ABSCISIC ACID-INDUCED MEMBRANE-POTENTIAL CHANGES IN BARLEY ALEURONE PROTOPLASTS - A POSSIBLE RELEVANCE FOR THE REGULATION OF RAB GENE-EXPRESSION

The effect of ABA on the membrane potential of barley (Hordeum vulgare cv. Himalaya) aleurone protoplasts was studied by measuring the distribution of the lipophilic cation tetraphenylphosphonium (TPP+). The resting membrane potential (E(m)) according to our measurements with TPP+ is about -53 mV and is in agreement with membrane potential values as measured with intracellular microelectrodes (about -55 mV). The TPP+-measurements could demonstrate a clear dependence of the resting E(m) on the external pH (pH(e)). Stimulation of the protoplasts with ABA induced a transient hyperpolarization of the membrane to -62 mV as measured with TPP+. The hyperpolarization was ABA-concentration dependent. Inhibition of the Hf-ATPases with the specific proton pump inhibitors diethylstilbestrol (DES) or Micanozole effectively prevented hyperpolarization. This indicates that the hyperpolarization is consistent with the activation of plasma membrane H+-ATPases. The K+-inward rectifier inhibitor BaCl2 was able to prolong the hyperpolarization. This result suggests that the hyperpolarization causes the opening of K+-channels. The ABA-induced proton-pump activation may be involved in ABA-induced gene-expression, as DES was able to inhibit this gene expression. BaCl2 did only show a slight inhibitory effect on ABA-induced gene-expression.