Natural variation of immune epitopes reveals intrabacterial antagonism

Plants and animals detect biomolecules termed Microbe-Associated Molecular Patterns (MAMPs) and induce immunity. Agricultural production is severely impacted by pathogens which can be controlled by transferring immune receptors. However, most studies use a single MAMP epitope and the impact of diverse multi-copy MAMPs on immune induction is unknown. Here we characterized the epitope landscape from five proteinaceous MAMPs across 4,228 plant-associated bacterial genomes. Despite the diversity sampled, natural variation was constrained and experimentally testable. Immune perception in both Arabidopsis and tomato depended on both epitope sequence and copy number variation. For example, Elongation Factor Tu is predominantly single copy and 92% of its epitopes are immunogenic. Conversely, 99.9% of bacterial genomes contain multiple Cold Shock Proteins and 46% carry a non-immunogenic form. We uncovered a new mechanism for immune evasion, intrabacterial antagonism, where a non-immunogenic Cold Shock Protein blocks perception of immunogenic forms encoded in the same genome. These data will lay the foundation for immune receptor deployment and engineering based on natural variation.

LuBiA (Luciferase-Based Binding Assay): Glowing Peptides as Sensitive Probes to Study Ligand-Receptor Interactions

The quantitative and qualitative biochemical description of molecular interactions is fundamental to the study of ligand/receptor pairs and their structure/function relationships. Bioactive peptides often are active at (sub-)nanomolar concentrations, indicating they have a high affinity for their sites of action, notably binding sites on receptors. Since such receptor proteins are commonly of low abundance, highly sensitive detection methods are required to study these ligand/receptor interactions. We present a protocol for an inexpensive luminescence-based detection setup in which the peptide ligand of interest is extended with the 11-amino acid HiBiT tag. This tag can be quantified easily down to fmol amounts by its ability to reconstitute the enzymatic activity of LgBiT, a truncated version of the Oplophorus gracilirostris luciferase.

Plant cell surface immune receptors-Novel insights into function and evolution

Plants use surface resident and intracellular immune receptors to provide robust immunity against microbial infections. The contribution of the two receptor types to plant immunity differs spatially and temporally. The ongoing identification of new plant cell surface immune receptors and their microbial-derived immunogenic ligands reveal a previously unexpected complexity of plant surface sensors involved in the detection of specific microbial species. Comparative analyses of the plant species distribution of cell surface immune receptors indicate that plants harbor larger sets of genus- or species-specific surface receptors in addition to very few widespread pattern sensors. Leucine-rich repeat surface and intracellular immune sensors emerge as two polymorphic receptor classes whose evolutionary trajectories appear to be linked. This is consistent with their functional cooperativity in providing full plant immunity.

Identification of a damage-associated molecular pattern (DAMP) receptor and its cognate peptide ligand in sweet potato (Ipomoea batatas)

Sweet potato (Ipomoea batatas) is an important tuber crop, but also target of numerous insect pests. Intriguingly, the abundant storage protein in tubers, sporamin, has intrinsic trypsin protease inhibitory activity. In leaves, sporamin is induced by wounding or a volatile homoterpene and enhances insect resistance. While the signalling pathway leading to sporamin synthesis is partially established, the initial event, perception of a stress-related signal is still unknown. Here, we identified an IbLRR-RK1 that is induced upon wounding and herbivory, and related to peptide-elicitor receptors (PEPRs) from tomato and Arabidopsis. We also identified a gene encoding a precursor protein comprising a peptide ligand (IbPep1) for IbLRR-RK1. IbPep1 represents a distinct signal in sweet potato, which might work in a complementary and/or parallel pathway to the previously described hydroxyproline-rich systemin (HypSys) peptides to strengthen insect resistance. Notably, an interfamily compatibility in the Pep/PEPR system from Convolvulaceae and Solanaceae was identified.

Evolution of lipochitooligosaccharide binding to a LysM-RLK for nodulation in Medicago truncatula

Lysin motif receptor like kinases (LysM-RLKs) are involved in the perception of chitooligosaccharides (COs) and related lipochitooligosaccharides (LCOs) in plants. Expansion and divergence of the gene family during evolution have led to various roles in symbiosis and defence. By studying proteins of the LYR-IA subclass of LysM-RLKs of the Poaceae, we show here that they are high affinity LCO binding proteins with a lower affinity for COs, consistent with a role in LCO perception to establish arbuscular mycorrhiza (AM). In Papilionoid legumes whole genome duplication has resulted in two LYR-IA paralogs, MtLYR1 and MtNFP in Medicago truncatula, with MtNFP playing an essential role in the root nodule symbiosis with nitrogen-fixing rhizobia. We show that MtLYR1 has retained the ancestral LCO binding characteristic and is dispensable for AM. Domain swapping between the three Lysin motifs (LysMs) of MtNFP and MtLYR1 and mutagenesis in MtLYR1 suggest that the MtLYR1 LCO binding site is on the second LysM, and that divergence in MtNFP led to better nodulation, but surprisingly with decreased LCO binding. These results suggest that divergence of the LCO binding site has been important for the evolution of a role of MtNFP in nodulation with rhizobia.

