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Abstract

DL-alpha-Difluoromethylornithine (DFMO), an inhibitor of the polyamine biosynthetic enzyme ornithine decarboxylase (ODCase; EC 4.1.1.17), strongly retards the growth of several species of phytopathogenic fungi in vitro. Such inhibition can be completely reversed by putrescine or spermidine, confirming the essentiality of polyamines for growth of fungal hyphae. We now have discovered that DFMO can protect a range of plants against a wide range of fungi. For example, DFMO can protect bean plants (Phaseolus vulgaris Linnaeus cv. Pinto) against infection by uredospores of the bean rust fungus, Uromyces phaseoli Linnaeus, race O. Unifoliolate leaves of 10-day-old greenhouse-grown seedlings were sprayed with 400 ul per leaf of DFMO at various concentrations in 0.01% Tween 20 at pH 7.0 before or after inoculation with uredospores of Uromyces. After 16 hr in darkness in dew chambers to facilitate spore germination, plants were transferred to the greenhouse, arranged randomly, and examined for local lesions 7 days later. All concentrations of DFMO 0.50 mM or higher gave complete protection against the pathogen; at lower concentrations, postinoculation treatments with DFMO were generally more effective than preinoculation. The appearance of lesions on plants treated with lower concentration of DFMO was retarded 2-6 days. DFMO also confered protection on unsprayed parts of treated plants, indicating the translocation of some protective effect from sprayed areas. DL-alpha-Difluoromethylarginine, an analogous inhibitor of arginine decarboxylase (ADCase; EC 4.1.1.19), which is the rate-limiting enzyme in an alternative pathway for polyamine biosynthesis in higher plants, confers little protection even at 5mM. This emphasizes ornithine decarboxylase as the biochemical locus of choice for the prevention of plant diseases by inhibiting polyamine metabolism. DFMO has also been shown to be an effective synthetic fungicide for the following: protects tomato plants against Verticillium wilt fungus; protects wheat against stem rust fungus; protects wheat against powdery mildew fungus; protects Tendergreen bean plants against powdery mildew fungus; protects the MacIntosh apple leaf against the powdery mildew fungus; protects Ogle oats against leaf rust fungus; and protects corn against the corn rust fungus.

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