Plant Receptor-like Kinases Research Articles

Cell Signalling: The Merry Lives of BAK1

Plant receptor-like kinases characterised by leucine-rich repeats have been shown to play dual roles in seemingly unrelated biological processes, inviting comparison with TOLL-like receptors of animals.

TECHNICAL ADVANCE: Functional immobilization of plant receptor‐like kinase onto microbeads towards receptor array construction and receptor‐based ligand fishing

SummaryDespite the large number of receptor‐like kinases (RLKs) in plants, few of their specific ligands are known. Ligand fishing is one of the most challenging post‐genomic technologies. Here, we report a strategy for functional immobilization of plant RLKs on microbeads via covalent linkage. Because of the high density of immobilized RLKs, ligand‐receptor interaction can be visualized at high sensitivity using fluorescent‐labeled ligands under the confocal laser scanning microscope. Moreover, using a receptor‐based affinity chromatography system with RLK microbeads, the ligand of the receptor was directly retrieved at high purity from complex natural samples. Our RLK microbeads and receptor‐based ligand fishing approach is a feasible alternative to conventional forward genetics and bioassay‐based biochemical purification for identification of novel ligand–receptor pairs in plants.

Multiple receptor-like kinase cDNAs from liverwort Marchantia polymorpha and two charophycean green algae, Closterium ehrenbergii and Nitella axillaris: Extensive gene duplications and gene shufflings in the early evolution of streptophytes

Plant receptor-like kinases (RLKs) comprise a large family with more than several hundred members in vascular plants. The RLK family is thought to have diverged specifically in the plant kingdom, and no family member has been identified in other lineages except for animals and Plasmodium, both of which have RLK related families of small size. To know the time of divergence of RLK family members by gene duplications and domain shufflings, comprehensive isolations of RLK cDNAs were performed from a nonvascular plant, liverwort Marchantia polymorpha and two charophycean green algae, Closterium ehrenbergii, and Nitella axillaris, thought to be the closest relatives to land plants. We obtained twenty-nine, fourteen, and thirteen RLK related cDNAs from M. polymorpha, C. ehrenbergii, and N. axillaris, respectively. The amino acid sequences of these RLKs were compared with those of vascular plants, and phylogenetic trees were inferred by GAMT, a genetic algorithm-based maximum likelihood (ML) method that outputs multiple trees, together with best one. The inferred ML trees revealed ancient gene duplications generating subfamilies with different domain organizations, which occurred extensively at least before the divergence of vascular and nonvascular plants. Rather it remains possible that the extensive gene duplications occurred during the early evolution of streptophytes. Multicellular-specific somatic embryogenesis receptor kinase (SERK) involved in somatic embryogenesis was found in a unicellular alga C. ehrenbergii, suggesting the evolution of SERK by gene recruitment of a unicellular gene.

Overexpression of CRK13, an Arabidopsis cysteine‐rich receptor‐like kinase, results in enhanced resistance to Pseudomonas syringae

Protein kinases play important roles in relaying information from perception of a signal to the effector genes in all organisms. Cysteine-rich receptor-like kinases (CRKs) constitute a sub-family of plant receptor-like kinases (RLKs) with more than 40 members that contain the novel C-X8-C-X2-C motif (DUF26) in the extracellular domains. Here we report molecular characterization of one member of this gene family, CRK13. Expression of this gene is induced more quickly and strongly in response to the avirulent compared with the virulent strains of Pseudomonas syringae, and peaks within 4 h after pathogen infection. In response to dexamethasone (DEX) treatment, plants expressing the CRK13 gene from a DEX-inducible promoter exhibited all tested features of pathogen defense activation, including rapid tissue collapse, accumulation of high levels of several defense-related gene transcripts including PR1, PR5 and ICS1, and accumulation of salicylic acid (SA). In addition, these plants suppressed growth of virulent pathogens by about 20-fold compared with the wild-type Col-0. CRK13-conferred pathogen resistance is salicylic acid-dependent. Gene expression analysis using custom cDNA microarrays revealed a remarkable overlap between the expression profiles of the plants overexpressing CRK13 and the plants treated with Pst DC3000 (avrRpm1). Our studies suggest that upregulation of CRK13 leads to hypersensitive response-associated cell death, and induces defense against pathogens by causing increased accumulation of salicylic acid.

