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Physiology and Biochemistry

Inhibition of Growth and Sitosterol-Induced Sexual Reproduction in Phytophthora cactorum by Steroidal Alkaloids. W. David Nes, Research chemist, Plant Physiology and Chemistry Research Unit, Western Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, Berkeley, CA 94710; Patrick K. Hanners(2), George A. Bean(3), and Glenn W. Patterson(4). (2)(3)(4)Undergraduate research assistant and professors, respectively, Department of Botany, University of Maryland, College Park 20742. Phytopathology 72:447-450. Accepted for publication 3 July 1981. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1982. DOI: 10.1094/Phyto-72-447.

As part of a study designed to examine the influence of polycyclic isopentenoid structure in fungi normally auxotrophic or heterotrophic for sterols, we have investigated the effects of 14 synthetic and naturally occurring sterols, nine steroidal alkaloids, and one sapogenin on radial growth, hyphal morphology, and oospore production in Phytophthora cactorum, a pathogenic fungus unable to synthesize sterols. When the 24 polycyclic isopentenoids were tested individually, only the steroidal alkaloids and the sapogenin were fungistatic. The extent of growth inhibition was dependent on the concentration of steroidal alkaloid and sapogenin added to the medium and on whether the compound was added in the presence or absence of sitosterol. Certain sterols stimulated growth or had no effect on growth relative to that of the control. In cultures treated with sterols that promoted growth, 24-ethyl sterols gave maximal stimulation of hyphal extension. In some of the steroidal alkaloid treatments, growth inhibition was annulled by adding sitosterol to medium containing azasteroid. Although the addition of sitosterol (10 μg/ml) appeared to protect the mycelium and allow for continued vegetative growth in media containing up to 15 μg of azasteroid per milliliter, sitosterol-induced oospore production was nearly or completely inhibited by the azasteroid. The extent of inhibition was dependent on the concentration and combination of the sterol-steroidal alkaloid pairs in the medium. The results are interpreted to imply that the molecular feature of the steroidal alkaloid responsible for inhibiting sitosterol-induced sexual reproduction is the imino function present in the ring structure.