Elicitor activity of a fungal endopolygalacturonase in tobacco requires a functional catalytic site and cell wall localization

Receptor kinase LecRK-I.9 regulates cell wall remodelling during lateral root formation in Arabidopsis

Assembling and remodelling the cell wall is essential for plant development. Cell wall dynamics are controlled by cell wall proteins, polysaccharide biosynthesis, and a variety of sensor and receptor systems. LecRK-I.9, an Arabidopsis thaliana plasma membrane-localized lectin receptor kinase, was previously shown to be involved in cell wall-plasma membrane contacts and to play roles in plant-pathogen interactions, but until now its role in development was not known. LecRK-I.9 is transcribed at a high level in root tissues including the pericycle. Comparative transcript profiling of a loss-of-function mutant versus the wild type identified LecRK-I.9 as a regulator of cell wall metabolism. Consistently, lecrk-I.9 mutants displayed an increased pectin methylesterification level correlated with decreased pectin methylesterase and increased polygalacturonase activities. Also, LecRK-I.9 negatively impacted lateral root development through the direct or indirect regulation of genes encoding (i) cell wall remodelling proteins during early events of lateral root initiation, and (ii) cell wall signalling peptides (CLE2 and CLE4) repressing lateral root emergence and growth. Furthermore, low nitrate reduced LecRK-I.9 expression in roots, particularly in the lateral root emergence zone: even in these conditions, the control of CLE2 and CLE4 expression is maintained. Altogether, the results show that LecRK-I.9 is a key player in negatively regulating both pre-branch site formation and lateral root emergence.

A plant mechanism of hijacking pathogen virulence factors to trigger innate immunity

Polygalacturonase-inhibiting proteins (PGIPs) interact with pathogen-derived polygalacturonases to inhibit their virulence-associated plant cell wall-degrading activity but stimulate immunity-inducing oligogalacturonide production. Here we show that interaction between Phaseolus vulgaris PGIP2 (PvPGIP2) and Fusarium phyllophilum polygalacturonase (FpPG) enhances substrate binding, resulting in inhibition of the enzyme activity of FpPG. This interaction promotes FpPG-catalyzed production of long-chain immunoactive oligogalacturonides, while diminishing immunosuppressive short oligogalacturonides. PvPGIP2 binding creates a substrate binding site on PvPGIP2-FpPG, forming a new polygalacturonase with boosted substrate binding activity and altered substrate preference. Structure-based engineering converts a putative PGIP that initially lacks FpPG-binding activity into an effective FpPG-interacting protein. These findings unveil a mechanism for plants to transform pathogen virulence activity into a defense trigger and provide proof of principle for engineering PGIPs with broader specificity.

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.

Expression of a Hyperthermophilic Cellobiohydrolase in Transgenic Nicotiana tabacum by Protein Storage Vacuole Targeting

Plant expression of microbial Cell Wall Degrading Enzymes (CWDEs) is a valuable strategy to produce industrial enzymes at affordable cost. Unfortunately, the constitutive expression of CWDEs may affect plant fitness to variable extents, including developmental alterations, sterility and even lethality. In order to explore novel strategies for expressing CWDEs in crops, the cellobiohydrolase CBM3GH5, from the hyperthermophilic bacterium Caldicellulosiruptor saccharolyticus, was constitutively expressed in N. tabacum by targeting the enzyme both to the apoplast and to the protein storage vacuole. The apoplast targeting failed to isolate plants expressing the recombinant enzyme despite a large number of transformants being screened. On the opposite side, the targeting of the cellobiohydrolase to the protein storage vacuole led to several transgenic lines expressing CBM3GH5, with an enzyme yield of up to 0.08 mg g DW-1 (1.67 Units g DW-1) in the mature leaf tissue. The analysis of CBM3GH5 activity revealed that the enzyme accumulated in different plant organs in a developmental-dependent manner, with the highest abundance in mature leaves and roots, followed by seeds, stems and leaf ribs. Notably, both leaves and stems from transgenic plants were characterized by an improved temperature-dependent saccharification profile.

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.

