation of reactive oxygen species in rubber tree mesophyll protoplasts. Altogether, these data indicate the involvement of C. gloeosporioides CgCP1 in conidiation and the interaction with rubber tree.

IMA Genome-F 7: Draft genome sequences for Ceratocystis fagacearum, C. harringtonii, Grosmannia penicillata, and Huntiella bhutanensis

Draft genomes for the fungi Ceratocystis fagacearum, C. harringtonii, Grosmannia penicillata, and Huntiella bhutanensis are presented. Ceratocystis fagacearum is a major causal agent of vascular wilt of oaks and other trees in the family Fagaceae. Ceratocystis harringtonii, previously known as C. populicola, causes disease in Populus species in the USA and Canada. Grosmannia penicillata is the causal agent of bluestain of sapwood on various conifers, including Picea spp. and Pinus spp. in Europe. Huntiella bhutanensis is a fungus in Ceratocystidaceae and known only in association with the bark beetle Ips schmutzenhorferi that infests Picea spinulosa in Bhutan. The availability of these genomes will facilitate further studies on these fungi.

A cerato-platanin-like protein HaCPL2 from Heterobasidion annosum sensu stricto induces cell death in Nicotiana tabacum and Pinus sylvestris

The cerato-platanin family is a group of small secreted cysteine-rich proteins exclusive for filamentous fungi. They have been shown to be involved in the interactions between fungi and plants. Functional characterization of members from this family has been performed mainly in Ascomycota, except Moniliophthora perniciosa. Our previous phylogenetic analysis revealed that recent gene duplication of cerato-platanins has occurred in Basidiomycota but not in Ascomycota, suggesting higher functional diversification of this protein family in Basidiomycota than in Ascomycota. In this study, we identified three cerato-platanin homologues from the basidiomycete conifer pathogen Heterobasidion annosum sensu stricto. Expression of the homologues under various conditions as well as their roles in the H. annosum s.s.-Pinus sylvestris (Scots pine) pathosystem was investigated. Results showed that HaCPL2 (cerato-platanin-like protein 2) had the highest sequence similarity to cerato-platanin from Ceratocystis platani and hacpl2 was significantly induced during nutrient starvation and necrotrophic growth. The treatment with recombinant HaCPL2 induced cell death, phytoalexin production and defense gene expression in Nicotiana tabacum. Eliciting and cell death-inducing ability accompanied by retardation of apical root growth was also demonstrated in Scots pine seedlings. Our results suggest that HaCPL2 might contribute to the virulence of H. annosum s.s. by promoting plant cell death.

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.

¹H, ¹⁵N, and ¹³C resonance assignment of cerato-populin, a fungal PAMP from Ceratocystis populicola

Plant pathogenic fungi secrete several non-catalytic proteins involved in various aspects of the pathogenesis process. Amongst these, cerato-populin (Pop1) produced by Ceratocystis populicola; a protein orthologous of cerato-platanin (CP), the core member of the CP family. These two proteins interact with host and non-host plants. In plane leaves they induce synthesis of phytoalexins, disruption of intercellular and intracellular leaf tissue, cell plasmolysis, programmed cell death, over-expression of defence-related genes, H2O2 and NO production, activation of MAPK cascade and plant resistance. All these features point to CP and Pop1 as defence inducers, though Pop1 shows a reduced efficiency. Pop1/CP similarity is 73%. CD spectroscopy highlights some secondary structure differences between Pop1 and CP. Indeed, the region between the first two cysteines (C20-C57), that in CP includes the β2-strand and it is involved in GlcNAc (N-acetyl-D-glucosamine) interaction, in Pop1 is predicted to be fully disordered.

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.

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.

Functional diversification of cerato-platanins in Moniliophthora perniciosa as seen by differential expression and protein function specialization

Cerato-platanins (CP) are small, cysteine-rich fungal-secreted proteins involved in the various stages of the host-fungus interaction process, acting as phytotoxins, elicitors, and allergens. We identified 12 CP genes (MpCP1 to MpCP12) in the genome of Moniliophthora perniciosa, the causal agent of witches' broom disease in cacao, and showed that they present distinct expression profiles throughout fungal development and infection. We determined the X-ray crystal structures of MpCP1, MpCP2, MpCP3, and MpCP5, representative of different branches of a phylogenetic tree and expressed at different stages of the disease. Structure-based biochemistry, in combination with nuclear magnetic resonance and mass spectrometry, allowed us to define specialized capabilities regarding self-assembling and the direct binding to chitin and N-acetylglucosamine (NAG) tetramers, a fungal cell wall building block, and to map a previously unknown binding region in MpCP5. Moreover, fibers of MpCP2 were shown to act as expansin and facilitate basidiospore germination whereas soluble MpCP5 blocked NAG6-induced defense response. The correlation between these roles, the fungus life cycle, and its tug-of-war interaction with cacao plants is discussed.

