A lectin S-domain receptor kinase mediates lipopolysaccharide sensing in Arabidopsis thaliana.

The sensing of microbe-associated molecular patterns (MAMPs) triggers innate immunity in animals and plants. Lipopolysaccharide (LPS) from Gram-negative bacteria is a potent MAMP for mammals, with the lipid A moiety activating proinflammatory responses via Toll-like receptor 4 (TLR4). Here we found that the plant Arabidopsis thaliana specifically sensed LPS of Pseudomonas and Xanthomonas. We isolated LPS-insensitive mutants defective in the bulb-type lectin S-domain-1 receptor-like kinase LORE (SD1-29), which were hypersusceptible to infection with Pseudomonas syringae. Targeted chemical degradation of LPS from Pseudomonas species suggested that LORE detected mainly the lipid A moiety of LPS. LORE conferred sensitivity to LPS onto tobacco after transient expression, which demonstrated a key function in LPS sensing and indicated the possibility of engineering resistance to bacteria in crop species.

LORE receptor homomerization is required for 3-hydroxydecanoic acid-induced immune signaling and determines the natural variation of immunosensitivity within the Arabidopsis genus.

The S-domain-type receptor-like kinase (SD-RLK) LIPOOLIGOSACCHARIDE-SPECIFIC REDUCED ELICITATION (LORE) from Arabidopsis thaliana is a pattern recognition receptor that senses medium-chain 3-hydroxy fatty acids, such as 3-hydroxydecanoic acid (3-OH-C10:0), to activate pattern-triggered immunity. Here, we show that LORE homomerization is required to activate 3-OH-C10:0-induced immune signaling. Fluorescence lifetime imaging in Nicotiana benthamiana demonstrates that AtLORE homomerizes via the extracellular and transmembrane domains. Co-expression of AtLORE truncations lacking the intracellular domain exerts a dominant negative effect on AtLORE signaling in both N. benthamiana and A. thaliana, highlighting that homomerization is essential for signaling. Screening for 3-OH-C10:0-induced reactive oxygen species production revealed natural variation within the Arabidopsis genus. Arabidopsis lyrata and Arabidopsis halleri do not respond to 3-OH-C10:0, although both possess a putative LORE ortholog. Both LORE orthologs have defective extracellular domains that bind 3-OH-C10:0 to a similar level as AtLORE, but lack the ability to homomerize. Thus, ligand binding is independent of LORE homomerization. Analysis of AtLORE and AlyrLORE chimera suggests that the loss of AlyrLORE homomerization is caused by several amino acid polymorphisms across the extracellular domain. Our findings shed light on the activation mechanism of LORE and the loss of 3-OH-C10:0 perception within the Arabidopsis genus.

Bacterial rhamnolipids and their 3-hydroxyalkanoate precursors activate Arabidopsis innate immunity through two independent mechanisms.

Plant innate immunity is activated upon perception of invasion pattern molecules by plant cell-surface immune receptors. Several bacteria of the genera Pseudomonas and Burkholderia produce rhamnolipids (RLs) from l-rhamnose and (R)-3-hydroxyalkanoate precursors (HAAs). RL and HAA secretion is required to modulate bacterial surface motility, biofilm development, and thus successful colonization of hosts. Here, we show that the lipidic secretome from the opportunistic pathogen Pseudomonas aeruginosa, mainly comprising RLs and HAAs, stimulates Arabidopsis immunity. We demonstrate that HAAs are sensed by the bulb-type lectin receptor kinase LIPOOLIGOSACCHARIDE-SPECIFIC REDUCED ELICITATION/S-DOMAIN-1-29 (LORE/SD1-29), which also mediates medium-chain 3-hydroxy fatty acid (mc-3-OH-FA) perception, in the plant Arabidopsis thaliana HAA sensing induces canonical immune signaling and local resistance to plant pathogenic Pseudomonas infection. By contrast, RLs trigger an atypical immune response and resistance to Pseudomonas infection independent of LORE. Thus, the glycosyl moieties of RLs, although abolishing sensing by LORE, do not impair their ability to trigger plant defense. Moreover, our results show that the immune response triggered by RLs is affected by the sphingolipid composition of the plasma membrane. In conclusion, RLs and their precursors released by bacteria can both be perceived by plants but through distinct mechanisms.

The power of patterns: new insights into pattern-triggered immunity.