Impact of arbuscular mycorrhizal fungi (AMF) on gene expression of some cell wall and membrane elements of wheat (Triticum aestivum L.) under water deficit using transcriptome analysis

Mycorrhizal symbiotic relationship is one of the most common collaborations between plant roots and the arbuscular mycorrhizal fungi (AMF). The first barrier for establishing this symbiosis is plant cell wall which strongly provides protection against biotic and abiotic stresses. The aim of this study was to investigate the gene expression changes in cell wall of wheat root cv. Chamran after inoculation with AMF, Funneliformis mosseae under two different irrigation regimes. To carry out this investigation, total RNA was extracted from the roots of mycorrhizal and non-mycorrhizal plants, and analyzed using RNA-Seq in an Illumina Next-Seq 500 platform. The results showed that symbiotic association between wheat and AMF and irrigation not only affect transcription profile of the plant growth, but also cell wall and membrane components. Of the 114428 genes expressed in wheat roots, the most differentially expressed genes were related to symbiotic plants under water stress. The most differentially expressed genes were observed in carbohydrate metabolic process, lipid metabolic process, cellulose synthase activity, membrane transports, nitrogen compound metabolic process and chitinase activity related genes. Our results indicated alteration in cell wall and membrane composition due to mycorrhization and irrigation regimes might have a noteworthy effect on the plant tolerance to water deficit.

Green Production and Biotechnological Applications of Cell Wall Lytic Enzymes

: Energy demand is constantly growing, and, nowadays, fossil fuels still play a dominant role in global energy production, despite their negative effects on air pollution and the emission of greenhouse gases, which are the main contributors to global warming. An alternative clean source of energy is represented by the lignocellulose fraction of plant cell walls, the most abundant carbon source on Earth. To obtain biofuels, lignocellulose must be efficiently converted into fermentable sugars. In this regard, the exploitation of cell wall lytic enzymes (CWLEs) produced by lignocellulolytic fungi and bacteria may be considered as an eco-friendly alternative. These organisms evolved to produce a variety of highly specific CWLEs, even if in low amounts. For an industrial use, both the identification of novel CWLEs and the optimization of sustainable CWLE-expressing biofactories are crucial. In this review, we focus on recently reported advances in the heterologous expression of CWLEs from microbial and plant expression systems as well as some of their industrial applications, including the production of biofuels from agricultural feedstock and of value-added compounds from waste materials. Moreover, since heterologous expression of CWLEs may be toxic to plant hosts, genetic strategies aimed in converting such a deleterious effect into a beneficial trait are discussed.

Glycans as Modulators of Plant Defense Against Filamentous Pathogens

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

Separation and identification of candidate protein elicitors from the cultivation medium of Leptosphaeria maculans inducing resistance in Brassica napus

The Dothideomycete Leptosphaeria maculans, a worldwide fungal pathogen of oilseed rape (Brassica napus), secretes a broad spectrum of molecules into the cultivation medium during growth in vitro. Here, candidate elicitor molecules, which induce resistance in B. napus to L. maculans, were identified in the cultivation medium. The elicitation activity was indicated by increased transcription of pathogenesis-related gene 1 (PR1) and enhanced resistance of B. napus plants to the invasion of L. maculans. The elicitation activity was significantly lowered when the cultivation medium was heated to 80°C. Active components were further characterized by specific cleavage with the proteolytic enzymes trypsin and proteinase K and with glycosidases α-amylase and β-glucanase. The elicitor activity was eliminated by proteolytic digestion while glycosidases had no effect. The filtered medium was fractionated by either ion-exchange chromatography or isoelectric focusing. Mass spectrometry analysis of the most active fractions obtained by both separation procedures revealed predominantly enzymes that can be involved in the degradation of plant cell wall polysaccharides. This is the first study searching for L. maculans-specific secreted elicitors with a potential to be used as defense-activating agents in the protection of B. napus against L. maculans in agriculture. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:918-928, 2016.

The pectic disaccharides lepidimoic acid and β-d-xylopyranosyl-(1→3)-d-galacturonic acid occur in cress-seed exudate but lack allelochemical activity

BACKGROUND AND AIMS

Cress-seed (Lepidium sativum) exudate exerts an allelochemical effect, promoting excessive hypocotyl elongation and inhibiting root growth in neighbouring Amaranthus caudatus seedlings. We investigated acidic disaccharides present in cress-seed exudate, testing the proposal that the allelochemical is an oligosaccharin-lepidimoic acid (LMA; 4-deoxy-β-l-threo-hex-4-enopyranuronosyl-(1→2)-l-rhamnose).

