Arabidopsis thaliana pattern recognition receptors for bacterial elongation factor Tu and flagellin can be combined to form functional chimeric receptors

Medicago truncatula SOBIR1 controls pathogen immunity and specificity in the Rhizobium-legume symbiosis

Medicago truncatula Nod Factor Perception (MtNFP) plays a role in both the Rhizobium-Legume (RL) symbiosis and plant immunity, and evidence suggests that the immune-related function of MtNFP is relevant for symbiosis. To better understand these roles of MtNFP, we sought to identify new interacting partners. We screened a yeast-2-hybrid cDNA library from Aphanomyces euteiches infected and noninfected M. truncatula roots. The M. truncatula leucine-rich repeat (LRR) receptor-like kinase SUPPRESSOR OF BIR1 (MtSOBIR1) was identified as an interactor of MtNFP and was characterised for kinase activity, and potential roles in symbiosis and plant immunity. We showed that the kinase domain of MtSOBIR1 is active and can transphosphorylate the pseudo-kinase domain of MtNFP. MtSOBIR1 could functionally complement Atsobir1 and Nbsobir1/sobir1-like mutants for defence activation, and Mtsobir1 mutants were defective in immune responses to A. euteiches. For symbiosis, we showed that Mtsobir1 mutant plants had both a strong, early infection defect and defects in the defence suppression in nodules, and both effects were plant genotype- and rhizobial strain-specific. This work highlights a conserved function for MtSOBIR1 in activating defence responses to pathogen attack, and potentially novel symbiotic functions of downregulating defence in association with the control of symbiotic specificity.

Natural variation in the pattern-triggered immunity response in plants: Investigations, implications and applications

The pattern-triggered immunity (PTI) response is triggered at the plant cell surface by the recognition of microbe-derived molecules known as microbe- or pathogen-associated molecular patterns or molecules derived from compromised host cells called damage-associated molecular patterns. Membrane-localized receptor proteins, known as pattern recognition receptors, are responsible for this recognition. Although much of the machinery of PTI is conserved, natural variation for the PTI response exists within and across species with respect to the components responsible for pattern recognition, activation of the response, and the strength of the response induced. This review describes what is known about this variation. We discuss how variation in the PTI response can be measured and how this knowledge might be utilized in the control of plant disease and in developing plant varieties with enhanced disease resistance.

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.

Feeding Assay to Study the Effect of Phytocytokines on Direct and Indirect Defense in Maize

Phytocytokines mediate defense against pests and pathogens. Many methods have been developed to study the physiological responses triggered by phytocytokines in dicot plants. Here, we describe a detailed peptide feeding protocol to study the effect of phytocytokines on direct and indirect anti-herbivore defense in maize. This method relies on peptide uptake by the excised maize seedling or leaves via the transpiration stream. The headspace volatiles from plant samples are then analyzed by proton transfer reaction time-of-flight mass spectrometry (PTR-TOF-MS), or by gas chromatography-mass spectrometry (GC-MS). The samples can also be further processed to evaluate phytocytokine-induced defense gene expression or phytohormone production.

The emerging frontier of plant immunity's core hubs

The ever-growing world population, increasingly frequent extreme weather events and conditions, emergence of novel devastating crop pathogens and the social strive for quality food products represent a huge challenge for current and future agricultural production systems. To address these challenges and find realistic solutions, it is becoming more important by the day to understand the complex interactions between plants and the environment, mainly the associated organisms, but in particular pathogens. In the past several years, research in the fields of plant pathology and plant-microbe interactions has enabled tremendous progress in understanding how certain receptor-based plant innate immune systems function to successfully prevent infections and diseases. In this review, we highlight and discuss some of these new ground-breaking discoveries and point out strategies of how pathogens counteract the function of important core convergence hubs of the plant immune system. For practical reasons, we specifically place emphasis on potential applications that can be detracted by such discoveries and what challenges the future of agriculture has to face, but also how these challenges could be tackled.

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.

Conservation and divergence of flg22, pep1 and nlp20 in activation of immune response and inhibition of root development

Many pattern-recognition receptors (PRRs) and their corresponding ligands have been identified. However, it is largely unknown how similar and different these ligands are in inducing plant innate immunity and affecting plant development. In this study, we examined three well characterized ligands in Arabidopsis thaliana, namely flagellin 22 (flg22), plant elicitor peptide 1 (pep1) and a conserved 20-amino-acid fragment found in most necrosis and ethylene-inducing peptide 1-like proteins (nlp20). Our quantitative analyses detected the differences in amplitude in the early immune responses of these ligands, with nlp20-induced responses typically being slower than those mediated by flg22 and pep1. RNA sequencing showed the shared differentially expressed genes (DEGs) was mostly enriched in defense response, whereas nlp20-regulated genes represent only a fraction of those genes differentially regulated by flg22 and pep1. The three elicitors all inhibited primary root growth, especially pep1, which inhibited both auxin transport and signaling pathway. In addition, pep1 significantly inhibited the cell division and genes involved in cell cycle. Compared with flg22 and nlp20, pep1 induced much stronger expression of its receptor in roots, suggesting a potential positive feedback regulation in the activation of immune response. Despite PRRs and their co-receptor BAK1 were necessary for both PAMP induced immune response and root growth inhibition, bik1 mutant only showed impaired defense response but relatively normal root growth inhibition, suggesting BIK1 acts differently in these two biological processes.

Emerging principles in the design of bioengineered made-to-order plant immune receptors

Crop yield and global food security are under constant threat from plant pathogens with the potential to cause epidemics. Traditional breeding for disease resistance can be too slow to counteract these emerging threats, resulting in the need to retool the plant immune system using bioengineered made-to-order immune receptors. Efforts to engineer immune receptors have focused primarily on nucleotide-binding domain and leucine-rich repeat (NLR) immune receptors and proof-of-principles studies. Based upon a near-exhaustive literature search of previously engineered plant immune systems we distil five emerging principles in the design of bioengineered made-to-order plant NLRs and describe approaches based on other components. These emerging principles are anticipated to assist the functional understanding of plant immune receptors, as well as bioengineering novel disease resistance specificities.

