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Cytological Characterization of Elicitin-Induced Protection in Tobacco Plants Infected by Phytophthora parasitica or Phytoplasma

First author: Laboratoire de Phytopharmacie et Biochimie des Interactions Cellulaires, UMR INRA-Université de Bourgogne, and Service Commun de Microscopie Electronique, INRA, BP 86510, 21065 Dijon-cedex France; second author: Recherche en Sciences de la Vie et de la Santé, Pavillon Charles-Eugène Marchand, Université Laval, Sainte-Foy, Québec G1K 7P4 Canada; third and fifth authors: Laboratoire des Phytoplasmes, UMR Plante-Microbe-Environnement, INRA-Université de Bourgogne, BP 86510, 21065 Dijon-cedex France; fourth and seventh authors: Laboratoire de Phytopharmacie et Biochimie des Interactions Cellulaires, UMR INRA-Université de Bourgogne, BP 86510, 21065 Dijon-cedex France; and sixth author: UMR Diversité et Génome des Plantes Cultivées, UMR DGPC, Résistance des Plantes, IRD, BP 64501, 34394 Montpellier-cedex France

Elicitins, small proteins secreted by Phytophthora and Pythium spp., display the ability to induce plant resistance toward pathogens. Ultrastructural investigations of cryptogein-treated tobacco plants evidenced host defense responses such as (i) formation of a calcium pectate gel in intercellular spaces of parenchymas, (ii) impregnation of pectin by phenolic compounds in intercellular spaces of phloem bundles, and (iii) accumulation of phloem proteins (P proteins) in the lumen of leaf sieve elements. These cytological modifications lead to the enhancement of physical barriers that prevent pathogen ingress and restrict host tissue colonization when cryptogein-treated tobacco plants were challenged with the pathogen Phytophthora parasitica. Wall appositions also were observed at most sites of penetration of hyphae. Moreover, growing hyphae exhibited severe morphological damages, suggesting a modified toxic environment. The same induction of P proteins in mature sieve tubes of tobacco leaves was obtained with oligandrin treatment, another elicitin. Cryptogein or oligandrin treatment prevented symptom expression in phytoplasma-infected tobacco plants in contrast with nontreated tobacco plants. Moreover, P protein plugs and occlusion of pore sites by callose were evidenced in sieve elements of treated plants. Both these phloem modifications might prevent the in planta movement of phloem-restricted microorganisms.

Additional keywords: elicitor disease resistance, histochemistry, microscopy, Nicotiana tabacum.