METHODS

Cress-seed exudate was variously treated [heating, ethanolic precipitation, solvent partitioning, high-voltage paper electrophoresis and gel-permeation chromatography (GPC)], and the products were bioassayed for effects on dark-grown Amaranthus seedlings. Two acidic disaccharides, including LMA, were isolated and characterized by electrophoresis, thin-layer chromatography (TLC) and nuclear magnetic resonance (NMR) spectroscopy, and then bioassayed.

KEY RESULTS

Cress-seed exudate contained low-Mr, hydrophilic, heat-stable material that strongly promoted Amaranthus hypocotyl elongation and inhibited root growth, but that separated from LMA on electrophoresis and GPC. Cress-seed exudate contained ∼250 µmLMA, whose TLC and electrophoretic mobilities, susceptibility to mild acid hydrolysis and NMR spectra are reported. A second acidic disaccharide, present at ∼120 µm, was similarly characterized, and shown to be β-d-xylopyranosyl-(1→3)-d-galacturonic acid (Xyl→GalA), a repeat unit of xylogalacturonan. Purified LMA and Xyl→GalA when applied at 360 and 740 µm, respectively, only slightly promoted Amaranthus hypocotyl growth, but equally promoted root growth and thus had no effect on the hypocotyl:root ratio, unlike total cress-seed exudate.

CONCLUSIONS

LMA is present in cress seeds, probably formed by rhamnogalacturonan lyase action on rhamnogalacturonan-I during seed development. Our results contradict the hypothesis that LMA is a cress allelochemical that appreciably perturbs the growth of potentially competing seedlings. Since LMA and Xyl→GalA slightly promoted both hypocotyl and root elongation, their effect could be nutritional. We conclude that rhamnogalacturonan-I and xylogalacturonan (pectin domains) are not sources of oligosaccharins with allelochemical activity, and the biological roles (if any) of the disaccharides derived from them are unknown. The main allelochemical principle in cress-seed exudate remains to be identified.

Carbohydrates in plant immunity and plant protection: roles and potential application as foliar sprays

Increasing interest is devoted to carbohydrates for their roles in plant immunity. Some of them are elicitors of plant defenses whereas other ones act as signaling molecules in a manner similar to phytohormones. This review first describes the main classes of carbohydrates associated to plant immunity, their role and mode of action. More precisely, the state of the art about perception of "PAMP, MAMP, and DAMP (Pathogen-, Microbe-, Damage-Associated Molecular Patterns) type" oligosaccharides is presented and examples of induced defense events are provided. A particular attention is paid to the structure/activity relationships of these compounds. The role of sugars as signaling molecules, especially in plant microbe interactions, is also presented. Secondly, the potentialities and limits of foliar sprays of carbohydrates to stimulate plant immunity for crop protection against diseases are discussed, with focus on the roles of the leaf cuticle and phyllosphere microflora.

Analysis of papaya cell wall-related genes during fruit ripening indicates a central role of polygalacturonases during pulp softening

Papaya (Carica papaya L.) is a climacteric fleshy fruit that undergoes dramatic changes during ripening, most noticeably a severe pulp softening. However, little is known regarding the genetics of the cell wall metabolism in papayas. The present work describes the identification and characterization of genes related to pulp softening. We used gene expression profiling to analyze the correlations and co-expression networks of cell wall-related genes, and the results suggest that papaya pulp softening is accomplished by the interactions of multiple glycoside hydrolases. The polygalacturonase cpPG1 appeared to play a central role in the network and was further studied. The transient expression of cpPG1 in papaya results in pulp softening and leaf necrosis in the absence of ethylene action and confirms its role in papaya fruit ripening.

