Acclimation vs Shock response to water deficit: what is the role of ABA?

While ABA has been widely demonstrated to play a fundamental role in controlling the rapid response of plants to physical changes in the environment, there is no clear evidence of its putative intervention in the ability; of plant cell to maintain cellular homeostasis under persisiting stress conditions. To compare rapid and long-term response to water stress, a potato cell population was treated abruptly with 20% PEG or acclimated by gradual transfer into a medium cointaining increasing concentration of this osmotic compound. Acclimation to water stress was accomplished by means of a set of metabolic and molecular cellular modifications, including proline accumulation, alteration in membrane lipid composition, synthesis of de novo proteins, changes in gene expression, which permitted active cellular growth at otherwise inhibiting conditions. The two cellular systems were different in the cellular level of endogenous ABA, which did not vary during acclimation, but increased five-fold in shocked cells. Tolerance to otherwise non permitting cellular growth conditions was then acquired through the activation of ABA- indipendent genes. This was confirmed by the observation that several heterologous lea genes (D-Il, rab 17 and rab 28, Em) were not up- regulated in acclimated cells. Differential Display Reverse Transcription-PCR (DDRT-PCR) is being current1y used as a strategy to clone genes specifically related to the ability of plant cells to increase tolerance through acclimation. Data on the first genes cloned and their role in the acclimation process will be presented and discussed.