Regulation of the ABA-sensitive Arabidopsis potassium channel gene GORK in response to water stress

Abstract The phytohormone abscisic acid (ABA) regulates many stress-related processes in plants. In this context ABA mediates the responsiveness of plants to environmental stresses such as drought, cold or salt. In response to water stress, ABA induces stomatal closure by activating Ca 2+ , K + and anion channels in guard cells. To understand the signalling pathways that regulate these turgor control elements, we studied the transcriptional control of the K + release channel gene GORK that is expressed in guard cells, roots and vascular tissue. GORK transcription was up-regulated upon onset of drought, salt stress and cold. The wilting hormone ABA that integrates responses to these stimuli induced GORK expression in seedlings in a timeand concentration-dependent manner and this induction was dependent on extracellular Ca 2+ . ABA-responsive expression of GORK was impaired in the ABA-insensitive mutants abi1-1 and abi2-1, indicating that these protein phosphatases are regulators of GORK expression. Application of ABA to suspensioncultured cells for 2 min followed by a 4 h chase was su⁄cient to manifest transcriptional activation of the K + channel gene. As predicted for a process involved in drought adaptation, only 122 4 h after the release of the stress hormone, GORK mRNA slowly decreased. In contrast to other tissues, GORK expression as well as K + out channel activity in guard cells is ABA insensitive, allowing the plant to adjust stomatal movement and water status control separately