Host and nonhost resistance in Medicago-Colletotrichum interactions

Medicago truncatula lines resistant (A17) or susceptible (F83005.5) to the alfalfa pathogen Colletotrichum trifolii were used to compare defense reactions induced upon inoculation with C. trifolii or with the nonadapted pathogens C. lindemuthianum and C. higginsianum. Nonadapted Colletotrichum spp. induced a hypersensitive response (HR)-like reaction similar to the one induced during the host-incompatible interaction. Molecular analyses indicated an induction of PR10 and chalcone synthase genes in host and nonhost interactions but delayed responses were observed in the F83005.5 line. The clste12 penetration-deficient C. lindemuthianum mutant induced an HR and defense gene expression, showing that perception of nonadapted strains occurs before penetration of epidermal cells. Cytological and transcriptomic analyses performed upon inoculation of near-isogenic M. truncatula lines, differing only at the C. trifolii resistance locus, Ct1, with the nonadapted Colletotrichum strain, showed that nonhost responses are similar in the two lines. These included a localized oxidative burst, accumulation of fluorescent compounds, and transient expression of a small number of genes. Host interactions were characterized by a group of defense and signaling-related genes induced at 3 days postinoculation, associated with an accumulation of salicylic acid. Together, these results show that M. truncatula displays a rapid and transient response to nonadapted Colletotrichum strains and that this response is not linked to the C. trifolii resistance locus.

Functional characterization of a gene family encoding Polygalacturonases in Phytophthora parasitica

Phytophthora parasitica is an oomycete plant pathogen that causes severe disease in a wide variety of plant species. In our previous study, we discovered a multigene family encoding endopolygalacturonases (endoPG) in Phytophthora parasitica. Here, we screened the genomic library of Phytophthora parasitica for the genes encoding endoPG named pppg2 through pppg10, and analyzed their functions. Results obtained by real-time quantitative reverse transcriptase-polymerase chain reaction demonstrated that some of these genes are highly induced during plant infection, which suggests their important roles in the pathogenesis of Phytophthora parasitica. Analysis by in-gel activity assay of recombinant proteins obtained from Pichia pastoris indicated that each of these genes encodes a functional endoPG. Investigation of the function of pppg genes in planta by a Potato virus X agroinfection system in tobacco revealed that each pppg caused specific effects, varying from no symptoms to dwarfism, necrosis, leaf curl, silvery leaf, and cracks in leaf stalks. Appearance of these effects depends on the expression of a pppg protein with a normal active site in the apoplast. These results indicated that each pppg plays a distinct role in the decomposition of plant cell wall.

Transgenic expression of a fungal endo-polygalacturonase increases plant resistance to pathogens and reduces auxin sensitivity

Polygalacturonases (PGs), enzymes that hydrolyze the homogalacturonan of the plant cell wall, are virulence factors of several phytopathogenic fungi and bacteria. On the other hand, PGs may activate defense responses by releasing oligogalacturonides (OGs) perceived by the plant cell as host-associated molecular patterns. Tobacco (Nicotiana tabacum) and Arabidopsis (Arabidopsis thaliana) plants expressing a fungal PG (PG plants) have a reduced content of homogalacturonan. Here, we show that PG plants are more resistant to microbial pathogens and have constitutively activated defense responses. Interestingly, either in tobacco PG or wild-type plants treated with OGs, resistance to fungal infection is suppressed by exogenous auxin, whereas sensitivity to auxin of PG plants is reduced in different bioassays. The altered plant defense responses and auxin sensitivity in PG plants may reflect an increased accumulation of OGs and subsequent antagonism of auxin action. Alternatively, it may be a consequence of perturbations of cellular physiology and elevated defense status as a result of altered cell wall architecture.

Cladosporium fulvum CfHNNI1 induces hypersensitive necrosis, defence gene expression and disease resistance in both host and nonhost plants

