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Cytological Analysis of Defense-Related Mechanisms Induced in Pea Root Tissues in Response to Colonization by Nonpathogenic Fusarium oxysporum Fo47

The ability of nonpathogenic Fusarium oxysporum, strain Fo47, to trigger plant defense reactions was investigated using Ri T-DNA-transformed pea roots. Cytological investigations of strain Fo47-inoculated roots showed that the fungus grew actively at the root surface and colonized a number of epidermal and cortical cells, inducing marked host cell metabolic changes. In roots inoculated with pathogenic F. oxysporum f. sp. pisi, the pathogen multiplied abundantly through much of the tissues, whereas in Fo47-inoculated roots, fungal growth was restricted to the epidermis and the outer cortex. Invading cells of strain Fo47 suffered from serious alterations, a phenomenon that was not observed in control roots in which F. oxysporum f. sp. pisi grew so actively that the vascular stele was invaded within a few days. Strain Fo47 establishment in the root tissues resulted in a massive elaboration of hemispherical wall appositions and in the deposition of an electron-opaque material frequently encircling pathogen hyphae and accumulating in the noninfected xylem vessels. This suggests that the host roots were signaled to defend themselves through the rapid stimulation of a general cascade of nonspecific defense responses. The specific relationship established between strain Fo47 and the root tissues is discussed in relation to other types of plant-fungus interactions, including pathogenic and symbiotic associations.