Alterazioni delle membrane biologiche indotte da stress idrico: le proteine

The most important functions of the plant cell depend on the functionality of its membranes which is impaired if proteins and lipids are damaged. When water content reaches a value below 20% membrane dry weight, proteins undergo functional and quali-quantitative alterations. Functional alterations involve changes in protein mobility and in the conformational status and they may be analysed by electron spin resonance (ESR) technique. Following water stress membrane proteins have a lower mobility (as it is shown by the higher rotational correlation time in comparison with the control values) and the oxidation of the SH-groups occurs. As a consequence, the functionality of the proteins which need conformational changes to exert their action is impaired. Under water stress there is a reduction of preformed polyribosome and /or the preferential synthesis of "stress proteins" causing quali-quantitative alterations of membrane proteins. For this reason, water deficit determines changes in total membrane proteins, polypeptide composition as well as in the protein/lipid, hydrophilic protein/hydrophobic protein and protein/chlorophyll ratios. However, plants possess defence mechanisms which allow them to maintain structural integrity of membrane and to avoid damages caused by activated oxygen species such as superoxide anion radical and hydrogen peroxide, whose production increases under water deficit conditions. Antioxidants such as ascorbic acid and glutathione as well as the enzymes related to their metabolism (ascorbate peroxidase, glutathione reductase and dehydroascorbate reductase) remove these toxic species by the glutathione/ascorbate cycle before membranes and the SH-groups of proteins are damaged.