Nonhost resistance as a durable and broad-spectrum defence strategy is of great potential for agricultural applications. We have previously isolated a cDNA showing homology with genes encoding bZIP transcription factors from tomato leaf mould pathogen Cladosporium fulvum. Upon expression, the cDNA results in necrosis in C. fulvum host tomato and nonhost tobacco plants and is thus named CfHNNI1 (for C . f ulvum host and nonhost plant necrosis inducer 1). In the present study we report the induction of necrosis in a variety of nonhost plant species belonging to three families by the transient in planta expression of CfHNNI1 using virus-based vectors. Additionally, transient expression of CfHNNI1 also induced expression of the HR marker gene LeHSR203 and greatly reduced the accumulation of recombinant Potato virus X. Stable CfHNNI1 transgenic tobacco plants were generated in which the expression of CfHNNI1 is under the control of the pathogen-inducible hsr203J promoter. When infected with the oomycetes pathogen Phytophthora parasitica var. nicotianae, these transgenic plants manifested enhanced expression of CfHNNI1 and subsequent accumulation of CfHNNI1 protein, resulting in high expression of the HSR203J and PR genes, and strong resistance to the pathogen. The CfHNNI1 transgenic plants also exhibited induced resistance to Pseudomonas syringae pv. tabaci and Tobacco mosaic virus. Furthermore, CfHNNI1 was highly expressed and the protein was translocated into plant cells during the incompatible interactions between C. fulvum and host and nonhost plants. Our results demonstrate that CfHNNI1 is a potential general elicitor of hypersensitive response and nonhost resistance.

Resistance to Botrytis cinerea induced in Arabidopsis by elicitors is independent of salicylic acid, ethylene, or jasmonate signaling but requires PHYTOALEXIN DEFICIENT3

Oligogalacturonides (OGs) released from plant cell walls by pathogen polygalacturonases induce a variety of host defense responses. Here we show that in Arabidopsis (Arabidopsis thaliana), OGs increase resistance to the necrotrophic fungal pathogen Botrytis cinerea independently of jasmonate (JA)-, salicylic acid (SA)-, and ethylene (ET)-mediated signaling. Microarray analysis showed that about 50% of the genes regulated by OGs, including genes encoding enzymes involved in secondary metabolism, show a similar change of expression during B. cinerea infection. In particular, expression of PHYTOALEXIN DEFICIENT3 (PAD3) is strongly up-regulated by both OGs and infection independently of SA, JA, and ET. OG treatments do not enhance resistance to B. cinerea in the pad3 mutant or in underinducer after pathogen and stress1, a mutant with severely impaired PAD3 expression in response to OGs. Similarly to OGs, the bacterial flagellin peptide elicitor flg22 also enhanced resistance to B. cinerea in a PAD3-dependent manner, independently of SA, JA, and ET. This work suggests, therefore, that elicitors released from the cell wall during pathogen infection contribute to basal resistance against fungal pathogens through a signaling pathway also activated by pathogen-associated molecular pattern molecules.

Characterization of necrosis and ethylene-inducing proteins (NEP) in the basidiomycete Moniliophthora perniciosa, the causal agent of witches' broom in Theobroma cacao

The hemibiotrophic basidiomycete Moniliophthora perniciosa causes witches' broom disease of Theobroma cacao. Analysis of the M. perniciosa draft genome led to the identification of three putative genes encoding necrosis and ethylene-inducing proteins (MpNEPs), which are apparently located on the same chromosome. MpNEP1 and 2 have highly similar sequences and are able to induce necrosis and ethylene emission in tobacco and cacao leaves. MpNEP1 is expressed in both biotrophic and saprotrophic mycelia, the protein behaves as an oligomer in solution and is very sensitive to temperature. MpNEP2 is expressed mainly in biotrophic mycelia, is present as a monomer in solution at low concentrations (<40 microM) and is able to recover necrosis activity after boiling. These differences indicate that similar NEPs can have distinct physical characteristics and suggest possible complementary roles during the disease development for both proteins. This is the first report of NEP1-like proteins in a basidiomycete.

A polygalacturonase-inhibiting protein from grapevine reduces the symptoms of the endopolygalacturonase BcPG2 from Botrytis cinerea in Nicotiana benthamiana leaves without any evidence for in vitro interaction

