Search

Plant pattern recognition receptors (PRRs) facilitate recognition of microbial patterns and mediate activation of plant immunity. Arabidopsis thaliana RLP42 senses fungal endopolygalacturonases (PGs) and triggers plant defence through complex formation with SOBIR1 and SERK co-receptors. Here, we show that a conserved 9-amino-acid fragment pg9(At) within PGs is sufficient to activate RLP42-dependent plant immunity. Structure-function analysis reveals essential roles of amino acid residues within the RLP42 leucine-rich repeat and island domains for ligand binding and PRR complex assembly. Sensitivity to pg9(At), which is restricted to A. thaliana and exhibits scattered accession specificity, is unusual for known PRRs. Arabidopsis arenosa and Brassica rapa, two Brassicaceae species closely related to A. thaliana, respectively perceive immunogenic PG fragments pg20(Aa) and pg36(Bra), which are structurally distinct from pg9(At). Our study provides evidence for rapid evolution of polymorphic PG sensors with distinct pattern specificities within a single plant family.

Parasitic plants pose a major biotic threat to plant growth and development and lead to losses in crop productivity of billions of USD annually. By comparison with “normal” autotrophic plants, parasitic plants live a heterotrophic lifestyle and rely on water, solutes and to a greater (holoparasitic plants) or lesser extent (hemiparasitic plants) on sugars from other host plants. Most hosts are unable to detect an infestation by plant parasites or unable to fend off these parasitic invaders. However, a few hosts have evolved defense strategies to avoid infestation or protect themselves actively post-attack often leading to full or partial resistance. Here, we review the current state of our understanding of the defense strategies to plant parasitism used by host plants with emphasis on the active molecular resistance mechanisms. Furthermore, we outline the perspectives and the potential of future studies that will be indispensable to develop and breed resistant crops.

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 flg22Atum of A. tumefaciens. In the wild grape species Vitis riparia, we discovered FLS2XL, a previously unknown form of FLS2, that provides exquisite sensitivity to typical flg22 and to flg22Atum. As exemplified by ectopic expression in tobacco, FLS2XL can limit crown gall disease caused by A. tumefaciens.

The root cap protects the stem cell niche of angiosperm roots from damage. In Arabidopsis, lateral root cap (LRC) cells covering the meristematic zone are regularly lost through programmed cell death, while the outermost layer of the root cap covering the tip is repeatedly sloughed. Efficient coordination with stem cells producing new layers is needed to maintain a constant size of the cap. We present a signalling pair, the peptide IDA-LIKE1 (IDL1) and its receptor HAESA-LIKE2 (HSL2), mediating such communication. Live imaging over several days characterized this process from initial fractures in LRC cell files to full separation of a layer. Enhanced expression of IDL1 in the separating root cap layers resulted in increased frequency of sloughing, balanced with generation of new layers in a HSL2-dependent manner. Transcriptome analyses linked IDL1-HSL2 signalling to the transcription factors BEARSKIN1/2 and genes associated with programmed cell death. Mutations in either IDL1 or HSL2 slowed down cell division, maturation and separation. Thus, IDL1-HSL2 signalling potentiates dynamic regulation of the homeostatic balance between stem cell division and sloughing activity.

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.

As part of their immune system, plants have pattern recognition receptors (PRRs) that can detect a broad range of microbe-associated molecular patterns (MAMPs). Here, we identified a PRR of Arabidopsis thaliana with specificity for the bacterial MAMP eMax from xanthomonads. Response to eMax seems to be restricted to the Brassicaceae family and also varied among different accessions of Arabidopsis. In crosses between sensitive accessions and the insensitive accession Shakhdara, eMax perception mapped to RECEPTOR-LIKE PROTEIN1 (RLP1). Functional complementation of rlp1 mutants required gene constructs that code for a longer version of RLP1 that we termed ReMAX (for receptor of eMax). ReMAX/RLP1 is a typical RLP with structural similarity to the tomato (Solanum lycopersicum) RLP Eix2, which detects fungal xylanase as a MAMP. Attempts to demonstrate receptor function by interfamily transfer of ReMAX to Nicotiana benthamiana were successful after using hybrid receptors with the C-terminal part of ReMAX replaced by that of Eix2. These results show that ReMAX determines specificity for eMax. They also demonstrate hybrid receptor technology as a promising tool to overcome problems that impede interfamily transfer of PRRs to enhance pathogen detection in crop plants.

