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 pl ant defense reactions was investigated using Ri T-DNA-transformed pea roots . Cytological investigations of strain Fo47-inoculated roots showed that th e fungus grew actively at the root surface and colonized a number of epider mal and cortical cells, inducing marked host cell metabolic changes, in roo ts inoculated with pathogenic F. oxysporum f. sp, pisi, the pathogen multip lied abundantly through much of the tissues, whereas in Fo47-inoculated roo ts, fungal growth was restricted to the epidermis and the outer cortex. Inv ading cells of strain Fo47 suffered from serious alterations, a phenomenon that was not observed in control roots in which F: oxysporum f. sp. pisi gr ew so actively that the vascular stele was invaded within a few days. Strai n Fo47 establishment in the root tissues resulted in a massive elaboration of hemispherical wall appositions and in the deposition of an electron-opaq ue material frequently encircling pathogen hyphae and accumulating in the n oninfected xylem vessels. This suggests that the host roots were signaled t o defend themselves through the rapid stimulation of a general cascade of n onspecific defense responses. The specific relationship established between strain Fo47 and the root tissues is discussed in relation to other types o f plant-fungus interactions, including pathogenic and symbiotic association s.