Stéphan Dorey

Spatial and Temporal Induction of Cell Death, Defense Genes, and Accumulation of Salicylic Acid in Tobacco Leaves Reacting Hypersensitively to a Fungal Glycoprotein Elicitor

We have analyzed the spatial and temporal expression of defense responses induced in attached tobacco leaves treated with a Phytophthora megasperma glycoprotein that was previously shown to be an efficient elicitor of the hypersensitive reaction. The infiltrated tissue (zone 1), the surrounding tissue (zone 2), which is 5 mm in width, and tissue at a distance >2 cm from zone 2 (zone 3) were analyzed separately. Cell death occurred only in zone 1 and was completed by 14 h. Defense gene expression was induced in zones 1 and 2 with striking differential patterns, but not in zone 3. There was a rapid (1 to 4 h) induction of genes of the phenylpropanoid, salicylic acid, and ses-quiterpenoid pathways in zones 1 and 2. However, it was strong and transient in the former and of lesser extent but sustained in the latter. High amounts of scopoletin, a phenylpropanoid metabolite, were found synthesized in zone 2. Pathogenesis-related (PR) transcripts and the corresponding PR proteins accumulated in high amounts in zo...

Bacterial rhamnolipids are novel MAMPs conferring resistance to Botrytis cinerea in grapevine.

Rhamnolipids produced by the bacteria Pseudomonas aeruginosa are known as very efficient biosurfactant molecules. They are used for a wide range of industrial applications, especially in food, cosmetics and pharmaceutical formulations as well as in bioremediation of pollutants. In this paper, the role of rhamnolipids as novel molecules triggering defence responses and protection against the fungus Botrytis cinerea in grapevine is presented. The effect of rhamnolipids was assessed in grapevine using cell suspension cultures and vitro-plantlets. Ca(2+) influx, mitogen-activated protein kinase activation and reactive oxygen species production form part of early signalling events leading from perception of rhamnolipids to the induction of plant defences that include expression of a wide range of defence genes and a hypersensitive response (HR)-like response. In addition, rhamnolipids potentiated defence responses induced by the chitosan elicitor and by the culture filtrate of B. cinerea. We also demonstrated that rhamnolipids have direct antifungal properties by inhibiting spore germination and mycelium growth of B. cinerea. Ultimately, rhamnolipids efficiently protected grapevine against the fungus. We propose that rhamnolipids are acting as microbe-associated molecular patterns (MAMPs) in grapevine and that the combination of rhamnolipid effects could participate in grapevine protection against grey mould disease.

Tobacco Class I and II Catalases Are Differentially Expressed During Elicitor-Induced Hypersensitive Cell Death and Localized Acquired Resistance

Expression of tobacco class I (CAT1) and class II (CAT2) catalases was analyzed in leaves reacting hypersensitively to tobacco mosaic virus (TMV) or to a fungal glycoprotein elicitor. In TMV-infected plants, Cat1 transcript levels declined rapidly while Cat2 transcripts accumulated strongly. The spatial and temporal changes in catalase transcripts, proteins, and activity during the hypersensitive reaction (HR) were further investigated in tobacco leaves infiltrated with a glycoprotein elicitor. Two functionally different zones were discriminated: the infiltrated tissue in which cells undergo the HR, called the HR-zone 1; and the surrounding tissue showing strong induced defense responses, called the LAR (Localized Acquired Resistance)- zone 2. Levels of Cat1 and Cat2 mRNA and proteins and catalase activity decreased in the HR-zone 1. In the LAR-zone 2, Cat1 transcripts became rapidly undetectable, but levels of Cat2 mRNA and protein and catalase activity increased. Catalase expression in elicitorinfiltrat...

RHAMNOLIPID BIOSURFACTANTS AS NEW PLAYERS IN ANIMAL AND PLANT DEFENSE AGAINST MICROBES

Rhamnolipids are known as very efficient biosurfactant molecules. They are used in a wide range of industrial applications including food, cosmetics, pharmaceutical formulations and bioremediation of pollutants. The present review provides an overview of the effect of rhamnolipids in animal and plant defense responses. We describe the current knowledge on the stimulation of plant and animal immunity by these molecules, as well as on their direct antimicrobial properties. Given their ecological acceptance owing to their low toxicity and biodegradability, rhamnolipids have the potential to be useful molecules in medicine and to be part of alternative strategies in order to reduce or replace pesticides in agriculture.

