Quorum sensing activity in Ophiostoma ulmi: effects of fusel oils and branched chain amino acids on yeast-mycelial dimorphism.

AIMS: For Ophiostoma (Ceratocystis) ulmi, the ability to undergo morphological change is a crucial factor for its virulence. To gain an understanding of quorum-sensing activity in O. ulmi as it relates to yeast-mycelium dimorphism control, this study examines the effects of branched-chain amino acids as well as their fusel alcohols and fusel acids as quorum sensing molecules. METHODS AND RESULTS: In a defined medium containing glucose, proline and salts, O. ulmi grew as yeasts when the culture was inoculated with a high density of spores (2 × 10(7) CFU ml(-1) ) and as mycelia when inoculated with a low spore density (4 × 10(5) CFU ml(-1) ). The cultures displaying yeast morphology secreted a quorum-sensing factor that shifted the morphology from mycelia to yeast. This quorum-sensing molecule was lipophilic and extractable by organic solvents from the spent medium. Using GC/MS analysis, it was determined that the major compound in the extract was 2-methyl-1-butanol. A similar effect was observed when the branched-chain amino acids (fusel alcohol precursors) were used as the nitrogen source. E, E-farnesol had no effect on the morphology of O. ulmi. CONCLUSIONS: Addition of the branched-chain amino acids or one of the compounds detected in the spent medium, 2-methyl-1-butanol or 4-hydroxyphenylacetic acid, or methylvaleric acid, decreased germ tube formation by more than 50%, thus demonstrating a quorum sensing molecule behaviour in O. ulmi cultures. SIGNIFICANCE AND IMPACT OF THE STUDY: This study presents advances in the investigation of dimorphism in O. ulmi, complementing the existing scientific basis, for studying, understanding and controlling this phenomenon.