Prolyl-4-hydroxylase activity in excised elongating and nonelongating regions of bean hypocotyl

Abstract

Apical (elongating) and basal (nonelongating) segments of bean hypocotyls were labelled with [4-3H] proline and in vivo prolyl-4-hydroxylase activity, and peptidyl proline synthesis and hydroxylation were determined. Prolyl-4-hydroxylase activity was negligible in newly excised segments. However, if segments were aerated before labelling, hydroxylase activity, uptake, and incorporation of [4-3H]proline were all significantly increased. 3H loss was negligible in the presence of α, α′-dipyridyl or when sections were labelled with [2,3-3H]proline. In basal sections aerated for 24 h, prolyl-4-hydroxylase activity and the proportion of peptidyl proline hydroxylated were twice that observed in apical sections. The data indicated that peptidyl proline biosynthesis was similar in nonelongating and elongating segments. When apical sections were aerated in solutions of NaCl, mannitol, or benzimidazole, at concentrations inhibitory to elongation, there was an increased release of 3H from [4-3H]proline. However, a similar increase in 3H release occurred in the [2,3-3H]proline-labelled controls. The results suggest that neither increased prolyl-4-hydroxylase activity or peptidyl proline biosynthesis are an obligatory requirement for cessation of growth in excised bean hypocotyls.