The plant immune system features numerous immune receptors localized on the cell surface to monitor the apoplastic space for danger signals from a broad range of plant colonizers. Recent discoveries shed light on the enormous complexity of molecular signals sensed by these receptors, how they are generated and removed to maintain cellular homeostasis and immunocompetence, and how they are shaped by host-imposed evolutionary constraints. Fine-tuning receptor sensing mechanisms at the molecular, cellular and physiological level is critical for maintaining a robust but adaptive host barrier to commensal, pathogenic, and symbiotic colonizers alike. These receptors are at the core of any plant-colonizer interaction and hold great potential for engineering disease resistance and harnessing beneficial microbiota to keep crops healthy.

Remodeling of Lipid A in Pseudomonas syringae pv. phaseolicola In Vitro.

Pseudomonas species infect a variety of organisms, including mammals and plants. Mammalian pathogens of the Pseudomonas family modify their lipid A during host entry to evade immune responses and to create an effective barrier against different environments, for example by removal of primary acyl chains, addition of phosphoethanolamine (P-EtN) to primary phosphates, and hydroxylation of secondary acyl chains. For Pseudomonas syringae pv. phaseolicola (Pph) 1448A, an economically important pathogen of beans, we observed similar lipid A modifications by mass spectrometric analysis. Therefore, we investigated predicted proteomes of various plant-associated Pseudomonas spp. for putative lipid A-modifying proteins using the well-studied mammalian pathogen Pseudomonas aeruginosa as a reference. We generated isogenic mutant strains of candidate genes and analyzed their lipid A. We show that the function of PagL, LpxO, and EptA is generally conserved in Pph 1448A. PagL-mediated de-acylation occurs at the distal glucosamine, whereas LpxO hydroxylates the secondary acyl chain on the distal glucosamine. The addition of P-EtN catalyzed by EptA occurs at both phosphates of lipid A. Our study characterizes lipid A modifications in vitro and provides a useful set of mutant strains relevant for further functional studies on lipid A modifications in Pph 1448A.

The Arabidopsis leucine-rich repeat receptor-like kinase MIK2 is a crucial component of early immune responses to a fungal-derived elicitor.

Fusarium spp. cause severe economic damage in many crops, exemplified by Panama disease of banana or Fusarium head blight of wheat. Plants sense immunogenic patterns (termed elicitors) at the cell surface to initiate pattern-triggered immunity (PTI). Knowledge of fungal elicitors and corresponding plant immune-signaling is incomplete but could yield valuable sources of resistance. We characterized Arabidopsis thaliana PTI responses to a peptide elicitor fraction present in several Fusarium spp. and employed a forward-genetic screen using plants containing a cytosolic calcium reporter to isolate fusarium elicitor reduced elicitation (fere) mutants. We mapped the causal mutation in fere1 to the leucine-rich repeat receptor-like kinase MDIS1-INTERACTING RECEPTOR-LIKE KINASE 2 (MIK2) and confirmed a crucial role of MIK2 in fungal elicitor perception. MIK2-dependent elicitor responses depend on known signaling components and transfer of AtMIK2 is sufficient to confer elicitor sensitivity to Nicotiana benthamiana. Arabidopsis senses Fusarium elicitors by a novel receptor complex at the cell surface that feeds into common PTI pathways. These data increase mechanistic understanding of PTI to Fusarium and place MIK2 at a central position in Arabidopsis elicitor responses.

Analysis of the Structure and Biosynthesis of the Lipopolysaccharide Core Oligosaccharide of Pseudomonas syringae pv. tomato DC3000.

Lipopolysaccharide (LPS), the major component of the outer membrane of Gram-negative bacteria, is important for bacterial viability in general and host-pathogen interactions in particular. Negative charges at its core oligosaccharide (core-OS) contribute to membrane integrity through bridging interactions with divalent cations. The molecular structure and synthesis of the core-OS have been resolved in various bacteria including the mammalian pathogen Pseudomonas aeruginosa. A few core-OS structures of plant-associated Pseudomonas strains have been solved to date, but the genetic components of the underlying biosynthesis remained unclear. We conducted a comparative genome analysis of the core-OS gene cluster in Pseudomonas syringae pv. tomato (Pst) DC3000, a widely used model pathogen in plant-microbe interactions, within the P. syringae species complex and to other plant-associated Pseudomonas strains. Our results suggest a genetic and structural conservation of the inner core-OS but variation in outer core-OS composition within the P. syringae species complex. Structural analysis of the core-OS of Pst DC3000 shows an uncommonly high phosphorylation and presence of an O-acetylated sugar. Finally, we combined the results of our genomic survey with available structure information to estimate the core-OS composition of other Pseudomonas species.

