Effects of a brassinosteroid on growth and electrogenic proton extrusion in Azuki bean epicotyls

Brassinolide, a plant hormone newly isolated from pollen, promotes growth of the stem of a number of plant species. Similar effects are induced by a brassinosteroid (BR), the synthetic 24-epibrassinolide. In this paper the effects of BR on acid secretion and transmembrane electrical potential difference in Azuki bean ( Vigna angularis , Ohwi and Ohashi cv. Takara) epicotyls were determined in short term experiments and compared with the effects on growth. At concentrations between 10^-7 to 10^-5 M , BR stimulates, similarly to indole-3-acetic acid (IAA), growth and H⁺ extrusion and hyperpolarizes the transmembrane electric potential (PD). These effects of BR, as well as those of IAA, are suppressed by inhibitors of RNA and protein synthesis. All these effects of BR and IAA appear roughly additive, even when both hormones are present at their optimal concentrations. The data are interpreted as showing that the action of BR on growth is at least in part mediated by its capability to activate electrogenic proton extrusion. The additivity of the effects of BR and IAA suggests that the primary mechanism of action of the two hormones is different.