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The plant hormone cytokinin stimulates target caulonemata of Funaria to form buds that develop into the leafy gametophyte. Previous reports have shown that increases in intracellular Ca(2+) occur during hormone-activated budding concomitant with an alteration in the polarity of the organelles in the bud site. In order to ascertain the involvement of voltage-dependent Ca(2+) channels in this phenomenon, we have employed dihydropyridines (DHP), compounds noted for their ability to alter Ca(2+) flux through potential-sensitive channels. Addition of the DHP agonists (+)202-791 and CGP 28392 (100 micromolar) induces bud initials on every target cell including the tip cell. Application of the DHP antagonist (-)202-791, in the presence of cytokinin (1 micromolar benzyladenine), inhibits budding 96%. Similarly, nifedipine blocks cytokinin-induced budding 87% and its effect on budding can be inactivated with a pulse of ultraviolet light. These results are consistent with the idea that cytokinin induces the budding response by increasing Ca(2+) entry through voltage-operated channels. We suggest that cytokinin activation of Ca(2+) channels is the first action of the hormone and that subsequent cytokinin-induced mechanisms are operating to maintain budding, since DHP-induced initials rarely develop into complete buds.

The Effect of 1,4-Dihydropyridines on the Initiation and Development of Gametophore Buds in the Moss Funaria