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 (GA₃) also induces elongation growth and we have used confocal laser scanning microscopy to study the effects of GA₃ on pH_i employing the pH-indicator dyes, 2′,7′-bis(2-carboxyethyl)-5-(and-6) carboxyfluorescein and carboxy-semi-naphthorhodafluor-1. We confirm that GA₃ 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 GA₃ 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 GA₃ 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.