Cuscuta spp. are widespread obligate holoparasitic plants with a broad host spectrum. Rootless Cuscuta penetrates host stems with so called haustoria to form a direct connection to the host vascular tissue ( Dawson et al., 1994 ; Lanini and Kogan, 2005; Kaiser et al., 2015 ). This connection allows a steady uptake of water, assimilates and essential nutrients from the host plant and therefore enables Cuscuta growth and proliferation. To quantify the parasites' ability to grow on potential host plants one can use the quantitative growth assay ( Hegenauer et al., 2016 ) described herein, which exclusively utilizes fresh weight measurement as readout.

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.

Dodders (Cuscuta spp.) are holoparasitic plants that enwind stems of host plants and penetrate those by haustoria to connect to the vascular bundles. Having a broad host plant spectrum, Cuscuta spp infect nearly all dicot plants – only cultivated tomato as one exception is mounting an active defense specifically against C. reflexa. In a recent work we identified a pattern recognition receptor of tomato, “Cuscuta Receptor 1“ (CuRe1), which is critical to detect a “Cuscuta factor” (CuF) and initiate defense responses such as the production of ethylene or the generation of reactive oxygen species. CuRe1 also contributes to the tomato resistance against C. reflexa. Here we point to the fact that CuRe1 is not the only relevant component for full tomato resistance but it requires additional defense mechanisms, or receptors, respectively, to totally fend off the parasite.

Resistance is not, after all, futile The parasitic plant known as dodder attaches to its hosts and sucks the life out of them. Oddly, the common tomato stands tall when under attack. Hegenauer et al. have leveraged that difference to identify part of the molecular defense system that protects tomato plants (see the Perspective by Ntoukakis and Gimenez-Ibanez). In a process analogous to defenses mounted against microbial infection, the host plant perceives a small-peptide signal from the parasitic plant and initiates defense responses. The candidate receptor isolated from the tomato plant provided partial protection when transferred to two other susceptible plant species. Science , this issue p. 478 ; see also p. 442

The pattern recognition receptor FLAGELLIN SENSING2 (FLS2) renders plant cells responsive to subnanomolar concentrations of flg22, the active epitope of bacterial flagellin. We recently observed that a preparation of the peptide IDL1, a signal known to regulate abscission processes via the receptor kinases HAESA and HAESA-like2, apparently triggered Arabidopsis thaliana cells in an FLS2-dependent manner. However, closer investigation revealed that this activity was due to contamination by a flg22-type peptide, and newly synthesized IDL1 peptide was completely inactive in FLS2 signaling. This raised alert over contamination events occurring in the process of synthesis or handling of peptides. Two recent reports have suggested that FLS2 has further specificities for structurally unrelated peptides derived from CLV3 and from Ax21. We thus scrutinized these peptides for activity in Arabidopsis cells as well. While responding to

A sulfated peptide activates a rice immune receptor., Surveillance of the extracellular environment by immune receptors is of central importance to eukaryotic survival. The rice receptor kinase XA21, which confers robust resistance to most strains of the Gram-negative bacterium Xanthomonas oryzae pv. oryzae (Xoo), is representative of a large class of cell surface immune receptors in plants and animals. We report the identification of a previously undescribed Xoo protein, called RaxX, which is required for activation of XA21-mediated immunity. Xoo strains that lack RaxX, or carry mutations in the single RaxX tyrosine residue (Y41), are able to evade XA21-mediated immunity. Y41 of RaxX is sulfated by the prokaryotic tyrosine sulfotransferase RaxST. Sulfated, but not nonsulfated, RaxX triggers hallmarks of the plant immune response in an XA21-dependent manner. A sulfated, 21–amino acid synthetic RaxX peptide (RaxX21-sY) is sufficient for this activity. Xoo field isolates that overcome XA21-mediated immunity encode an alternate raxX allele, suggesting that coevolutionary interactions between host and pathogen contribute to RaxX diversification. RaxX is highly conserved in many plant pathogenic Xanthomonas species. The new insights gained from the discovery and characterization of the sulfated protein, RaxX, can be applied to the development of resistant crop varieties and therapeutic reagents that have the potential to block microbial infection of both plants and animals.

