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Published online by Cambridge University Press: 05 December 2011
Plants have evolved a multiplicity of defence mechanisms against pathogen attack. Their modes of action may be to (i) kill the pathogen directly, (ii) block the action of enzymes required for infection, or (iii) erect barriers to pathogen growth. Some of these reactions proceed via free radical intermediates and make use of either atmospheric oxygen 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 production of free radicals by plants in defensive processes and the utilisation of free radicals by pathogens in offensive reactions are considered and particular attention is given to the use of electron paramagnetic resonance (EPR) spectroscopy for the direct observation of such free radical reactions.