Inoculation with Ophiostoma novo-ulmi subsp. americana affects photosynthesis, nutrition and oxidative stress in in vitro Ulmus minor plants

Dutch Elm Disease (DED) has dramatically reduced the number of field elm (Ulmus minor Mill.) populations in the northern hemisphere. In order to understand some aspects of the relationship between its causal agent (the fungus Ophiostoma sp.) and the host, in vitro elm plants were inoculated, at the base of the stem, either with blastospores (SI-plants) or with extracellular filtrate (FI-plants) of O. novo-ulmi subsp. americana. Both SI-plants and FI-plants showed senescence symptoms such as reduced growth, chlorosis and leaf senescence, with more severe symptoms in SI-plants. Chlorophyll contents and fluorescence as well as soluble sugars and starch contents decreased more severely in SI-plants. SI-plant had reduced K, Ca and Mg levels in leaves, while K increased in roots. FI-plant and, mostly, SI-plants showed increased H2O2 production in leaves during the first days, as well as progressive membrane degradation (increases of solute and electrolyte leakages) and lipid peroxidation. Catalase (CAT) and guaiacol peroxidase (POX) activities were stimulated mainly during first days, while superoxide dismutase (SOD) activity was stimulated later. The effects of inoculation with O. novo-ulmi subsp. americana (with both blastospores and culture filtrate) in elm oxidative stress, nutrient balances and photosynthesis and its evolution with time are characterized. These data provide a new insight into the effects, at the host physiological level, of the interaction between elm and Ophiostoma. Also, the correlations among the parameters determined in this study, as well as the implications of using in vitro cultures to assay host/pathogen relationships, are discussed.

► Ulmus minor in vitro plants inoculated with Ophiostoma novo-ulmi subsp. americana spores (SI) and culture filtrate (FI). ► Both SI and FI induced reduced growth, chlorosis and leaf senescence. ► FI and mostly SI induced decrease in chlorophyll contents and fluorescence, nutritional imbalance, membrane degradation and oxidative stress.