Signalling during epidermal development

The process of L1 specification early in plant embryogenesis, and subsequent maintenance and elaboration of epidermal organization, are fundamental to plant growth and fitness. To occur in a co-ordinated fashion, these processes require considerable cell-cell cross-talk. It is perhaps then unsurprising that several classes of plant RLKs (receptor-like kinases), as well as other membrane-localized signalling components, have been implicated both in epidermal specification and in patterning events governing the distribution of epidermal cell types. However, despite our growing knowledge of the roles of these signalling molecules, remarkably little is understood regarding their function at the cellular level. In particular the potential role of regulated proteolytic cleavage in controlling the activity of signalling molecules at the plant plasma membrane has remained largely unaddressed despite its massive importance in signalling in animal systems. Because of the relative physical accessibility of their expression domains, molecules involved in epidermal development present opportunities for investigating mechanisms of cell-cell signalling in planta. Advances in understanding the potential regulatory processing of membrane-localized signalling molecules during epidermal development will be examined using parallels with animal systems to highlight potential future directions for this field of research.

The Role and Regulation of Receptor-Like Kinases in Plant Defense

Receptor-like kinases (RLKs) in plants are a large superfamily of proteins that are structurally similar. RLKs are involved in a diverse array of plant responses including development, growth, hormone perception and the response to pathogens. Current studies have focused attention on plant receptor-like kinases as an important class of sentinels acting in plant defense responses. RLKs have been identified that act in both broad-spectrum, elicitor-initiated defense responses and as dominant resistance (R) genes in race-specific pathogen defense. Most defense-related RLKs are of the leucine-rich repeat (LRR) subclass although new data are highlighting other classes of RLKs as important players in defense responses. As our understanding of RLK structure, activation and signaling has expanded, the role of the ubiquitin/proteasome system in the regulation of these receptors has emerged as a central theme.

Induced Endocytosis of the Receptor Kinase FLS2

Receptor-like kinases (RLKs) that function as pattern-recognition receptors (PRRs) play a key role in plant immune responses. The receptor recognizing flagellin in Arabidopsis, FLS2, is encoded by a membrane resident RLK. FLS2 is involved in pre-invasive immunity against bacterial infection. Recent observations revealed that upon flagellin perception FLS2 accumulates in intracellular mobile vesicles and is then degraded. Reminiscent of ligand-induced receptor endocytosis in animals, FLS2 internalization is Wortmannin-sensitive. Mutation of the potentially phosphorylated residue threonine-867 impaired FLS2 endocytosis and flagellin-triggered responses. Furthermore, mutation of a PEST-motif abolished FLS2 endocytosis and downstream flagellin-elicited responses were affected. Thus, FLS2 endocytosis likely involves phosphorylation and ubiquitination events and appears to be interconnected with flagellin signaling. Similarly, TLR4, the mammalian PRR recognizing bacterial lipopolysaccharides (LPS) is internalized in a ligand specific manner. In this addendum, we discuss endocytic processes of plant RLKs focussing on FLS2 and provide a brief comparison with TLR4 endocytosis.

Domain-Specific Positive Selection Contributes to the Evolution of Arabidopsis Leucine-Rich Repeat Receptor-Like Kinase (LRR RLK) Genes

Leucine-rich repeat receptor-like kinases (LRR RLKs) comprise the largest group within the plant receptor-like kinase (RLK) superfamily, and the Arabidopsis genome alone contains over 200 LRR RLK genes. Although there is clear evidence for diverse roles played by individual LRR RLK genes in Arabidopsis growth and development, the evolutionary mechanism for this functional diversification is currently unclear. In this study, we focused on the LRRII RLK subfamily to investigate the molecular mechanisms that might have led to the functional differentiation of Arabidopsis LRR RLK genes. Phylogenetic analysis of 14 genes in this subfamily revealed three well-supported groups (I, II, and III). RT-PCR analysis did not find many qualitative differences in expression among these 14 genes in various Arabidopsis tissues, suggesting that evolution of regulatory sequences did not play a major role in their functional divergence. We analyzed substitution patterns in the predicted ligand-binding regions of these genes to examine if positive selection has acted to produce novel ligand-binding specificities, using the nonsynonymous/synonymous rate ratio (d (N)/d (S)) as an indicator of selective pressure. Estimates of d (N)/d (S) ratios from multiple methods indicate that nonsynonymous substitutions accumulated during divergence of the three lineages. Positive selection is likely to have occurred along the lineages ancestral to groups II and III. We suggest that positive selection on the ligand-binding sites of LRRII RLKs promoted diversification of ligand-binding specificities and thus contributed to the functional differentiation of Arabidopsis LRRII RLK genes during evolution.

