The Vitis vinifera receptor VvLYK6 negatively regulates chitin-triggered immune responses and promotes fungal infections

Recognition of microbial challenges leads to enhanced immunity at both the local and systemic levels. In Arabidopsis, EFR and PEPR1/PEPR2 act as the receptor for the bacterial elongation factor EF-Tu (elf18 epitope) and for the endogenous PROPEP-derived Pep epitopes, respectively. The PEPR pathway has been described to mediate defence signalling following microbial recognition. Here we show that PROPEP2/PROPEP3 induction upon pathogen challenges is robust against jasmonate, salicylate, or ethylene dysfunction. Comparative transcriptome profiling between Pep2- and elf18-treated plants points to co-activation of otherwise antagonistic jasmonate- and salicylate-mediated immune branches as a key output of PEPR signalling. Accordingly, as well as basal defences against hemibiotrophic pathogens, systemic immunity is reduced in pepr1 pepr2 plants. Remarkably, PROPEP2/PROPEP3 induction is essentially restricted to the pathogen challenge sites during pathogen-induced systemic immunity. Localized Pep application activates genetically separable jasmonate and salicylate branches in systemic leaves without significant PROPEP2/PROPEP3 induction. Our results suggest that local PEPR activation provides a critical step in connecting local to systemic immunity by reinforcing separate defence signalling pathways.

Plants use a plethora of sophisticated detection systems to recognize a variety of attackers and to subsequently initiate defense responses. A well-known paradigm in this context is the perception of microbe-associated molecular patterns (MAMPs) by pattern recognition receptors (PRRs), a process referred to as pattern triggered immunity (PTI). Additionally, plants also recognize endogenous molecules to induce similar defense responses. These molecules are believed to be released upon enemy attack and are therefore referred to as danger-associated molecular patterns (DAMPs). The best-investigated DAMP so far is systemin, a short peptide capable of inducing defense responses in tomato and required for full-strength defense against insect herbivores. More recently, a family of eight peptides has been discovered in Arabidopsis, named Arabidopsis thaliana danger peptides (AtPeps) 1-8. These AtPeps have been shown to be capable of inducing PTI-like responses and to be expressed upon the detection of various biotic stresses, therefore being considered as DAMPs. Moreover, two PRRs, named Pep-Receptor 1 (PEPR1) and Pep-Receptor 2 (PEPR2) have been identified to perceive AtPeps and to induce defense responses upon receptor-ligand interaction. Despite of eliciting PTI responses and being expressed upon the detection of biotic stress, no direct beneficial involvement of the AtPep-PEPR system to plant defense against attackers has been described so far. Taking advantage of a mutant deficient in both PEPRs and thus fully impaired in AtPep-PEPR signaling, we investigated the potential contribution of a functional AtPep-PEPR system to plant defense responses. In a first approach, we investigated the potential interplay between MAMP and DAMP signaling, especially in the context of DAMPs being believed to act as endogenous amplifiers of MAMP-induced PTI. Doing so, we identified that the AtPep-triggered production of reactive oxygen species (ROS) is strongly enhanced upon previous MAMP detection, indeed indicating a role of the AtPep-PEPR signaling system as an enhancer of MAMP-triggered defense signaling. In a second approach, we compared the AtPep-PEPR system to systemins – well described DAMPs in tomato with generally similar molecular features to AtPeps. Following up the lead that systemins are important mediators of defense responses against herbivorous insects, we checked whether a similar role would apply to the AtPep-PEPR system. Here, we could show that the AtPep-PEPR system is indeed induced by herbivore feeding and strongly interacts with the plant hormone jasmonic acid (JA) to orchestrate defense responses. Accordingly, mutants deficient in AtPep-PEPR signaling are strongly impaired in defense responses against the generalist herbivore Spodoptera littoralis, underlining the importance of AtPep signaling in plant defense against herbivores. Thirdly, we followed up a lead that the expression of some AtPeps as well as both PEPRs is induced upon virus infection. Assessing the potential contribution of the AtPep-PEPR system to plant defense against viruses, we could not observe an increased susceptibility of plants deficient in both PEPRs. However, mutants in BAK1 (BRI1 Associated Kinase 1), a co-receptor required for full-strength AtPep-triggered signaling and many other PRRs, showed a clearly increased susceptibility to all viruses tested. Therefore, we established a first potential line of evidence for a role of PTI in plant defense against viruses. All in all, we provide several lines of evidence that show the contribution of a functional AtPep-PEPR signaling system to plant defense. Therefore, we underline the pivotal importance of DAMP signaling to plant immunity against a plethora of biotic invaders.