CORK1, A LRR-Malectin Receptor Kinase, Is Required for Cellooligomer-Induced Responses in Arabidopsis thaliana

Cell wall integrity (CWI) maintenance is central for plant cells. Mechanical and chemical distortions, pH changes, and breakdown products of cell wall polysaccharides activate plasma membrane-localized receptors and induce appropriate downstream responses. Microbial interactions alter or destroy the structure of the plant cell wall, connecting CWI maintenance to immune responses. Cellulose is the major polysaccharide in the primary and secondary cell wall. Its breakdown generates short-chain cellooligomers that induce Ca²⁺-dependent CWI responses. We show that these responses require the malectin domain-containing CELLOOLIGOMER-RECEPTOR KINASE 1 (CORK1) in Arabidopsis and are preferentially activated by cellotriose (CT). CORK1 is required for cellooligomer-induced cytoplasmic Ca²⁺ elevation, reactive oxygen species (ROS) production, mitogen-associated protein kinase (MAPK) activation, cellulose synthase phosphorylation, and the regulation of CWI-related genes, including those involved in biosynthesis of cell wall material, secondary metabolites and tryptophan. Phosphoproteome analyses identified early targets involved in signaling, cellulose synthesis, the endoplasmic reticulum/Golgi secretory pathway, cell wall repair and immune responses. Two conserved phenylalanine residues in the malectin domain are crucial for CORK1 function. We propose that CORK1 is required for CWI and immune responses activated by cellulose breakdown products.

Differential root and shoot magnetoresponses in Arabidopsis thaliana

The geomagnetic field (GMF) is one of the environmental stimuli that plants experience continuously on Earth; however, the actions of the GMF on plants are poorly understood. Here, we carried out a time-course microarray experiment to identify genes that are differentially regulated by the GMF in shoot and roots. We also used qPCR to validate the activity of some genes selected from the microarray analysis in a dose-dependent magnetic field experiment. We found that the GMF regulated genes in both shoot and roots, suggesting that both organs can sense the GMF. However, 49% of the genes were regulated in a reverse direction in these organs, meaning that the resident signaling networks define the up- or downregulation of specific genes. The set of GMF-regulated genes strongly overlapped with various stress-responsive genes, implicating the involvement of one or more common signals, such as reactive oxygen species, in these responses. The biphasic dose response of GMF-responsive genes indicates a hormetic response of plants to the GMF. At present, no evidence exists to indicate any evolutionary advantage of plant adaptation to the GMF; however, plants can sense and respond to the GMF using the signaling networks involved in stress responses.

Perception of Agrobacterium tumefaciens flagellin by FLS2XL confers resistance to crown gall disease

Bacterial flagella are perceived by the innate immune systems of plants¹ and animals² alike, triggering resistance. Common to higher plants is the immunoreceptor FLAGELLIN-SENSING 2 (FLS2)³, which detects flagellin via its most conserved epitope, flg22. Agrobacterium tumefaciens, which causes crown gall disease in many crop plants, has a highly diverged flg22 epitope and evades immunodetection by plants so far studied. We asked whether, as a next step in this game of 'hide and seek', there are plant species that have evolved immunoreceptors with specificity for the camouflaged flg22^Atum of A. tumefaciens. In the wild grape species Vitis riparia, we discovered FLS2^XL, a previously unknown form of FLS2, that provides exquisite sensitivity to typical flg22 and to flg22^Atum. As exemplified by ectopic expression in tobacco, FLS2^XL can limit crown gall disease caused by A. tumefaciens.

Anion channel SLAH3 is a regulatory target of chitin receptor-associated kinase PBL27 in microbial stomatal closure

In plants, antimicrobial immune responses involve the cellular release of anions and are responsible for the closure of stomatal pores. Detection of microbe-associated molecular patterns (MAMPs) by pattern recognition receptors (PRRs) induces currents mediated via slow-type (S-type) anion channels by a yet not understood mechanism. Here, we show that stomatal closure to fungal chitin is conferred by the major PRRs for chitin recognition, LYK5 and CERK1, the receptor-like cytoplasmic kinase PBL27, and the SLAH3 anion channel. PBL27 has the capacity to phosphorylate SLAH3, of which S127 and S189 are required to activate SLAH3. Full activation of the channel entails CERK1, depending on PBL27. Importantly, both S127 and S189 residues of SLAH3 are required for chitin-induced stomatal closure and anti-fungal immunity at the whole leaf level. Our results demonstrate a short signal transduction module from MAMP recognition to anion channel activation, and independent of ABA-induced SLAH3 activation.