Plants and animals employ innate immune systems to cope with microbial infection. Pattern-triggered immunity relies on the recognition of microbe-derived patterns by pattern recognition receptors (PRRs). Necrosis and ethylene-inducing peptide 1-like proteins (NLPs) constitute plant immunogenic patterns that are unique, as these proteins are produced by multiple prokaryotic (bacterial) and eukaryotic (fungal, oomycete) species. Here we show that the leucine-rich repeat receptor protein (LRR-RP) RLP23 binds in vivo to a conserved 20-amino-acid fragment found in most NLPs (nlp20), thereby mediating immune activation in Arabidopsis thaliana. RLP23 forms a constitutive, ligand-independent complex with the LRR receptor kinase (LRR-RK) SOBIR1 (Suppressor of Brassinosteroid insensitive 1 (BRI1)-associated kinase (BAK1)-interacting receptor kinase 1), and recruits a second LRR-RK, BAK1, into a tripartite complex upon ligand binding. Stable, ectopic expression of RLP23 in potato (Solanum tuberosum) confers nlp20 pattern recognition and enhanced immunity to destructive oomycete and fungal plant pathogens, such as Phytophthora infestans and Sclerotinia sclerotiorum. PRRs that recognize widespread microbial patterns might be particularly suited for engineering immunity in crop plants.

By comparison with plant-microbe interaction, little is known about the interaction of parasitic plants with their hosts. Plants of the genus Cuscuta belong to the family of Cuscutaceae and comprise about 200 species, all of which live as stem holoparasites on other plants. Cuscuta spp. possess no roots nor fully expanded leaves and the vegetative portion appears to be a stem only. The parasite winds around plants and penetrates the host stems via haustoria, forming direct connections to the vascular bundles of their hosts to withdraw water, carbohydrates and other solutes. Besides susceptible hosts, a few plants exist that exhibit an active resistance against infestation by Cuscuta spp. For example, cultivated tomato (Solanum lycopersicum) fends off Cuscuta reflexa by means of a hypersensitive-type response occurring in the early penetration phase. This report on the plant-plant dialogue between Cuscuta spp. and its host plants focuses on the incompatible interaction of Cuscuta reflexa with tomato.

Dodder or Cuscutaceae are holoparasitic plants subsisting on other dicotyledonous plants. The infection process is initiated by adherence of Cuscuta prehaustoria to the host surface, followed by penetration attempts by hyphae. In the case of a successful infection, these organs connect the parasite's vascular tissue to that of the host. Here we show that contact of Cuscuta reflexa prehaustoria to tomato induces the expression of a new arabinogalactan protein (AGP), attAGP, in the tomato precisely at the site of dodder attack. We show that attAGP is a plasma membrane-bound cell wall-localized protein. Using the RNAi technique and attAGP-targeted virus-induced gene silencing, we observed a correlation between attAGP expression level and force of attachment of the parasite to host tomatoes. If the expression level of attAGP was reduced, the C. reflexa attachment capability was significantly reduced, too. We conclude that C. reflexa infection induced a signal in the host leading to expression of tomato attAGP, which promotes the parasite's adherence.

A suppressive subtractive hybridization technique was used to identify genes, which were induced during the early phases of the interaction between dodder (Cuscuta reflexa), a phanerogamic parasite, and its incompatible host plant tomato. One of the identified genes encodes a tomato xyloglucan endotransglycosylase/hydrolase (XTH)--an enzyme involved in cell wall elongation and restructuring. The corresponding LeXTH1 mRNA accumulated 6 h after attachment of the parasite. In contrast, wounding did not influence the expression level. Subsequent to LeXTH1 mRNA accumulation, an increase in XTH activity at the infection sites as well as in adjacent tissues was observed. The effect of IAA on LeXTH1 expression was analyzed because the concentration of this phytohormone is known to increase in the tomato tissue during the interaction with the parasite. LeXTH1 mRNA accumulation was in fact induced by external application of auxin. However, in the auxin-insensitive tomato mutant diageotropica, Cuscuta induced LeXTH1-mRNA accumulated with a time course similar to wild type tomato. Thus, auxin appears not to be an essential signal for infection-induced LeXTH1 activation. Our data suggest a role for xyloglucan transglycosylation in defence reactions associated with the incompatible tomato- Cuscuta interaction.