Plant Immunity: AvrPto Targets the Frontline

The first layer of innate immunity in plants is initiated through the perception of microbe-associated molecular patterns (MAMPs) or damage-associated molecular patterns (DAMPs) by pattern recognition receptors. MAMP/DAMP perception initiates downstream defense responses, a process which ultimately leads to pattern triggered immunity, as reviewed in the first chapter of this thesis. In the second chapter of this thesis, based on a deep-sequencing expression profiling approach, a number of hitherto overlooked genes have been identified that are induced in wild type Arabidopsis seedlings upon treatment with both the MAMP, flg22, and the DAMP, AtPep1. This implies the possible involvement of the corresponding gene products in innate immunity. Four of them, named PP2-B13, ACLP1, SERP1 and GRP89, respectively, were studied in more detail. Homozygous mutant lines for the genes encoding these proteins were obtained and analyzed. The mutants pp2-b13, aclp1, serp1 and grp89 exhibited an increased susceptibility to infection by the virulent pathogen P. syringae pv. tomato DC3000 and also by its avirulent hrcC mutant. Furthermore, it was observed that the aclp1 mutant was deficient in ethylene production upon flg22 treatment, while the mutants pp2-b13, serp1 and grp89 were deficient in reactive oxygen species production. As mentioned, in addition to MAMPs, plants can sense and recognize DAMPs, i.e. endogenous elicitors which activate the immune system in response to biotic and also abiotic stimuli. So far, eight peptides have been described as DAMPs or endogenous danger peptides in Arabidopsis thaliana, named AtPeps1-8. These peptides are derived from precursor proteins called the AtPROPEPs. The leucine-rich-repeat receptor kinases, AtPEPR1 and AtPEPR2, act as the receptors for the AtPep peptides. In the third chapter of this thesis, promoter-GUS reporter constructs were used to study the expression pattern of the genes encoding the AtPROPEPs as well as the AtPEPRs under biotic and abiotic stress, including AtPep1, flg22, Methyl jasmonate, and NaCl treatments. We found that the genes for the two AtPEPR receptors were differentially regulated in response to MAMPs (flg22) and DAMPs (AtPEP1). In addition, we showed that the activation pattern of the genes encoding the eight AtPROPEPs was totally different, despite the similarity of the members of the Pep family. This allowed us to classify the activity of the AtPROPEP promoters, based on their differential response to biotic and abiotic stimuli.

Introduction to a Virtual Special Issue on cell biology at the plant-microbe interface.

Switching on Plant Immune Signaling Systems Using Microbe-Associated Molecular Patterns

Plasma membrane-borne pattern recognition receptors, which recognize microbe-associated molecular patterns and endogenous damage-associated molecular patterns, provide the first line of defense in innate immunity. In plants, leucine-rich repeat receptor kinases fulfill this role, as exemplified by FLS2 and EFR, the receptors for the microbe-associated molecular patterns flagellin and elongation factor Tu. Here we examined the perception of the damage-associated molecular pattern peptide 1 (AtPep1), an endogenous peptide of Arabidopsis identified earlier and shown to be perceived by the leucine-rich repeat protein kinase PEPR1. Using seedling growth inhibition, elicitation of an oxidative burst and induction of ethylene biosynthesis, we show that wild type plants and the pepr1 and pepr2 mutants, affected in PEPR1 and in its homologue PEPR2, are sensitive to AtPep1, but that the double mutant pepr1/pepr2 is completely insensitive. As a central body of our study, we provide electrophysiological evidence that at the level of the plasma membrane, AtPep1 triggers a receptor-dependent transient depolarization through activation of plasma membrane anion channels, and that this effect is absent in the double mutant pepr1/pepr2. The double mutant also fails to respond to AtPep2 and AtPep3, two distant homologues of AtPep1 on the basis of homology screening, implying that the PEPR1 and PEPR2 are responsible for their perception too. Our findings provide a basic framework to study the biological role of AtPep1-related danger signals and their cognate receptors.

