Production of Reactive Oxygen Intermediates (O2 ˙−, H2O2, and ˙OH) by Maize Roots and Their Role in Wall Loosening and Elongation Growth

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Abstract
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) 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)(.-), H(2)O(2), and (.)OH 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 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 produced by endogenous cell wall peroxidase in the presence of NADH and H(2)O(2). (4) Inhibition of endogenous (.)OH formation by O(2)(.-) or (.)OH 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, and to respond to (.)OH by wall loosening, in passing through the growing zone. Moreover, inhibitor studies indicate that (.)OH formation is essential for normal root growth.