Production of reactive oxygen intermediates (O2•-, H2O2, and •OH) by maize roots and their role in wall loosening and elongation growth

Cell extension in the growing zone of plant roots typically takes place with a maximum local growth rate of 50% length increase per hour. The biochemical mechanism of this dramatic growth process is still poorly understood. Here we test the hypothesis that the wall-loosening reaction controlling root elongation is effected by the production of reactive oxygen intermediates, initiated by a NAD(P)H oxidase-catalyzed formation of superoxide radicals (O-2(.-)) at the plasma membrane and culminating in the generation of polysaccharide-cleaving hydroxyl radicals ((OH)-O-.) by cell wall peroxidase. The following results were obtained using primary roots of maize (Zea mays) seedlings as experimental material. (1) Production of O-2(.-), H2O2, and (OH)-O-. can be demonstrated in the growing zone using specific histochemical assays and electron paramagnetic resonance spectroscopy. (2) Auxin-induced inhibition of growth is accompanied by a reduction of O-2(.-) production. (3) Experimental generation of (OH)-O-. in the cell walls with the Fenton reaction causes wall loosening (cell wall creep), specifically in the growing zone. Alternatively, wall loosening can be induced by (OH)-O-. produced by endogenous cell wall peroxidase in the presence of NADH and H2O2.(4) Inhibition of endogenous (OH)-O-. formation by O-2(.-) or (OH)-O-. scavengers, or inhibitors of NAD(P)H oxidase or peroxidase activity; suppress elongation growth. These results show that juvenile root cells transiently express the ability to generate (OH)-O-., and to respond to (OH)-O-. by wall loosening, in passing through the growing zone. Moreover, inhibitor studies indicate that (OH)-O-. formation is essential for normal root growth.