Controlled synthesis of the DSF cell–cell signal is required for biofilm formation and virulence in Xanthomonas campestris

Virulence of the black rot pathogen Xanthomonas campestris pv. campestris (Xcc) is regulated by cell–cell signalling involving the diffusible signal factor DSF. Synthesis and perception of DSF require products of genes within the rpf cluster (for regulation of pathogenicity factors). RpfF directs DSF synthesis whereas RpfC and RpfG are involved in DSF perception. Here we have examined the role of the rpf/DSF system in biofilm formation in minimal medium using confocal laser-scanning microscopy of GFP-labelled bacteria. Wild-type Xcc formed microcolonies that developed into a structured biofilm. In contrast, an rpfF mutant (DSF-minus) and an rpfC mutant (DSF overproducer) formed only unstructured arrangements of bacteria. A gumB mutant, defective in xanthan biosynthesis, was also unable to develop the typical wild-type biofilm. Mixed cultures of gumB and rpfF mutants formed a typical biofilm in vitro. In contrast, in mixed cultures the rpfC mutant prevented the formation of the structured biofilm by the wild-type and did not restore wild-type biofilm phenotypes to gumB or rpfF mutants. These effects on structured biofilm formation were correlated with growth and disease development by Xcc strains in Nicotiana benthamiana leaves. These findings suggest that DSF signalling is finely balanced during both biofilm formation and virulence

Plant Antimicrobial Agents and Their Effects on Plant and Human Pathogens

To protect themselves, plants accumulate an armoury of antimicrobial secondary metabolites. Some metabolites represent constitutive chemical barriers to microbial attack (phytoanticipins) and others inducible antimicrobials (phytoalexins). They are extensively studied as promising plant and human disease-controlling agents. This review discusses the bioactivity of several phytoalexins and phytoanticipins defending plants against fungal and bacterial aggressors and those with antibacterial activities against pathogens affecting humans such as Pseudomonas aeruginosa and Staphylococcus aureus involved in respiratory infections of cystic fibrosis patients. The utility of plant products as “antibiotic potentiators” and “virulence attenuators” is also described as well as some biotechnological applications in phytoprotection

Bases biochimiques et cellulaires des interactions plante-pathogène dans le système Chondrus crispus-Acrochaete operculata

Chondrus crispus (Gigartinales, Rhodophyceae) is a marine red algae with an isomorphic life history, comprising morphologically similar haploid gametophytes and diploid sporophytes. The algae is exploited for the industrial production of its cell wall polysaccharides, known as carrageenans, which are used as texturing agents. C. crispus sporophytes synthesize lambda-carrageenans whereas the gametophytes have kappa-carrageenans in the medulla and inner cortex and iota-carrageenans in the outer cortex and the epidermis. The green algal endophyte Acroaete operculata is able to completely invade the sporophytic phase of the red algae Chondrus crispus, whereas it does not penetrate beyond the outer cell layers of the gametophytic phase. Our results show that oligosaccharidic signals are involved in cell-cell recognition in the Chondrus crispus-Acrochaete operculata host-pathogen association and mediate a cross talk between both partners. We have shown that the virulente of the green algal pathogen is mediated by the recognition of carrageenan oligosaccharides released from its red algal host: kappa-carrageenan oligosaccharides inhibit A. operculata virulence while lambda-carrageenan oligosaccharides enhance its pathogenicity. Recognition of A. operculata by C. crispus is shown to also involve an oligosaccharidic signal. The signal transduction events including phospholipases C and A2 activation, ion fluxes, phosphorylation and/or dephosphorylation of proteins, NADPH oxidase, MAP kinases and lipoxygenase activation, are induced by this A. operculata oligosaccharide. This signal appears to be present in the non-virulent form of the pathogen whereas it is absent from the virulent form. This signal was also involved in the regulation of the UV-fluorescent compounds at the sites of attempted penetration by A. operculata zoosphores in C. crispus gametophytes. In the other hand, the alginate oligosaccharides elicit, an oxidative burst, induce the resistance, the protection and the accumulation of UV-fluorescent compounds biosynthesis around the infection sites by A. operculata zoosphores in C. crispus sporophytes. These oligosaccharidic signals induce the production of mycosporine-like amino acids and also halogenated volatile compounds in C. crispus. This study emphasize the potential role of oligosaccharidic signals in activating protection against pathogen in plants.Chondrus crispus (Gigartinales, Rhodophycées) est une algue rouge qui se caractérise par un cycle de vie isomorphe, dans lequel le sporophyte (diploïde) et les gamétophytes (haploïdes) sont virtuellement identiques sur le plan morphologique. L'algue est exploitée pour la production industrielle des polysaccharides de sa paroi cellulaire, nommés carraghénanes, comme agents de texture. Le sporophyte contient des carraghénanes de type lambda, tandis que le gamétophyte contient exclusivement des carraghénanes de type kappa, dans l'épiderme et une partie du cortex et iota dans la deuxième partie du cortex ainsi que la medulla. L'endophyte algue verte Acrochaete operculata est capable d'envahir complètement la phase sporophytique de l'algue rouge, tandis qu'il ne pénètre pas dans la phase gamétophytique, au delà des premières couches de cellules.Nos résultats montrent que les signaux oligosacchariques sont impliqués dans l'interaction Chondrus crispus-Acrochaete operculata. Nous avons montré que la virulence de l'algue verte pathogène est médiée par la reconnaissance des oligosaccharides libérés par son hôte : l'oligokappa-carraghénane inhibe la virulence d'A. operculata alors que l'oligolambda-carraghénane augmente sa pathogénicité. La reconnaissance d'A. operculata par C. crispus implique aussi des signaux oligosaccharidiques. Les événements de transduction du signal, incluant l'activation des phospholipases C et A2, des flux ioniques, des phosphorylations des protéines, de la NADPH oxydase, des MAP kinases et de la lipoxygénase, sont induits par cet oligosaccharide d'A. operculata. Ce signal apparaît être présent dans la forme non virulente du pathogène tandis qu'il est absent dans la forme virulente. Ce signal est aussi impliqué dans la régulation de la biosynthèse des composés fluorescents dans l'UV autour des sites de pénétration par les zoospores d'A. operculata chez les gamétophytes de C. crispus. Par ailleurs, les oligoalginates élicitent un burst oxydatif, induisent la résistance, une protection durable et l'accumulation de composés auotofluorescents autour des sites d'infection par les zoospores d'A. operculata dans le sporophyte de C. crispus. Ces signaux oligosaccharidiques induisent la protection de composés de type "mycosporines-like amino acids" et aussi de composés halogénés volatils chez C. crispus. Cette étude accentue le rôle potentiel des signaux oligosaccharidiques dans l'activation de la protection des plantes contre les agents pathogènes

