In Arabidopsis suspension cells a rapid plasma membrane depolarization is triggered by abscisic acid (ABA). Activation of anion channels was shown to be a component leading to this ABA-induced plasma membrane depolarization. Using experiments employing combined voltage clamping, continuous measurement of extracellular pH, we examined whether plasma membrane H 1 -ATPases could also be involved in the depolarization. We found that ABA causes simultaneously cell depolarization and medium alkalinization, the second effect being abolished when ABA is added in the presence of H 1 pump inhibitors. Inhibition of the proton pump by ABA is thus a second component leading to the plasma membrane depolarization. The ABA-induced depolarization is therefore the result of two different processes: activation of anion channels and inhibition of H 1 -ATPases. These two processes are independent because impairing one did not suppress the depolarization. Both processes are however dependent on the [Ca 21 ] cyt increase induced by ABA since increase in [Ca 21 ] cyt enhanced anion channels and impaired H 1 -ATPases.Abscisic acid (ABA) induces the depolarization of the plasma membrane (Thiel et al., 1992). This depolarization has been interpreted as the consequence of the activation of anion channels in stomatal guard cells of Vicia faba (Blatt, 1990;Schroeder and Keller, 1992;Thiel et al., 1992;Ward et al., 1995), Nicotiana benthamiana and Commelina communis (Schwartz et al., 1995;Leonhardt et al., 1999). In addition, we demonstrated that the extracellular perception of ABA in Arabidopsis suspension cells was necessary for the activation of anion channels inducing the plasma membrane depolarization (Ghelis et al., 2000a), and recently we showed that this anion channel stimulation induced by extracellular ABA perception was dependent on phospholipase D activities (Hallouin et al., 2002). In guard cells that are the most studied plant cell model used for the dissection of ABA signaling pathways (Assmann, 1993;Schroeder et al., 2001), two distinct anion channels, rapid anion channels (R-type) and slow anion channels (S-type), were proposed to participate in the plasma membrane depolarization (Schroeder and Keller, 1992;. Both R-type and S-type channels have been suggested to contribute to an initial phase of the depolarization, while maintenance of the depolarized state of the plasma membrane was only attributed to the S-type anion channels (Schroeder and Keller, 1992). The mechanisms by which ABA activates anion channels are not entirely understood . In V. faba guard cells, activation of anion channels by ABA can be observed without variation of the cytoplasm calcium concentration, suggesting that the ABA-induced anion efflux is calcium-independent (Schwarz and Schroeder, 1998). However, numerous data support the calcium dependence of the anion channel activation in response to ABA. Some of the anion channels involved in a long-term plasma membrane depolarization are Ca 21 -sensitive and therefore are activated by an increase in cyto...
Summary• Fusaric acid (FA) is a toxin produced by Fusarium species. Most studies on FA have reported toxic effects (for example, alteration of cell growth, mitochondrial activity and membrane permeability) at concentrations greater than 10 − 5 M . FA participates in fungal pathogenicity by decreasing plant cell viability. However, FA is also produced by nonpathogenic Fusarii , potential biocontrol agents of vascular wilt fusaria. The aim of this study was to determine whether FA, at nontoxic concentrations, could induce plant defence responses.• Nontoxic concentrations of FA were determined from cell-growth and O 2 -uptake measurements on suspensions of Arabidopsis thaliana cells. Ion flux variations were analysed from electrophysiological and pH measurements. H 2 O 2 and cytosolic calcium were quantified by luminescence techniques.• FA at nontoxic concentrations (i.e. below 10 − 6 M ) was able to induce the synthesis of phytoalexin, a classic delayed plant response to pathogen. FA could also induce rapid responses putatively involved in signal transduction, such as the production of reactive oxygen species, and an increase in cytosolic calcium and ion channel current modulations.• FA can thus act as an elicitor at nanomolar concentrations.
In Arabidopsis thaliana suspension cells, ABA was previously shown to promote the activation of anion channels and the reduction of proton pumping that both contribute to the plasma membrane depolarization. These two ABA responses were shown to induce two successive [Ca(2+)](cyt) spikes. As reactive oxygen species (ROS) have emerged as components of ABA signaling pathways especially by promoting [Ca(2+)](cyt) variations, we studied whether ROS were involved in the regulation of anion channels and proton pumps activities. Here we demonstrated that ABA induced ROS production which triggered the second of the two [Ca(2+)](cyt) increases observed in response to ABA. Blocking ROS generation using diphenyleneiodonium (DPI) impaired the proton pumping reduction, the anion channel activation and the RD29A gene expression in response to ABA. Furthermore, H(2)O(2) was shown to activate anion channels and to inhibit plasma membrane proton pumping, as did ABA. However, ROS partially mimicked ABA's effects since H(2)O(2) treatment elicited anion channel activation but not the subsequent expression of the RD29A gene as did ABA. This suggests that expression of the RD29A gene in response to ABA results from the activation of multiple concomitant signaling pathways: blocking of one of them would impair gene expression whereas stimulating only one would not. We conclude that ROS are a central messenger of ABA in the signaling pathways leading to the plasma membrane depolarization induced by ABA.
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