Abstract: Reactive oxygen species (ROS) produced as intermediates in the
reduction of O_{2} to H_{2}O (superoxide
radical, hydrogen peroxide, hydroxyl radical), are generally regarded as
harmful products of oxygenic metabolism causing cell damage in plants, animals
and microorganisms. However, oxygen radical chemistry can also play useful
roles if it takes place outside of the protoplast. In plants, the production of
these ROS initiated by the plasma membrane NAD(P)H oxidase can be used for
controlled polymer breakdown leading to wall loosening during extension growth.
Backbone cleavage of cell wall polysaccharides can be accomplished by hydroxyl
radicals produced from hydrogen peroxide and superoxide in a reaction catalyzed
by cell wall peroxidase. Growing plant organs such as coleoptiles or roots of
maize seedlings produce these ROS specifically in the apoplast of actively
growing tissues, e.g. in the epidermis of the coleoptile and the growing zone
of the root. Auxin promotes the release of hydroxyl radicals when inducing
elongation growth. Experimental generation of hydroxyl radicals 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 ROS 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 the generation of hydroxyl radicals,
initiated by the plasma membrane NAD(P)H oxidase, plays a central role.