Grape Transcriptome Response to Powdery Mildew Infection: Comparative Transcriptome Profiling of Chinese Wild Grapes Provides Insights Into Powdery Mildew Resistance

Erysiphe necator, the fungal pathogen of grape powdery mildew disease, poses a great threat to the grape market and the wine industry. To better understand the molecular basis of grape responses to E. necator, we performed comparative transcriptome profiling on two Chinese wild grape accessions with varying degrees of resistance to E. necator. At 6-, 24-, and 96-h postinoculation of E. necator, 2,856, 2,678, and 1,542 differentially expressed genes (DEGs) were identified in the susceptible accession Vitis pseudoreticulata 'Hunan-1', and at those same time points, 1,921, 2,498, and 3,249 DEGs, respectively, were identified in the resistant accession V. quinquangularis 'Shang-24'. 'Hunan-1' had a substantially larger fraction of down-regulated genes than 'Shang-24' at every infection stage. Analysis of DEGs revealed that up-regulated genes were mostly associated with defense response and disease resistance-related metabolite biosynthesis, and such signaling genes were significantly suppressed in 'Hunan-1'. Interestingly, fatty acid biosynthesis- and elongation-related genes were suppressed by the fungus in the 'Shang-24' accession but somehow induced in the 'Hunan-1' accession, consistent with the concept that E. necator is likely to be a fatty acid auxotroph that requires lipids from the host. Moreover, genes involved in biosynthesis and signaling of phytohormones, such as jasmonic acid and cytokinin, as well as genes encoding protein kinases and nucleotide-binding domain leucine-rich repeat proteins, differentially responded to E. necator in the two wild grapes. The variation of gene regulation associated with nutrient uptake by the fungus and with signaling transduction and pathogen recognition suggests a multilayered regulatory network that works in concert to assist in the establishment of fungal pathogen infections.

The Arabidopsis MIK2 receptor elicits immunity by sensing a conserved signature from phytocytokines and microbes

Sessile plants encode a large number of small peptides and cell surface-resident receptor kinases, most of which have unknown functions. Here, we report that the Arabidopsis receptor kinase MALE DISCOVERER 1-INTERACTING RECEPTOR-LIKE KINASE 2 (MIK2) recognizes the conserved signature motif of SERINE-RICH ENDOGENOUS PEPTIDEs (SCOOPs) from Brassicaceae plants as well as proteins present in fungal Fusarium spp. and bacterial Comamonadaceae, and elicits various immune responses. SCOOP signature peptides trigger immune responses and altered root development in a MIK2-dependent manner with a sub-nanomolar sensitivity. SCOOP12 directly binds to the extracellular leucine-rich repeat domain of MIK2 in vivo and in vitro, indicating that MIK2 is the receptor of SCOOP peptides. Perception of SCOOP peptides induces the association of MIK2 and the coreceptors SOMATIC EMBRYOGENESIS RECEPTOR KINASE 3 (SERK3) and SERK4 and relays the signaling through the cytosolic receptor-like kinases BOTRYTIS-INDUCED KINASE 1 (BIK1) and AVRPPHB SUSCEPTIBLE1 (PBS1)-LIKE 1 (PBL1). Our study identifies a plant receptor that bears a dual role in sensing the conserved peptide motif from phytocytokines and microbial proteins via a convergent signaling relay to ensure a robust immune response.

Peptide signals generated during plant microbe interactions can trigger immune responses in plants. Here the authors show that SCOOP12, a member of a family of peptides present in Brassicaceae plants, and SCOOP12-like motifs in Fusarium fungi, can trigger immune responses following perception by the MIK2 receptor kinase.

Genome editing for resistance against plant pests and pathogens

The conventional breeding of crops struggles to keep up with increasing food needs and ever-adapting pests and pathogens. Global climate changes have imposed another layer of complexity to biological systems, increasing the challenge to obtain improved crop cultivars. These dictate the development and application of novel technologies, like genome editing (GE), that assist targeted and fast breeding programs in crops, with enhanced resistance to pests and pathogens. GE does not require crossings, hence avoiding the introduction of undesirable traits through linkage in elite varieties, speeding up the whole breeding process. Additionally, GE technologies can improve plant protection by directly targeting plant susceptibility (S) genes or virulence factors of pests and pathogens, either through the direct edition of the pest genome or by adding the GE machinery to the plant genome or to microorganisms functioning as biocontrol agents (BCAs). Over the years, GE technology has been continuously evolving and more so with the development of CRISPR/Cas. Here we review the latest advancements of GE to improve plant protection, focusing on CRISPR/Cas-based genome edition of crops and pests and pathogens. We discuss how other technologies, such as host-induced gene silencing (HIGS) and the use of BCAs could benefit from CRISPR/Cas to accelerate the development of green strategies to promote a sustainable agriculture in the future.

Receptors in the Induction of the Plant Innate Immunity

Plants adjust amplitude and duration of immune responses via different strategies to maintain growth, development, and resistance to pathogens. Pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI) play vital roles. Pattern recognition receptors, comprising a large number of receptor-like protein kinases and receptor-like proteins, recognize related ligands and trigger immunity. PTI is the first layer of the innate immune system, and it recognizes PAMPs at the plasma membrane to prevent infection. However, pathogens exploit effector proteins to bypass or directly inhibit the PTI immune pathway. Consistently, plants have evolved intracellular nucleotide-binding domain and leucine-rich repeat-containing proteins to detect pathogenic effectors and trigger a hypersensitive response to activate ETI. PTI and ETI work together to protect plants from infection by viruses and other pathogens. Diverse receptors and the corresponding ligands, especially several pairs of well-studied receptors and ligands in PTI immunity, are reviewed to illustrate the dynamic process of PTI response here.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Integrity of the Post-LRR Domain Is Required for TIR-NB-LRR Function

Plants trigger appropriate defense responses, notably, through intracellular nucleotide-binding (NB) and leucine-rich repeat (LRR)-containing receptors (NLRs) that detect secreted pathogen effector proteins. In NLR resistance genes, the toll/interleukin-1 receptor (TIR)-NB-LRR proteins (TNLs) are an important subfamily, out of which approximately half the members carry a post-LRR (PL) domain of unknown role. We first investigated the requirement of the PL domain for TNL-mediated immune response by mutating the most conserved amino acids across PL domains of Arabidopsis thaliana TNLs. We identified several amino acids in the PL domain of RPS4, required for its ability to trigger a hypersensitive response to AvrRps4 in a Nicotiana tabacum transient assay. Mutating the corresponding amino acids within the PL domain of the tobacco TNL gene N also affected its function. Consequently, our results indicate that the integrity of the PL domain at conserved positions is crucial for at least two unrelated TNLs. We then tested the PL domain specificity for function by swapping PL domains between the paralogs RPS4 and RPS4B. Our results suggest that the PL domain is involved in their TNL pair specificity, 'off state' stability, and NLR complex activation. Considering genetically paired Arabidopsis TNLs, we finally compared the PL and TIR domains of their sensor and executor sequences, respectively. While TIR and PL domains from executors present complete motifs, sensors showed a lack of conservation with degenerated motifs. We here provide a contribution to the functional analysis of the PL domain in order to decipher its role for TNL function.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Perception of unrelated microbe-associated molecular patterns triggers conserved yet variable physiological and transcriptional changes in Brassica rapa ssp. pekinensis