The Medicago truncatula LysM receptor-like kinase LYK9 plays a dual role in immunity and the arbuscular mycorrhizal symbiosis

Plant -specific lysin-motif receptor-like kinases (LysM-RLKs) are implicated in the perception of N-acetyl glucosamine-containing compounds, some of which are important signal molecules in plant-microbe interactions. Among these, both lipo-chitooligosaccharides (LCOs) and chitooligosaccharides (COs) are proposed as arbuscular mycorrhizal (AM) fungal symbiotic signals. COs can also activate plant defence, although there are scarce data about CO production by pathogens, especially nonfungal pathogens. We tested Medicago truncatula mutants in the LysM-RLK MtLYK9 for their abilities to interact with the AM fungus Rhizophagus irregularis and the oomycete pathogen Aphanomyces euteiches. This prompted us to analyse whether A. euteiches can produce COs. Compared with wild-type plants, Mtlyk9 mutants had fewer infection events and were less colonised by the AM fungus. By contrast, Mtlyk9 mutants were more heavily infected by A. euteiches and showed more disease symptoms. Aphanomyces euteiches was also shown to produce short COs, mainly CO II, but also CO III and CO IV, and traces of CO V, both ex planta and in planta. MtLYK9 thus has a dual role in plant immunity and the AM symbiosis, which raises questions about the functioning and the ancestral origins of such a receptor protein.

Reduction of geomagnetic field (GMF) to near null magnetic field (NNMF) affects Arabidopsis thaliana root mineral nutrition

The Earth magnetic field (or geomagnetic field, GMF) is a natural component of our planet and variations of the GMF are perceived by plants with a still uncharacterized magnetoreceptor. The purpose of this work was to assess the effect of near null magnetic field (NNMF, ∼40 nT) on Arabidopsis thaliana Col0 root ion modulation. A time-course (from 10 min to 96 h) exposure of Arabidopsis to NNMF was compared to GMF and the content of some cations (NH₄⁺, K⁺, Ca²⁺ and Mg²⁺) and anions (Cl^-, SO₄⁼, NO₃^- and PO₄⁼) was evaluated by capillary electrophoresis. The expression of several cation and anion channel- and transporter-related genes was assessed by gene microarray. A few minutes after exposure to NNMF, Arabidopsis roots responded with a significant change in the content and gene expression of all nutrient ions under study, indicating the presence of a plant magnetoreceptor that responds immediately to MF variations by modulating channels, transporters and genes involved in mineral nutrition. The response of Arabidopsis to reduced MF was a general reduction of plant ion uptake and transport. Our data suggest the importance to understand the nature and function of the plant magnetoreceptor for future space programs involving plant growth in environments with a reduced MF.

The systemin receptor SYR1 enhances resistance of tomato against herbivorous insects

The discovery in tomato of systemin, the first plant peptide hormone^1,2, was a fundamental change for the concept of plant hormones. Numerous other peptides have since been shown to play regulatory roles in many aspects of the plant life, including growth, development, fertilization and interactions with symbiotic organisms^3-6. Systemin, an 18 amino acid peptide derived from a larger precursor protein ⁷ , was proposed to act as the spreading signal that triggers systemic defence responses observed in plants after wounding or attack by herbivores^1,7,8. Further work culminated in the identification of a leucine-rich repeat receptor kinase (LRR-RK) as the systemin receptor 160 (SR160)^9,10. SR160 is a tomato homologue of Brassinosteroid Insensitive 1 (BRI1), which mediates the regulation of growth and development in response to the steroid hormone brassinolide^11-13. However, a role of SR160/BRI1 as systemin receptor could not be corroborated by others^14-16. Here, we demonstrate that perception of systemin depends on a pair of distinct LRR-RKs termed SYR1 and SYR2. SYR1 acts as a genuine systemin receptor that binds systemin with high affinity and specificity. Further, we show that presence of SYR1, although not decisive for local and systemic wound responses, is important for defence against insect herbivory.

LYR3, a high-affinity LCO-binding protein of Medicago truncatula, interacts with LYK3, a key symbiotic receptor

LYR3, LYK3, and NFP are lysin motif-containing receptor-like kinases (LysM-RLKs) from Medicago truncatula, involved in perception of symbiotic lipo-chitooligosaccharide (LCO) signals. Here, we show that LYR3, a high-affinity LCO-binding protein, physically interacts with LYK3, a key player regulating symbiotic interactions. In vitro, LYR3 is phosphorylated by the active kinase domain of LYK3. Fluorescence lifetime imaging/Förster resonance energy transfer (FLIM/FRET) experiments in tobacco protoplasts show that the interaction between LYR3 and LYK3 at the plasma membrane is disrupted or inhibited by addition of LCOs. Moreover, LYR3 attenuates the cell death response, provoked by coexpression of NFP and LYK3 in tobacco leaves.