Plant innate immunity is a potential basal defense system existing in plant kingdom. This system provides powerful weapons to the host plants to fight against viral, bacterial, fungal, and oomycete pathogens and serves as a surveillance system against invasion of pathogens. It is not active in normal healthy plants and it requires specific signals to get activated. Pathogen-associated molecular patterns (PAMPs) act as alarm/danger signals to trigger the plant innate immune responses. When pathogens land on the plant’s surface, plants read the molecular fingerprints/signatures of pathogens (PAMPs) by binding the PAMPs with cognate pattern-recognition receptors (PRRs) residing in plant cell plasma membrane and trigger several defense signaling systems. Pathogens contain a wide array of PAMPs of diverse chemical structures and every pathogen contains or secretes multiple PAMPs. Each PAMP may regulate induction of different defense genes. The time of induction, intensity of induction, and duration of induction of the defense signals may vary depending on PAMPs. Amount of PAMP available in the plant-pathogen interaction site may determine the intensity of induced gene expression. Each PAMP may regulate distinctly different signaling pathway(s). Sometimes different PAMPs may induce the same signaling system, but the intensity of the defense signaling gene expression may differ. The same PAMP may behave differently in different plant system. A single PAMP may not be able to activate all the defense signaling-related genes and several PAMPs may be required to activate the complex signaling systems. PAMPs may act synergistically or antagonistically in inducing defense signaling. Some PAMPs have additive effect, while others show antagonistic effect between them. The PAMPs are perceived as danger signals by PRRs and the PAMP-PRR complex activates the plant innate immunity. PAMPs trigger phosphorylation of PRRs. Fine control of membrane-resident PRR activity is essentially achieved by a combination of proper endoplasmic reticulum (ER) folding, degradation and trafficking of PRRs. Strict elimination of the misfolded PRR occurs in the absence of the identified ER folding machineries, which would avoid precocious immune activation. Pre-recognition membrane traffic of PRRs from the ER to their functional sites, together with post-recognition internalization is crucial for PRR function. The signals generated by PAMPs are perceived by PRRs and several second messengers are involved in transmission of the signals downstream of the PRRs. Highly complex networks of signaling pathways are activated by the PAMP-PRR signaling system.KeywordsInnate immunityPAMPsPRRsPAMP-PRR signaling complex • Second messengers • Trafficking of PRRs

The Arabidopsis mutant wrky33 is highly susceptible to Botrytis cinerea. We identified >1680 Botrytis-induced WRKY33 binding sites associated with 1576 Arabidopsis genes. Transcriptional profiling defined 318 functional direct target genes at 14 hr post inoculation. Comparative analyses revealed that WRKY33 possesses dual functionality acting either as a repressor or as an activator in a promoter-context dependent manner. We confirmed known WRKY33 targets involved in hormone signaling and phytoalexin biosynthesis, but also uncovered a novel negative role of abscisic acid (ABA) in resistance towards B. cinerea 2100. The ABA biosynthesis genes NCED3 and NCED5 were identified as direct targets required for WRKY33-mediated resistance. Loss-of-WRKY33 function resulted in elevated ABA levels and genetic studies confirmed that WRKY33 acts upstream of NCED3/NCED5 to negatively regulate ABA biosynthesis. This study provides the first detailed view of the genome-wide contribution of a specific plant transcription factor in modulating the transcriptional network associated with plant immunity.DOI: http://dx.doi.org/10.7554/eLife.07295.001

Plants are sessile organisms that are under constant attack from microbes. They rely on both preformed defenses, and their innate immune system to ward of the microbial pathogens. Preformed defences include for example the cell wall and cuticle, which act as physical barriers to microbial colonization. The plant immune system is composed of surveillance systems that perceive several general microbe elicitors, which allow plants to switch from growth and development into a defense mode, rejecting most potentially harmful microbes. The elicitors are essential structures for pathogen survival and are conserved among pathogens. The conserved microbe-specific molecules, referred to as microbe- or pathogen-associated molecular patterns (MAMPs or PAMPs), are recognized by the plant innate immune systems pattern recognition receptors (PRRs). General elicitors like flagellin (Flg), elongation factor Tu (EF-Tu), peptidoglycan (PGN), lipopolysaccharides (LPS), Ax21 (Activator of XA21-mediated immunity in rice), fungal chitin, and β-glucans from oomycetes are recognized by plant surface localized PRRs. Several of the MAMPs and their corresponding PRRs have, in recent years, been identified. This review focuses on the current knowledge regarding important MAMPs from bacteria, fungi, and oomycetes, their structure, the plant PRRs that recognizes them, and how they induce MAMP-triggered immunity (MTI) in plants.