Evidence for lectin activity of a plant receptor-like protein kinase by application of neoglycoproteins and bioinformatic algorithms

Detection of genes for putative receptor-like protein kinases, which contain an extracellular domain related to leguminous lectins, in plant genomes inspired the hypothesis that this part acts as sensor. Initial support for this concept came from proof for protein kinase activity. The next step, focusing on the protein of lombardy poplar (Populus nigra var. italica), is scrutiny for lectin activity. Consequently, we first pinpointed sets of high-scoring sequence pairs by extensive databank search. The calculations resulted in P-values in the range from 10(-14) to 10(-18) exclusively for leguminous lectins, the Pterocarpus angolensis agglutinin being front runner with P=3 x 10(-18) and thus most suitable template for modeling. The superimposition of the two folds gave notable similarity in the region responsible for binding carbohydrate and Ca(2+)/Mn(2+)-ions. Binding activity toward carbohydrates was detected by assaying a panel of (neo)glycoproteins as polyvalent probes, especially for alpha-l-rhamnose and glycans of asialofetuin. It was strictly dependent on Ca(2+)-ions, enhanced by Mn(2+)-ions and reached a K(D)-value of 34.3 nM for the neoglycoprotein with rhamnose as ligand. These results give further research direction to define physiological ligands, plant/bacterial rhamnose-containing saccharides and rhamnose-mimetic glycans or peptides being potential candidates.

Regulation of Plant Symbiosis Receptor Kinase through Serine and Threonine Phosphorylation

We studied the biochemical properties of a plant receptor-like kinase to gain insights into the regulatory mechanism of this largest class of plant kinases. SYMRK (symbiosis receptor kinase) is required for early signal transduction leading to plant root symbioses with nitrogen-fixing rhizobia and phosphate-acquiring arbuscular mycorrhizal fungi. Amino acid substitutions in positions critical for activity of other related kinases cause a nonsymbiotic plant phenotype, suggesting that SYMRK kinase activity is required for symbiosis. SYMRK is capable of intermolecular autophosphorylation. Nonphosphorylated SYMRK is less active than the phosphorylated version, suggesting the phosphorylation status of SYMRK determines its activity. Three Ser/Thr residues were identified as residues required for full kinase activation through targeted mutagenesis. Using quadrupole time-of-flight mass spectrometry analysis, two of these were confirmed to be phosphorylated in vitro. These crucial phosphorylation sites are conserved among various plant receptor-like kinases as well as animal Pelle/interleukin-1 receptor associated kinase. Despite the distinct domain architecture of receptor-like kinases versus Pelle/interleukin-1 receptor associated kinase, our results suggest the existence of conserved activation mechanisms.

Calcium/Calmodulin Up-regulates a Cytoplasmic Receptor-like Kinase in Plants

Calcium/calmodulin-dependent kinases play an important role in protein phosphorylation in eukaryotes. However, not much is known about calcium/calmodulin-dependent protein phosphorylation and its role in signal transduction in plants. By using a protein-protein interaction-based approach, we have isolated a novel plant-specific calmodulin-binding receptor-like cytoplasmic kinase (CRCK1) from Arabidopsis thaliana, as well as its ortholog from Medicago sativa (alfalfa). CRCK1 does not show high homology to calcium/calmodulin-dependent protein kinases in animals. In contrast, it shows high homology in the kinase domain to serine/threonine receptor-like kinases in plants. However, it contains neither a transmembrane domain nor an extracellular domain. Calmodulin binds to CRCK1 in a calcium-dependent manner with an affinity of approximately 20.5 nm. The calmodulin-binding site in CRCK1 is located in amino acids 160-183, which overlap subdomain II of the kinase domain. CRCK1 undergoes autophosphorylation in the presence of Mg2+ at the threonine residue(s). The Km and Vmax values of CRCK1 for ATP are 1 microm and 33.6 pmol/mg/min, respectively. Calcium/calmodulin stimulates the kinase activity of CRCK1, which increases the Vmax of CRCK1 approximately 9-fold. The expression of CRCK1 is increased in response to stresses such as cold and salt and stress molecules such as abscisic acid and hydrogen peroxide. These results indicate the presence of a calcium/calmodulin-regulated receptor-like cytoplasmic kinase in plants. Furthermore, these results also suggest that calcium/calmodulin-regulated protein phosphorylation involving CRCK1 plays a role in stress signal transduction in plants.

