Aspects of the Cellular and Molecular Basis of Cold Tolerance in Plants

Abstract

The diversity of the reactions of plants to low temperatures is illustrated by the classical distinction between chilling-sensitive and freezing-sensitive species. In an attempt to improve our understanding of the cellular and molecular basis underlying this diversity, we have investigated the fluidity of specific membranes in relation to cold resistance in Eucalyptus gunnii, a freezing-sensitive tree, and the changes in protein synthesis induced by cold acclimation in soybean (Glycine max), a chilling-sensitive species. We have been able to obtain isolated vacuoles and to purify, by free-flow electrophoresis, specific membranes (tonoplast and plasmalemma) from two cell lines of Eucalyptus gunnii which exhibit differential frost tolerance. The lateral and rotational mobilities of lipids in these different systems were studied by two biophysical techniques: fluorescence recovery after photobleaching (FRAP) and fluorescence polarisation. Our results show, for the first time, that intrinsically the tonoplast exhibits a higher fluidity than the plasma membrane. In addition, the membranes from the frost tolerant line or from acclimated lines were always more fluid than those from the sensitive line. These data strongly suggest a correlation between membrane fluidity and freezing tolerance in Eucalyptus. Through a progressive exposure to low temperatures, soybean plants were acclimated to a temperature of 8°C. As is the case with freezing-sensitive plants, acclimation improved the cold tolerance of chilling-sensitive plants. In order to assess changes in protein synthesis related to cold acclimation, proteins were labelled in vivo with ³⁵S methionine, separated by two dimensional gel electrophoresis and the derived autoradiograms were subjected to computer analysis. The comparison of soluble proteins stimulated during acclimation and at low temperatures during the post-acclimation phase revealed that no new polypeptides were synthesised. The effects of acclimation on protein synthesis are essentially quantitative. Of the soluble proteins, the synthesis of a polypeptide homologous to the heat shock protein 70 family is stimulated. This polypeptide could protect soybean proteins from denaturation and aggregation at low, non-freezing temperatures.