Ultrastructural and cytochemical characterization of elicitor-induced structural responses in tomato root tissues infected by Fusarium oxysporum f.sp. radicis-lycopersici

Abstract

Commercial chitosan and laminarin, as well as β-glucans, isolated from either Phytophthora megasperma f.sp. glycinea or Saccharomyces cerevisiae, were applied to decapitated tomato (Lycopersicon esculentum Mill.) plants and evaluated for their potential to induce defense mechanisms in root tissues infected by Fusarium oxysporum f.sp. radicis-lycopersici. A significant decrease in disease incidence was monitored in elicitor-treated plants as compared to water-treated plants. No difference was detected in the capacity of the elicitors under study to confer enhanced protection against pathogen attack. Ultrastructural investigations of the infected root tissues from watertreated (control) plants showed a rapid colonization of all tissues including the vascular stele. Fungal ingress was lways associated with marked host cell disorganization and cell wall alteration. In root tissues from elicitortreated plants, restriction of fungal growth to the epidermis and the outer cortex, decrease in pathogen viability, and formation of numerous wall appositions at sites of attempted penetration were the main features of the hostpathogen interaction. The wall appositions were found to vary greatly in their appearance from multi-textured to multi-layered structures, from elongated deposits to hemispherical protuberances. Application of various goldcomplexed probes to root tissue sections revealed that callose, pectin and phenolic-like compounds (likely lignin) were the main components of the newly-formed barriers. By contrast, cellulose appeared confined to outer or intermediate layers resembling the host cell wall in terms of structure and architecture. In the absence of fungal challenge, the cytologically visible consequences of elicitation were restricted to a discrete deposition of electron-opaque substances in the vacuoles of some cells, and wall appositions were not detected. The key importance of fungal challenge in the elaboration of defense mechanisms is discussed in relation to the possibility that an alarm ignal provided by the pathogen itself is required for the expression of resistance in plants previously sensitized by an exogenous elicitor.