Lipo-chitooligosaccharidic nodulation factors and their perception by plant receptors

Lipo-chitooligosaccharides produced by nitrogen-fixing rhizobia are signaling molecules involved in the establishment of an important agronomical and ecological symbiosis with plants. These compounds, known as Nod factors, are biologically active on plant roots at very low concentrations indicating that they are perceived by specific receptors. This article summarizes the main strategies developed for the syntheses of bioactive Nod factors and their derivatives in order to better understand their mode of perception. Different Nod factor receptors and LCO-binding proteins identified by genetic or biochemical approaches are also presented, indicating perception mechanisms that seem to be more complicated than expected, probably involving multi-component receptor complexes.

Lipo-chitooligosaccharidic symbiotic signals are recognized by LysM receptor-like kinase LYR3 in the legume Medicago truncatula

While chitooligosaccharides (COs) derived from fungal chitin are potent elicitors of defense reactions, structurally related signals produced by certain bacteria and fungi, called lipo-chitooligosaccharides (LCOs), play important roles in the establishment of symbioses with plants. Understanding how plants distinguish between friend and foe through the perception of these signals is a major challenge. We report the synthesis of a range of COs and LCOs, including photoactivatable probes, to characterize a membrane protein from the legume Medicago truncatula. By coupling photoaffinity labeling experiments with proteomics and transcriptomics, we identified the likely LCO-binding protein as LYR3, a lysin motif receptor-like kinase (LysM-RLK). LYR3, expressed heterologously, exhibits high-affinity binding to LCOs but not COs. Homology modeling, based on the Arabidopsis CO-binding LysM-RLK AtCERK1, suggests that LYR3 could accommodate the LCO in a conserved binding site. The identification of LYR3 opens up ways for the molecular characterization of LCO/CO discrimination.

An experimental system to study responses of Medicago truncatula roots to chitin oligomers of high degree of polymerization and other microbial elicitors

A fully acetylated, soluble CO preparation of mean DP of ca. 7 was perceived with high sensitivity by M. truncatula in a newly designed versatile root elicitation assay. The root system of legume plants interacts with a large variety of microorganisms, either pathogenic or symbiotic. Understanding how legumes recognize and respond specifically to pathogen-associated or symbiotic signals requires the development of standardized bioassays using well-defined preparations of the corresponding signals. Here we describe the preparation of chitin oligosaccharide (CO) fractions from commercial chitin and their characterization by a combination of liquid-state and solid-state nuclear magnetic resonance spectroscopy. We show that the CO fraction with highest degree of polymerization (DP) became essentially insoluble after lyophilization. However, a fully soluble, fully acetylated fraction with a mean DP of ca. 7 was recovered and validated by showing its CERK1-dependent activity in Arabidopsis thaliana. In parallel, we developed a versatile root elicitation bioassay in the model legume Medicago truncatula, using a hydroponic culture system and the Phytophthora β-glucan elicitor as a control elicitor. We then showed that M. truncatula responded with high sensitivity to the CO elicitor, which caused the production of extracellular reactive oxygen species and the transient induction of a variety of defense-associated genes. In addition, the bioassay allowed detection of elicitor activity in culture filtrates of the oomycete Aphanomyces euteiches, opening the way to the analysis of recognition of this important legume root pathogen by M. truncatula.

Ginkgo biloba responds to herbivory by activating early signaling and direct defenses

Background

Ginkgo biloba (Ginkgoaceae) is one of the most ancient living seed plants and is regarded as a living fossil. G. biloba has a broad spectrum of resistance or tolerance to many pathogens and herbivores because of the presence of toxic leaf compounds. Little is known about early and late events occurring in G. biloba upon herbivory. The aim of this study was to assess whether herbivory by the generalist Spodoptera littoralis was able to induce early signaling and direct defense in G. biloba by evaluating early and late responses.

Methodology/Principal Findings

Early and late responses in mechanically wounded leaves and in leaves damaged by S. littoralis included plasma transmembrane potential (Vm) variations, time-course changes in both cytosolic calcium concentration ([Ca²⁺]_cyt) and H₂O₂ production, the regulation of genes correlated to terpenoid and flavonoid biosynthesis, the induction of direct defense compounds, and the release of volatile organic compounds (VOCs). The results show that G. biloba responded to hebivory with a significant Vm depolarization which was associated to significant increases in both [Ca²⁺]_cyt and H₂O₂. Several defense genes were regulated by herbivory, including those coding for ROS scavenging enzymes and the synthesis of terpenoids and flavonoids. Metabolomic analyses revealed the herbivore-induced production of several flavonoids and VOCs. Surprisingly, no significant induction by herbivory was found for two of the most characteristic G. biloba classes of bioactive compounds; ginkgolides and bilobalides.

Conclusions/Significance

By studying early and late responses of G. biloba to herbivory, we provided the first evidence that this “living fossil” plant responds to herbivory with the same defense mechanisms adopted by the most recent angiosperms.