BASES BIOCHIMIQUES ET CELLULAIRES DES INTERACTIONS PLANTE-PATHOGENE DANS LE SYSTEME CHONDRUS-CRISPUS-ACROHAETE OPERCULATA

PARIS-BIUSJ-Physique recherche (751052113) / SudocCentre Technique Livre Ens. Sup. (774682301) / SudocSudocFranceF

The Chondrus crispus-Acrochaete operculata host-pathogen association, a novel model in glycobiology and applied phycopathology

A review is presented of ongoing research on the oligosaccharide signals involved in cell-cell recognition in the Chondrus crispus-Acrochaete operculata host-pathogen association. In this pathosystem, the host gametophytes are resistant to the pathogen, whereas the sporophytic generation is susceptible to infection. The virulence of the green algal pathogen is mediated by the recognition of carrageenan oligosaccharides released from its red algal host: kappa-carrageenan oligosaccharides inhibit A. operculata virulence while lambda carrageenan oligosaccharides enhance its pathogenicity. It appears that the recognition of A. operculata by C. crispus also involves an oligosaccharidic signal. This signal is present in the non-virulent form of the pathogen whereas it is absent from the virulent form. Altogether this pathosystem offers a unique model to investigate the recognition of oligosaccharide signals in plant-pathogen interactions. The possible applications of this research to develop new strategies for disease control in maricultured algal crops are discussed

The Innate Immunity of a Marine Red Alga Involves Oxylipins from Both the Eicosanoid and Octadecanoid Pathways

The oxygenated derivatives of fatty acids, known as oxylipins, are pivotal signaling molecules in animals and terrestrial plants. In animal systems, eicosanoids regulate cell differentiation, immune responses, and homeostasis. In contrast, terrestrial plants use derivatives of C18 and C16 fatty acids as developmental or defense hormones. Marine algae have emerged early in the evolution of eukaryotes as several distinct phyla, independent from the animal and green-plant lineages. The occurrence of oxylipins of the eicosanoid family is well documented in marine red algae, but their biological roles remain an enigma. Here we address the hypothesis that they are involved with the defense mechanisms of the red alga Chondrus crispus. By investigating its association with a green algal endophyte Acrochaete operculata, which becomes invasive in the diploid generation of this red alga, we showed that (1) when challenged by pathogen extracts, the resistant haploid phase of C. crispus produced both C20 and C18 oxylipins, (2) elicitation with pathogen extracts or methyl jasmonate activated the metabolism of C20 and C18 polyunsaturated fatty acids to generate hydroperoxides and cyclopentenones such as prostaglandins and jasmonates, and (3) C20 and C18 hydroperoxides as well as methyl jasmonate did induce shikimate dehydrogenase and Phe ammonialyase activities in C. crispus and conferred an induced resistance to the diploid phase, while inhibitors of fatty acid oxidation reduced the natural resistance of the haploid generation. The dual nature of oxylipin metabolism in this alga suggests that early eukaryotes featured both animal- (eicosanoids) and plant-like (octadecanoids) oxylipins as essential components of innate immunity mechanisms

A Siderophore Analog of Fimsbactin from Acinetobacter Hinders Growth of the Phytopathogen Pseudomonas syringae and Induces Systemic Priming of Immunity in Arabidopsis thaliana

Siderophores produced in soil by plant growth-promoting rhizobacteria (PGPRs) play several roles, including nutrient mobilizers and can be useful as plants defense elicitors. We investigated the role of a synthetic mixed ligand bis-catechol-mono-hydroxamate siderophore (SID) that mimics the chemical structure of a natural siderophore, fimsbactin, produced by Acinetobacter spp. in the resistance against the phytopathogen Pseudomonas syringaepv tomato DC3000 (Pst DC3000), in Arabidopsis thaliana. We first tested the antibacterial activity of SID against Pst DC3000 in vitro. After confirming that SID had antibacterial activity against Pst DC3000, we tested whether the observed in vitro activity could translate into resistance of Arabidopsis to Pst DC3000, using bacterial loads as endpoints in a plant infection model. Furthermore, using quantitative polymerase chain reaction, we explored the molecular actors involved in the resistance of Arabidopsis induced by SID. Finally, to assure that SID would not interfere with PGPRs, we tested in vitro the influence of SID on the growth of a reference PGPR, Bacillus subtilis. We report here that SID is an antibacterial agent as well as an inducer of systemic priming of resistance in A. thaliana against Pst DC3000, and that SID can, at the same time, promote growth of a PGPR