A receptor-like kinase from Arabidopsis thaliana is a calmodulin-binding protein.

Screening a cDNA expression library with a radiolabelled calmodulin (CaM) probe led to the isolation of AtCaMRLK, a receptor-like kinase (RLK) of Arabidopsis thaliana. AtCaMRLK polypeptide sequence shows a modular organization consisting of the four distinctive domains characteristic of receptor kinases: an amino terminal signal sequence, a domain containing seven leucine-rich repeats, a single putative membrane-spanning segment and a protein kinase domain. Using truncated versions of the protein and a synthetic peptide, we demonstrated that a region of 23 amino acids, located near the kinase domain of AtCaMRLK, binds CaM in a calcium-dependent manner. Real-time binding experiments showed that AtCaMRLK interacted in vitro with AtCaM1, a canonical CaM, but not with AtCaM8, a divergent isoform of the Ca2+ sensor. The bacterially expressed kinase domain of the protein was able to autophosphorylate and to phosphorylate the myelin basic protein, using Mn2+ preferentially to Mg2+ as an ion activator. Site-directed mutagenesis of the conserved lysine residue (Lys423) to alanine, in the kinase subdomain II, resulted in a complete loss of kinase activity. CaM had no influence on the autophosphorylation activity of AtCaMRLK. AtCaMRLK was expressed in reproductive and vegetative tissues of A. thaliana, except in leaves. Disruption in the AtCaMRLK coding sequence by insertion of a DsG transposable element in an Arabidopsis mutant did not generate a discernible phenotype. The CaM-binding motif of AtCaMRLK was found to be conserved in several other members of the plant RLK family, suggesting a role for Ca2+/CaM in the regulation of RLK-mediated pathways.

Symbiosis Receptor Identified

Two groups (Stracke et al. and Endre et al. ) report the cloning of a receptor involved in the symbiotic relationship between fungi and plants and between bacteria and plants. The fungi or bacteria supply essential nutrients in exchange for carbon from the plants, allowing the plants to live in nutrient-poor environments. Stracke et al. cloned the gene mutated in Lotus japonicus that results in the inability of the plants to be colonized by either arbuscular mycorrhizal fungi or the bacteria that result in the formation of root nodules in the host plant. Endre et al. cloned the gene from a mutant alfalfa that cannot form root nodules or be colonized by arbuscular micorrhizal fungi. Each group identified a leucine-rich repeat (LRR) transmembrane protein with a putative intracellular kinase domain. These LRR-containing receptors are a very large family in plants, with Arabidopsis having 174 members. In plants and animals, LRR-containing receptors have been implicated in developmental processes and immune signaling. Thus, symbiosis signaling can be added to the list of functions of this family of proteins (see Spaink). S. Stracke, C Kistner, S. Yoshida, L. Mulder, S. Sato, T. Kaneko, S. Tabata, N. Sandal, J. Stougaard, K. Szczyglowski, M. Parniske, A plant receptor-like kinase required for both bacterial and fungal symbiosis. Nature 417 , 959-962 (2002). [Online Journal] G. Endre, A. Kereszt, Z. Kevel, S. Mihacea, P. Kaló, G. B. Kiss, A receptor kinase gene regulating symbiotic nodule development. Nature 417 , 962-966 (2002). [Online Journal] H. P. Spaink, A receptor in symbiotic dialogue. Nature 417 , 910-911 (2002). [Online Journal]

A plant receptor-like kinase required for both bacterial and fungal symbiosis

Most higher plant species can enter a root symbiosis with arbuscular mycorrhizal fungi, in which plant carbon is traded for fungal phosphate. This is an ancient symbiosis, which has been detected in fossils of early land plants. In contrast, the nitrogen-fixing root nodule symbioses of plants with bacteria evolved more recently, and are phylogenetically restricted to the rosid I clade of plants. Both symbioses rely on partially overlapping genetic programmes. We have identified the molecular basis for this convergence by cloning orthologous SYMRK ('symbiosis receptor-like kinase') genes from Lotus and pea, which are required for both fungal and bacterial recognition. SYMRK is predicted to have a signal peptide, an extracellular domain comprising leucine-rich repeats, a transmembrane and an intracellular protein kinase domain. Lotus SYMRK is required for a symbiotic signal transduction pathway leading from the perception of microbial signal molecules to rapid symbiosis-related gene activation. The perception of symbiotic fungi and bacteria is mediated by at least one common signalling component, which could have been recruited during the evolution of root nodule symbioses from the already existing arbuscular mycorrhiza symbiosis.

