Involvement of Membrane Damage in Stress-Induced Oxidative Deactivation of Ribulose Bisphosphate Carboxylase From Lemna minor

Abstract

Previous studies have shown that Lemna minor L. responds to an osmotic shock by producing an enzyme system capable of oxidising ribulose bisphosphate carboxylase (RuBP carboxylase; EC 4.1.1.39) to an acidic and catalytically inactive form. The induction of the oxidase system which occurs when Lemna is subjected to osmotic shock in the light is delayed by darkness and inhibited in the presence of protein synthesis inhibitors. When Lemna fronds are deprived of calcium, the oxidase system is induced in a manner identical to that observed during osmotic shock. Furthermore, a similar phenomenon is observed when the plants are incubated in the presence of membrane-damaging compounds such as filipin, ethanol or Triton X-100. This effect is reversed by the addition of compounds that are known to stabilise cellular membranes, namely calcium, polyamines and sterols.

Our results suggest that osmotic shock, calcium starvation and other stress situations damage Lemna cellular membranes, releasing the oxidase system. The presumed membrane damage and the induction of the oxidase system may be somewhat related, as shown by the observation that, in the presence of certain membrane-damaging situations, membrane stabilisers not only protect Lemna membranes but also prevent the stress-induction of the oxidase system.