Induction of RAB18 gene expression and activation of K+ outward rectifyingchannels depend on an extracellular perception of ABA in Arabidopsis thaliana suspension cells

Important progress has been made regarding the characterization of the ABA signalling components using genetic and molecular approaches (Leung and Gir audat, 1998). However, we do not yet know the mechanism of ABA perception. Conflicting results concerning the site of ABA perception have been publish ed. The prevailing view is that since ABA controls many responses, differen t sites of perception for ABA might exist. In order to establish the cellul ar localisation of the ABA receptors in Arabidopsis thaliana suspension cel ls, we developed two physiological tests based upon the capacity of imperme ant ABA-BSA conjugate to mimic permeant free ABA effects. We show that puri fied ABA-BSA conjugate is able to trigger RAB18 gene expression and that th is response is strictly due to the natural (+)-ABA enantiomer. The rate of RAB18 gene expression was independent of the level of ABA uptake by the cel ls. Using the voltage-clamp technique we show that ABA-BSA, similarly to AB A, evokes a membrane depolarization and activates time-and voltage-dependen t outward rectifying currents (ORC). We demonstrate that these ORC are due to a K+ efflux as assessed by tail currents and specific inhibition by both tetraethylammonium (TEA) and Ba2+. These observations provide evidence in favour of an extracellular site for ABA perception.