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 th e most destructive plant pathogens studied in the 20th century. Extensive r esearch programs have provided insights into die biology of these interesti ng 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 poss essed characteristics that would suggest that it was a wilt toxin. Injectio n of, or exposure of elm cuttings to purified CU, resulted in wilting and o ther symptoms associated with inoculation of the pathogen into trees. In th is review, we present an analysis of the physiological and biochemical stud ies on cerato-ulmin, and discuss the more recent molecular genetic studies related to the protein and its role in Dutch elm disease. The complete sequ ence (100 amino acids) of this protein was ascertained. This led to the ide ntification of cerato-ulmin as a hydrophobin and to the characterization of the gene for cerato-ulmin. Null mutants (cu(-)) of Ophiostoma novo-ulmi we re generated through gene disruption and utilized in greenhouse trials. Res ults from these trials demonstrated that cerato-ulmin was not directly invo lved in the virulence of the pathogen. All of the epidemiological data, how ever, indicated a correlation between cerato-ulmin and the pathology of Dut ch 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 producti on and fitness of the pathogen in natural populations. We suggest that the critical evaluation and consideration of these recent data offer opportunit ies in developing biological control strategies for Dutch elm disease.