INDUCTION OF RESISTANCE AGAINST FUSARIUM-WILT OF TOMATO BY COMBINATION OF CHITOSAN WITH AN ENDOPHYTIC BACTERIAL STRAIN - ULTRASTRUCTURE ANDCYTOCHEMISTRY OF THE HOST RESPONSE

The potential of Bacillus pumilus (PGPR strain SE 34), either alone or in combination with chitosan, for inducing defense reactions in tomat o (Lycopersicon esculentum Mill.) plants inoculated with the vascular fungus, Fusarium oxysporum f. sp. radicis-lycopersici was studied by l ight and transmission electron microscopy and further investigated by gold cytochemistry. The key importance of fungal challenge in the elab oration of defense mechanisms is discussed in relation to the possibil ity that an alarm signal provided by the pathogen itself is required f or the expression of resistance in plants previously sensitized by bio tic agents. Ultrastructural investigations of the infected root tissue s from water-treated (control) plants showed a rapid colonization of a ll tissues including the vascular stele. In root tissues from bacteriz ed tomato plants grown in the absence of chitosan, the limited fungal development coincided with marked changes in the host physiology. The main facets of the altered host metabolism concerned the induction of a structural response at sites of fungal entry and the abnormal accumu lation of electron-dense substances in the colonized areas. A substant ial increase in the extent and magnitude of the cellular changes induc ed by B. pumilus was observed when chitosan was supplied to bacterized tomato plants. These changes were characterized by a considerable enl argement of the callose-enriched wall appositions deposited onto the i nner cell wall surface in the epidermis and the outer cortex. The use of the wheat germ agglutinin-ovomucoid-gold complex provided evidence that the wall-bound chitin component in Fusarium cells colonizing bact erized tomato roots was not substantially altered. One of the most-typ ical fungal cell reactions, observed only when bacterized tomato plant s were grown in the presence of chitosan, was the formation of abnorma l chitin-enriched deposits between the retracted plasma membrane and t he cell wall. Results of the present study provide the first evidence that combination of biocontrol approaches is a promising step towards elaborating integrated pest management programmes.