Functional expression of the gene cu, encoding the phytotoxic hydrophobin cerato-ulmin, enables Ophiostoma quercus, a nonpathogen on elm, to cause symptoms of Dutch elm disease

We studied the involvement of the phytotoxic hydrophobin cerato-ulmin (CU) in pathogenesis and virulence of Dutch elm disease (DED) by expressing its encoding gene (cu) in Ophiostoma quercus, a nonpathogenic species on elm cl osely related to the DED pathogens O, ulmi and O, novoulmi. The production of the toxin was quantitatively determined in culture filtrates and in myce lial extracts of the transformants. Production of CU in vitro was associate d with the ability to cause typical DED symptoms, consisting of foliar yell ow and wilting and vascular tissue discoloration on a moderately resistant elm genotype, The presence of CU was monitored by enzyme-linked immunosorbe nt assay in symptomatic leaves of plants inoculated with O, quercus transfo rmants expressing CU and found to be associated with wilt symptoms. In gene ral, the virulence of the cu-expressing transformants, as measured in terms of vascular discoloration and percentage of defoliation, was lower than th at of the mildly pathogenic isolate E2 of O, ulmi. However, one transforman t (C39) displayed a virulence level intermediate between that of E2 and 182 , a highly virulent isolate of O, novo-ulmi. Our results indicate that CU p roduction influences virulence in nonaggressive strains of Ophiostoma fungi .