The cell wall-localized atypical ß-1,3 glucanase ZERZAUST controls tissue morphogenesis in Arabidopsis thaliana.

Orchestration of cellular behavior in plant organogenesis requires integration of intercellular communication and cell wall dynamics. The underlying signaling mechanisms are poorly understood. Tissue morphogenesis in Arabidopsis depends on the receptor-like kinase STRUBBELIG. Mutations in ZERZAUST were previously shown to result in a strubbelig-like mutant phenotype. Here, we report on the molecular identification and functional characterization of ZERZAUST We show that ZERZAUST encodes a putative GPI-anchored ß-1,3 glucanase suggested to degrade the cell wall polymer callose. However, a combination of in vitro, cell biological and genetic experiments indicate that ZERZAUST is not involved in the regulation of callose accumulation. Nonetheless, Fourier-transformed infrared-spectroscopy revealed that zerzaust mutants show defects in cell wall composition. Furthermore, the results indicate that ZERZAUST represents a mobile apoplastic protein, and that its carbohydrate-binding module family 43 domain is required for proper subcellular localization and function whereas its GPI anchor is dispensable. Our collective data reveal that the atypical ß-1,3 glucanase ZERZAUST acts in a non-cell-autonomous manner and is required for cell wall organization during tissue morphogenesis.

Altered glycosylation of exported proteins, including surface immune receptors, compromises calcium and downstream signaling responses to microbe-associated molecular patterns in Arabidopsis thaliana.

BACKGROUND: Calcium, as a second messenger, transduces extracellular signals into cellular reactions. A rise in cytosolic calcium concentration is one of the first plant responses after exposure to microbe-associated molecular patterns (MAMPs). We reported previously the isolation of Arabidopsis thaliana mutants with a "changed calcium elevation" (cce) response to flg22, a 22-amino-acid MAMP derived from bacterial flagellin. RESULTS: Here, we characterized the cce2 mutant and its weaker allelic mutant, cce3. Besides flg22, the mutants respond with a reduced calcium elevation to several other MAMPs and a plant endogenous peptide that is proteolytically processed from pre-pro-proteins during wounding. Downstream defense-related events such flg22-induced mitogen-activated protein kinase activation, accumulation of reactive oxygen species and growth arrest are also attenuated in cce2/cce3. By genetic mapping, next-generation sequencing and allelism assay, CCE2/CCE3 was identified to be ALG3 (Asparagine-linked glycosylation 3). This encodes the α-1,3-mannosyltransferase responsible for the first step of core oligosaccharide Glc3Man9GlcNAc2 glycan assembly on the endoplasmic reticulum (ER) luminal side. Complementation assays and glycan analysis in yeast alg3 mutant confirmed the reduced enzymatic function of the proteins encoded by the cce2/cce3 alleles - leading to accumulation of M5(ER), the immature five mannose-containing oligosaccharide structure found in the ER. Proper protein glycosylation is required for ER/Golgi processing and trafficking of membrane proteins to the plasma membrane. Endoglycosidase H-insensitivity of flg22 receptor, FLS2, in the cce2/cce3 mutants suggests altered glycan structures in the receptor. CONCLUSION: Proper glycosylation of MAMP receptors (or other exported proteins) is required for optimal responses to MAMPs and is important for immune signaling of host plants.

Barley disease susceptibility factor RACB acts in epidermal cell polarity and positioning of the nucleus.