Pattern-triggered immunity (PTI) includes the different transcriptional and physiological responses that enable plants to ward off microbial invasion. Surface-localized pattern-recognition receptors (PRRs) recognize conserved microbe-associated molecular patterns (MAMPs) and initiate a branched signaling cascade that culminate in an effective restriction of pathogen growth. In the model species Arabidopsis thaliana, early PTI events triggered by different PRRs are broadly conserved although their nature or intensity is dependent on the origin and features of the detected MAMP. In order to provide a functional basis for disease resistance in leafy vegetable crops, we surveyed the conservation of PTI events in Brassica rapa ssp. pekinensis. We identified the PRR homologs present in B. rapa genome and found that only one of the two copies of the bacterial Elongation factor-Tu receptor (EFR) might function. We also characterized the extent and unexpected specificity of the transcriptional changes occurring when B. rapa seedlings are treated with two unrelated MAMPs, the bacterial flagellin flg22 peptide and the fungal cell wall component chitin. Finally, using a MAMP-induced protection assay, we could show that bacterial and fungal MAMPs elicit a robust immunity in B. rapa, despite significant differences in the kinetic and amplitude of the early signaling events. Our data support the relevance of PTI for crop protection and highlight specific functional target for disease resistance breeding in Brassica crops.

Molecular Characterization of Differences between the Tomato Immune Receptors Flagellin Sensing 3 and Flagellin Sensing 2

Plants mount defense responses by recognizing indicators of pathogen invasion, including microbe-associated molecular patterns (MAMPs). Flagellin, from the bacterial pathogen Pseudomonas syringae pv. tomato (Pst), contains two MAMPs, flg22 and flgII-28, that are recognized by tomato (Solanum lycopersicum) receptors Flagellin sensing2 (Fls2) and Fls3, respectively, but to what degree each receptor contributes to immunity and whether they promote immune responses using the same molecular mechanisms are unknown. Here, we characterized CRISPR/Cas9-generated Fls2 and Fls3 tomato mutants and found that the two receptors contribute equally to disease resistance both on the leaf surface and in the apoplast. However, we observed striking differences in certain host responses mediated by the two receptors. Compared to Fls2, Fls3 mediated a more sustained production of reactive oxygen species and an increase in transcript abundance of 44 tomato genes, with two genes serving as specific reporters for the Fls3 pathway. Fls3 had greater in vitro kinase activity than Fls2 and could transphosphorylate a substrate. Using chimeric Fls2/Fls3 proteins, we found no evidence that a single receptor domain is responsible for the Fls3-sustained reactive oxygen species, suggesting involvement of multiple structural features or a nullified function of the chimeric construct. This work reveals differences in certain immunity outputs between Fls2 and Fls3, suggesting that they might use distinct molecular mechanisms to activate pattern-triggered immunity in response to flagellin-derived MAMPs.

Chitin Triggers Calcium-Mediated Immune Response in the Plant Model Physcomitrella patens

A characteristic feature of a plant immune response is the increase of the cytosolic calcium (Ca²⁺) concentration following infection, which results in the downstream activation of immune response regulators. The bryophyte Physcomitrella patens has been shown to mount an immune response when exposed to bacteria, fungi, or chitin elicitation, in a manner similar to the one observed in Arabidopsis thaliana. Nevertheless, whether the response of P. patens to microorganism exposure is Ca²⁺ mediated is currently unknown. Here, we show that P. patens plants treated with chitin oligosaccharides exhibit Ca²⁺ oscillations, and that a calcium ionophore can stimulate the expression of defense-related genes. Treatment with chitin oligosaccharides also results in an inhibition of growth, which can be explained by the depolymerization of the apical actin cytoskeleton of tip growing cells. These results suggest that chitin-triggered calcium oscillations are conserved and were likely present in the common ancestor of bryophytes and vascular plants.

The BIR2/BIR3-Associated Phospholipase Dγ1 Negatively Regulates Plant Immunity

Plants have evolved effective strategies to defend themselves against pathogen invasion. Starting from the plasma membrane with the recognition of microbe-associated molecular patterns (MAMPs) via pattern recognition receptors, internal cellular signaling pathways are induced to ultimately fend off the attack. Phospholipase D (PLD) hydrolyzes membrane phospholipids to produce phosphatidic acid (PA), which has been proposed to play a second messenger role in immunity. The Arabidopsis (Arabidopsis thaliana) PLD family consists of 12 members, and for some of these, a specific function in resistance toward a subset of pathogens has been shown. We demonstrate here that Arabidopsis PLDγ1, but not its close homologs PLDγ2 and PLDγ3, is specifically involved in plant immunity. Genetic inactivation of PLDγ1 resulted in increased resistance toward the virulent bacterium Pseudomonas syringae pv. tomato DC3000 and the necrotrophic fungus Botrytis cinerea As pldγ1 mutant plants responded with elevated levels of reactive oxygen species to MAMP treatment, a negative regulatory function for this PLD isoform is proposed. Importantly, PA levels in pldγ1 mutants were not affected compared to stressed wild-type plants, suggesting that alterations in PA levels are not likely the cause for the enhanced immunity in the pldγ1 line. Instead, the plasma-membrane-attached PLDγ1 protein colocalized and associated with the BAK1-INTERACTING RECEPTOR-LIKE KINASES BIR2 and BIR3, which are known negative regulators of pattern-triggered immunity. Moreover, complex formation of PLDγ1 and BIR2 was further promoted upon MAMP treatment. Hence, we propose that PLDγ1 acts as a negative regulator of plant immune responses in complex with immunity-related proteins BIR2 and BIR3.

Peptide signaling for drought-induced tomato flower drop

The premature abscission of flowers and fruits limits crop yield under environmental stress. Drought-induced flower drop in tomato plants was found to be regulated by phytosulfokine (PSK), a peptide hormone previously known for its growth-promoting and immune-modulating activities. PSK formation in response to drought stress depends on phytaspase 2, a subtilisin-like protease of the phytaspase subtype that generates the peptide hormone by aspartate-specific processing of the PSK precursor in the tomato flower pedicel. The mature peptide acts in the abscission zone where it induces expression of cell wall hydrolases that execute the abscission process. Our results provide insight into the molecular control of abscission as regulated by proteolytic processing to generate a small plant peptide hormone.

Deviating from the Beaten Track: New Twists in Brassinosteroid Receptor Function

A key feature of plants is their plastic development tailored to the environmental conditions. To integrate environmental signals with genetic growth regulatory programs, plants rely on a number of hormonal pathways, which are intimately connected at multiple levels. Brassinosteroids (BRs), a class of plant sterol hormones, are perceived by cell surface receptors and trigger responses instrumental in tailoring developmental programs to environmental cues. Arguably, BR signalling is one of the best-characterized plant signalling pathways, and the molecular composition of the core signal transduction cascade seems clear. However, BR research continues to reveal new twists to re-shape our view on this key signalling circuit. Here, exciting novel findings pointing to the plasma membrane as a key site for BR signalling modulation and integration with other pathways are reviewed and new inputs into the BR signalling pathway and emerging “non-canonical” functions of the BR receptor complex are highlighted. Together, this new evidence underscores the complexity of plant signalling integration and serves as a reminder that highly-interconnected signalling pathways frequently comprise non-linear aspects which are difficult to convey in classical conceptual models.

