Cerato-ulmin, a Hydrophobin Secreted by the Causal Agents of Dutch Elm Disease, Is a Parasitic Fitness Factor

Abstract

Dutch elm disease is caused by the aggressiveOphiostoma novo-ulmiand the nonaggressiveO. ulmi.Both secrete the protein cerato-ulmin (CU). To determine what role CU plays in the pathology of Dutch elm disease, we constructed a CU overexpression mutant of the nonaggressiveO. ulmiH5. Stable integration of a single copy of thecugene from the aggressiveO. novo-ulmiinto the genome of the nonaggressive isolate resulted in increased secretion of CU protein. Trials with American elm,Ulmus americana,suggested no alteration of virulence of this overexpressing transformant. Using aggressive and nonaggressive wild types, thecuoverexpressing mutant, and our cu−mutant (Bowdenet al.,1996), we have demonstrated that CU production is correlated with an altered phenotype and more hydrophobic and adherent yeast-like cells. Our results also demonstrate that CU has a role in protecting infectious propagules from desiccation. These biological roles for CU would affect transmission of Dutch elm disease, and we therefore propose that this hydrophobin acts as a parasitic fitness factor.