RHO GTPases are regulators of cell polarity and immunity in eukaryotes. In plants, RHO-like RAC/ROP GTPases are regulators of cell shaping, hormone responses, and responses to microbial pathogens. The barley (Hordeum vulgare L.) RAC/ROP protein RACB is required for full susceptibility to penetration by Blumeria graminis f.sp. hordei (Bgh), the barley powdery mildew fungus. Disease susceptibility factors often control host immune responses. Here we show that RACB does not interfere with early microbe-associated molecular pattern-triggered immune responses such as the oxidative burst or activation of mitogen-activated protein kinases. RACB also supports rather than restricts expression of defence-related genes in barley. Instead, silencing of RACB expression by RNAi leads to defects in cell polarity. In particular, initiation and maintenance of root hair growth and development of stomatal subsidiary cells by asymmetric cell division is affected by silencing expression of RACB. Nucleus migration is a common factor of developmental cell polarity and cell-autonomous interaction with Bgh RACB is required for positioning of the nucleus near the site of attack from Bgh We therefore suggest that Bgh profits from RACB's function in cell polarity rather than from immunity-regulating functions of RACB.

Microbe-associated molecular pattern-induced calcium signaling requires the receptor-like cytoplasmic kinases, PBL1 and BIK1.

BACKGROUND: Plant perception of conserved microbe-derived or damage-derived molecules (so-called microbe- or damage-associated molecular patterns, MAMPs or DAMPs, respectively) triggers cellular signaling cascades to initiate counteracting defence responses. Using MAMP-induced rise in cellular calcium levels as one of the earliest biochemical readouts, we initiated a genetic screen for components involved in early MAMP signaling in Arabidopsis thaliana. RESULTS: We characterized here the "changed calcium elevation 5" (cce5) mutant, where five allelic cce5 mutants were isolated. They all show reduced calcium levels after elicitation with peptides representing bacteria-derived MAMPs (flg22 and elf18) and endogenous DAMP (AtPep1), but a normal response to chitin octamers. Mapping, sequencing of the mutated locus and complementation studies revealed CCE5 to encode the receptor-like cytoplasmic kinase (RLCK), avrPphB sensitive 1-like 1 (PBL1). Kinase activities of PBL1 derived from three of the cce5 alleles are abrogated in vivo. Validation with T-DNA mutants revealed that, besides PBL1, another RLCK, Botrytis-induced kinase 1 (BIK1), is also required for MAMP/DAMP-induced calcium elevations. CONCLUSIONS: Hence, PBL1 and BIK1 (but not two related RLCKs, PBS1 and PBL2) are required for MAMP/DAMP-induced calcium signaling. It remains to be investigated if the many other RLCKs encoded in the Arabidopsis genome affect early calcium signal transduction - perhaps in dependence on the type of MAMP/DAMP ligands. A future challenge would be to identify the substrates of these various RLCKs, in order to elucidate their signaling role between the receptor complexes at the plasma membrane and downstream cellular signaling components.

Ca2+ signalling in plant immune response: from pattern recognition receptors to Ca2+ decoding mechanisms.

Ca2+ is a ubiquitous second messenger for cellular signalling in various stresses and developmental processes. Here, we summarize current developments in the roles of Ca2+ during plant immunity responses. We discuss the early perception events preceding and necessary for triggering cellular Ca2+ fluxes, the potential Ca2+-permeable channels, the decoding of Ca2+ signals predominantly via Ca2+-dependent phosphorylation events and transcriptional reprogramming. To highlight the complexity of the cellular signal network, we briefly touch on the interplay between Ca2+-dependent signalling and selected major signalling mechanisms--with special emphasis on reactive oxygen species at local and systemic levels.

Interplay between calcium signalling and early signalling elements during defence responses to microbe- or damage-associated molecular patterns.

While diverse microbe- or damage-associated molecular patterns (MAMPs/DAMPs) typically trigger a common set of intracellular signalling events, comparative analysis between the MAMPs flg22 and elf18 revealed MAMP-specific differences in Ca(2+) signalling, defence gene expression and MAMP-mediated growth arrest in Arabidopsis thaliana. Such MAMP-specific differences are, in part, controlled by BAK1, a kinase associated with several receptors. Whereas defence gene expression and growth inhibition mediated by flg22 were reduced in bak1 mutants, BAK1 had no or minor effects on the same responses elicited by elf18. As the residual Ca(2+) elevations induced by diverse MAMPs/DAMPs (flg22, elf18 and Pep1) were virtually identical in bak1 mutants, a differential BAK1-mediated signal amplification to attain MAMP/DAMP-specific Ca(2+) amplitudes in wild-type plants may be hypothesized. Furthermore, abrogation of reactive oxygen species (ROS) accumulation, either in the rbohD mutant or through inhibitor application, led to loss of a second Ca(2+) peak, demonstrating a feedback effect of ROS on Ca(2+) signalling. Conversely, mpk3 mutants showed a prolonged accumulation of ROS but this did not significantly impinge on the overall Ca(2+) response. Thus, fine-tuning of MAMP/DAMP responses involves interplay between diverse signalling elements functioning both up- or downstream of Ca(2+) signalling.