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.

Interactions and Coadaptation in Plant Metaorganisms

Plants associate with a wide diversity of microorganisms. Some microorganisms engage in intimate associations with the plant host, collectively forming a metaorganism. Such close coexistence with plants requires specific adaptations that allow microorganisms to overcome plant defenses and inhabit plant tissues during growth and reproduction. New data suggest that the plant immune system has a broader role beyond pathogen recognition and also plays an important role in the community assembly of the associated microorganism. We propose that core microorganisms undergo coadaptation with their plant host, notably in response to the plant immune system allowing them to persist and propagate in their host. Microorganisms, which are vertically transmitted from generation to generation via plant seeds, putatively compose highly adapted species and may have plant-beneficial functions. The extent to which plant domestication has impacted the underlying genetics of plant-microbe associations remains poorly understood. We propose that the ability of domesticated plants to select and maintain advantageous microbial partners may have been affected. In this review, we discuss factors that impact plant metaorganism assembly and function. We underline the importance of microbe-microbe interactions in plant tissues, as they are still poorly studied but may have a great impact on plant health.

An EFR-Cf-9 chimera confers enhanced resistance to bacterial pathogens by SOBIR1- and BAK1-dependent recognition of elf18

The transfer of well-studied native and chimeric pattern recognition receptors (PRRs) to susceptible plants is a proven strategy to improve host resistance. In most cases, the ectodomain determines PRR recognition specificity, while the endodomain determines the intensity of the immune response. Here we report the generation and characterization of the chimeric receptor EFR-Cf-9, which carries the ectodomain of the Arabidopsis thaliana EF-Tu receptor (EFR) and the endodomain of the tomato Cf-9 resistance protein. Both transient and stable expression of EFR-Cf-9 triggered a robust hypersensitive response (HR) upon elf18 treatment in tobacco. Co-immunoprecipitation and virus-induced gene silencing studies showed that EFR-Cf-9 constitutively interacts with SUPPRESSOR OF BIR1-1 (SOBIR1) co-receptor, and requires both SOBIR1 and kinase-active BRI1-ASSOCIATED KINASE1 (BAK1) for its function. Transgenic plants expressing EFR-Cf-9 were more resistant to the (hemi)biotrophic bacterial pathogens Pseudomonas amygdali pv. tabaci (Pta) 11528 and Pseudomonas syringae pv. tomato DC3000, and mounted an HR in response to high doses of Pta 11528 and P. carotovorum. Taken together, these data indicate that the EFR-Cf-9 chimera is a valuable tool for both investigating the molecular mechanisms responsible for the activation of defence responses by PRRs, and for potential biotechnological use to improve crop disease resistance.

Comparing Arabidopsis receptor kinase and receptor protein-mediated immune signaling reveals BIK1-dependent differences

Pattern recognition receptors (PRRs) sense microbial patterns and activate innate immunity against attempted microbial invasions. The leucine-rich repeat receptor kinases (LRR-RK) FLS2 and EFR, and the LRR receptor protein (LRR-RP) receptors RLP23 and RLP42, respectively, represent prototypical members of these two prominent and closely related PRR families. We conducted a survey of Arabidopsis thaliana immune signaling mediated by these receptors to address the question of commonalities and differences between LRR-RK and LRR-RP signaling. Quantitative differences in timing and amplitude were observed for several early immune responses, with RP-mediated responses typically being slower and more prolonged than those mediated by RKs. Activation of RLP23, but not FLS2, induced the production of camalexin. Transcriptomic analysis revealed that RLP23-regulated genes represent only a fraction of those genes differentially expressed upon FLS2 activation. Several positive and negative regulators of FLS2-signaling play similar roles in RLP23 signaling. Intriguingly, the cytoplasmic receptor kinase BIK1, a positive regulator of RK signaling, acts as a negative regulator of RP-type immune receptors in a manner dependent on BIK1 kinase activity. Our study unveiled unexpected differences in two closely related receptor systems and reports a new negative role of BIK1 in plant immunity.

Screening and identification of a non-peptide antagonist for the peptide hormone receptor in Arabidopsis

Intercellular signaling mediated by peptide hormones and membrane-localized receptor kinases plays crucial roles in plant developmental processes. Because of their diverse functions, agonistic or antagonistic modulation of peptide signaling holds enormous promise for agricultural applications. Here we established a high-throughput screening system using a bead-immobilized receptor kinase and fluorescent-labeled peptide ligand to identify small molecules that bind peptide hormone receptors in competition with natural ligands. We used the Arabidopsis CLE9-BAM1 ligand-receptor pair to screen a library of ≈30,000 chemicals and identified NPD12704 as an antagonist for BAM1. NPD12704 also inhibited CLV3 binding to BAM1 but only minimally interfered with CLV3 binding to CLV1, the closest homolog of BAM1, demonstrating preferential receptor specificity. Treatment of clv1-101 mutant seedlings with NPD12704 enhanced the enlarged shoot apical meristem phenotype. Our results provide a technological framework enabling high-throughput identification of small non-peptide chemicals that specifically control receptor kinase–mediated peptide hormone signaling in plants.

Hidefumi Shinohara and colleagues used the CLE9-BAM1 ligand-receptor pair as a model system for screening peptide hormone receptor-binding small molecules in plants. They identified the small molecule NPD12704 as an antagonist for BAM1 and demonstrated the specific regulatory activity of NPD12704 in shoot apical meristem.

The rice XA21 ectodomain fused to the Arabidopsis EFR cytoplasmic domain confers resistance to Xanthomonas oryzae pv. oryzae

Rice (Oryza sativa) plants expressing the XA21 cell-surface receptor kinase are resistant to Xanthomonas oryzae pv. oryzae (Xoo) infection. We previously demonstrated that expressing a chimeric protein containing the ELONGATION FACTOR Tu RECEPTOR (EFR) ectodomain and the XA21 endodomain (EFR:XA21) in rice does not confer robust resistance to Xoo. To test if the XA21 ectodomain is required for Xoo resistance, we produced transgenic rice lines expressing a chimeric protein consisting of the XA21 ectodomain and EFR endodomain (XA21:EFR) and inoculated these lines with Xoo. We also tested if the XA21:EFR rice plants respond to a synthetic sulfated 21 amino acid derivative (RaxX21-sY) of the activator of XA21-mediated immunity, RaxX. We found that five independently transformed XA21:EFR rice lines displayed resistance to Xoo as measured by lesion length analysis, and showed that five lines share characteristic markers of the XA21 defense response (generation of reactive oxygen species and defense response gene expression) after treatment with RaxX21-sY. Our results indicate that expression of the XA21:EFR chimeric receptor in rice confers resistance to Xoo. These results suggest that the endodomain of the EFR and XA21 immune receptors are interchangeable and the XA21 ectodomain is the key determinant conferring robust resistance to Xoo.

