Laboratory assays on non-enzymatic processes associated with iron and esca tracheomycotic fungi

The tracheomycotic fungi Phaeomoniella chlamydospora and Phaeoacremonium aleophilum are considered the main pathogens causing esca of grapevine, a widely spread wood disease that affects vine yield and longevity. These pathogens can produce toxins and extracellular enzymes in a complex infection process, not yet fully understood. The possibility that fungi employ also non-enzymatic means in such process was investigated: biological tests in the laboratory were carried out to examine the role of iron in promoting non-enzymatic processes with the production of radicals. P. chlamydospora and P. aleophilum produced siderophores, and reduced ferric iron; although both fungi lack of specific enzymatic activity, they were able to degrade crystalline cellulose, but only in the presence of iron. It was therefore hypothesized that the activation of iron-dependent cellulose-degrading mechanisms that supplement the degrading activity of the enzymes, may have taken place with the production of hydroxyl radicals; this hypothesis was confirmed by a specific spectrophotometric test as capacity of the radicals to break down 2-deoxi-D-ribose. The present study demonstrated that P. chlamydospora and P. aleophilum can activate iron-dependent non-enzymatic mechanisms, amplifying their pathogenic capacity. These findings, if confirmed by further study, may furnish a new way to explain the nature of the vine/pathogen relationship in term of wood degradation and/or foliar symptoms formation, leading to a better understanding of the esca complex infection process.