[초청강연] What is going on in guard cells: Genetic dissection of ABA and Ca2+ signaling

Abscisic acid (ABA) plays a central role in the protection of plants from various environmental stresses. Guard cells are responsible for controlling CO2 uptake and water loss by regulating the size of stomatal pores. During drought stress, a rapid response of a plant is to close stomatal pores. This process is mediated by ABA. In stomatal guard cells, reactive oxygen species (ROS) have been suggested to function in ABA signaling. Despite extensive studies, molecular components working downstream of ROS in ABA signaling remain to be elucidated. In order to identify and characterize MAPK cascades mediating guard cell ABA/ROS signaling, we identified two MAPK genes, GCMAPK3 and GCMAPK4, that are preferentially and highly expressed in guard cells. To provide direct genetic evidence, RNAi-based gene silencing plant lines were generated in which both genes are silenced. In parallel, Arabidopsis single and double mutants carrying deleterious point mutations in these genes were generated. Interestingly, ABA-induced stomatal closure was strongly impaired in the RNAi lines in which both GCMAPK3 and GCMAPK4 transcripts were significantly silenced. Consistent with this result, the Arabidopsis mutants carrying point mutations in both genes showed a strong ABA-insensitive response in stomatal movement assays. Together, these results provide genetic evidence that GCMAPK3 and GCMAPK4 function in guard cell ABA signaling, and there is functional redundancy in these genes. In addition, cell-type specific microarray analysis allows us to identify genes that are highly and preferentially expressed in guard cells. We are currently characterizing promoters of guard cell-specific genes which should provide a basis for cell type-specific gene disruption, development and signal transduction studies, and molecular engineering of plants.It has long been a question how universal Ca2+ signal elicits the specific cellular activities in response to various stimuli. In stomatal guard cells, cytosolic Ca2+ has been shown to regulate stomatal movements. For example, ABA induces increases in the cytosolic Ca2+ concentration which result in stomatal closure, and Ca2+ oscillations encode necessary information for stomatal movements. Furthermore, it was shown that Ca2+ oscillation amplitude and frequency control gene expression in mammalian cells. However, molecular components mediating Ca2+ oscillation-regulated cellular responses including gene expression remain largely unknown in both kingdoms. Our effort to genetically dissect Ca2+ signal transduction mechanisms using Arabidopsis guard cells will be presented.