Hydroxyl radical-induced cell-wall loosening in vitro and in vivo:: implications for the control of elongation growth

Hydroxyl radicals (OH) are capable of unspecifically cleaving, cell-wall polysaccharides in a site-specific reaction. I investigated the hypothesis that cell-wall loosening underlying the elongation growth of plant organs is controlled by apoplastically produced OH attacking load-beairing cell-wall matrix polymers. Isolated cell walls (operationally, frozen/thawed abraded segments from coleoptiles or hypocotyls, respectively) from maize, cucumber, soybeans sunflower or Scots pine seedlings were pre-loaded with catalytic Cu or Fe ions and then incubated in a mixture: of ascorbate + H2O2 for generating OH in the walls. This treatment induced irreversible wall extension (creep) in walls stretched in an extensiometer. The reaction could be promoted by acid pH and inhibited by several OH scavengers. Generation of OH by the same reaction in living coleoptile or hypocotyl segments caused elongation growth. Auxin-induced elongation growth of maize coleoptiles could be inhibited by OH scavengers. Auxin promoted the production of superoxide radicals (O-2(-)), an OH precursor, in the growth-controlling outer epidermis of maize coleoptiles. It is concluded that OH fulfils basic criteria for a wall-loosening factor acting in auxin-mediated elongation growth of plant species with widely differing cell-wall polysaccharide compositions.