Wilt diseases caused by vascular pathogens include some of the most damaging stresses affecting trees. Dutch elm disease (DED), caused by the fungus Ophiostoma novo-ulmi, destroyed most of North American and European elm populations in the 20th century. The highly susceptible English elm, also known as Atinian clone, suffered the highest mortality rates during the last pandemic event, probably due to its lack of genetic diversity. To study the DED pathosystem, we inoculated English elm ramets with O. novo-ulmi and evaluated xylem anatomy, molecular response, and disease symptoms. The high DED susceptibility of the clone was linked to xylem structure. The transcript levels changed significantly for 1,696 genes during O. novo-ulmi invasion.Genes covering different steps of the plant immune system were identified, many of which showed homology with Arabidopsis thaliana genes involved in systemic acquired resistance.Induction of several pathogenesis-related proteins and repression of fasciclin-like arabinogalactan proteins and other cell wall biosynthesis pathways evidence unbalanced costs between growth and defence mechanisms far from the inoculation point. This study sheds light on elm molecular defence mechanisms against DED.
KEYWORDSDutch elm disease, microarrays, plant immune system, systemic acquired resistance, Ulmus minor, vascular pathogen
| INTRODUCTIONTrees are exposed to many instances of environmental interaction and stress throughout their life cycle. These include pests and diseases that can cause dramatic mortality events in forests and loss of foundation species and iconic trees (e.g., Ellison et al., 2005). Diseases affecting tree vascular systems are usually very harmful due to the crucial role of these tissues in transporting water, nutrients, hormones, and other compounds. Dutch elm disease (DED) is a vascular wilt disease caused by the pathogenic fungi Ophiostoma ulmi planted in a research plot at Puerta de Hierro Forest Breeding Centre (Madrid, Spain; 40.456238 N, 3.751823 W) These observations were performed at sampling times (1, 3, 7, 14, and 21 dpi) and at 28, 60, and 120 dpi.
| Characterization of anatomical structureAnatomical parameters were measured in two control ramets. Two additional ramets inoculated with the pathogen were sampled to visualize xylem responses to infection by O. novo-ulmi.