In the version of this Article originally published, Fig. 6b, which is composed of individual pictures of six plants, inadvertently and erroneously displayed the same image of one Col-0 wt plant twice. This has been corrected so that Fig. 6b now shows two different representative plants for the Col-0 wt control.

The plant parasite Cuscuta reflexa induces various responses in compatible and incompatible host plants. The visual reactions of both types of host plants including obvious morphological changes require the recognition of Cuscuta ssp. A consequently initiated signaling cascade is triggered which leads to a tolerance of the infection or, in the case of some incompatible host plants, to resistance. Calcium (Ca(2+)) release is the major second messenger during signal transduction. Therefore, we have studied Ca(2+) spiking in tomato and tobacco during infection with C. reflexa. In our recently published study Ca(2+) signals were monitored as bioluminescence in aequorin-expressing tomato plants after the onset of C. reflexa infestation. Signals at the attachment sites were observed from 30 to 48 h after infection. In an assay with leaf disks of aequorin-expressing tomato which were treated with different C. reflexa plant extracts it turned out that the substance that induced Ca(2+) release in the host plant was closely linked to the parasite's haustoria.

As part of their innate immune system plants carry a number of pattern recognition receptors (PRRs) that can detect a broad range of microbe-associated molecular patterns (MAMPs). In a recently published article (1) we described a novel, proteinaceous MAMP termed eMax (enigmatic MAMP of Xanthomonas) that derives from Xanthomonas and gets recognized by the receptor-like protein ReMAX (RECEPTOR OF eMax) of Arabidopsis thaliana. ReMAX has no ortholog in Nicotiana benthamiana and this species does not respond to eMax even when transformed with ReMAX. However, interfamily transfer of eMax perception was successful with a chimeric form of ReMAX where the C-terminal part of the protein was replaced by the corresponding part of the tomato RLP EIX2 (ETHYLENE INDUCING XYLANASE2). In this addendum we describe the difficulties with the purification and identification of the MAMP eMax and we present data demonstrating that functionality of ReMAX, much like that of related RLPs, depends on the presence of the receptor kinase SOBIR (SUPPRESSOR OF BIR1-1).

The flagellin receptor of Arabidopsis thaliana, At-FLAGELLIN SENSING2 (FLS2), has become a model for mechanistic and functional studies on plant immune receptors. Here, we started out with a comparison of At-FLS2 and the orthologous tomato (Solanum lycopersicum) receptor Sl-FLS2. Both receptors specifically responded to picomolar concentrations of the genuine flg22 ligand but proved insensitive to >106-fold higher concentrations of CLV3 peptides that have recently been reported as a second type of ligand for At-FLS2. At-FLS2 and Sl-FLS2 exhibit species-specific differences in the recognition of shortened or sequence-modified flg22 ligands. To map the sites responsible for these species-specific traits on the FLS2 receptors, we performed domain swaps, substituting subsets of the 28 leucine-rich repeats (LRRs) in At-FLS2 with the corresponding LRRs from Sl-FLS2. We found that the LRRs 7 to 10 of Sl-FLS2 determine the high affinity of Sl-FLS2 for the core part RINSAKDD of flg22. In addition, we discovered importance of the LRRs 19 to 24 for the responsiveness to C-terminally modified flagellin peptides. These results indicate that ligand perception in FLS2 is a complex molecular process that involves LRRs from both the outermost and innermost LRRs of the FLS2 ectodomain.

