COUNTERACTIVE EFFECTS OF ABA AND GA(3) ON EXTRACELLULAR AND INTRACELLULAR PH AND MALATE IN BARLEY ALEURONE

Barley (Hordeum vulgare L.) aleurone layers are known to constitutively acidify their surroundings, primarily by L-malic acid release (J. Mikola, M. Virtanen [1980] Plant Physiol 66: S-142). Here we demonstrate the antagonistic effects of the plant hormones gibberellic acid (GA(3)) and abscisic acid (ABA) on the regulation of extracellular pH (pHe) of barley aleurone layers. We observed a strong correlation between ABA-induced enhancement of extracellular acidification and an ABA-induced increase in L-malic acid release. In addition, ABA caused an increase in intracellular L-malate level. GA(3) caused a slight decrease in intracellular L-malate level and was able to inhibit the ABA-induced increase in L-malate intracellular concentration and release. In addition, this ABA-induced L-malate release could be completely inhibited by GA(3). The ABA-induced release of L-malic acid could not account for the total ABA-induced pH(e) decrease, suggesting the existence of an additional mechanism involved in the regulation of pH(e). It has been reported that ABA induces an intracellular pH (pH(i)) increase, possibly due to the activation of plasma membrane proton pumps (R. Van der Veen, S. Heimovaara-Dijkstra, M. Wang [1992] Plant Physiol 100: 699-705). A pH(i) increase, such as that caused by ABA, might be correlated with the intracellular L-malate increase as suggested by the pH stat model of D.D. Davies ([1986] Physiol Plant 67: 702-706). We studied if the effects of GA(3) on L-malate concentration were correlated with changes in pH(i) and found that GA(3) caused a pH(i) decrease and that GA(3) and ABA could interfere in the regulation of pH(i). In addition, we were able to mimic the effect of both hormones on L-malate release by bringing about artificial pH(i) changes with the weak acid 5,5-dimethyl-2,4-oxazolidinedione and the weak base methylamine. The physiological meaning of the effects of GA(3) and ABA on the regulation of both pH(e) and pH(i) during grain germination are discussed.