Plant Protection Institute, Centre for Agricultural Research, Hungarian Academy of Sciences, P.O. Box 102, 1525 Budapest, Hungary.
Pretreatment of tobacco leaves with low concentrations (5 to 10 mM) of H2O2 suppressed hypersensitive-type necrosis associated with resistance to Tobacco mosaic virus (TMV) or Pseudomonas syringae pv. phaseolicola. The same pretreatment resulted in suppression of normosensitive necrosis associated with susceptibility to Botrytis cinerea. This type of H2O2-mediated, induced disease symptom resistance correlated with enhanced host antioxidant capacity, i.e., elevated enzymatic activities of catalase (CAT), ascorbate peroxidase (APX), and guaiacol peroxidase (POX) after viral and bacterial infections. Induction of genes that encode the antioxidants superoxide dismutase (SOD), CAT, and APX was also enhanced early after TMV infection. Artificial application of SOD and CAT suppressed necroses caused by viral, bacterial, or fungal pathogens similarly as H2O2 pretreatment, implying that H2O2-mediated symptom resistance operates through enhancement of plant antioxidant capacity. Pathogen multiplication was not significantly affected in H2O2-pretreated plants. Salicylic acid (SA), a central component of plant defense, does not seem to function in this type of H2O2-mediated symptom resistance, indicated by unchanged levels of free and bound SA and a lack of early up-regulation of an SA glucosyltransferase gene in TMV-infected H2O2-pretreated tobacco. Taken together, H2O2-mediated, induced resistance to necrotic symptoms in tobacco seems to depend on enhanced antioxidant capacity.