RESISTANCE RESPONSES OF MYCORRHIZAL RI T-DNA-TRANSFORMED CARROT ROOTSTO INFECTION BY FUSARIUM-OXYSPORUM F SP CHRYSANTHEMI

The influence exerted by arbuscular mycorrhizal fungi in stimulating p lant defense reactions was investigated using an in vitro system in wh ich Ri T-DNA-transformed carrot roots were infected with Fusarium oxys porum f. sp. chrysanthemi. This experimental model, based on the use o f root-organ culture to obtain typical mycorrhizal infections in axeni c 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, t he pathogen multiplied abundantly through much of the tissue, includin g the vascular stele, whereas in mycorrhizal roots, pathogen growth wa s restricted to the epidermis and the outer cortex. The accumulation o f an unusual material in both colonized and noncolonized host cells an d the coating of most intercellular spaces with similar substances wer e among the most typical features of host reactions. The newly formed deposits were free of cellulosic or pectic substances, as judged by go ld labeling. According to their texture and electron density, the poss ibility that such deposits may be infused with phenolics has been sugg ested. Support for this hypothesis came from the observation that the deposited material often interacted physically with the walls of invad ing hyphae exhibiting morphological changes and cytological alteration s. These observations lead us to suggest that this material, in additi on to acting as a barrier to fungal spread, also may display a fungito xic activity. A few electron-opaque structures resembling the deposits found in mycorrhizal-infected carrot roots were seen in some cells an d intercellular spaces of noninfected, mycorrhizal carrot roots. By co ntrast, they were absent in cells of infected, nonmycorrhizal carrot r oots. Cytochemical labeling of chitin using wheat germ agglutinin, a l ectin with N-acetylglucosamine-binding specificity, in conjunction wit h gold-complexed ovomucoid, showed that disorganization of Fusarium hy phae colonizing mycorrhizal roots correlated with chitin degradation. These observations provided indirect evidence for the production of pl ant chitinases. However, the finding that some chitin molecules still occurred over cell walls of empty hyphae was taken as an indication th at chitinases were probably not among the first determinants in the se quence of events leading to the establishment of plant resistance. Thu s, restriction of pathogen growth together with an increase in hyphal alterations and accumulation of new plant products appears to be a typ ical feature observed in mycorrhizal carrot roots. Because these react ions were not seen in nonmycorrhizal roots, it is likely that mycorrhi zal infection is responsible, at least in part, for the activation of the plant defense system that, in turn, provides enhanced protection a gainst pathogen attack.