EFFECT OF TAXOL AND RELATED-COMPOUNDS ON GROWTH OF PLANT-PATHOGENIC FUNGI

Taxol, currently in use as therapy for refractory ovarian cancer, has no known function in the plant(Taxus brevifolia) from which it is isolated. In an effort to explore its biological role, we investigated the effect of Taxol and related compounds on the growth of plant pathogenic fungal hyphae. Taxol and cephalomannine inhibited the growth of several fungi, especially Phytophthora and Pythium species (Oomycetes) and Rhizoctonia solani (Basidiomycetes). Baccatin III had no effect on the growth of any fungus tested. Strains of Aspergillus (Deuteromycetes) and Fusarium (Ascomycetes) were not inhibited by any taxane. As little as 1.0 nmol of Taxol or cephalomannine severely retarded radial growth of Phytophthora species on solid medium. A dose-dependent effect was also evident in liquid culture assays down to 0.1 mu M Taxol, with total inhibition of mycelial growth at 1.0 mu M. At lower concentrations of Taxol, the fungal mycelium reached stationary stage 4 days later than the controls, indicating growth retardation rather than death. When hyphae were transferred to Taxol-free media, growth resumed as normal. Active doses are roughly 600 times lower than an estimated concentration of Taxol in Toxus needles. It is proposed that Taxol may be a preformed antifungal defense compound in Taxus species.