THE INVOLVEMENT OF OXYGEN-DERIVED FREE-RADICALS IN PLANT-PATHOGEN INTERACTIONS

Plants have evolved a multiplicity of defence mechanisms against patho gen attack. Their modes of action may be to (i) kill the pathogen dire ctly, (ii) block the action of enzymes required for infection, or (iii ) erect barriers to pathogen growth. Some of these reactions proceed v ia free radical intermediates and make use of either atmospheric oxyge n or reactive oxygen species. This paper reviews the various types of reaction involving oxygen-derived free radicals that are initiated in plant tissue when it is invaded by pathogenic organisms. Both the prod uction of free radicals by plants in defensive processes and the utili sation of free radicals by pathogens in offensive reactions are consid ered and particular attention is given to the use of electron paramagn etic resonance (EPR) spectroscopy for the direct observation of such f ree radical reactions.