Review Article

Abstract: The phytohormone abscisic acid (ABA) plays a major role as an endogenous messenger in the regulation of plant’s water status. ABA is generated as a signal during a plant’s life cycle to control seed germination and further developmental processes and in response to abiotic stress imposed by salt, cold, drought, and wounding. The action of ABA can target specifical-ly guard cells for induction of stomatal closure but may also sig-nal systemically for adjustment towards severe water shortage. At the molecular level, the responses are primarily mediated by regulation of ion channels and by changes in gene expression. In the last years, the molecular complexity of ABA signal trans-duction surfaced more and more. Many proteins and a plethora of “secondary ” messengers that regulate or modulate ABA-re-sponses have been identified by analysis of mutants including gene knock-out plants and by applying RNA interference tech-nology together with protein interaction analysis. The complex-ity possibly reflects intensive cross-talk with other signal path-ways and the role of ABA to be part of and to integrate several responses. Despite the missing unifying concept, it is becoming clear that ABA action enforces a sophisticated regulation at all levels. Key words: Abscisic acid, Arabidopsis, signal transduction, ABA biosynthesis, protein-protein interaction, abiotic stress, in vivo imaging