Comparative analysis of defence responses induced by the endophytic plant growth-promoting rhizobacterium Burkholderia phytofirmans strain PsJN and the non-host bacterium Pseudomonas syringae pv. pisi in grapevine cell suspensions

Plant growth-promoting rhizobacteria (PGPR) are beneficial microorganisms that colonize the rhizosphere of many plant species and confer beneficial effects, such as an increase in plant growth. PGPR are also well known as inducers of systemic resistance to pathogens in plants. However, the molecular mechanisms involved locally after direct perception of these bacteria by plant cells still remain largely unknown. Burkholderia phytofirmans strain PsJN is an endophytic PGPR that colonizes grapevine and protects the plant against the grey mould disease caused by Botrytis cinerea. This report focuses on local defence events induced by B. phytofirmans PsJN after perception by the grapevine cells. It is demonstrated that, after addition to cell suspension cultures, the bacteria were tightly attaching to plant cells in a way similar to the grapevine non-host bacteria Pseudomonas syringae pv. pisi. B. phytofirmans PsJN perception led to a transient and monophasic extracellular alkalinization but no accumulation of reactive oxygen species or cell death were detected. By contrast, challenge with P. syringae pv. pisi induced a sustained and biphasic extracellular alkalinization, a two phases oxidative burst, and a HR-like response. Perception of the PGPR also led to the production of salicylic acid (SA) and the expression of a battery of defence genes that was, however, weaker in intensity compared with defence gene expression triggered by the non-host bacteria. Some defence genes up-regulated after B. phytofirmans PsJN challenge are specifically induced by exogenous treatment with SA or jasmonic acid, suggesting that both signalling pathways are activated by the PGPR in grapevine.

Uncovering plant-pathogen crosstalk through apoplastic proteomic studies.

Plant pathogens have evolved by developing different strategies to infect their host, which in turn have elaborated immune responses to counter the pathogen invasion. The apoplast, including the cell wall and extracellular space outside the plasma membrane, is one of the first compartments where pathogen-host interaction occurs. The plant cell wall is composed of a complex network of polysaccharides polymers and glycoproteins and serves as a natural physical barrier against pathogen invasion. The apoplastic fluid, circulating through the cell wall and intercellular spaces, provides a means for delivering molecules and facilitating intercellular communications. Some plant-pathogen interactions lead to plant cell wall degradation allowing pathogens to penetrate into the cells. In turn, the plant immune system recognizes microbial- or damage-associated molecular patterns (MAMPs or DAMPs) and initiates a set of basal immune responses, including the strengthening of the plant cell wall. The establishment of defense requires the regulation of a wide variety of proteins that are involved at different levels, from receptor perception of the pathogen via signaling mechanisms to the strengthening of the cell wall or degradation of the pathogen itself. A fine regulation of apoplastic proteins is therefore essential for rapid and effective pathogen perception and for maintaining cell wall integrity. This review aims to provide insight into analyses using proteomic approaches of the apoplast to highlight the modulation of the apoplastic protein patterns during pathogen infection and to unravel the key players involved in plant-pathogen interaction.

The grapevine flagellin receptor VvFLS2 differentially recognizes flagellin‐derived epitopes from the endophytic growth‐promoting bacterium Burkholderia phytofirmans and plant pathogenic bacteria