A High-Sensitivity, Microtiter-Based Plate Assay for Plant Pattern-Triggered Immunity

The first step in the plant immune response to pathogen challenge involves the perception of conserved epitopes, called microbe-associated molecular patterns (MAMPs), by cell-surface pattern recognition receptors (PRRs). Given the key roles that MAMPs and PRRs play in plant innate immunity, great effort has been expended to identify these molecules. Current methods for assaying these immune responses are often limited in their resolution and throughput, and consequently, there is a need for medium- to high-throughput methodologies. Here, we describe the development of a 96-well microtiter plate-based assay for plant pattern-triggered immunity that measures the activity of plant peroxidase (POX) enzymes produced in response to treatment with bacterial MAMPs. The system has been optimized to minimize both the amount of plant tissue and MAMPs required and displays up to three orders of magnitude greater sensitivity than the traditional luminol-based reactive oxygen species assay when measuring the plant response to treatment with the bacterial MAMP flg22, reaching detection limits in the picomolar range. This high sensitivity opens the possibility of evaluating the immune-eliciting effects of weaker elicitors. The throughput and material requirements of the assay make it ideal for screens involving quantitative measurement of the plant innate immune response to MAMPs.

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.

Dynamic N-glycoproteome analysis of maize seedling leaves during de-etiolation using Concanavalin A lectin affinity chromatography and a nano-LC-MS/MS-based iTRAQ approach

The identification of N -glycosylated proteins with information about changes in the level of N -glycosylation during de-etiolation provides a database that will aid further research on plant N -glycosylation and de-etiolation. N-glycosylation is one of the most prominent and abundant protein post-translational modifications in all eukaryotes and in plants it plays important roles in development, stress tolerance and immune responses. Because light-induced de-etiolation is one of the most dramatic developmental processes known in plants, seedlings undergoing de-etiolation are an excellent model for investigating dynamic proteomic profiles. Here, we present a comprehensive, quantitative N-glycoproteomic profile of maize seedlings undergoing 12 h of de-etiolation obtained using Concanavalin A (Con A) lectin affinity chromatography enrichment coupled with a nano-LC-MS/MS-based iTRAQ approach. In total, 1084 unique N-glycopeptides carrying 909 N-glycosylation sites and corresponding to 609 proteins were identified and quantified, including 186 N-glycosylation sites from 162 proteins that were significantly regulated over the course of the 12 h de-etiolation period. Based on hierarchical clustering analysis, the significantly regulated N-glycopeptides were divided into seven clusters that showed different N-glycosylation patterns during de-etiolation. We found no obvious difference in the enriched MapMan bincode categories for each cluster, and these clustered significantly regulated N-glycoproteins (SRNPs) are enriched in miscellaneous, protein, cell wall and signaling, indicating that although the N-glycosylation regulation patterns of these SRNPs might differ, they are involved in similar biological processes. Overall, this study represents the first large-scale quantitative N-glycoproteome of the model C4 plant, maize, which is one of the most important cereal and biofuel crops. Our results greatly expand the maize N-glycoproteomic database and also shed light on the potential roles of N-glycosylation modification during the greening of maize leaves.

A deletion affecting an LRR-RLK gene co-segregates with the fruit flat shape trait in peach

In peach, the flat phenotype is caused by a partially dominant allele in heterozygosis (Ss), fruits from homozygous trees (SS) abort a few weeks after fruit setting. Previous research has identified a SSR marker (UDP98-412) highly associated with the trait, found suitable for marker assisted selection (MAS). Here we report a ∼10 Kb deletion affecting the gene PRUPE.6G281100, 400 Kb upstream of UDP98-412, co-segregating with the trait. This gene is a leucine-rich repeat receptor-like kinase (LRR-RLK) orthologous to the Brassinosteroid insensitive 1-associated receptor kinase 1 (BAK1) group. PCR markers suitable for MAS confirmed its strong association with the trait in a collection of 246 cultivars. They were used to evaluate the DNA from a round fruit derived from a somatic mutation of the flat variety 'UFO-4', revealing that the mutation affected the flat associated allele (S). Protein BLAST alignment identified significant hits with genes involved in different biological processes. Best protein hit occurred with AtRLP12, which may functionally complement CLAVATA2, a key regulator that controls the stem cell population size. RT-PCR analysis revealed the absence of transcription of the partially deleted allele. The data support PRUPE.6G281100 as a candidate gene for flat shape in peach.

Structural Characterization of Maize SIRK1 Kinase Domain Reveals an Unusual Architecture of the Activation Segment

Kinases are primary regulators of plant metabolism and excellent targets for plant breeding. However, most kinases, including the abundant receptor-like kinases (RLK), have no assigned role. SIRK1 is a leucine-rich repeat receptor-like kinase (LRR-RLK), the largest family of RLK. In Arabidopsis thaliana, SIRK1 (AtSIRK1) is phosphorylated after sucrose is resupplied to sucrose-starved seedlings and it modulates the sugar response by phosphorylating several substrates. In maize, the ZmSIRK1 expression is altered in response to drought stress. In neither Arabidopsis nor in maize has the function of SIRK1 been completely elucidated. As a first step toward the biochemical characterization of ZmSIRK1, we obtained its recombinant kinase domain, demonstrated that it binds AMP-PNP, a non-hydrolysable ATP-analog, and solved the structure of ZmSIRK1- AMP-PNP co-crystal. The ZmSIRK1 crystal structure revealed a unique conformation for the activation segment. In an attempt to find inhibitors for ZmSIRK1, we screened a focused small molecule library and identified six compounds that stabilized ZmSIRK1 against thermal melt. ITC analysis confirmed that three of these compounds bound to ZmSIRK1 with low micromolar affinity. Solving the 3D structure of ZmSIRK1-AMP-PNP co-crystal provided information on the molecular mechanism of ZmSIRK1 activity. Furthermore, the identification of small molecules that bind this kinase can serve as initial backbone for development of new potent and selective ZmSIRK1 antagonists.