Over the last decade, there has been an increasing awareness of the importance of cell-to-cell communication in plants by peptide ligands and membrane-bound receptors due to striking aberrant phenotypes caused by synthetic peptides, overexpression of peptides, or mutations in peptide or receptor genes. The number of experimentally confirmed peptide-receptor signaling modules represents only a tiny fraction of the number of genes encoding putative ligands and receptors in Arabidopsis thaliana. Thus, a major challenge in plant biology is to identify new partners of plant signaling peptides. Here we present an overview of methods and prerequisites for ligand–receptor matchmaking with examples from current literature.

FLS2 and EFR are pattern recognition receptors in Arabidopsis thaliana perceiving the bacterial proteins flagellin and Elongation factor Tu (EF-Tu). Both receptors belong to the > 200 membered protein family of Leucine-Rich Repeat Receptor Kinases (LRR-RKs) in Arabidopsis. FLS2 and EFR are engaged in the activation of a common intracellular signal output and they belong to the same subfamily of LRR-RKs, sharing structural features like the intracellular kinase domain and the ectodomain organized in LRRs. On the amino acid sequence level, however, they are only < 50 % identical even in their kinase domains. In our recently published paper1 we demonstrated that it is possible to create chimeric receptors of EFR and FLS2 which are fully functional in ligand binding and receptor activation. Chimeric receptors consisting of the complete EFR ectodomain and the FLS2 kinase domain proved to be sensitive to elf18, the minimal peptide required for EF-Tu recognition, similar to the native EFR. In chimeric receptors where parts of the FLS2 ectodomain were swapped into the EFR LRR-domain, the receptor function was strongly affected even in cases with only small fragments exchanged. In this addendum we want to address problems and limits but also possibilities and chances of studying receptor functions using a chimeric approach.

Cuscuta reflexa induces a variety of reaction in its hosts. Some of these are visual reactions, and it is clear that these morphological changes are preceded by events at the molecular level, where signal transduction is one of the early processes. Calcium (Ca(2+)) release is the major second messenger during signal transduction, and we therefore studied Ca(2+) spiking in tomato during infection with C. reflexa. Bioluminescence in aequorin-expressing tomato was monitored for 48 h after the onset of Cuscuta infestation. Signals at the attachment sites were observed from 30 to 48 h. Treatment of aequorin-expressing tomato leaf disks with Cuscuta plant extracts suggested that the substance that induced Ca(2+) release from the host was closely linked to parasite haustoria.

Cuscuta spp represent a unique group of holoparasitic dicotyledonous plants which can infect nearly all dicotyledonous species. Lacking roots and leaves, these parasitic plants are completely dependent on nutrients, carbohydrates and water from host plants. The physical connection between parasite and host is medi- ated by specific organs, called haustoria, which connect the vascular tissues from both plants. Here we review the fascinating life cycle of Cuscuta spp and also the ecological aspects and problems related to Cuscuta spp infestations. For preven- tion purposes, different biotechnological approaches including hosts which show a resistance to Cuscuta spp are suggested.

Background: Plant infestation with parasitic weeds like Cuscuta reflexa induces morphological as well as biochemical changes in the host and the parasite. These modifications could be caused by a change in protein or gene activity. Using a comparative macroarray approach Cuscuta genes specifically upregulated at the host attachment site were identified. Results: One of the infestation specific Cuscuta genes encodes a cysteine protease. The protein and its intrinsic inhibitory peptide were heterologously expressed, purified and biochemically characterized. The haustoria specific enzyme was named cuscutain in accordance with similar proteins from other plants, e.g. papaya. The role of cuscutain and its inhibitor during the host parasite interaction was studied by external application of an inhibitor suspension, which induced a significant reduction of successful infection events. Conclusions: The study provides new information about molecular events during the parasitic plant - host interaction. Inhibition of cuscutain cysteine proteinase could provide means for antagonizing parasitic plants. Background Parasitic weeds such as Cuscuta reflexa are obligate holoparasites with low host specificity. The plants are found in areas with relatively mild climates around the world. In farming regions, these parasites cause substantial damage to many commercially important crops such as sugar beet, alfalfa, pepper, cucumber, tomato potato or allium [1]. Currently, an effective control of Cuscuta outbreaks is based on preventive strategies including control of seed contamination and application of herbicides prior to seed emergence. The use of herbicides on infected plants with an established host parasite interaction only appears to be successful and not harmful to