• The role of flagellin perception in the context of plant beneficial bacteria still remains unclear. Here, we characterized the flagellin sensing system flg22-FLAGELLIN SENSING 2 (FLS2) in grapevine, and analyzed the flagellin perception in the interaction with the endophytic plant growth-promoting rhizobacterium (PGPR) Burkholderia phytofirmans. • The functionality of the grapevine FLS2 receptor, VvFLS2, was demonstrated by complementation assays in the Arabidopsis thaliana fls2 mutant, which restored flg22-induced H₂O₂ production and growth inhibition. Using synthetic flg22 peptides from different bacterial origins, we compared recognition specificities between VvFLS2 and AtFLS2. • In grapevine, flg22-triggered immune responses are conserved and led to partial resistance against Botrytis cinerea. Unlike flg22 peptides derived from Pseudomonas aeruginosa or Xanthomonas campestris, flg22 peptide derived from B. phytofirmans triggered only a small oxidative burst, weak and transient defense gene induction and no growth inhibition in grapevine. Although, in Arabidopsis, all the flg22 epitopes exhibited similar biological activities, the expression of VvFLS2 into the fls2 background conferred differential flg22 responses characteristic for grapevine. • These results demonstrate that VvFLS2 differentially recognizes flg22 from different bacteria, and suggest that flagellin from the beneficial PGPR B. phytofirmans has evolved to evade this grapevine immune recognition system.

Rhamnolipids Elicit Defense Responses and Induce Disease Resistance against Biotrophic, Hemibiotrophic, and Necrotrophic Pathogens That Require Different Signaling Pathways in Arabidopsis and Highlight a Central Role for Salicylic Acid

Plant resistance to phytopathogenic microorganisms mainly relies on the activation of an innate immune response usually launched after recognition by the plant cells of microbe-associated molecular patterns. The plant hormones, salicylic acid (SA), jasmonic acid, and ethylene have emerged as key players in the signaling networks involved in plant immunity. Rhamnolipids (RLs) are glycolipids produced by bacteria and are involved in surface motility and biofilm development. Here we report that RLs trigger an immune response in Arabidopsis (Arabidopsis thaliana) characterized by signaling molecules accumulation and defense gene activation. This immune response participates to resistance against the hemibiotrophic bacterium Pseudomonas syringae pv tomato, the biotrophic oomycete Hyaloperonospora arabidopsidis, and the necrotrophic fungus Botrytis cinerea. We show that RL-mediated resistance involves different signaling pathways that depend on the type of pathogen. Ethylene is involved in RL-induced resistance to H. arabidopsidis and to P. syringae pv tomato whereas jasmonic acid is essential for the resistance to B. cinerea. SA participates to the restriction of all pathogens. We also show evidence that SA-dependent plant defenses are potentiated by RLs following challenge by B. cinerea or P. syringae pv tomato. These results highlight a central role for SA in RL-mediated resistance. In addition to the activation of plant defense responses, antimicrobial properties of RLs are thought to participate in the protection against the fungus and the oomycete. Our data highlight the intricate mechanisms involved in plant protection triggered by a new type of molecule that can be perceived by plant cells and that can also act directly onto pathogens.

Deciphering the Role of Phytoalexins in Plant-Microorganism Interactions and Human Health

Phytoalexins are low molecular weight antimicrobial compounds that are produced by plants as a response to biotic and abiotic stresses. As such they take part in an intricate defense system which enables plants to control invading microorganisms. In this review we present the key features of this diverse group of molecules, namely their chemical structures, biosynthesis, regulatory mechanisms, biological activities, metabolism and molecular engineering.

Relationship Between Localized Acquired Resistance (LAR) and the Hypersensitive Response (HR): HR Is Necessary for LAR to Occur and Salicylic Acid Is Not Sufficient to Trigger LAR

In tobacco plants reacting hypersensitively to pathogen infection, localized acquired resistance (LAR) is induced in a sharp zone surrounding hypersensitive response (HR) lesions. Using a fungal glycoprotein inducing HR and LAR when infiltrated at 50 nM into tobacco leaves, we have shown previously that a plant signal(s) is released by HR cells and diffuses to induce LAR. Here we address two questions: does LAR occur when HR is not induced, and is salicylic acid the (or one of the) mobile LAR signal? We found that application to tobacco leaves of 0.25 nM glycoprotein triggered defense responses without HR and without an H2O2 burst. The analyzed responses include changes in expression of O-methyltransferase (OMT), 3-hydroxy-3-methylglutarylCoA reductase, pathogenesis-related (PR) proteins, and changes in levels of the signal salicylic acid. No defense responses and no increased resistance to tobacco mosaic virus infection were found beyond the elicitor-infiltrated tissue, providing strong evidence that the...