GIBBERELLIC-ACID INDUCES CYTOPLASMIC ACIDIFICATION IN MAIZE COLEOPTILES

Indole-3-acetic acid (IAA), fusicoccin and weak acids all lower the cytoplasmic pH (pH(i)) and induce elongation growth of maize (Zea mays L.) coleoptiles. Gibberellic acid (GA(3)) also induces elongation growth and we have used confocal laser scanning microscopy to study the effects of GA(3) on pH(i) employing the pH-indicator dyes, 2',7'-bis(2-carboxyethyl)-5-(and-6) carboxyfluorescein and carboxy-semi-naphthorhodafluor-I. We confirm that GA(3) induces growth significantly in light-grown but only slightly or not at all in dark-grown coleoptiles. The growth induced by IAA treatment was similar in light- and dark-grown coleoptiles. The pH(i) decreased by up to 0.6 units during the first 7 min of GA(3) or IAA treatment of both light- and dark-grown coleoptiles. Gibberellic acid inhibited IAA-induced growth of dark-grown coleoptiles. Hence, in dark-grown coleoptiles GA(3) may activate either directly or indirectly reactions that interfere with the signalling pathway leading to elongation growth. The possible role of pH(i) in growth is discussed.