THE ORIGIN OF THE OXIDATIVE BURST IN PLANTS

A large number of publications recently have drawn strong analogies be tween the production of active oxygen species in plant cells and the ' 'oxidative burst'' of the phagocyte, even to the point of constructing elaborate models involving receptor mediated G-protein activation of a plasmalemma NADPH oxidase in plant cells. However there are potentia lly other active oxygen species generating systems at the plant cell s urface. The present work examines these alternatives with particular e mphasis on the rapid production of active oxygen species, in common wi th a number of other systems, by suspension-cultured cells of French b ean on exposure to an elicitor preparation from the fungal pathogen Co lletotrichum lindemuthianum. The cells show a rapid increase in oxygen uptake which is followed shortly afterwards by the appearance of a bu rst of these active oxygen species, as measured by a luminescence assa y, which is probably all accounted for by hydrogen peroxide. An essent ial factor in this production of H2O2 appears to be a transient alkali nization of the apoplast where the pH rises to 7.0-7.2. Dissipation of this pH change with a number of treatments, including ionophores and strong buffers, substantially inhibits the oxidative burst. Little evi dence was found for enhanced activation of a membrane-bound NADPH oxid ase. However the production of H2O2 under alkaline conditions can be m odelled in vitro with a number of peroxidases, one of which, an M(r) 4 6,000 wall-bound cationic peroxidase, is able to sustain H2O2 producti on at neutral pH unlike the other peroxidases which only show low leve ls of this reaction under such conditions and have pH optima at values greater than 8.0. On the basis of such comparative pH profiles betwee n the cells and the purified peroxidase and further inhibition studies a direct production of H2O2 from the wall peroxidase in French bean c ells is proposed. These experiments may mimic some of the responses to plant pathogens, particularly the hypersensitive response, which is a n important feature of resistance. A cell wall peroxidase-origin for t he oxidative burst is clearly different from a model consisting of rec eptor activation of a plasmalemma-localised NADPH oxidase generating s uperoxide. An alternative simple and rapid mechanism thus exists for t he generation of H2O2 which does not require such multiple proteinaceo us components.