The cryptic-growth response of maize coleoptiles and its relationship to H2O2-dependent cell wall stiffening

Auxin-mediated elongation growth of maize ( Zea mays L.) coleoptile segments can be nullified by lowering the turgor pressure by 0.45 MPa. Under these conditions irreversible segment length (l_in) measured after freezing/thawing increases steadily over a period of 8 h although the in vivo length (l_tot) remains constant. This phenomenon, designated as 'cryptic growth', is an indication of a wall-stiffening process which appears to be an intrinsic component of irreversible cell wall extension. Using a range of metabolic inhibitors it is demonstrated that cryptic growth is caused by a temperature-sensitive biochemical process in the cell wall which depends on the presence of O₂ and active peroxidase, but not on ATP and protein synthesis. Inhibition of cryptic growth by anaerobic conditions can be alleviated by extermal H₂O₂. Moreover, cryptic growth can be partially inhibited by the antioxidant ascorbate. It is concluded that cryptic growth represents a wall-stiffening reaction mediated by peroxidase-catalyzed, H₂O₂-dependent cross-linking of phenolic residues of wall polymers. The experimental demonstration of a wall-stiffening reaction in a rapidly growing organ supports the concept that irreversible cell elongation (growth) is caused by an interplay of two chemorheological reactions, a turgor-dependent wall-loosening reaction and a separate wall-stiffening reaction which fixes the viscoelastically extended wall structure through oxidative cross-linking and thus conferring irreversibility to wall extension.