Genome-Editing Technologies for Enhancing Plant Disease Resistance

One of the greatest challenges for agricultural science in the 21st century is to improve yield stability through the progressive development of superior cultivars. The increasing numbers of infectious plant diseases that are caused by plant-pathogens make it ever more necessary to develop new strategies for plant disease resistance breeding. Targeted genome engineering allows the introduction of precise modifications directly into a commercial variety, offering a viable alternative to traditional breeding methods. Genome editing is a powerful tool for modifying crucial players in the plant immunity system. In this work, we propose and discuss genome-editing strategies and targets for improving resistance to phytopathogens. First of all, we present the opportunities to rewrite the effector-target sequence for avoiding effector-target molecular interaction and also to modify effector-target promoters for increasing the expression of target genes involved in the resistance process. In addition, we describe potential approaches for obtaining synthetic R-genes through genome-editing technologies (GETs). Finally, we illustrate a genome editing flowchart to modify the pathogen recognition sites and engineer an R-gene that mounts resistance to some phylogenetically divergent pathogens. GETs potentially mark the beginning of a new era, in which synthetic biology affords a basis for obtaining a reinforced plant defense system. Nowadays it is conceivable that by modulating the function of the major plant immunity players, we will be able to improve crop performance for a sustainable agriculture.

The pattern-recognition receptor CORE of Solanaceae detects bacterial cold-shock protein

Plants and animals recognize microbial invaders by detecting microbe-associated molecular patterns (MAMPs) by cell surface receptors. Many plant species of the Solanaceae family detect the highly conserved nucleic acid binding motif RNP-1 of bacterial cold-shock proteins (CSPs), represented by the peptide csp22, as a MAMP. Here, we exploited the natural variation in csp22 perception observed between cultivated tomato (Solanum lycopersicum) and Solanum pennellii to map and identify the leucine-rich repeat (LRR) receptor kinase CORE (cold shock protein receptor) of tomato as the specific, high-affinity receptor site for csp22. Corroborating its function as a genuine receptor, heterologous expression of CORE in Arabidopsis thaliana conferred full sensitivity to csp22 and, importantly, it also rendered these plants more resistant to infection by the bacterial pathogen Pseudomonas syringae pv. tomato DC3000. Our study also confirms the biotechnological potential of enhancing plant immunity by interspecies transfer of highly effective pattern-recognition receptors such as CORE to different plant families.

Detection of the plant parasite Cuscuta reflexa by a tomato cell surface receptor

Parasitic plants are a constraint on agriculture worldwide. Cuscuta reflexa is a stem holoparasite that infests most dicotyledonous plants. One exception is tomato, which is resistant to C. reflexa We discovered that tomato responds to a small peptide factor occurring in Cuscuta spp. with immune responses typically activated after perception of microbe-associated molecular patterns. We identified the cell surface receptor-like protein CUSCUTA RECEPTOR 1 (CuRe1) as essential for the perception of this parasite-associated molecular pattern. CuRe1 is sufficient to confer responsiveness to the Cuscuta factor and increased resistance to parasitic C. reflexa when heterologously expressed in otherwise susceptible host plants. Our findings reveal that plants recognize parasitic plants in a manner similar to perception of microbial pathogens.

XA21-specific induction of stress-related genes following Xanthomonas infection of detached rice leaves

The rice XA21 receptor kinase confers robust resistance to the bacterial pathogen Xanthomonas oryzaepv. oryzae (Xoo). We developed a detached leaf infection assay to quickly and reliably measure activation of the XA21-mediated immune response using genetic markers. We used RNA sequencing of elf18 treated EFR:XA21:GFP plants to identify candidate genes that could serve as markers for XA21 activation. From this analysis, we identified eight genes that are up-regulated in both in elf18 treated EFR:XA21:GFP rice leaves and Xoo infected XA21 rice leaves. These results provide a rapid and reliable method to assess bacterial-rice interactions.

Multiple Xanthomonas euvesicatoria Type III Effectors Inhibit flg22-Triggered Immunity

Xanthomonas euvesicatoria is the causal agent of bacterial spot disease in pepper and tomato. X. euvesicatoria bacteria interfere with plant cellular processes by injecting effector proteins into host cells through the type III secretion (T3S) system. About 35 T3S effectors have been identified in X. euvesicatoria 85-10, and a few of them were implicated in suppression of pattern-triggered immunity (PTI). We used an Arabidopsis thaliana pathogen-free protoplast-based assay to identify X. euvesicatoria 85-10 effectors that interfere with PTI signaling induced by the bacterial peptide flg22. Of 33 tested effectors, 17 inhibited activation of a PTI-inducible promoter. Among them, nine effectors also interfered with activation of an abscisic acid-inducible promoter. However, effectors that inhibited flg22-induced signaling did not affect phosphorylation of mitogen-activated protein (MAP) kinases acting downstream of flg22 perception. Further investigation of selected effectors revealed that XopAJ, XopE2, and XopF2 inhibited activation of a PTI-inducible promoter by the bacterial peptide elf18 in Arabidopsis protoplasts and by flg22 in tomato protoplasts. The effectors XopF2, XopE2, XopAP, XopAE, XopH, and XopAJ inhibited flg22-induced callose deposition in planta and enhanced disease symptoms caused by attenuated Pseudomonas syringae bacteria. Finally, selected effectors were found to localize to various plant subcellular compartments. These results indicate that X. euvesicatoria bacteria utilize multiple T3S effectors to suppress flg22-induced signaling acting downstream or in parallel to MAP kinase cascades and suggest they act through different molecular mechanisms.

Directed Evolution of FLS2 towards Novel Flagellin Peptide Recognition

Microbe-associated molecular patterns (MAMPs) are molecules, or domains within molecules, that are conserved across microbial taxa and can be recognized by a plant or animal immune system. Although MAMP receptors have evolved to recognize conserved epitopes, the MAMPs in some microbial species or strains have diverged sufficiently to render them unrecognizable by some host immune systems. In this study, we carried out in vitro evolution of the Arabidopsis thaliana flagellin receptor FLAGELLIN-SENSING 2 (FLS2) to isolate derivatives that recognize one or more flagellin peptides from bacteria for which the wild-type Arabidopsis FLS2 confers little or no response. A targeted approach generated amino acid variation at FLS2 residues in a region previously implicated in flagellin recognition. The primary screen tested for elevated response to the canonical flagellin peptide from Pseudomonas aeruginosa, flg22. From this pool, we then identified five alleles of FLS2 that confer modest (quantitatively partial) recognition of an Erwinia amylovora flagellin peptide. Use of this Erwinia-based flagellin peptide to stimulate Arabidopsis plants expressing the resulting FLS2 alleles did not lead to a detectable reduction of virulent P. syringae pv. tomato growth. However, combination of two identified mutations into a single allele further increased FLS2-mediated responses to the E. amylovora flagellin peptide. These studies demonstrate the potential to raise the sensitivity of MAMP receptors toward particular targets.

