GIBBERELLIC-ACID INDUCES CYTOPLASMIC ACIDIFICATION IN MAIZE COLEOPTILES

Indole-3-acetic acid (IAA), fusicoccin and weak acids all lower the cy toplasmic pH (pH(i)) and induce elongation growth of maize (Zea mays L .) coleoptiles. Gibberellic acid (GA(3)) also induces elongation growt h and we have used confocal laser scanning microscopy to study the eff ects of GA(3) on pH(i) employing the pH-indicator dyes, 2',7'-bis(2-ca rboxyethyl)-5-(and-6) carboxyfluorescein and carboxy-semi-naphthorhoda fluor-I. We confirm that GA(3) induces growth significantly in light-g rown but only slightly or not at all in dark-grown coleoptiles. The gr owth induced by IAA treatment was similar in light- and dark-grown col eoptiles. The pH(i) decreased by up to 0.6 units during the first 7 mi n of GA(3) or IAA treatment of both light- and dark-grown coleoptiles. Gibberellic acid inhibited IAA-induced growth of dark-grown coleoptil es. Hence, in dark-grown coleoptiles GA(3) may activate either directl y or indirectly reactions that interfere with the signalling pathway l eading to elongation growth. The possible role of pH(i) in growth is d iscussed.