Biological roles for cerato-ulmin, a hydrophobin secreted by the elm pathogens, Ophiostoma ulmi and O-novo-ulmi

The causal agents of Dutch elm disease, species of Ophiostoma, are among the most destructive plant pathogens studied in the 20th century. Extensive research programs have provided insights into die biology of these interesting fungi. Approximately 25 years ago, cerato-ulmin, a secreted protein, was isolated and partially characterized. Cerato-ulmin production was directly correlated to aggressive forms of Ophiostoma. The hydrophobic protein possessed characteristics that would suggest that it was a wilt toxin. Injection of, or exposure of elm cuttings to purified CU, resulted in wilting and other symptoms associated with inoculation of the pathogen into trees. In this review, we present an analysis of the physiological and biochemical studies on cerato-ulmin, and discuss the more recent molecular genetic studies related to the protein and its role in Dutch elm disease. The complete sequence (100 amino acids) of this protein was ascertained. This led to the identification of cerato-ulmin as a hydrophobin and to the characterization of the gene for cerato-ulmin. Null mutants (cu(-)) of Ophiostoma novo-ulmi were generated through gene disruption and utilized in greenhouse trials. Results from these trials demonstrated that cerato-ulmin was not directly involved in the virulence of the pathogen. All of the epidemiological data, however, indicated a correlation between cerato-ulmin and the pathology of Dutch elm disease. Working with wild isolates of aggressive and nonaggressive strains, as well as the cu- mutant, and a transgenic over expression mutant, experimental results provided a correlation between cerato-ulmin production and fitness of the pathogen in natural populations. We suggest that the critical evaluation and consideration of these recent data offer opportunities in developing biological control strategies for Dutch elm disease.