Evolutionary Dynamics of the Leucine-Rich Repeat Receptor-Like Kinase (LRR-RLK) Subfamily in Angiosperms

Gene duplications are an important factor in plant evolution, and lineage-specific expanded (LSE) genes are of particular interest. Receptor-like kinases expanded massively in land plants, and leucine-rich repeat receptor-like kinases (LRR-RLK) constitute the largest receptor-like kinases family. Based on the phylogeny of 7,554 LRR-RLK genes from 31 fully sequenced flowering plant genomes, the complex evolutionary dynamics of this family was characterized in depth. We studied the involvement of selection during the expansion of this family among angiosperms. LRR-RLK subgroups harbor extremely contrasting rates of duplication, retention, or loss, and LSE copies are predominantly found in subgroups involved in environmental interactions. Expansion rates also differ significantly depending on the time when rounds of expansion or loss occurred on the angiosperm phylogenetic tree. Finally, using a dN/dS-based test in a phylogenetic framework, we searched for selection footprints on LSE and single-copy LRR-RLK genes. Selective constraint appeared to be globally relaxed at LSE genes, and codons under positive selection were detected in 50% of them. Moreover, the leucine-rich repeat domains, and specifically four amino acids in them, were found to be the main targets of positive selection. Here, we provide an extensive overview of the expansion and evolution of this very large gene family.

An improved microtiter plate assay to monitor the oxidative burst in monocot and dicot plant cell suspension cultures

BACKGROUND

A screening method for elicitor and priming agents does not only allow detecting new bioactive substances, it can also be used to understand structure-function relationships of known agents by testing different derivatives of them. This can not only provide new lead compounds for the development of novel, more environment-benign, bio-based agro-chemicals, it may eventually also lead to a better understanding of defense mechanisms in plants. Reactive oxygen species (ROS) are sensitive indicators of these mechanisms but current assay formats are not suitable for multiplex screening, in particularly not in the case of monocot systems.

RESULTS

Here we describe continuous monitoring of ROS in 96-well microtiter plates using the chemiluminescent probe L012, a luminol derivative producing chemiluminescence when oxidised by ROS like hydrogen peroxide, superoxide, or hydroxyl radical that can thus be used as an indicator for these ROS. We were able to measure ROS in both monocot (Oryza sativa) and dicot (Medicago truncatula) cell suspension cultures and record dose dependencies for the carbohydrate elicitors and priming agents ulvan and chitosan at low substrate concentrations (0.3-2.5 µg/ml). The method was optimized in terms of cell density, L012 concentration, and pre-incubation time. In contrast to the single peak observed using a cuvette luminometer, the improved method revealed a double burst in both cell systems during the 90-min measuring period, probably due to the detection of multiple ROS rather than only H2O2.

CONCLUSION

We provide a medium throughput screening method for monocot and dicot suspension-cultured cells that enables direct comparison of monocot and dicot plant systems regarding their reaction to different signaling molecules.

The phylogenetically-related pattern recognition receptors EFR and XA21 recruit similar immune signaling components in monocots and dicots

During plant immunity, surface-localized pattern recognition receptors (PRRs) recognize pathogen-associated molecular patterns (PAMPs). The transfer of PRRs between plant species is a promising strategy for engineering broad-spectrum disease resistance. Thus, there is a great interest in understanding the mechanisms of PRR-mediated resistance across different plant species. Two well-characterized plant PRRs are the leucine-rich repeat receptor kinases (LRR-RKs) EFR and XA21 from Arabidopsis thaliana (Arabidopsis) and rice, respectively. Interestingly, despite being evolutionary distant, EFR and XA21 are phylogenetically closely related and are both members of the sub-family XII of LRR-RKs that contains numerous potential PRRs. Here, we compared the ability of these related PRRs to engage immune signaling across the monocots-dicots taxonomic divide. Using chimera between Arabidopsis EFR and rice XA21, we show that the kinase domain of the rice XA21 is functional in triggering elf18-induced signaling and quantitative immunity to the bacteria Pseudomonas syringae pv. tomato (Pto) DC3000 and Agrobacterium tumefaciens in Arabidopsis. Furthermore, the EFR:XA21 chimera associates dynamically in a ligand-dependent manner with known components of the EFR complex. Conversely, EFR associates with Arabidopsis orthologues of rice XA21-interacting proteins, which appear to be involved in EFR-mediated signaling and immunity in Arabidopsis. Our work indicates the overall functional conservation of immune components acting downstream of distinct LRR-RK-type PRRs between monocots and dicots.

Pests and diseases cause significant agricultural losses. Plants recognize pathogen-derived molecules via plasma membrane-localized immune receptors (called pattern recognition receptors or PRRs), resulting in pathogen resistance. In recent years, the transfer of PRRs across plant species has emerged as a promising biotechnological approach to improve crop disease resistance. Successful transfers of PRRs suggest that immune signaling components are conserved across plant species. In this study, we demonstrate that the PRR XA21 from the monocot plant rice is functional in the dicot plant Arabidopsis thaliana (Arabidopsis) and that it confers quantitatively enhanced resistance to bacteria. Furthermore, we show that the rice XA21 and the Arabidopsis EFR, which are evolutionary-distant but phylogenetically closely related, recruit similar signaling components for their function, revealing an overall conservation of immune pathways across monocots and dicots. These findings demonstrate evolutionary conservation of downstream signaling from PRRs and indicate that transfer of PRRs is possible between different plant families, but also between monocots and dicots.

Design and characterization of a modular membrane protein anchor to functionalize the moss Physcomitrella patens with extracellular catalytic and/or binding activities

Heterologous enzymes and binding proteins were secreted by the moss Physcomitrella patens or anchored extracellularly on its cell membrane in order to functionalize the apoplast as a biochemical reaction compartment. This modular membrane anchoring system utilizes the signal peptide and the transmembrane segment of the somatic embryogenesis receptor-like kinase (SERK), which were identified in a comprehensive bioinformatic analysis of the P. patens genome. By fusing the soluble enzyme NanoLuc luciferase to the signal peptide, its secretion capability was confirmed in vivo. The membrane localization of hybrid proteins comprising the SERK signal peptide, NanoLuc or other functional modules, the SERK transmembrane anchor, and a C-terminal GFP reporter was demonstrated using fluorescence microscopy as well as site-specific proteolytic release of the extracellular enzyme domain. Our membrane anchoring system enables the expression of various functional proteins in the apoplast of P. patens, empowering this photoautotrophic organism for biotechnological applications.

Tools and Strategies to Match Peptide-Ligand Receptor Pairs

Peptide signals have emerged as an important class of regulators in cell-to-cell communication in plants. Several families of small, secreted proteins with a conserved C-terminal Pro-rich motif have been identified as functional peptide signals in Arabidopsis thaliana. These proteins are presumed to be trimmed proteolytically and undergo posttranslational modifications, such as hydroxylation of Pro residues and glycosylation, to form mature, bioactive signals. Identification and matching of such ligands with their respective receptors remains a major challenge since the genes encoding them often show redundancy and low expression restricted to a few cells or particular developmental stages. To overcome these difficulties, we propose the use of ectopic expression of receptor genes in suitable plant cells like Nicotiana benthamiana for testing ligand candidates in receptor output assays and in binding studies. As an example, we used the IDA peptide HAE/HSL2 receptor signaling system known to regulate floral organ abscission. We demonstrate that the oxidative burst response can be employed as readout for receptor activation by synthetic peptides and that a new, highly sensitive, nonradioactive labeling approach can be used to reveal a direct correlation between peptide activity and receptor affinity. We suggest that these approaches will be of broad value for the field of ligand-receptor studies in plants.