Differential timing of defense-related responses induced by cerato-platanin and cerato-populin, two non-catalytic fungal elicitors

The cerato-platanin (CP) family consists of fungal-secreted proteins involved in various stages of the host-fungus interaction and acting as phytotoxins and elicitors of defense responses. The founder member of this family is CP, a non-catalytic protein with a six-stranded double-ψβ-barrel fold. Cerato-populin (Pop1) is an ortholog showing low sequence identity with CP. CP is secreted by Ceratocystis platani, the causal agent of the canker stain of plane. Pop1 is secreted by Ceratocystis populicola, a pathogen of poplar. CP and Pop1 have been suggested to act as PAMPs (pathogen-associated molecular patterns) because they induce phytoalexin synthesis, transcription of defense-related genes, restriction of conidia growth and cell death in various plants. Here, we treated plane leaves with CP or Pop1, and monitored defense responses to define the role of these elicitors in the plant interactions. Both CP and Pop1 were able to induce mitogen-activated protein kinases (MAPKs) phosphorylation, production of reactive oxygen species and nitric oxide, and overexpression of defense related genes. The characteristic DNA fragmentation and the cytological features indicate that CP and Pop1 induce cell death by a mechanism of programmed cell death. Therefore, CP and Pop1 can be considered as two novel, non-catalytic fungal PAMPs able to enhance primary defense. Of particular interest is the observation that CP showed faster activity compared to Pop1. The different timing in defense activation could potentially be due to the structural differences between CP and Pop1 (i.e. different hydrophobic index and different helix content) therefore constituting a starting point in unraveling their structure-function relationships.

BcSpl1, a cerato-platanin family protein, contributes to Botrytis cinerea virulence and elicits the hypersensitive response in the host

Proteins belonging to the cerato-platanin family are small proteins with phytotoxic activity. A member of this family, BcSpl1, is one of the most abundant proteins in the Botrytis cinerea secretome. Expression analysis of the bcspl1 gene revealed that the transcript is present in every condition studied, showing the highest level in planta at the late stages of infection. Expression of a second cerato-platanin gene found in the B. cinerea genome, bcspl2, was not detected in any condition. Two bcspl1 knock-out mutants were generated and both showed reduced virulence in a variety of hosts. • bcspl1 was expressed in Pichia pastoris and the recombinant protein was able to cause a fast and strong necrosis when infiltrated in tomato, tobacco and Arabidopsis leaves, in a dose-dependent manner. The BcSpl1-treated plant tissues showed symptoms of the hypersensitive response such as induction of reactive oxygen species, electrolyte leakage, cytoplasm shrinkage, and cell autofluorescence, as well as the induction of defense genes considered to be markers of the hypersensitive response. The Arabidopsis bak1 mutation partially prevented the induction of necrosis in this plant by BcSpl1. Two different BcSpl1-derived 40-amino acids peptides were also active in inducing necrosis.

The structure of the elicitor Cerato-platanin (CP), the first member of the CP fungal protein family, reveals a double ψβ-barrel fold and carbohydrate binding

Cerato-platanin (CP) is a secretion protein produced by the fungal pathogen Ceratocystis platani, the causal agent of the plane canker disease and the first member of the CP family. CP is considered a pathogen-associated molecular pattern because it induces various defense responses in the host, including production of phytoalexins and cell death. Although much is known about the properties of CP and related proteins as elicitors of plant defense mechanisms, its biochemical activity and host target(s) remain elusive. Here, we present the three-dimensional structure of CP. The protein, which exhibits a remarkable pH and thermal stability, has a double ψβ-barrel fold quite similar to those found in expansins, endoglucanases, and the plant defense protein barwin. Interestingly, although CP lacks lytic activity against a variety of carbohydrates, it binds oligosaccharides. We identified the CP region responsible for binding as a shallow surface located at one side of the β-barrel. Chemical shift perturbation of the protein amide protons, induced by oligo-N-acetylglucosamines of various size, showed that all the residues involved in oligosaccharide binding are conserved among the members of the CP family. Overall, the results suggest that CP might be involved in polysaccharide recognition and that the double ψβ-barrel fold is widespread in distantly related organisms, where it is often involved in host-microbe interactions.

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

Proteins belonging to the newly identified Cerato-platanin (CP) family have been shown to have elicitor activity in inducing disease resistance responses in various plants. In this study, we characterized a gene, MgSM1, from Magnaporthe grisea, encoding a putative small protein belonging to the CP family. MgSM1 was constitutively expressed not only in different fungal growth stages but also during its infection process in rice plants. Agrobacterium-mediated transient expression of MgSM1 in Arabidopsis resulted in hypersensitive response in the infiltrated local leaves and enhanced disease resistance against Botrytis cinerea and Pseudomonas syringae pv. tomato (Pst) DC3000 in upper leaves of plants, accompanyed by up-regulated expression of defense genes (PR-1, PR-5 and PDF1.2). Transgenic Arabidopsis plants expressing MgSM1 under control of a dexamethasone (DEX)-inducible promoter were generated. Expression of MgSM1 in transgenic plants was induced by exogenous application of DEX. MgSM1-expressing plants showed normal growth with application of <10 microm DEX. After DEX induction, the MgSM1-expressing plants showed enhanced disease resistance against B. cinerea, Alternaria brassicicola and Psto DC3000 as well as up-regulated expression of some of defense genes. Moreover, accumulation of reactive oxygen species was observed in MgSM1-expressing plants. These results collectively suggest that ectopic expression of MgSM1 in transgenic plants confers broad-spectrum resistance against different types of pathogens. Our study also provides a novel strategy to generate environment-friendly crops with enhanced broad-spectrum resistance through ectopic expression of microbe-derived disease resistance-inducing proteins.