Six endopolygalacturonases from Botrytis cinerea (BcPG1 to BcPG6) as well as mutated forms of BcPG1 and BcPG2 were expressed transiently in leaves of Nicotiana benthamiana using agroinfiltration. Expression of BcPG1, BcPG2, BcPG4, BcPG5, and mutant BcPG1-D203A caused symptoms, whereas BcPG3, BcPG6, and mutant BcPG2-D192A caused no symptoms. Expression of BcPG2 caused the most severe symptoms, including wilting and necrosis. BcPG2 previously has been shown to be essential for B. cinerea virulence. The in vivo effect of this enzyme and the inhibition by a polygalacturonase-inhibiting protein (PGIP) was examined by coexpressing Bcpg2 and the Vvpgipl gene from Vitis vinifera in N. benthamiana. Coinfiltration resulted in a substantial reduction of the symptoms inflicted by the activity of BcPG2 in planta, as evidenced by quantifying the variable chlorophyll fluorescence yield. In vitro, however, no interaction between pure VvPGIP1 and pure BcPG2 was detected. Specifically, VvPGIP1 neither inhibited BcPG2 activity nor altered the degradation profile of polygalacturonic acid by BcPG2. Furthermore, using surface plasmon resonance technology, no physical interaction between VvPGIP1 and BcPG2 was detected in vitro. The data suggest that the in planta environment provided a context to support the interaction between BcPG2 and VvPGIP1, leading to a reduction in symptom development, whereas neither of the in vitro assays detected any interaction between these proteins.

Cellulose binding domains of a Phytophthora cell wall protein are novel pathogen-associated molecular patterns

The cellulose binding elicitor lectin (CBEL) from Phytophthora parasitica nicotianae contains two cellulose binding domains (CBDs) belonging to the Carbohydrate Binding Module1 family, which is found almost exclusively in fungi. The mechanism by which CBEL is perceived by the host plant remains unknown. The role of CBDs in eliciting activity was investigated using modified versions of the protein produced in Escherichia coli or synthesized in planta through the potato virus X expression system. Recombinant CBEL produced by E. coli elicited necrotic lesions and defense gene expression when injected into tobacco (Nicotiana tabacum) leaves. CBEL production in planta induced necrosis. Site-directed mutagenesis on aromatic amino acid residues located within the CBDs as well as leaf infiltration assays using mutated and truncated recombinant proteins confirmed the importance of intact CBDs to induce defense responses. Tobacco and Arabidopsis thaliana leaf infiltration assays using synthetic peptides showed that the CBDs of CBEL are essential and sufficient to stimulate defense responses. Moreover, CBEL elicits a transient variation of cytosolic calcium levels in tobacco cells but not in protoplasts. These results define CBDs as a novel class of molecular patterns in oomycetes that are targeted by the innate immune system of plants and might act through interaction with the cell wall.

Polygalacturonase inhibiting proteins: players in plant innate immunity?

Polygalacturonase-inhibiting proteins (PGIPs) are extracellular leucine-rich repeat (LRR) proteins that recognize and inhibit fungal polygalacturonases (PGs). The PG-PGIP interaction favours the accumulation of elicitor-active oligogalacturonides and causes the activation of defence responses. Small gene families encode PGIP isoforms that differ in affinity and specificity for PGs secreted by different pathogens. The consensus motif within the LRR structure of PGIPs is the same as that of the extracellular receptors of the plant innate immune system. Structural and functional evidence suggest that PGIPs are versatile proteins involved in innate immunity and that they are capable of recognizing different surface motifs of functionally related but structurally variable PGs.

An endopolygalacturonase from Sclerotinia sclerotiorum induces calcium-mediated signaling and programmed cell death in soybean cells

A basic endopolygalacturonase (PG) isoform, produced early by Sclerotinia sclerotiorum when infecting soybean seedlings, was used to examine the signaling role of the enzyme in aequorin-expressing soybean cells. A cytosolic Ca2+ elevation was induced, with a rapid increase (phase 1) and a very slow decrease (phase 2) of Ca2+ concentration, indicating the involvement of Ca2+ ions in PG signaling. Within 1 h of PG-cell contact a remarkable level of cell death was recorded, significantly higher than the control cell culture turnover. The observed morphological and biochemical changes were indicative of the activation of programmed cell death; in particular, cytochrome c release in the cytoplasm and activation of both caspase 9-like and caspase 3-like proteases were found. When a polygalacturonase-inhibiting protein (PGIP) and the PG were simultaneously applied to cells, both the Ca2+ increase and cell death were annulled. The possible roles of prolonged sustained cytosolic Ca2+ concentrations in inducing cell death and of the PG-PGIP interaction in preventing PG signaling are discussed.

