Effect of Steric and Nuclear Changes in Steroids and Triterpenoids on Sexual Reproduction in Phytophthora cactorum.

The comparative biological activity of 21 naturally occurring or synthetically derived steroids, 7 tetracyclic and pentacylic triterpenoids, and antheridiol incubated with cultures of Phytophthora cactorum has been examined. There was greater dependence on precise steric features of the sterol side chain than on the extent of nuclear unsaturation in inducing oospore formation. There was no significant effect on oospore formation by changing nuclear unsaturation in ring B from Delta(5) to Delta(7) or to Delta(5,7). Converting the unsaturated sterol to its corresponding stanol resulted in a significant reduction in the number of oospores produced. The effectiveness of sterols bearing different side chains in inducing oospores was found to be in the following relative order: 24alpha-ethyl = trans-Delta(22)-24alpha-ethyl > trans-Delta(22)-24beta-ethyl = 24alpha-E-ethylidene = 24alpha-methyl > 24beta-methyl = trans-Delta(22)-24beta-methyl = 26-methyl = saturated C(7) side chain and C-20 R (17-alphaH, 20-alphaH, right-handed conformer) = cis-Delta(22)-C(7) side chain and C-20 R > saturated C(7) side chain and C-20 S (17-alphaH, 20-betaH, right-handed conformer) > no sterol = 29-hydroxyporiferasterol = 20alpha-hydroxycholesterol = 24xi-hydroxy-24-vinylcholesterol. Of the sterols examined the most significant stereochemical criterion for the induction of oospore formation was absence of bulk on the front face of C-20. This follows from the observation that 20-isocholesterol and 20alpha-hydroxycholesterol, in which a methyl and hydroxy group, respectively, project to the front in the right handed conformation, were inactive in stimulating production of oospores. None of the triterpenoids studied induced oospore formation to any significant degree. Oospore formation was not induced by antheridiol nor 29-hydroxyporiferasterol in combination or added separately to growing cultures of P. cactorum in the concentration range 0.01 - 10.0 milligrams per liter.