ß-D-XYLOSIDASE 4 modulates systemic immune signaling in Arabidopsis thaliana.

Pectin- and hemicellulose-associated structures of plant cell walls participate in defense responses against pathogens of different parasitic lifestyles. The resulting immune responses incorporate phytohormone signaling components associated with salicylic acid (SA) and jasmonic acid (JA). SA plays a pivotal role in systemic acquired resistance (SAR), a form of induced resistance that - after a local immune stimulus - confers long-lasting, systemic protection against a broad range of biotrophic invaders. ß-D-XYLOSIDASE 4 (BXL4) protein accumulation is enhanced in the apoplast of plants undergoing SAR. Here, two independent Arabidopsis thaliana mutants of BXL4 displayed compromised systemic defenses, while local resistance responses to Pseudomonas syringae remained largely intact. Because both phloem-mediated and airborne systemic signaling were abrogated in the mutants, the data suggest that BXL4 is a central component in SAR signaling mechanisms. Exogenous xylose, a possible product of BXL4 enzymatic activity in plant cell walls, enhanced systemic defenses. However, GC-MS analysis of SAR-activated plants revealed BXL4-associated changes in the accumulation of certain amino acids and soluble sugars, but not xylose. In contrast, the data suggest a possible role of pectin-associated fucose as well as of the polyamine putrescine as regulatory components of SAR. This is the first evidence of a central role of cell wall metabolic changes in systemic immunity. Additionally, the data reveal a so far unrecognized complexity in the regulation of SAR, which might allow the design of (crop) plant protection measures including SAR-associated cell wall components.

A cell wall extract from the endophytic fungus Piriformospora indica promotes growth of Arabidopsis seedlings and induces intracellular calcium elevation in roots.

Calcium (Ca2+), as a second messenger, is crucial for signal transduction processes during many biotic interactions. We demonstrate that cellular [Ca2+] elevations are early events in the interaction between the plant growth-promoting fungus Piriformospora indica and Arabidopsis thaliana. A cell wall extract (CWE) from the fungus promotes the growth of wild-type seedlings but not of seedlings from P. indica-insensitive mutants. The extract and the fungus also induce a similar set of genes in Arabidopsis roots, among them genes with Ca2+ signalling-related functions. The CWE induces a transient cytosolic Ca2+ ([Ca2+](cyt)) elevation in the roots of Arabidopsis and tobacco (Nicotiana tabacum) plants, as well as in BY-2 suspension cultures expressing the Ca2+ bioluminescent indicator aequorin. Nuclear Ca2+ transients were also observed in tobacco BY-2 cells. The Ca2+ response was more pronounced in roots than in shoots and involved Ca2+ uptake from the extracellular space as revealed by inhibitor studies. Inhibition of the Ca2+ response by staurosporine and the refractory nature of the Ca2+ elevation suggest that a receptor may be involved. The CWE does not stimulate H2O2 production and the activation of defence gene expression, although it led to phosphorylation of mitogen-activated protein kinases (MAPKs) in a Ca2+-dependent manner. The involvement of MAPK6 in the mutualistic interaction was shown for an mpk6 line, which did not respond to P. indica. Thus, Ca2+ is likely to be an early signalling component in the mutualistic interaction between P. indica and Arabidopsis or tobacco.

A mutation in Asparagine-Linked Glycosylation 12 (ALG12) leads to receptor misglycosylation and attenuated responses to multiple microbial elicitors.

Changes in cellular calcium levels are one of the earliest signalling events in plants exposed to pathogens or other exogenous factors. In a genetic screen, we identified an Arabidopsis thaliana 'changed calcium elevation 1' (cce1) mutant with attenuated calcium response to the bacterial flagellin flg22 peptide and several other elicitors. Whole-genome resequencing revealed a mutation in asparagine-linked glycosylation 12 that encodes the mannosyltransferase responsible for adding the eighth mannose residue in an α-1,6 linkage to the dolichol-PP-oligosaccharide N-glycosylation glycan tree precursors. While properly targeted to the plasma membrane, misglycosylation of several receptors in the cce1 background suggests that N-glycosylation is required for proper functioning of client proteins.