A receptor-like protein kinase with a lectin-like domain from lombardy poplar: gene expression in response to wounding and characterization of phosphorylation activity

Plant receptor-like protein kinases (RLKs) are thought to be involved in various cellular processes mediated via signal transduction pathways. To clarify the initial step in such a signal transduction pathway in woody plants, we cloned a cDNA encoding PnLPK (a P opulus n igra var. italica lectin-like protein kinase) from lombardy poplar. The C-terminal region of the predicted PnLPK protein includes a protein kinase catalytic domain consisting of the 12 subdomains typical of the eukaryotic protein kinase superfamily. Following the signal peptide at the N-terminus, a domain that shows homology to legume lectins retains the putative Mn(2+)- and Ca(2+)-binding amino acids, which are highly conserved among lectin-related proteins. Because a putative hydrophobic transmembrane domain was localized between the lectin-like domain and the protein kinase domain, PnLPK was determined to be a member of the plant RLK subfamily with a lectin-like domain. Transcripts of the PnLPK gene accumulate in roots, mature leaves and calli of lombardy poplar, whereas only trace amounts of the transcripts are detectable in stems, young leaves and apical buds. Wounding of the young leaves increased the amount of PnLPK mRNA, but none of several phytohormones tested had any effect on the transcription of PnLPK. When incubated in the presence of divalent metal cations such as Mn(2+), the C-terminal catalytic domain of PnLPK showed significantly higher autophosphorylation activity than the full-length PnLPK protein. The phosphorylation activity of PnLPK was also detected using beta-casein as substrate. Phosphoamino acid analysis indicated that PnLPK is a serine/threonine kinase.

Plant receptor-like kinase gene family: diversity, function, and signaling.

Plant receptor-like kinases (RLKs) are transmembrane proteins with putative amino-terminal extracellular domains and carboxyl-terminal intracellular kinase domains, with striking resemblance in domain organization to the animal receptor tyrosine kinases such as epidermal growth factor receptor. The recently sequenced Arabidopsis genome contains more than 600 RLK homologs, representing nearly 2.5% of the annotated protein-coding genes in Arabidopsis. Although only a handful of these genes have known functions and fewer still have identified ligands or downstream targets, the studies of several RLKs such as CLAVATA1, Brassinosteroid Insensitive 1, Flagellin Insensitive 2, and S-locus receptor kinase provide much-needed information on the functions mediated by members of this large gene family. RLKs control a wide range of processes, including development, disease resistance, hormone perception, and self-incompatibility. Combined with the expression studies and biochemical analysis of other RLKs, more details of RLK function and signaling are emerging.

Plant Receptor-Like Kinase Gene Family: Diversity, Function, and Signaling

Plant Receptor-Like Kinase Gene Family: Diversity, Function, and Signaling

Receptor-like kinases from Arabidopsis form a monophyletic gene family related to animal receptor kinases.

Plant receptor-like kinases (RLKs) are proteins with a predicted signal sequence, single transmembrane region, and cytoplasmic kinase domain. Receptor-like kinases belong to a large gene family with at least 610 members that represent nearly 2.5% of Arabidopsis protein coding genes. We have categorized members of this family into subfamilies based on both the identity of the extracellular domains and the phylogenetic relationships between the kinase domains of subfamily members. Surprisingly, this structurally defined group of genes is monophyletic with respect to kinase domains when compared with the other eukaryotic kinase families. In an extended analysis, animal receptor kinases, Raf kinases, plant RLKs, and animal receptor tyrosine kinases form a well supported group sharing a common origin within the superfamily of serine/threonine/tyrosine kinases. Among animal kinase sequences, Drosophila Pelle and related cytoplasmic kinases fall within the plant RLK clade, which we now define as the RLK/Pelle family. A survey of expressed sequence tag records for land plants reveals that mosses, ferns, conifers, and flowering plants have similar percentages of expressed sequence tags representing RLK/Pelle homologs, suggesting that the size of this gene family may have been close to the present-day level before the diversification of land plant lineages. The distribution pattern of four RLK subfamilies on Arabidopsis chromosomes indicates that the expansion of this gene family is partly a consequence of duplication and reshuffling of the Arabidopsis genome and of the generation of tandem repeats.