Biochemical and phylogenetic analysis of CEBiP-like LysM domain-containing extracellular proteins in higher plants

The chitin elicitor-binding protein (CEBiP) from rice was the first plant lysin motif (LysM) protein for which the biological and biochemical function had been established. It belongs to a plant-specific family of extracellular LysM proteins (LYMs) for which we analyzed the phylogeny. LYMs are present in vascular plants only, where an early gene duplication event might have resulted in two types which were retained in present day genomes. LYMs consist of a signal peptide, three consecutive LysMs, separated by cysteine pairs, and a C-terminal region without any known signature, whose length allows the distinction between the two types, and which may be followed by a glycosylphosphatidylinositol (GPI) anchor motif. We analyzed a representative of each type, MtLYM1 and MtLYM2, from Medicago truncatula at the biochemical level and with respect to their expression patterns and observed some similarities but also marked differences. MtLYM1 and MtLYM2 proved to be very different with regard to abundance and apparent molecular mass on SDS-PAGE. Both undergo several post-translational modifications, including N-glycosylation and the addition of a GPI anchor, which would position the proteins at the outer face of the plasma membrane. Only MtLYM2, but not MtLYM1, showed specific binding to biotinylated N-acetylchitooctaose in a manner similar to CEBiP, which belongs to the same type. We postulate that LYM2-type proteins likely function in the perception of chitin-related molecules, whereas possible functions of LYM1-type proteins remain to be elucidated.

Flavone synthase II (CYP93B16) from soybean (Glycine max L.)

Flavonoids are a very diverse group of plant secondary metabolites with a wide array of activities in plants, as well as in nutrition and health. All flavonoids are derived from a limited number of flavanone intermediates, which serve as substrates for a variety of enzyme activities, enabling the generation of diversity in flavonoid structures. Flavonoids can be characteristic metabolites, like isoflavonoids for legumes. Others, like flavones, occur in nearly all plants. Interestingly, there exist two fundamentally different enzymatic systems able to directly generate flavones from flavanones, flavone synthase (FNS) I and II. We describe an inducible flavone synthase activity from soybean (Glycine max) cell cultures, generating 7,4'-dihydroxyflavone (DHF), which we classified as FNS II. The corresponding full-length cDNA (CYP93B16) was isolated using known FNS II sequences from other plants. Functional expression in yeast allowed the detailed biochemical characterization of the catalytic activity of FNS II. A direct conversion of flavanones such as liquiritigenin, naringenin, and eriodictyol into the corresponding flavones DHF, apigenin and luteolin, respectively, was demonstrated. The enzymatic reaction of FNSII was stereoselective, favouring the (S)- over the (R)-enantiomer. Phylogenetic analyses of the subfamily of plant CYP93B enzymes indicate the evolution of a gene encoding a flavone synthase which originally catalyzed the direct conversion of flavanones into flavones, via early gene duplication into a less efficient enzyme with an altered catalytic mechanism. Ultimately, this allowed the evolution of the legume-specific isoflavonoid synthase activity.

Identification of a multigene family encoding putative beta-glucan-binding proteins in Medicago truncatula

Branched 1,6-1,3-beta-glucans from Phytophthora sojae cell walls represent pathogen-associated molecular patterns (PAMPs) that have been shown to mediate the activation of plant defence reactions in many legumes. In soybean, a receptor protein complex containing a high affinity beta-glucan-binding protein (GBP) was identified and investigated in detail. In the model legume Medicago truncatula, used for functional genomic studies of various plant-microbe interactions, a high-affinity beta-glucan-binding site was characterized biochemically. However, to date, none of the genes encoding GBPs from M. truncatula have been described. Here, we report the identification of four full-length clones encoding putative beta-glucan-binding proteins from M. truncatula, MtGBP1, 2, 3, and 4, composing a multigene family encoding GBP-related proteins in this plant. Differences in expression patterns as well as in regulation on treatment with two different biotic elicitors are demonstrated for the members of the GBP family and for a selection of defence-related genes.

Activation of members of a MAPK module in beta-glucan elicitor-mediated non-host resistance of soybean

Plants recognize microbial pathogens by discriminating pathogen-associated molecular patterns from self-structures. We study the non-host disease resistance of soybean (Glycine max L.) to the oomycete, Phytophthora sojae. Soybean senses a specific molecular pattern consisting of a branched heptaglucoside that is present in the oomycetal cell walls. Recognition of this elicitor may be achieved through a beta-glucan-binding protein, which forms part of a proposed receptor complex. Subsequently, soybean mounts a complex defense response, which includes the increase of the cytosolic calcium concentration, the production of reactive oxygen species, and the activation of genes responsible for the synthesis of phytoalexins. We now report the identification of two mitogen-activated protein kinases (MAPKs) and one MAPK kinase (MAPKK) that may function as signaling elements in triggering the resistance response. The use of specific antisera enabled the identification of GmMPKs 3 and 6 whose activity is enhanced within the signaling pathway leading to defense reactions. Elicitor specificity of MAPK activation as well as the sensitivity against inhibitors suggested these kinases as part of the beta-glucan signal transduction pathway. An upstream GmMKK1 was identified based on sequence similarity to other plant MAPKKs and its interaction with the MAPKs was analyzed. Recombinant GmMKK1 interacted predominantly with GmMPK6, with concomitant phosphorylation of the MAPK protein. Moreover, a preferential physical interaction between GmMKK1 and GmMPK6 was demonstrated in yeast. These results suggest a role of a MAPK cascade in mediating beta-glucan signal transduction in soybean, similar to other triggers that activate MAPKs during innate immune responses in plants.

