Influence of the Structure of the Sterol Molecule on Sterol-induced Reproduction in Phytophthora cactorum

Abstract

Summary: Production of oospores by Phytophthora cactorum in media supplemented with ten Δ5-sterols, differing in the structure of their side chains, has been measured. Spore production increased with increasing size of the substituent at C-24: H < CH3< C 2 H5. A 24α-methyl group conferred greater activity than a 24β-methyl, but there was less difference between Δ5-sterols with 24α- and 24β-ethyl groups. There was a greater difference between Δ7-sterols with 24α- and 24β-ethyl substituents. A double bond at C-22 had no effect on spore production when trans, but reduced activity when cis. Four Δ7-sterols have been compared with their corresponding Δ5-sterols. Fewer oospores were produced at low concentrations (0.2 to 1 mg 1−1) of each Δ7-sterol than with the Δ5-sterol, but at higher concentrations (10 to 30 mg 1−1), the numbers with the Δ7-sterol bore no consistent relation to the numbers with the Δ5-sterol. The difference between two sterols in promoting sexual reproduction in Phytophthora cactorum is not apparently related to differences in uptake into the mycelium, or to known effects on properties of membranes, but is believed to be due to differences in metabolism of the sterols, and to differences in the hormone activity of compounds derived from them.