Proteomic analysis of the metabolic adaptation of the biocontrol agent Pseudozyma flocculosa leading to glycolipid production

Pseudozyma flocculosa (Traquair, Shaw, and Jarvis) Boekhout and Traquair, a basidiomycetous yeast originally classified as Sporothrix flocculosa, is a powerful and specific antagonist of powdery mildew fungi whose mode of action appears to be unique among other related Pseudozyma species [1]. It is able to colonize powdery mildew colonies within a few hours following its contact with the pathogen and this ability was preceded and/or facilitated by the release of an antifungal glycolipid. This glycolipid, named flocculosin, was recently isolated and purified from solid cultures of P. flocculosa [2]. Extracellular glycolipids are produced by a large variety of microorganisms and may serve different purposes to confer fitness advantages. Interestingly, the structure of flocculosin is closely related to the cellobiose lipid, ustilagic acid, produced by the plant pathogen Ustilago maydis [2]. In a previous study, the absence of growth factors contained in yeast extract, combined with high carbon availability were described as stress conditions inducing the production of flocculosin in P. flocculosa cultures [3]. However, it is unclear how these conditions relate to the antagonistic behavior of the fungus in nature.In spite of recent technical advances in molecular biology, the molecular basis of metabolic changes of P. flocculosa under stress condition is poorly understood because of the limited knowledge about the genetics of Pseudozyma spp. in general and of P. flocculosa in particular. Recently, Marchand et al. [4] isolated and studied the expression of a putative homolog of a cyp1 gene involved in the biosynthesis of ustilagic acid in U. maydis [5]. They showed a direct correlation between cyp1 expression and flocculosin production in stressed cultures of P. flocculosa but did not observe major differences in cyp1 expression when the fungus was developing on healthy or powdery-mildew infected leaves. These findings offered a relative appreciation of gene expression w