Spatial and temporal expression patterns of Avr1b-1 and defense-related genes in soybean plants upon infection with Phytophthora sojae

The Avr1b locus is required for avirulence of the oomycete pathogen Phytophthora sojae on soybeans carrying resistance gene Rps1b. One of the Avr genes of the locus (Avr1b-1) was shown to encode an elicitor. We have analyzed the spatial and temporal expression patterns of Avr1b-1 in comparison to defense-related genes induced in soybean. Avr1b-1 expression was detectable mainly in close proximity to the site of infection, in wound-inoculated hypocotyls as well as in roots infected with zoospores. Usually, in compatible interactions, higher expression levels of Avr1b-1 were observed in roots when compared with incompatible P. sojae-soybean interactions, whereas neither the timing nor the amount of transcript accumulation of defense-related genes showed cultivar-specific differences. In contrast, the PsojNIP gene encoding a proposed virulence factor was expressed only during the necrotrophic phase in the compatible interaction.

Catalytic properties of the bifunctional soybean beta-glucan-binding protein, a member of family 81 glycoside hydrolases

The beta-glucan-binding protein (GBP) of soybean (Glycine max L.) has been shown to contain two different activities. As part of the plasma membrane-localized pathogen receptor complex, it binds a microbial cell wall elicitor, triggering the activation of defence responses. Additionally, the GBP is able to hydrolyze beta-1,3-glucans, as present in the cell walls of potential pathogens. The substrate specificity, the mode of action, and the stereochemistry of the catalysis have been elucidated. This defines for the first time the inverting mode of the catalytic mechanism of glycoside hydrolases belonging to family 81.

An ancient enzyme domain hidden in the putative beta-glucan elicitor receptor of soybean may play an active part in the perception of pathogen-associated molecular patterns during broad host resistance

A successful defense against potential pathogens requires that a host organism is able to discriminate between self and nonself structures. Soybean (Glycine max L.) exploits a specific molecular pattern, a 1,6-beta-linked and 1,3-beta-branched heptaglucoside (HG), present in cell walls of the oomycetal pathogen Phytophthora sojae, as a signal compound eliciting the onset of defense reactions. The specific and high affinity HG-binding site is contained in the beta-glucan-binding protein (GBP), which in turn is part of a proposed receptor complex. The ability to perceive and respond to Phytophthora cell wall-derived beta-glucan elicitors is exclusive to plants that belong to the Fabaceae. However, we propose that the presence of the GBP is essential, but not sufficient for beta-glucan elicitor-dependent disease resistance because genes encoding GBP-related proteins can be retrieved from many plant species. Furthermore, we show that the GBP is composed of two different carbohydrateactive protein domains, one containing the beta-glucan-binding site, and the other related to glucan endoglucosidases of fungal origin. The glucan hydrolase displays most likely an endo-specific mode of action, cleaving only 1,3-beta-d-glucosidic linkages of oligoglucosides consisting of at least four moieties. Thus, the intrinsic endo-1,3-beta-glucanase activity of the GBP is perfectly suited during initial contact with Phytophthora to release oligoglucoside fragments enriched in motifs that constitute ligands for the high affinity binding site present in the same protein. The concept of innate immunity in plants receives substantial support by this highly sophisticated system using ancient enzyme modules as an active part of the recognition mechanism.

The role of octadecanoids and functional mimics in soybean defense responses

Oxylipins of the jasmonate pathway and synthetic functional analogs have been analyzed for their elicitor-like activities in an assay based on the induced accumulation of glyceollins, the phytoalexins of soybean (Glycine max L.), in cell suspension cultures of this plant. Jasmonic acid (JA) and its methyl ester showed weak phytoalexin-inducing activity when compared to an early jasmonate biosynthetic precursor, 12-oxo-phytodienoic acid (OPDA), as well as to the bacterial phytotoxin coronatine and certain 6-substituted indanoyl-L-isoleucine methyl esters, which all were highly active. Interestingly, different octadecanoids and indanoyl conjugates induced the accumulation of transcripts of various defense-related genes to different degrees, indicating distinct induction competencies. Therefore, these signaling compounds and mimics were further analyzed for their effects on signal transduction elements, such as the transient enhancement of the cytosolic Ca2+ concentration and MAP kinase activation, which are known to be initiated by a soybean pathogen-derived beta-glucan elicitor. In contrast to the beta-glucan elicitor, none of the other compounds tested triggered these early signaling elements. Moreover, endogenous levels of OPDA and JA in soybean cells were shown to be unaffected after treatment with beta-glucans. Thus, OPDA and JA, which are functionally mimicked by coronatine and a variety of 6-substituted derivatives of indanoyl-L-isoleucine methyl ester, represent highly efficient signaling compounds of a lipid-based pathway not deployed in the beta-glucan elicitor-initiated signal transduction.

