ANTIFUNGAL ACTIVITY OF TOBACCO OSMOTIN HAS SPECIFICITY AND INVOLVES PLASMA-MEMBRANE PERMEABILIZATION

Osmotin protein is able to inhibit in vitro the growth of a number of unrelated pathogens. A survey of 31 isolates representing 18 fungal ge nera indicated that sensitivity may be determined at the genus level. Hyphal growth of Aspergillus flavus, Aspergillus parasitica, Rhizocton ia solani and Macrophomina phaseolina was highly resistant to osmotin whereas the growth of Bipolaris, Fusarium and Phytophthora species was very sensitive. Of all fungi tested Trichoderma longibrachiatum hypha l growth was most inhibited by osmotin treatment. Osmotin either induc ed spore lysis, inhibited spore germination or reduced germling viabil ity in seven fungal species that exhibited some degree of sensitivity in hyphal growth inhibition tests. The species-specific growth inhibit ion was correlated with the ability of osmotin to dissipate the fungal membrane pH gradient. Both growth inhibition and pH gradient dissipat ion by osmotin were sensitive to NaCl and other inorganic cations. Cel ls of T. longibrachiatum were insensitive to osmotin after plasmolysis , suggesting that the cell wall may be a component of the mechanism by which osmotin permeabilizes the plasma membrane and kills fungal cell s.