Abscissic acid specific expression of RAB18 involves activation of anion channels in Arabidopsis thaliana suspension cells.

The abscissic acid (ABA) transduction cascade following the plasmalemma perception was analyzed in intact Arabidopsis thaliana suspension cells. In response to impermeant ABA, anion currents were activated and K(+) inward rectifying currents were inhibited. Anion current activation was required for the ABA specific expression of RAB18. By contrast, specific inhibition of K(+) channels by tetraethylammonium or Ba(2+) did not affect RAB18 expression. Thus, outer plasmalemma ABA perception triggered two separated signaling pathways.

Ion currents involved in early Nod factor response in Medicago sativa root hairs: a discontinuous single-electrode voltage-clamp study.

Nod factor [NodRm-IV(Ac,S)], isolated from the bacterium Rhizobium meliloti, induces a well-known depolarization in Medicago sativa (cv Sitel) root hairs. Analysis of this membrane response using the discontinuous single-electrode voltage-clamp technique (dSEVC) shows that anion channel, K+ channel and H+-ATPase pump currents are involved in young growing root hairs. The early Nod-factor-induced depolarization is due to increase of the inward ion current and inhibition of the H+ pump. It involved an instantaneous inward anion current (IIAC) and/or a time-dependent inward K+ current (IRKC). These two ion currents are then down-regulated while the H+ pump is stimulated, allowing long-term rectification of the membrane potential (Em). Our results support the idea that the regulation of inward current plays a primary role in the Nod-factor-induced electrical response, the nature of the ions carried by these currents depending on the activated anion and/or K+ channels at the plasma membrane.

Induction of RAB18 gene expression and activation of K+ outward rectifying channels 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 Giraudat, 1998). However, we do not yet know the mechanism of ABA perception. Conflicting results concerning the site of ABA perception have been published. The prevailing view is that since ABA controls many responses, different sites of perception for ABA might exist. In order to establish the cellular localisation of the ABA receptors in Arabidopsis thaliana suspension cells, we developed two physiological tests based upon the capacity of impermeant ABA-BSA conjugate to mimic permeant free ABA effects. We show that purified ABA-BSA conjugate is able to trigger RAB18 gene expression and that this 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 cells. Using the voltage-clamp technique we show that ABA-BSA, similarly to ABA, evokes a membrane depolarization and activates time- and voltage-dependent 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.