Ecological Basis of the Interaction between Pseudozyma flocculosa and Powdery Mildew Fungi

In this work, we sought to understand how glycolipid production and the availability of nutrients could explain the ecology ofPseudozyma flocculosa and its biocontrol activity. For this purpose, we compared the development ofP. flocculosa to that of a close relative, the plant pathogenUstilago maydis , under different environmental conditions. This approach was further supported by measuring the expression ofcyp1 , a pivotal gene in the synthesis of unique antifungal cellobiose lipids of both fungi. On healthy cucumber and tomato plants, the expression ofcyp1 remained unchanged over time inP. flocculosa and was undetected inU. maydis . At the same time, green fluorescent protein (GFP) strains of both fungi showed only limited green fluorescence on control leaves. On powdery mildew-infected cucumber leaves,P. flocculosa induced a complete collapse of the pathogen colonies, but glycolipid production, as studied bycyp1 expression, was still comparable to that of controls. In complete contrast,cyp1 was upregulated nine times whenP. flocculosa was applied toBotrytis cinerea -infected leaves, but the biocontrol fungus did not develop very well on the pathogen. Analysis of the possible nutrients that could stimulate the growth ofP. flocculosa on powdery mildew structures revealed that the complex Zn/Mn played a key role in the interaction. Other related fungi such asU. maydis do not appear to have the same nutritional requirements and hence lack the ability to colonize powdery mildews. Whether production of antifungal glycolipids contributes to the release of nutrients from powdery mildew colonies is unclear, but the specificity of the biocontrol activity ofP. flocculosa toward Erysiphales does appear to be more complex than simple antibiosis.