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Cytology and Histology

Resistance Responses of Mycorrhizal Ri T-DNA-Transformed Carrot Roots to Infection by Fusarium oxysporum f. sp. chrysanthemi. Nicole Benhamou, Recherche en sciences de la vie et de la santé, Pavillon C.E. Marchand, Université Laval, Sainte-Foy, Québec, Canada, G1K 7P4; J. André Fortin(2), Chantal Hamel(3), Marc St-Arnaud(4), and Andrea Shatilla(5). (2)(3)(4)(5)Institut de recherche en biologie végétale, Université de Montréal, 4101 est rue Sherbrooke, Montréal H1X 2B2. Phytopathology 84:958-968. Accepted for publication 12 May 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/Phyto-84-958.

The influence exerted by arbuscular mycorrhizal fungi in stimulating plant defense reactions was investigated using an in vitro system in which Ri T-DNA-transformed carrot roots were infected with Fusarium oxysporum f. sp. chrysanthemi. This experimental model, based on the use of root-organ culture to obtain typical mycorrhizal infections in axenic conditions, was selected as a reliable means of studying the events associated with subsequent pathogen attack. Cytological investigations of root samples revealed striking differences in the extent of plant defense reactions between mycorrhizal and nonmycorrhizal carrot roots after inoculation with the fungal pathogen. In nonmycorrhizal roots, the pathogen multiplied abundantly through much of the tissue, including the vascular stele, whereas in mycorrhizal roots, pathogen growth was restricted to the epidermis and the outer cortex. The accumulation of an unusual material in both colonized and noncolonized host cells and the coating of most intercellular spaces with similar substances were among the most typical features of host reactions. The newly formed deposits were free of cellulosic or pectic substances, as judged by gold labeling. According to their texture and electron density, the possibility that such deposits may be infused with phenolics has been suggested. Support for this hypothesis came from the observation that the deposited material often interacted physically with the walls of invading hyphae exhibiting morphological changes and cytological alterations. These observations lead us to suggest that this material, in addition to acting as a barrier to fungal spread, also may display a fungitoxic activity. A few electron-opaque structures resembling the deposits found in mycorrhizal-infected carrot roots were seen in some cells and intercellular spaces of noninfected, mycorrhizal carrot roots. By contrast, they were absent in cells of infected, nonmycorrhizal carrot roots. Cytochemical labeling of chitin using wheat germ agglutinin, a lectin with N-acetylglucosamine-binding specificity, in conjunction with gold-complexed ovomucoid, showed that disorganization of Fusarium hyphae colonizing mycorrhizal roots correlated with chitin degradation. These observations provided indirect evidence for the production of plant chitinases. However, the finding that some chitin molecules still occurred over cell walls of empty hyphae was taken as an indication that chitinases were probably not among the first determinants in the sequence of events leading to the establishment of plant resistance. Thus, restriction of pathogen growth together with an increase in hyphal alterations and accumulation of new plant products appears to be a typical feature observed in mycorrhizal carrot roots. Because these reactions were not seen in nonmycorrhizal roots, it is likely that mycorrhizal infection is responsible, at least in part, for the activation of the plant defense system that, in turn, provides enhanced protection against pathogen attack.