The multifaceted functions of lipopolysaccharide in plant-bacteria interactions.

In Gram-negative bacteria, the cell envelope largely consists of lipopolysaccharide (LPS), a class of heterogeneous glycolipids. As a fundamental component of the outer membrane, LPS provides stability to the bacterial cell and forms a protective cover shielding it from hostile environments. LPS is not only fundamental to bacterial viability, but also makes a substantial contribution both directly and indirectly to multiple aspects of inter-organismic interactions. During infection of animal and plant hosts, LPS promotes bacterial virulence but simultaneously betrays bacteria to the host immune system. Moreover, dynamic remodulation of LPS structures allows bacteria to fine-tune OM properties and quickly adapt to diverse and often hostile environments, such as those encountered in host tissues. Here, we summarize recent insights into the multiple functions of LPS in plant-bacteria interactions and discuss what we can learn from the latest advances in the field of animal immunity. We further pinpoint open questions and future challenges to unravel the different roles of LPS in the dynamic interplay between bacteria and plant hosts at the mechanistic level.

Loss of wbpL disrupts O-polysaccharide synthesis and impairs virulence of plant-associated Pseudomonas strains.

Despite its importance for membrane stability and pathogenicity of mammalian pathogens, functions of the O-polysaccharide (OPS) of lipopolysaccharide (LPS) remain unclear in plant-associated bacteria. Genetic information about OPS biosynthesis in these bacteria is largely missing. Genome analysis of various plant-associated Pseudomonas strains revealed that one of the two known OPS biosynthesis clusters from Pseudomonas aeruginosa PAO1, the common polysaccharide antigen (CPA) gene cluster, is only conserved in some strains of the Pseudomonas fluorescens group. For the O-specific antigen (OSA) biosynthesis cluster, the putative genomic position could be identified, but orthologues of most functional important OSA biosynthesis enzymes could not be detected. Nevertheless, orthologues of the glycosyltransferase WbpL, required for initiation of CPA and OSA synthesis in P. aeruginosa PAO1, could be identified in the analysed Pseudomonas genomes. Knockout mutations of wbpL orthologues in Pseudomonas syringae pv. tomato DC3000 (Pst) and Pseudomonas cichorii ATCC10857/DSM50259 (Pci) resulted in strains lacking the OPS. Infection experiments of Arabidopsis thaliana plants revealed a reduced entry into the leaf apoplast after spray inoculation and a reduced apoplastic amplification of Pst ∆wbpL. Stab and spray inoculation of lettuce (Lactuca sativa) leaves with Pci ∆wbpL causes reduced infection symptoms compared to the wild-type strain. Furthermore, swarming motility was reduced in ∆wbpL mutants of Pst and Pci. This might be a possible reason for reduced bacterial titres after surface inoculation and reduced bacterial amplification in the plant. Our results imply that the presence of lipopolysaccharide OPS is required for efficient host colonization and full virulence of plant-pathogenic Pseudomonas bacteria.

Bacterial medium-chain 3-hydroxy fatty acid metabolites trigger immunity in Arabidopsis plants.

In plants, cell-surface immune receptors sense molecular non-self-signatures. Lipid A of Gram-negative bacterial lipopolysaccharide is considered such a non-self-signature. The receptor kinase LIPOOLIGOSACCHARIDE-SPECIFIC REDUCED ELICITATION (LORE) mediates plant immune responses to Pseudomonas and Xanthomonas but not enterobacterial lipid A or lipopolysaccharide preparations. Here, we demonstrate that synthetic and bacterial lipopolysaccharide-copurified medium-chain 3-hydroxy fatty acid (mc-3-OH-FA) metabolites elicit LORE-dependent immunity. The mc-3-OH-FAs are sensed in a chain length- and hydroxylation-specific manner, with free (R)-3-hydroxydecanoic acid [(R)-3-OH-C10:0] representing the strongest immune elicitor. By contrast, bacterial compounds comprising mc-3-OH-acyl building blocks but devoid of free mc-3-OH-FAs-including lipid A or lipopolysaccharide, rhamnolipids, lipopeptides, and acyl-homoserine-lactones-do not trigger LORE-dependent responses. Hence, plants sense low-complexity bacterial metabolites to trigger immune responses.