Deletion of a single amino acid residue from different 4-coumarate:CoA ligases from soybean results in the generation of new substrate specificities

Plant 4-coumarate:coenzyme A ligases, acyl-CoA ligases, peptide synthetases, and firefly luciferases are grouped in one family of AMP-binding proteins. These enzymes do not only use a common reaction mechanism for the activation of carboxylate substrates but are also very likely marked by a similar functional architecture. In soybean, four 4-coumarate:CoA ligases have been described that display different substrate utilization profiles. One of these (Gm4CL1) represented an isoform that was able to convert highly ring-substituted cinnamic acids. Using computer-based predictions of the conformation of Gm4CL1, a peptide motif was identified and experimentally verified to exert a critical influence on the selectivity toward differently ring-substituted cinnamate substrates. Furthermore, one unique amino acid residue present in the other isoenzymes of soybean was shown to be responsible for the incapability to accommodate highly substituted substrates. The deletion of this residue conferred the ability to activate sinapate and, in one case, also 3,4-dimethoxy cinnamate and was accompanied by a significantly better affinity for ferulate. The engineering of the substrate specificity of the critical enzymes that activate the common precursors of a variety of phenylpropanoid-derived secondary metabolites may offer a convenient tool for the generation of transgenic plants with desirably modified metabolite profiles.

Divergent members of a soybean (Glycine max L.) 4-coumarate:coenzyme A ligase gene family

4-Coumarate:CoA ligase (4CL) is involved in the formation of coenzyme A thioesters of hydroxycinnamic acids that are central substrates for subsequent condensation, reduction, and transfer reactions in the biosynthesis of plant phenylpropanoids. Previous studies of 4CL appear to suggest that many isoenzymes are functionally equivalent in supplying substrates to various subsequent branches of phenylpropanoid biosyntheses. In contrast, divergent members of a 4CL gene family were identified in soybean (Glycine max L.). We isolated three structurally and functionally distinct 4CL cDNAs encoding 4CL1, 4CL2, and 4CL3 and the gene Gm4CL3. A fourth cDNA encoding 4CL4 had high similarity with 4CL3. The recombinant proteins expressed in Escherichia coli possessed highly divergent catalytic efficiency with various hydroxycinnamic acids. Remarkably, one isoenzyme (4CL1) was able to convert sinapate; thus the first cDNA encoding a 4CL that accepts highly substituted cinnamic acids is available for further studies on branches of phenylpropanoid metabolism that probably lead to the precursors of lignin. Surprisingly, the activity levels of the four isoenzymes and steady-state levels of their transcripts were differently affected after elicitor treatment of soybean cell cultures with a beta-glucan elicitor of Phytophthora sojae, revealing the down-regulation of 4CL1 vs. up-regulation of 4CL3/4. A similar regulation of the transcript levels of the different 4CL isoforms was observed in soybean seedlings after infection with Phytophthora sojae zoospores. Thus, partitioning of cinnamic acid building units between phenylpropanoid branch pathways in soybean could be regulated at the level of catalytic specificity and the level of expression of the 4CL isoenzymes.

Induction of H(2)O(2) synthesis by beta-glucan elicitors in soybean is independent of cytosolic calcium transients

Soybean cell suspension cultures have been used to investigate the role of the elevation of the cytosolic Ca(2+) concentration in beta-glucan elicitors-induced defence responses, such as H(2)O(2) and phytoalexin production. The intracellular Ca(2+) concentration was monitored in transgenic cells expressing the Ca(2+)-sensing aequorin. Two lines of evidence showed that a transient increase of the cytosolic Ca(2+) concentration is not necessarily involved in the induction of H(2)O(2) generation: (i) a Bradyrhizobium japonicum cyclic beta-glucan induced the H(2)O(2) burst without increasing the cytosolic Ca(2+) concentration; (ii) two ion channel blockers (anthracene-9-carboxylate, A9C; 5-nitro-2-(3-phenylpropylamino)-benzoate, NPPB) could not prevent a Phytophthora soja beta-glucan elicitor-induced H(2)O(2) synthesis but did prevent a cytosolic Ca(2+) concentration increase. Moreover, A9C and NPPB inhibited P. sojae beta-glucan-elicited defence-related gene inductions as well as the inducible accumulation of phytoalexins, suggesting that the P. sojae beta-glucan-induced transient cytosolic Ca(2+) increase is not necessary for the elicitation of H(2)O(2) production but is very likely required for phytoalexin synthesis.