SummaryPattern‐triggered immunity (PTI) is activated in plants upon recognition by pattern recognition receptors (PRRs) of damage‐ and microbe‐associated molecular patterns (DAMPs and MAMPs) derived from plants or microorganisms, respectively. To understand better the plant mechanisms involved in the perception of carbohydrate‐based structures recognized as DAMPs/MAMPs, we have studied the ability of mixed‐linked β‐1,3/1,4‐glucans (MLGs), present in some plant and microbial cell walls, to trigger immune responses and disease resistance in plants. A range of MLG structures were tested for their capacity to induce PTI hallmarks, such as cytoplasmic Ca2+ elevations, reactive oxygen species production, phosphorylation of mitogen‐activated protein kinases and gene transcriptional reprogramming. These analyses revealed that MLG oligosaccharides are perceived by Arabidopsis thaliana and identified a trisaccharide, β‐d‐cellobiosyl‐(1,3)‐β‐d‐glucose (MLG43), as the smallest MLG structure triggering strong PTI responses. These MLG43‐mediated PTI responses are partially dependent on LysM PRRs CERK1, LYK4 and LYK5, as they were weaker in cerk1 and lyk4 lyk5 mutants than in wild‐type plants. Cross‐elicitation experiments between MLG43 and the carbohydrate MAMP chitohexaose [β‐1,4‐d‐(GlcNAc)6], which is also perceived by these LysM PRRs, indicated that the mechanism of MLG43 recognition could differ from that of chitohexaose, which is fully impaired in cerk1 and lyk4 lyk5 plants. MLG43 treatment confers enhanced disease resistance in A. thaliana to the oomycete Hyaloperonospora arabidopsidis and in tomato and pepper to different bacterial and fungal pathogens. Our data support the classification of MLGs as a group of carbohydrate‐based molecular patterns that are perceived by plants and trigger immune responses and disease resistance.

A Spätzle-Processing Enzyme Required for Toll Signaling Activation in Drosophila Innate Immunity

Proteins of the nucleotide-binding domain, leucine-rich repeat (NLR)-containing family recently gained attention as important components of the innate immune system. Although over 20 of these proteins are present in humans, only a few members including the cytosolic pattern recognition receptors NOD1, NOD2, and NLRP3 have been analyzed extensively. These NLRs were shown to be pivotal for mounting innate immune response toward microbial invasion. Here we report on the characterization of human NLRC5 and provide evidence that this NLR has a function in innate immune responses. We found that NLRC5 is a cytosolic protein expressed predominantly in hematopoetic cells. NLRC5 mRNA and protein expression was inducible by the double-stranded RNA analog poly(I.C) and Sendai virus. Overexpression of NLRC5 failed to trigger inflammatory responses such as the NF-kappaB or interferon pathways in HEK293T cells. However, knockdown of endogenous NLRC5 reduced Sendai virus- and poly(I.C)-mediated type I interferon pathway-dependent responses in THP-1 cells and human primary dermal fibroblasts. Taken together, this defines a function for NLRC5 in anti-viral innate immune responses.

Exciting Developments in the Immunology of Fungal Infections

Since signaling machineries for two modes of plant-induced immunity, pattern-triggered immunity (PTI) and effector-triggered immunity (ETI), extensively overlap, PTI and ETI signaling likely interact. In an Arabidopsis quadruple mutant, in which four major sectors of the signaling network, jasmonate, ethylene, PAD4, and salicylate, are disabled, the hypersensitive response (HR) typical of ETI is abolished when the Pseudomonas syringae effector AvrRpt2 is bacterially delivered but is intact when AvrRpt2 is directly expressed in planta These observations led us to discovery of a network-buffered signaling mechanism that mediates HR signaling and is strongly inhibited by PTI signaling. We named this mechanism the ETI-Mediating and PTI-Inhibited Sector (EMPIS). The signaling kinetics of EMPIS explain apparently different plant genetic requirements for ETI triggered by different effectors without postulating different signaling machineries. The properties of EMPIS suggest that information about efficacy of the early immune response is fed back to the immune signaling network, modulating its activity and limiting the fitness cost of unnecessary immune responses.