Hydroxyproline and Peroxidases in Cell Walls of Pisum sativum: Regulation by Ethylene

Abstract

Purified cell-wall preparations from the epicotyl of etiolated Pisum sativum contain covalently bound peroxidases and hydroxyproline-rich proteins. Towards the end of cell elongation there is a large rise in these wall components and thereafter a continuing slow rise which is associated with increasing age of tissue. Ethylene at concentrations of 0.1 ppm or more increases both peroxidase activity and hydroxyproline levels in the walls, the greatest response occurring in immature tissue including the apical hook. Growth of these tissues is highly sensitive to ethylene which causes an inhibition of elongation in extending cells and an enhanced lateral cell expansion. We suggest that the effects of ethylene on wall-bound peroxidase and hydroxyproline are implicated in the ethylene regulation of cell growth. The covalently bound wall peroxidase was found to be extremely stable and to contain unique isoenzymes which do not occur in either the cytoplasm or in the peroxidase which is ionically bound to walls. Ethylene increases peroxidase activity in both the cytoplasmic and the ionically bound wall fractions, but there is little or no increase in their hydroxyproline content. The possible relationships between covalently bound wall peroxidase and hydroxyproline are discussed and we speculate that this peroxidase may be involved in the hydroxylation of proline in the walls.