Fatty acids with certain structural characteristics are potent inhibitors of germination and inducers of cell death of powdery mildew spores

Abstract

Fatty acids with certain structural characteristics inhibited the germination of Erysiphe polygoni spores. Inhibition of germination was associated with disruption of cellular organization, which included rupture of vacuoles, loss of structural order and condensation of cellular contents and finally considerable shrinkage of the spores. These events could be invoked at a low concentration, i.e. 20μM, of the fatty acids and were irreversible after 15min. Cellular disorganization was detected in a relatively short time, approx. 1h. In examining a group of fatty acids with different structural characteristics, it was found that the presence of a cis double bond was important for the inhibitory activities. Saturated and trans -monounsaturated fatty acids were ineffective. Also, at 20μM, polyunsaturated fatty acids had marginal activities and not all cis -monounsaturated fatty acids were inhibitory. It was noted that the fatty acids with the highest activities shared a common feature,. i.e. a double bond 6 or 7 carbons from either end of the carbon chain. The results suggest that the presence of a Δ6, Δ7, ω6, or ω7 double bond by the cis -monounsaturated fatty acids is an important characteristic for the potent inhibitory activity. In contrast, the presence of an acid group was apparently not a critical requirement for activity, because twocis -monounsaturated fatty alcohols tested also exhibited similar potent inhibitory and disruptive effects.Palmitoleic acid-induced spore shrinkage could be partially prevented by BaCl2 and fully prevented by spermine. These are compounds known to block certain K+ channels, suggesting the involvement of K+ channels in palmitoleic acid effects. Also, when the spores were stained with the K+ binding ratiometric dye, tetra (acetoxymethyl) ester benzofuran isophthalate (PBFI-AM), disappearance of fluorescence at 340nm was observed in spores treated with palmitoleic acid, indicating a loss of K+ in these spores.