Evidence that hydroxyl radicals mediate auxin-induced extension growth

Reactive oxygen intermediates, i.e. the superoxide radical (O-2(.-)), hydrogen peroxide (H2O2) and the hydroxyl radical ((OH)-O-.), are generally regarded as harmful products of oxygenic metabolism causing cell damage in plants, animals and microorganisms. However, oxygen radical chemistry may also play a useful role in polymer breakdown leading to wall loosening during extension growth of plant cells controlled by the phytohormone auxin. Backbone cleavage of cell wall polysaccharides can be accomplished in vitro, by (OH)-O-. produced from H2O2 in a Fenton reaction or in a reaction catalyzed by peroxidase supplied with O-2 and NADH. Here, we show that coleoptile growth of maize seedlings is accompanied by the release of reactive oxygen intermediates in the cell wall. Auxin promotes release of O-2(.-) and subsequent generation of (OH)-O-. when inducing elongation growth. Experimental generation of (OH)-O-. in the wall causes an increase in wall extensibility in vitro and replaces auxin in inducing growth. Auxin-induced growth can be inhibited by scavengers of O-2(.-) H2O2 or (OH)-O-., or inhibitors interfering with the formation of these molecules in the cell wall. These results provide the experimental background for a novel hypothesis on the mechanism of plant cell growth in which (OH)-O-., produced from O-2(.-) and H2O2 by cell wall peroxidase, acts as a wall-loosening agent.