Bacterial-mediated induced resistance in cucumber: Beneficial effect of the endophytic bacterium Serratia plymuthica on the protection against infection by Pythium ultimum

The potential of the endophytic bacterium Serratia plymuthica strain RIGC4 in stimulating defense reactions in cucumber (Cucumis sativus) seedlings in oculated with the soilborne pathogen Pythium ultimum was explored at the ce llular level. Bacterial treatment prior to Pythium inoculation resulted in less seedling disease development as compared with that in nontreated contr ol plants, in which typical root symptoms were visible by 3 days after inoc ulation with the pathogen. Histological investigations of root samples reve aled striking differences in the extent of plant defense reactions between bacterized and nonbacterized plants. These observations were further confir med at the ultrastructural level with the demonstration that restriction of fungal colonization to the outermost root tissues of bacterized seedlings correlated with the deposition of enlarged callose-enriched wall apposition s at sites of potential pathogen penetration and the accumulation of an osm iophilic material in the colonized areas. Hyphae of the pathogen, surrounde d by this electron-opaque material, exhibited considerable changes includin g cytoplasm disorganization and, in many cases, loss of the protoplasm. How ever, labeling with the beta-1,4-exoglucanase resulted in a regular labelin g of Pythium cell walls, even at a time when these walls were entirely coat ed by the osmiophilic material. This material was also found to infiltrate into the invading hyphae to form either an internal coating of the cell wal l or a network of polymorphic droplets in the area previously occupied by t he cytoplasm. Cytochemical investigations revealed that callose, pectin, an d cellulose appeared in the wall appositions. In addition, glucosides, lipi ds, and phenolics were detected in the electron-dense aggregates forming th e core of most wall appositions. Finally, galactose residues were among the minor polysaccharidic compounds detected in the wall appositions. Evidence is provided in this study showing that treatment with S. plymuthica sensit izes susceptible cucumber plants to react more rapidly and more efficiently to Pythium attack through the formation of physical and chemical barriers at sites of potential fungal entry.