Mode of action of biocontrol agents: all that glitters is not gold

Pseudozyma flocculosa (syn: Sporothrix flocculosa) was first discovered and described in 1987 as an epiphytic yeast on powdery mildew-infected clover leaves. It was subsequently found to be a powerful antagonist of powdery mildews which prompted its study and development as a biocontrol agent (BCA). Most BCAs exert their activity through the manifestation of one or more of the following modes of action: competition, parasitism, antibiosis and induced resistance. In the case of P. flocculosa, in vitro bioassays, electron microscopy studies, and chemical analyses all pointed to a single mode of action: antibiosis. This conclusion was reinforced by the characterization and purification of an active molecule, flocculosin, and by the demonstration of its powerful antimicrobial activity. This was further supported by the discovery of a complex gene cluster regulating the synthesis of flocculosin, a molecule nearly identical to ustilagic acid - a compound produced by U. maydis under the control of a similar gene cluster. Despite this strong evidence, there is new evidence to indicate that flocculosin plays a secondary, if any, role in the antagonistic activity of P. flocculosa. The biocontrol process instead appears to be mediated by an intricate interaction involving nutrients produced by the plant, harvested by the phytopathogen and exploited by P. flocculosa. With the imminent completion of sequencing of the P. flocculosa genome, the recent publication of the barley powdery mildew genome (Blumeria graminis f. sp. hordei), and the availability of many plant genomes, the latest developments in DNA sequencing and transcriptomic analyses will allow unparalleled insight into the complex and delicate balance defining this unique tripartite interaction.