Grapevine (Vitis vinifera L.) is susceptible to many pathogens, such as Botrytis cinerea, Plasmopara viticola, Uncinula necator, and Eutypa lata. Phytochemicals are used intensively in vineyards to limit pathogen infections, but the appearance of pesticide-resistant pathogen strains and a desire to protect the environment require that alternative strategies be found. In the present study, the beta-1,3-glucan laminarin derived from the brown algae Laminaria digitata was shown both to be an efficient elicitor of defense responses in grapevine cells and plants and to effectively reduce B. cinerea and P. viticola development on infected grapevine plants. Defense reactions elicited by laminarin in grapevine cells include calcium influx, alkalinization of the extracellular medium, an oxidative burst, activation of two mitogen-activated protein kinases, expression of 10 defense-related genes with different kinetics and intensities, increases in chitinase and beta-1,3-glucanase activities, and the production of two phytoalexins (resveratrol and epsilon-viniferin). Several of these effects were checked and confirmed in whole plants. Laminarin did not induce cell death. When applied to grapevine plants, laminarin reduced infection by B. cinerea and P. viticola by approximately 55 and 75%, respectively. Our data describing a large set of defense reactions in grapevine indicate that the activation of defense responses using elicitors could be a valuable strategy to protect plants against pathogens.

Increasing interest is devoted to carbohydrates for their roles in plant immunity. Some of them are elicitors of plant defenses whereas other ones act as signaling molecules in a manner similar to phytohormones. This review first describes the main classes of carbohydrates associated to plant immunity, their role and mode of action. More precisely, the state of the art about perception of “PAMP, MAMP, and DAMP (Pathogen-, Microbe-, Damage-Associated Molecular Patterns) type” oligosaccharides is presented and examples of induced defense events are provided. A particular attention is paid to the structure/activity relationships of these compounds. The role of sugars as signaling molecules, especially in plant microbe interactions, is also presented. Secondly, the potentialities and limits of foliar sprays of carbohydrates to stimulate plant immunity for crop protection against diseases are discussed, with focus on the roles of the leaf cuticle and phyllosphere microflora.

Sulfated laminarin (PS3) has been shown previously to be an elicitor of plant defense reactions in tobacco and Arabidopsis and to induce protection against tobacco mosaic virus. Here, we have demonstrated the efficiency of PS3 in protecting a susceptible grapevine cultivar (Vitis vinifera cv. Marselan) against downy mildew (Plasmopara viticola) under glasshouse conditions. This induced resistance was associated with potentiated H2O2 production at the infection sites, upregulation of defense-related genes, callose and phenol depositions, and hypersensitive response-like cell death. Interestingly, similar responses were observed following P. viticola inoculation in a tolerant grapevine hybrid cultivar (Solaris). A pharmacological approach led us to conclude that both callose synthesis and jasmonic acid pathway contribute to PS3-induced resistance.

The phylogenetic relationships of 17 phytopathogenic mycoplasmalike organisms (MLOs) representing seven major taxonomic groups established on the basis of MLO 16s ribosomal DNA (rDNA) restriction patterns were examined by performing a sequence analysis of the 16s rDNA gene. The sequence data showed that the MLOs which we examined are members of a relatively homogeneous group that evolved monophyletically from a common ancestor. In agreement with results obtained previously with other MLOs, our results also revealed that the organisms are more closely related to AchoZepZasma Zaidhwii and other members of the anaeroplasma clade than to any other mollicutes. A phylogenetic tree based on 16s rDNAs showed that the MLOs which we examined can be divided into the following five primary clusters: (i) the aster yellows strain cluster; (ii) the apple proliferation strain cluster; (iii) the western-X disease strain cluster; (iv) the sugarcane white leaf strain cluster; and (v) the elm yellows strain cluster. The aster yellows, western-X disease, and elm yellows strain clusters were divided into two subgroups each. MLOs whose 16s rDNA sequences have been determined previously by other workers can be placed in one of the five groups. In addition to the overall division based on 16s rDNA sequence homology data, the primary clusters and subgroups could be further defined by a number of positions in the 16s rDNAs that exhibited characteristic compositions, especially in the variable regions of the gene.Nonhelical phytopathogenic mollicutes, which are most often referred to as mycoplasmalike organisms (MLOs), are wall-less, nonculturable prokaryotes that cause diseases in several hundred plant species (23). The first attempts to differentiate and classify these organisms were based on symptoms, host ranges, and insect vector relationships (4, 9, 21).The phylogenetic interrelatedness of the MLOs was poorly understood until recently, when molecular methods were introduced into plant mycoplasmology. Many MLO strains were differentiated and partially characterized by dot and Southern hybridization techniques, as well as by serological techniques. Most of this work has been reviewed by Kirkpatrick (10) and Lee and Davis (16). Closely related MLOs belonging to the aster yellows strain cluster could be classified by Southern blot analysis (17).Neither serological methods nor nucleic acid hybridization experiments performed with randomly cloned DNA fragments have revealed the phylogenetic or taxonomic positions of MLOs in relation to each other and to other microorganisms. In contrast, the 16s rRNA gene is a universal characteristic in prokaryotes and has both conserved and sufficiently variable regions. This gene is therefore suitable for phylogenetic and taxonomic classifications at various levels, including intrageneric differentiation (30,34 yellows agent (strain SAY), and the western-X disease agent (strain WX), as well as several Japanese MLOs, showed that these organisms exhibit levels of sequence homology of at least 89.4%. Thus...

β-(1→3)-Glucans can be found as structural polysaccharides in cereals, in algae or as exo-polysaccharides secreted on the surfaces of mushrooms or fungi. Research has now established that β-(1→3)-glucans can trigger different immune responses and act as efficient immunostimulating agents. They constitute prevalent sources of carbons for microorganisms after subsequent recognition by digesting enzymes. Nevertheless, mechanisms associated with both roles are not yet clearly understood. This review focuses on the variety of elucidated molecular interactions that involve these natural or synthetic polysaccharides and their receptors, i.e., Dectin-1, CR3, glycolipids, langerin and carbohydrate-binding modules.