COMPARATIVE-ANALYSIS OF SHORT-TERM AND LONG-TERM CHANGES IN GENE-EXPRESSION CAUSED BY LOW WATER POTENTIAL IN POTATO (SOLANUM-TUBEROSUM) CELL-SUSPENSION CULTURES

To dissect the cellular response to water stress and compare changes i nduced as a generalized response with those involved in tolerance/accl imation mechanisms, we analyzed changes in two-dimensional electrophor etic patterns of in vivo [S-35]methionine-labeled polypeptides of cult ured potato (Solanum tuberosum) cells after gradual and long exposure to polyethylene glycol (PEC)mediated low water potential versus those induced in cells abruptly exposed to the same stress intensity. Protei n synthesis was not inhibited by gradual stress imposition, and the ex pression of 17 proteins was induced in adapted cells. Some polypeptide s were inducible under mild stress conditions (5% PEC) and accumulated further when cells were exposed to a higher stress intensity (10 and 20% PEC). The synthesis of another set of polypeptides was up-regulate d only when more severe water-stress conditions were applied, suggesti ng that plant cells were able to monitor different levels of stress in tensity and modulate gene expression accordingly. In contrast, in pota to cells abruptly exposed to 20% PEC, protein synthesis was strongly i nhibited. Nevertheless, a large set of polypeptides was identified who se expression was increased. Most of these polypeptides were not induc ed in adapted cells, but many of them were common to those observed in abscisic acid (ABA)treated cells. These data, along with the finding that cellular ABA content increased in PEC-shocked cells but not in PE C-adapted cells, suggested that this hormone is mainly involved in the rapid response to stress rather than long-term adaptation. A further group of proteins included those induced after long exposure to both w ater stress and shock. Western blot analysis revealed that osmotin was one protein belonging to this common group. This class may represent induced proteins that accumulate specifically in response to low water potential and that are putatively involved in the maintenance of cell ular homeostasis under prolonged stress.