Changes in ribulose‐1,5‐bisphosphate carboxylase and its translatable small subunit mRNA levels during senescence of mustard (<i>Sinapis alba</i>) cotyledons

Abstract

Senescence of mustard ( Sinapis alba L.) cotyledons was studied by monitoring the levels of ribulose‐1,5‐bisphosphate carboxylase (RuBP carboxylase, EC 4.1.1.39) activity, enzyme protein and translatable mRNA of the small subunit (SSU‐mRNA) of RuBP carboxylase. In continuous white light (cWL) RuBP carboxylase began to decline 6 days after sowing and the decrease in SSU‐mRNA preceded the drop in enzyme protein. The decline in RuBP carboxylase was more pronounced when the seedlings were transferred to darkness. This dark‐mediated senescence was affected by phytochrome (P fr ) primarily by retardation of enzyme degradation and to a lesser extent by the maintenance of gene expression. When seedlings were reirradiated with cWL or red light after being kept for 6–8 days in darkness the levels of enzyme protein and SSU‐mRNA again increased. Thus, even after long periods of darkness the seedlings retained their ability to respond to light with renewed gene expression which was affected by P fr . Under short‐day conditions (8 h light/16 h dark), almost no decline of RuBP carboxylase occurred, whereas SSU‐mRNA decrease followed the same pattern as in cWL or darkness.