Mechanosensing and Sphingolipid-Docking Mediate Lipopeptide-Induced Immunity in Arabidopsis

Bacteria-derived lipopeptides are immunogenic triggers of host defenses in metazoans and plants. Root-associated rhizobacteria produce cyclic lipopeptides that activate systemically induced resistance (IR) against microbial infection in various plants. How these molecules are perceived by plant cells remains elusive. Here, we reveal that immunity activation inArabidopsis thalianaby the lipopeptide elicitor surfactin is mediated by docking into specific sphingolipid-enriched domains and relies on host membrane deformation and subsequent activation of mechanosensitive ion channels. This mechanism leads to host defense potentiation and resistance to the necrotrophB. cinereabut is distinct from host pattern recognition receptor-mediated immune activation and reminiscent of damage-induced plant immunity

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

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 {\dq}changed calcium elevation 5{\dq} (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

Pattern Recognition Receptors—Versatile Genetic Tools for Engineering Broad-Spectrum Disease Resistance in Crops

Infestations of crop plants with pathogens pose a major threat to global food supply. Exploiting plant defense mechanisms to produce disease-resistant crop varieties is an important strategy to control plant diseases in modern plant breeding and can greatly reduce the application of agrochemicals. The discovery of different types of immune receptors and a detailed understanding of their activation and regulation mechanisms in the last decades has paved the way for the deployment of these central plant immune components for genetic plant disease management. This review will focus on a particular class of immune sensors, termed pattern recognition receptors (PRRs), that activate a defense program termed pattern-triggered immunity (PTI) and outline their potential to provide broad-spectrum and potentially durable disease resistance in various crop species—simply by providing plants with enhanced capacities to detect invaders and to rapidly launch their natural defense program

Prominent examples of functions of the different LPS domains in mammalian and plant immunity^*.

<p>Prominent examples of functions of the different LPS domains in mammalian and plant immunity^{<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1005596#t001fn001" target="_blank">*</a>}.</p

Immune Sensing of Lipopolysaccharide in Plants and Animals: Same but Different - Fig 1

<p><b>Scheme of LPS structures of gram-negative bacteria (A) and the currently known LPS sensing systems in humans and cruciferous plants (B). A.</b> LPS structure showing a representative core region and LA from <i>Escherichia coli</i> K12 and <i>Pseudomonas aeruginosa</i>. The typical enterobacterial LA consists of a di-phosphorylated di-glucosamine with four primary and two secondary FAs (all C12/14) attached in an asymmetric fashion [<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1005596#ppat.1005596.ref003" target="_blank">3</a>]. <i>Pseudomonas</i> spp. mostly produce penta-acylated and symmetrically hexa-acylated LA with shorter fatty acids (C10/12) [<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1005596#ppat.1005596.ref036" target="_blank">36</a>]. The FAs are embedded in the OM, and the di-glucosamine is linked to the core oligosaccharide composed of about ten monosaccharides, which is conceptually subdivided into the rather variable outer core and the more conserved inner core region that usually contains heptose and the LPS-specific monosaccharide “Kdo” [<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1005596#ppat.1005596.ref003" target="_blank">3</a>,<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1005596#ppat.1005596.ref036" target="_blank">36</a>]. Often, an OPS consisting of repetitive units composed of several monosaccharides is attached to the core region. Dotted/grey lines indicate non-stoichiometric substitutions. Structures according to [<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1005596#ppat.1005596.ref036" target="_blank">36</a>,<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1005596#ppat.1005596.ref037" target="_blank">37</a>]. Abbreviations: Ara4N, 4-amino-4-deoxy-L-arabinose; Cm, carbamoyl; Etn, ethanolamine; Gal, galactose; GalN, galactosamine; Glc, glucose; GlcN, glucosamine; Hep, L-<i>glycero</i>-D-<i>manno</i>-heptose; Kdo, 3-deoxy-D-<i>manno</i>-oct-2-ulosonic acid; <i>P</i>, phosphate; Rha, L-rhamnose. <b>B.</b> In humans (left panel), LPS is sensed by different immune cells through different extra- and intracellular receptors [<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1005596#ppat.1005596.ref014" target="_blank">14</a>]. LPS is disaggregated from the bacterial membrane by the serum protein LBP and transferred to CD14, which occurs as a soluble (sCD14) and membrane-linked (mCD14) version. Dependent on the cell type, CD14 can trigger LPS signaling itself, such as calcium signaling and activation of NFAT transcription factors in dendritic cells, or further transfers LPS to the membrane-resident TLR4/MD-2 receptor complex. LA binding to a preformed TLR4/MD-2 hetero-dimer leads to association with another TLR4/MD-2-dimer and initiates intracellular signaling. Depending on the cellular localization (at the plasma membrane or in endosomes upon CD14-dependent endocytosis), TLR4/MD-2/LPS complexes activate production of either interferons or cytokines through distinct signaling adapters (TIRAP/MyD88 or TRIF/TRAM) [<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1005596#ppat.1005596.ref014" target="_blank">14</a>]. Intracellular LPS leads to oligomerization of caspase-4, activation of the non-canonical inflammasome and pyroptotic cell death [<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1005596#ppat.1005596.ref019" target="_blank">19</a>]. In plants (right panel), the bulb-type lectin S-domain-1 RLK LORE (LipoOligosaccharide-specific Reduced Elicitation) was identified as the first LPS receptor component in plants and mediates sensitive perception of <i>Pseudomonas</i> LA [<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1005596#ppat.1005596.ref030" target="_blank">30</a>]. If and how LPS is processed in the apoplast to make the membrane-embedded LA accessible for receptor binding, if LA directly binds to LORE or to an LPS-binding co-receptor and how the receptor complex and downstream signaling is activated, is yet unknown. In analogy to other SD-RLKs, LORE presumably forms dimers and is activated through mutual phosphorylation by the cytosolic kinase domain [<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1005596#ppat.1005596.ref038" target="_blank">38</a>]. Taken together, both mammals and plants sense LA as pathogen-associated molecular pattern (PAMP) but with distinct epitope specificities and through different types of receptors.</p

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