The involvement of lipoxygenase and antioxidants in pathogenesis of powdery mildew on tobacco plants

Tobacco plants (cv. Michal) exhibited between a four- to seven-fold increase in lipoxygenase activity as a result of infection with the powdery mildew fungal pathogen, Erysiphe cichoracearum. Isolated hyphae and conidia of the pathogen had a very low activity of the enzyme. Increased enzyme activity was detectable at 2 to 3 days after inoculation, reached a maximal level at 11 days after inoculation, and gradually declined thereafter. Elevated lipoxygenase activity in homogenates of affected leaves was associated with corresponding increments in peroxides, endogenic antioxidant substances and carotene-bleaching activity. Young mildewed leaves also exhibited a × 4·5, × 2·5 and × 2 increase in leakage of electrolytes, sugars and proteins, respectively, as against non-infected tissue. Exogenous application of the antioxidant α-tocopherol (Vitamin E) to leaves of normal tobacco plants reduced both lipoxygenase activity and mildew development. Tocopherol had no adverse effect on conidial germination in vitro. It is suggested that the marked increase in lipoxygenase activity induced in tobacco by Erysiphe cichoracearum might cause a distortion of host membranous structures which, in turn, would bring about two major physiological consequences: (a) transfer of solutes from mesophyll host cells into fungal haustoria in epidermal cells, and (b) enhanced senescence and necrotization of the invaded host tissue.