Effects of a brassinosteroid on growth and on H+-extrusion in isolated radish cotyledons: comparison with the effects of benzyladenine

We investigated the effect of 24-epibrassinolide (BR) on the cytokinin-bioassay based on growth of isolated radish (Raphanus sativus L. cv. Tondo Rosso Quarantino) cotyledons. BR stimulated growth of the cotyledons by about 50% when applied at saturating concentrations (3 μM). This effect was much lower than that of saturating concentrations of benzyladenine (BA) which was about 150% at 10 μM BA. The effects of saturating concentrations of BR and BA were additive. BR, but not BA, stimulated H+-secretion by the cotyledons (measured as acidification of the incubation medium) slightly but reproducibly. These results indicate that the modes of action of the two phytoregulators are at least partially different. BR-induced stimulation of H+-secretion was of similar amplitude to that induced by a concentration of fusicoccin (30 nM) such as to stimulate growth to the same extent as BR. These results suggest that BR-induced stimulation of radish cotyledon growth might depend, at least in part, on BR-induced acidification of the wall space, similar to that observed on Azuki bean epicotyls and maize roots.