The hepta-beta-glucoside elicitor-binding proteins from legumes represent a putative receptor family

The ability of legumes to recognize and respond to beta-glucan elicitors by synthesizing phytoalexins is consistent with the existence of a membrane-bound beta-glucan-binding site. Related proteins of approximately 75 kDa and the corresponding mRNAs were detected in various species of legumes which respond to beta-glucans. The cDNAs for the beta-glucan-binding proteins of bean and soybean were cloned. The deduced 75-kDa proteins are predominantly hydrophilic and constitute a unique class of glucan-binding proteins with no currently recognizable functional domains. Heterologous expression of the soybean beta-glucan-binding protein in tomato cells resulted in the generation of a high-affinity binding site for the elicitor-active hepta-beta-glucoside conjugate (Kd = 4.5 nM). Ligand competition experiments with the recombinant binding sites demonstrated similar ligand specificities when compared with soybean. In both soybean and transgenic tomato, membrane-bound, active forms of the glucan-binding proteins coexist with immunologically detectable, soluble but inactive forms of the proteins. Reconstitution of a soluble protein fraction into lipid vesicles regained beta-glucoside-binding activity but with lower affinity (Kd = 130 nM). We conclude that the beta-glucan elicitor receptors of legumes are composed of the 75 kDa glucan-binding proteins as the critical components for ligand-recognition, and of an as yet unknown membrane anchor constituting the plasma membrane-associated receptor complex.

Isolation of a French bean (Phaseolus vulgaris L.) homolog to the beta-glucan elicitor-binding protein of soybean (Glycine max L.)

A high-affinity membrane-bound beta-glucan elicitor-binding protein has been purified from microsomal preparations of French bean (Phaseolus vulgaris L.) roots. A 5900-fold purification was achieved by affinity chromatography of functionally solubilized membrane proteins. The beta-glucan-binding protein had an apparent molecular mass of 78 kDa when subjected to SDS-PAGE. Western blot analysis showed specific crossreactivity of this French bean protein with an antiserum raised against a synthetic peptide representing an internal 15 amino acid fragment of the beta-glucan-binding protein from soybean. Northern blot analysis with a cDNA probe of the soybean beta-glucan-binding protein gene revealed a crosshybridizing transcript of 2.4 kb in French bean. These results indicate that the beta-glucan-binding proteins of French bean and soybean are conserved homologs involved in beta-glucan elicitor recognition.

Maize glutathione-dependent formaldehyde dehydrogenase: protein sequence and catalytic properties

Glutathione-dependent formaldehyde dehydrogenase (FDH; EC 1.2.1.1) has been purified 3900-fold from maize cell-suspension cultures to a specific activity of 4.68 mumol (mg protein)-1 min-1. The homogeneous enzyme consisted of two identical subunits with a molecular mass of 42 kDa, and an isoelectric point of 5.8. Eight tryptic peptides were sequenced and gave a perfect fit to the protein sequence derived from maize Fdh cDNA (J. Fliegmann and H. Sandermann, 1997, Plant Mol Biol 34: 843-854). There was 62% identity with the eucaryotic FDH consensus sequence. Michaelis constants of approx. 20 microns (formaldehyde), approx. 50 microns (glutathione) and approx. 31 microns (NAD+) were determined for the maize enzyme as well as for FDH partially purified from dog lung. Besides S-hydroxymethylglutathione, pentanol-1, octanol-1, and omega-hydroxy-fatty acids served as substrates for both FDH preparations. The unusual substrate specificity indicates that FDH may be involved in the detoxification of long-chain lipid peroxidation products.

Maize glutathione-dependent formaldehyde dehydrogenase cDNA: a novel plant gene of detoxification

We have previously shown that intact plants and cultured plant cells can metabolize and detoxify formaldehyde through the action of a glutathione-dependent formaldehyde dehydrogenase (FDH), followed by C-1 metabolism of the initial metabolite (formic acid). The cloning and heterologous expression of a cDNA for the glutathione-dependent formaldehyde dehydrogenase from Zea mays L. is now described. The functional expression of the maize cDNA in Escherichia coli proved that the cloned enzyme catalyses the NAD(+)- and glutathione (GSH)-dependent oxidation of formaldehyde. The deduced amino acid sequence of 41 kDa was on average 65% identical with class III alcohol dehydrogenase from animals and less than 60% identical with conventional plant alcohol dehydrogenases (ADH) utilizing ethanol. Genomic analysis suggested the existence of a single gene for this cDNA. Phylogenetic analysis supports the convergent evolution of ethanol-consuming ADHs in animals and plants from formaldehyde-detoxifying ancestors. The high structural conservation of present-day glutathione-dependent FDH in microorganisms, plants and animals is consistent with a universal importance of these detoxifying enzymes.