Necrotizing activity of five Botrytis cinerea endopolygalacturonases produced in Pichia pastoris

Five Botrytis cinerea endopolygalacturonase enzymes (BcPGs) were individually expressed in Pichia pastoris, purified to homogeneity and biochemically characterized. While the pH optima of the five enzymes were similar (approximately pH 4.5) the maximum activity of individual enzymes differed significantly. For hydrolysis of polygalacturonic acid (PGA), the V(max,app) ranged from 10 to 900 U mg(-1), while the K(m,app) ranged from 0.16 to 0.6 mg ml(-1). Although all BcPGs are true endopolygalacturonases, they apparently have different modes of action. PGA hydrolysis by BcPG1, BcPG2 and BcPG4 leads to the transient accumulation of oligomers with DP < 7, whereas PGA hydrolysis by BcPG3 and BcPG6 leads to the immediate accumulation of monomers and dimers. The necrotizing activity (NA) of all BcPGs was tested separately in tomato, broad bean and Arabidopsis thaliana. They showed different NAs on these plants. BcPG1 and BcPG2 possessed the strongest NA as tissue collapse was observed within 10 min after infiltration of broad bean leaves. The amino acid (aa) D192A substitution in the active site of BcPG2 not only abolished enzyme activity but also the NA, indicating that the NA is dependent on enzyme activity. Furthermore, deletion of the Bcpg2 gene in B. cinerea resulted in a strong reduction in virulence on tomato and broad bean. Primary lesion formation was delayed by approximately 24 h and the lesion expansion rate was reduced by 50-85%. These data indicate that BcPG2 is an important virulence factor for B. cinerea.

Polygalacturonases, polygalacturonase-inhibiting proteins and pectic oligomers in plant-pathogen interactions

Polygalacturonases (PGs) are produced by fungal pathogens during early plant infection and are believed to be important pathogenicity factors. Polygalacturonase-inhibiting proteins (PGIPs) are plant defense proteins which reduce the hydrolytic activity of endoPGs and favor the accumulation of long-chain oligogalacturonides (OGs) which are elicitors of a variety of defense responses. PGIPs belong to the superfamily of leucine reach repeat (LRR) proteins which also include the products of several plant resistance genes. A number of evidence demonstrates that PGIPs efficiently inhibit fungal invasion.

Plant innate immunity – direct and indirect recognition of general and specific pathogen-associated molecules

Plants have the capacity to recognise and reject pathogens at various stages of their attempted colonisation of the plant. Non-specific rejection often arises as a consequence of the potential pathogen's attempt to breach the first lines of plant defence. Pathogens able to penetrate beyond this barrier of non-host resistance may seek a subtle and persuasive relationship with the plant. For some, this may be limited to molecular signals released outside the plant cell wall, but for others it includes penetration of the cell wall and the delivery of signal molecules to the plant cytosol. Direct or indirect recognition of these signals triggers host-specific resistance. Our understanding of host-specific resistance and its possible links to non-host-specific resistance has advanced significantly as more is discovered about the nature and function of the molecules underpinning both kinds of resistance.

Production of a cell wall-associated endopolygalacturonase by Colletotrichum lindemuthianum and pectin degradation during bean infection

The bean pathogen Colletotrichum lindemuthianum expresses two endopolygalacturonase genes, CLPG1 and CLPG2, during interaction with its host plant. However, only CLPG1 was found to be secreted to the extracellular medium during saprophytic growth of the fungus on pectin. To localize CLPG2, a FLAG epitope sequence was inserted in the C-terminal sequence of CLPG2 and the modified gene was introduced into C. lindemuthianum. Western blot analysis using a FLAG monoclonal antibody allowed the detection of CLPG2 in intracellular protein extracts and in the cell wall fraction, but not in the culture medium. Indirect immunofluorescence microscopy was performed to detect CLPG2 during saprophytic or parasitic growth. According to the expression pattern of CLPG2, it was found that CLPG2 accumulates in the fungal cell wall during growth on pectin medium and during appressorium formation, both in vitro and during interaction with the plant. Pectin degradation was not detected around the infection peg using the monoclonal antibody JIM7, specific for methyl-esterified galacturonan. However, extensive pectin dissolution was observed during the development of secondary hyphae.