Interaction of the Arabidopsis Receptor Protein Kinase Wak1 with a Glycine-rich Protein, AtGRP-3

The Arabidopsis wall-associated receptor kinase, Wak1, is a member of the Wak family (Wak1-5) that links the plasma membrane to the extracellular matrix. By the yeast two-hybrid screen, we found that a glycine-rich extracellular protein, AtGRP-3, binds to the extracellular domain of Wak1. Further in vitro binding studies indicated that AtGRP-3 is the only isoform among the six tested AtGRPs that specifically interacts with Waks, and the cysteine-rich carboxyl terminus of AtGRP-3 is essential for its binding to Wak1. We also show that Wak1 and AtGRP-3 form a complex with a molecular size of approximately 500 kDa in vivo in conjunction with the kinase-associated protein phosphatase, KAPP, that has been shown to interact with a number of plant receptor-like kinases. Binding of AtGRP-3 to Wak1 is shown to be crucial for the integrity of the complex. Wak1 and AtGRP-3 are both induced by salicylic acid treatment. Moreover, exogenously added AtGRP-3 up-regulates the expression of Wak1, AtGRP-3, and PR-1 (for pathogenesis-related) in protoplasts. Taken together, our data suggest that AtGRP-3 regulates Wak1 function through binding to the cell wall domain of Wak1 and that the interaction of Wak1 with AtGRP-3 occurs in a pathogenesis-related process in planta.

Oligopeptide signalling and the action of systemin.

In the 19th century, the German botanist Julius von Sachs postulated the existence of chemical messengers that are responsible for the regulation of plant growth and development. The first such messenger was identified in the 1930s as indole-3-acetic acid, an auxin. In the following years, four additional classes of phytohormones were discovered: cytokinins, gibberellins, abscisic acid and ethylene. For decades, these classical phytohormones have been and still are at the centre of research in this area and it was generally believed that plant hormone action is fundamentally different from that in animals. This view is changing, however, since over recent years brassinosteroids and jasmonates, resembling animal steroids and prostaglandins, respectively, have been added to the group of chemical messengers in plants. Furthermore, there is now compelling evidence for the existence of plant peptide hormones. Indirect evidence is derived from the observation that plants possess putative receptor proteins for (poly)peptide ligands. Over 40 receptor-like kinases (RLKs) have been identified in sequence databases (Xiaorong, 1998). These RLKs comprise three functional domains, an extracellular domain, the transmembrane domain and an intracellular catalytic kinase domain. They appear to be involved in processes as diverse as embryogenesis, morphogenesis, selfincompatibility, light and hormone signal transduction, and pathogen resistance. The extracellular domains of RLKs contain regions of leucine-rich repeats (LRRs) which are known to form amphipathic peptide surfaces involved in protein-protein interactions. Based on the sequences of the LRRs and their structural similarity to animal LRR motifs it seems likely that plant RLKs bind peptide ligands which, however, remain to be identified. LRRs are also typically found in resistance gene-encoded proteins that are hypothesized to be the receptors of race-specific polypeptide elicitors produced by phytopathogenic bacteria, fungi, and viruses (Baker et al., 1997). On the other hand, three classes of endogenous oligopeptides have been discovered clearly having signalling function in plants. These include ENOD40, phytosulfokines and systemin and will be the subject of this review article. In a first section, the most salient features of these peptides will be summarized and compared. Most of these data have been reviewed already and for references I refer the reader to recent reviews on systemin (Ryan and Pearce, 1998; Bowles, 1998) and enod40 (Barciszewski and Legocki, 1997; Franssen, 1998) and to the most recent paper on phytosulfokines (Matsubayashi et al., 1999). After this introductory section, the main focus of the article will be on very recent observations that shed light on the molecular mechanisms involved in systemin perception and in the subsequent activation of cellular responses.