Functional analysis of chimeric lysin motif domain receptors mediating Nod factor-induced defense signaling in Arabidopsis thaliana and chitin-induced nodulation signaling in Lotus japonicus

The expression of chimeric receptors in plants is a way to activate specific signaling pathways by corresponding signal molecules. Defense signaling induced by chitin from pathogens and nodulation signaling of legumes induced by rhizobial Nod factors (NFs) depend on receptors with extracellular lysin motif (LysM) domains. Here, we constructed chimeras by replacing the ectodomain of chitin elicitor receptor kinase 1 (AtCERK1) of Arabidopsis thaliana with ectodomains of NF receptors of Lotus japonicus (LjNFR1 and LjNFR5). The hybrid constructs, named LjNFR1-AtCERK1 and LjNFR5-AtCERK1, were expressed in cerk1-2, an A. thaliana CERK1 mutant lacking chitin-induced defense signaling. When treated with NFs from Rhizobium sp. NGR234, cerk1-2 expressing both chimeras accumulated reactive oxygen species, expressed chitin-responsive defense genes and showed increased resistance to Fusarium oxysporum. In contrast, expression of a single chimera showed no effects. Likewise, the ectodomains of LjNFR1 and LjNFR5 were replaced by those of OsCERK1 (Oryza sativa chitin elicitor receptor kinase 1) and OsCEBiP (O. sativa chitin elicitor-binding protein), respectively. The chimeras, named OsCERK1-LjNFR1 and OsCEBiP-LjNFR5, were expressed in L. japonicus NF receptor mutants (nfr1-1; nfr5-2) carrying a GUS (β-glucuronidase) gene under the control of the NIN (nodule inception) promoter. Upon chitin treatment, GUS activation reflecting nodulation signaling was observed in the roots of NF receptor mutants expressing both chimeras, whereas a single construct was not sufficient for activation. Hence, replacement of ectodomains in LysM domain receptors provides a way to specifically trigger NF-induced defense signaling in non-legumes and chitin-induced nodulation signaling in legumes.

Rapid bioassay to measure early reactive oxygen species production in Arabidopsis leave tissue in response to living Pseudomonas syringae

BACKGROUND

Arabidopsis thaliana and Pseudomonas syringae pathovar tomato (Pto) provide an excellent plant-bacteria model system to study innate immunity. During pattern-triggered immunity (PTI), cognate host receptors perceive pathogen-associated molecular patterns (PAMPs) as non-self molecules. Pto harbors many PAMPs; thus for experimental ease, many studies utilize single synthesized PAMPs such as flg22, a short protein peptide derived from Pseudomonas flagellin. Flg22 recognition by Arabidopsis Flagellin Sensing 2 (FLS2) initiates a plethora of signaling responses including rapid production of apoplastic reactive oxygen species (ROS). Assessing flg22-ROS has been instrumental in identifying novel PAMP-signaling components; but comparably little is known whether in Arabidopsis, ROS is produced in response to intact live Pto and whether this response can be used to dissect genetic requirements of the plant host and live bacterial pathogens in planta.

RESULTS

Here, we report of a fast and robust bioassay to quantitatively assess early ROS in Arabidopsis leaves, a tissue commonly used for pathogen infection assays, in response to living bacterial Pto strains. We establish that live Pto elicits a transient and dose-dependent ROS that differed in timing of initiation, amplitude and duration compared to flg22-induced ROS. Our control experiments confirmed that the detected ROS was dependent on the presence of the bacterial cells. Utilizing Arabidopsis mutants previously shown to be defective in flg22-induced ROS, we demonstrate that ROS elicited by live Pto was fully or in part dependent on RbohD and BAK1, respectively. Because fls2 mutants did not produce any ROS, flagellin perception by FLS2 is the predominant recognition event in live Pto-elicited ROS in Arabidopsis leaves. Furthermore using different Pto strains, our in planta results indicate that early ROS production appeared to be independent of the Type III Secretion System.

CONCLUSIONS

We provide evidence and necessary control experiments demonstrating that in planta, this ROS bioassay can be utilized to rapidly screen different Arabidopsis mutant lines and ecotypes in combination with different bacterial strains to investigate the genetic requirements of a plant host and its pathogen. For future experiments, this robust bioassay can be easily extended beyond Arabidopsis-Pto to diverse plant-pathosystems including crop species and their respective microbial pathogens.

Structure-function aspects of extracellular leucine-rich repeat-containing cell surface receptors in plants

Plants exploit several types of cell surface receptors for perception of extracellular signals, of which the extracellular leucine-rich repeat (eLRR)-containing receptors form the major class. Although the function of most plant eLRR receptors remains unclear, an increasing number of these receptors are shown to play roles in innate immunity and a wide variety of developmental processes. Recent efforts using domain swaps, gene shuffling analyses, site-directed mutagenesis, interaction studies, and crystallographic analyses resulted in the current knowledge on ligand binding and the mechanism of activation of plant eLRR receptors. This review provides an overview of eLRR receptor research, specifically summarizing the recent understanding of interactions among plant eLRR receptors, their co-receptors and corresponding ligands. The functions of distinct eLRR receptor domains, and their role in structure, ligand perception and multimeric complex formation are discussed. [Figure: see text] Bart P.H.J. Thomma (Corresponding author).

A two-hybrid-receptor assay demonstrates heteromer formation as switch-on for plant immune receptors

Receptor kinases sense extracellular signals and trigger intracellular signaling and physiological responses. However, how does signal binding to the extracellular domain activate the cytoplasmic kinase domain? Activation of the plant immunoreceptor Flagellin sensing2 (FLS2) by its bacterial ligand flagellin or the peptide-epitope flg22 coincides with rapid complex formation with a second receptor kinase termed brassinosteroid receptor1 associated kinase1 (BAK1). Here, we show that the receptor pair of FLS2 and BAK1 is also functional when the roles of the complex partners are reversed by swapping their cytosolic domains. This reciprocal constellation prevents interference by redundant partners that can partially substitute for BAK1 and demonstrates that formation of the heteromeric complex is the molecular switch for transmembrane signaling. A similar approach with swaps between the Elongation factor-Tu receptor and BAK1 also resulted in a functional receptor/coreceptor pair, suggesting that a "two-hybrid-receptor assay" is of more general use for studying heteromeric receptor complexes.