Elicitor-Induced Resistance in Tomato Plants Against Fungal Pathogens: Ultrastructure and Cytochemistry of the Induced Response

The introduction of gold cytochemistry in plant pathology has opened new avenues for accurately localizing molecules thought to play important biological functions in the plant metabolism. In an effort to understand the process associated with the induction of plant disease resistance, we investigated the effect of fungal elicitors on the cellular response of tomato root tissues during attack by Fusarium oxysporum f. sp. radicis-lycopersici, the causal agent of tomato crown and root rot. Chitosan was found to induce marked morphological and ultrastructural changes in the fungus grown in pure culture, including cytoplasm aggregation and abnormal deposition of an amorphous chitin-rich material. Examination of colonized root tissues at sites of fungal penetration revealed that a pre-treatment with chitosan (as well as with other elicitors) was associated with the expression of typical defense reactions. In the outer cortex, invading fungal cells suffered from serious damage and were frequently encircled by an electron-dense material. In the non-colonized inner cortex, strong host reactions were detected that were mainly associated with the deposition of two types of material that differed by their electron-density. Gold cytochemistry with purified β-1,3-glucanase and laccase showed that the more electron-dense material was of phenolic nature whereas the other material, occurring as deposits inserted between the phenolic aggregates, was made of callose. Application of various gold-complexed probes revealed that callose and phenolic-like compounds (likely lignin) were also the main components of newly-formed wall appositions. Results of these studies provide evidence that cytochemical approaches have the potential to significantly improve our knowledge of how plants defend themselves and how plant disease resistance can be directly enhanced by elicitors