Specificity of the cis-Isomers of Inhibitors of Uredospore Germination in the Rust Fungi.

The cis- but not the trans-isomers of the natural inhibitors, methyl ferulate and methyl 3,4-dimethoxycinnamate, inhibit germination of the uredospores of wheat-stem rust. Minimum concentrations for 50% inhibition are about 7 and 140 nM, respectively. Ultraviolet radiation activates the trans-isomers by partial conversion to the cis-forms, and partially inactivates the cis-form by the reverse transformation. About 100 molecules of the germination stimulant, n-nonyl alcohol, are required per molecule of methyl cis-ferulate to overcome inhibition and restore 50% germination. Seasonal and age differences in sensitivity of spores to inhibitor may be related to variations in content of stimulant.