Microbe-associated molecular patterns (MAMPs) are molecular signatures typical of whole classes of microbes, and their recognition plays a key role in innate immunity. Endogenous elicitors are similarly recognized as damage-associated molecular patterns (DAMPs). This review focuses on the diversity of MAMPs/DAMPs and on progress to identify the corresponding pattern recognition receptors (PRRs) in plants. The two best-characterized MAMP/PRR pairs, flagellin/FLS2 and EF-Tu/EFR, are discussed in detail and put into a phylogenetic perspective. Both FLS2 and EFR are leucine-rich repeat receptor kinases (LRR-RKs). Upon treatment with flagellin, FLS2 forms a heteromeric complex with BAK1, an LRR-RK that also acts as coreceptor for the brassinolide receptor BRI1. The importance of MAMP/PRR signaling for plant immunity is highlighted by the finding that plant pathogens use effectors to inhibit PRR complexes or downstream signaling events. Current evidence indicates that MAMPs, DAMPs, and effectors are all perceived as danger signals and induce a stereotypic defense response.

A Renaissance of Elicitors: Perception of Microbe-Associated Molecular Patterns and Danger Signals by Pattern-Recognition Receptors

Plant hormones have pivotal roles in the regulation of plant growth, development, and reproduction. Additionally, they emerged as cellular signal molecules with key functions in the regulation of immune responses to microbial pathogens, insect herbivores, and beneficial microbes. Their signaling pathways are interconnected in a complex network, which provides plants with an enormous regulatory potential to rapidly adapt to their biotic environment and to utilize their limited resources for growth and survival in a cost-efficient manner. Plants activate their immune system to counteract attack by pathogens or herbivorous insects. Intriguingly, successful plant enemies evolved ingenious mechanisms to rewire the plant’s hormone signaling circuitry to suppress or evade host immunity. Evidence is emerging that beneficial root-inhabiting microbes also hijack the hormone-regulated immune signaling network to establish a prolonged mutualistic association, highlighting the central role of plant hormones in the regulation of plant growth and survival.

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Hormonal Modulation of Plant Immunity

Jasmonates: biosynthesis, perception, signal transduction and action in plant stress response, growth and development. An update to the 2007 review in Annals of Botany

Elicitor signal transduction leading to production of plant secondary metabolites.

Infection of plants by necrotizing pathogens or colonization of plant roots with certain beneficial microbes causes the induction of a unique physiological state called "priming." The primed state can also be induced by treatment of plants with various natural and synthetic compounds. Primed plants display either faster, stronger, or both activation of the various cellular defense responses that are induced following attack by either pathogens or insects or in response to abiotic stress. Although the phenomenon has been known for decades, most progress in our understanding of priming has been made over the past few years. Here, we summarize the current knowledge of priming in various induced-resistance phenomena in plants.

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Priming: Getting Ready for Battle

The importance of nitric oxide (NO) in innate and adaptive immunity in mammals is well recognised. NO exerts antimicrobial properties against invaders but also displays immunoregulatory functions in which S-nitrosylation represents a signalling process of major importance. Over the last two decades, a growing body of evidence suggests that NO is also a major component of plant immunity. Our understanding of its role in plant defence has been enriched by the identification and functional analysis of S-nitrosylated proteins. The recent identification of new S-nitrosylated proteins including the chaperone-like enzyme cell division cycle 48 (CDC48), histone deacetylases (HDACs) and calmodulin (CaM) reveals that NO could act as a modulator of epigenetic changes and targeting of ubiquitinated proteins for degradation. These findings also expand our understanding of the mechanisms controlling NO synthesis and its crosstalks with calcium signalling in plant immunity.

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NO Signalling in Plant Immunity

Salicylic Acid and Induced Plant Defenses

> Les micro-organismes ont conquis l’ensemble des niches écologiques de la biosphère et sont les êtres vivants les plus répandus à la surface de la terre. Les plantes et les animaux ont donc appris à vivre et à cohabiter avec eux, qu’ils leur soient bénéfiques ou nuisibles. Comme les animaux, les plantes possèdent une immunité innée leur permettant de percevoir les micro-organismes agressifs. Des avancées majeures ont récemment permis d’identifier, chez les plantes, des récepteurs de l’immunité qui reconnaissent les signaux de danger qui sont associés à une invasion microbienne [1]. Ces signaux sont de deux types : soit des motifs moléculaires directement associés aux agents pathogènes, les PAMP (pathogen-associated molecular pattern), soit des molécules libérées lors de lésions tissulaires de l’hôte, les DAMP (danger-associated molecular pattern).

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Lutter contre les infections bactériennes : le système immunitaire des plantes est aussi très efficace !

Plant defense stimulators (PDS) rely on the activation of plant innate immunity in order to protect crops against various pests. These molecules are thought to be a safer alternative to classical plant protection products (PPP). Given that innate immune systems share common features in plants and vertebrates, PDS can potentially cross-react with innate immunity of non-target organisms. To test this hypothesis, we studied effects of the commercial PDS Stifenia (FEN560) which is composed of crushed Fenugreek seeds. We tested various concentrations of Stifenia (0.03 to 1 mg.mL-1) on human peripheral blood mononuclear cells (PBMC) and checked 20 hours later cell metabolic activity using XTT assay, cell death by flow cytometry analysis and IL-1β inflammatory cytokine released in the culture medium using ELISA. Stifenia induced a general decrease of the cell metabolic activity which was concomitant with a dose-dependent release of IL-1β. Our results highlight the activation of human immune cells. The inflammatory effect of Stifenia was partially inhibited by pan-caspase inhibitor. Accordingly, Stifenia induced the release of p20 caspase-1 fragment into the culture medium suggesting the involvement of the NLRP3 inflammasome. Furthermore, we observed that Stifenia can induce cell death. We also tested the effect of Stifenia on Zebrafish larvae. After 24 hours of exposure, Stifenia induced a dose-dependent IL-1β and TNFα gene expression. The human-cell-based approach developed in this work revealed a high sensitivity concerning inflammatory properties of a plant protection product. These tests could be routinely used to screen the potential adverse effects of this type of compounds. Finally, our results suggest a potential danger of using extensively certain PDS for crop protection.

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Inflammatory Effects of the Plant Protection Product Stifenia (FEN560) on Vertebrates

Nitric oxide is regarded as a key signaling messenger in several organisms. Its physiological relevance is partly due to its capacity to induce posttranslational modifications of proteins through its direct or indirect reaction with specific amino acid residues. Among them, S-nitrosylation has been shown to be involved in a broad range of cellular signaling pathways both in animals and plants. The identification of S-nitrosylated proteins has been made possible by the development of the Biotin-Switch Technique (BST) in the early 2000s. Here, we describe the BST protocol we routinely use to check in vitro S-nitrosylation of recombinant proteins induced by NO donors.

Olivier Lamotte

Papers

NO Signalling in Plant Immunity

Salicylic Acid and Induced Plant Defenses

Lutter contre les infections bactériennes : le système immunitaire des plantes est aussi très efficace !

Inflammatory Effects of the Plant Protection Product Stifenia (FEN560) on Vertebrates

Analysis of Recombinant Protein S-Nitrosylation Using the Biotin-Switch Technique.