THE CRYPTIC-GROWTH RESPONSE OF MAIZE COLEOPTILES AND ITS RELATIONSHIPTO H2O2-DEPENDENT CELL-WALL STIFFENING

Auxin-mediated elongation growth of maize (Zea mays L.) coleoptile seg ments can be nullified by lowering the turgor pressure by 0.45 MPa. Un der these conditions irreversible segment length (l(irr)) measured aft er freezing/thawing increases steadily over a period of 8 h although t he in vivo length (l(tot)) remains constant. This phenomenon, designat ed as 'cryptic growth', is an indication of a wall-stiffening process which appears to be an intrinsic component of irreversible cell wall e xtension. Using a range of metabolic inhibitors it is demonstrated tha t cryptic growth is caused by a temperature-sensitive biochemical proc ess in the cell wall which, depends on the presence of O-2 and active peroxidase, but not on ATP and protein synthesis. Inhibition of crypti c growth by anaerobic conditions can be alleviated by external H2O2. M oreover, cryptic growth can be partially inhibited by the antioxidant ascorbate. It is concluded that cryptic growth represents a wall-stiff ening reaction mediated by peroxidase-catalyzed, H2O2-dependent cross- linking of phenolic residues of wall polymers. The experimental demons tration of a wall-stiffening reaction in a rapidly growing organ suppo rts the concept that irreversible cell elongation (growth) is caused b y an interplay of two chemorheological reactions, a turgor-dependent w all-loosening reaction and a separate wall-stiffening reaction which f ixes the viscoelastically extended wall structure through oxidative cr oss-linking and thus conferring irreversibility to wall extension.