Specificity and mode of action of the antifungal fatty acid cis-9-heptadecenoic acid produced by Pseudozyma flocculosa

Abstract

cis-9-Heptadecenoic acid (CHDA), an antifungal fatty acid produced by the biocontrol agent Pseudozyma flocculosa, was studied for its effects on growth and/or spore germination in fungi. Inhibition of growth and/or germination varied considerably and revealed CHDA sensitivity groups within tested fungi. Analysis of lipid composition in these fungi demonstrated that sensitivity was related primarily to a low intrinsic sterol content and that a high level of unsaturation of phospholipid fatty acids was not as involved as hypothesized previously. Our data indicate that CHDA does not act directly with membrane sterols, nor is it utilized or otherwise modified in fungi. A structural mechanism of CHDA, consistent with the other related antifungal fatty acids produced by P. flocculosa, is proposed in light of its activity and specificity. The probable molecular events implicated in the sensitivity of fungi to CHDA are (i) partitioning of CHDA into fungal membranes; (ii) a variable elevation in fluidity dependent on the buffering capability (sterol content) in fungi; and (iii) higher membrane disorder causing conformational changes in membrane proteins, increased membrane permeability and, eventually, cytoplasmic disintegration.

FIG. 1

Proposed model of activity of CHDA, an antifungal compound produced by P. flocculosa. (A) Susceptible fungi. CHDA partitions into the hydrophobic region of fungal membranes, producing significant changes in the packing of lipid molecules by inducing disorder in neighboring acyl chains due to its high motional freedom. The resulting change in membrane dynamics would affect the activity of membrane-bound proteins. These effects result in alteration of membrane potentials which leads to its collapse, as reported for susceptible fungi exposed to P. flocculosa (17). (B) Resistant fungi. Sterols buffer stress-induced fluctuations in membrane fluidity by ordering fatty acyl chains. This would maintain more optimal activity of membrane-bound enzymes and proteins by minimizing the impact of the CHDA-induced disordering effect. Minimal membrane alterations are induced, and resistant fungi can overcome this stress, as observed with I. bolleyi (2). This model is consistent with the activity of the other related antifungal fatty acids produced by P. flocculosa, namely 6-methyl-9-heptadecenoic acid, whereby its methyl branch would cause an even greater disturbance in the lipid environment which would explain its greater activity in vitro (1, 5).