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Inhibition of Cytokinin-Regulated Responses by Calmodulin-Binding Compounds ¹

School of Biological Sciences, Flinders University of South Australia, Bedford Park, SA 5042 Australia

Two classes of compounds which bind to calmodulin in a calcium-dependent manner (neuroleptic drugs and local anaesthetics) were used to investigate the possible involvement of a calcium-dependent regulator protein in the action of the plant hormones cytokinins.

The cytokinin-induced synthesis of betacyanin in Amaranthus tricolor seedlings was used as one test system. The calmodulin antagonists inhibited betacyanin synthesis with the following order of potency: fluphenazine > trifluoperazine = pimozide > chlorpromazine > dibucaine > penfluridol > haloperidol > tetracaine, over a concentration range (IC₅₀) of 0.1 to 0.6 millimolar. Red light and fusicoccin increase betacyanin accumulation and are synergistic with cytokinins. These red light- and fusicoccin-dependent inductions were inhibited by lower concentrations of the drugs than cytokinin-dependent induction, and the order of potency of the drugs was not precisely the same. The results are compatible with the hypothesis that cytokinins may act, at least in part, by changing ion fluxes, with the additional involvement of a calmodulin.

The second cytokinin-regulated response studied was growth in soybean callus culture. In this system, inhibition was observed with lower concentrations of drugs than in cytokinin-dependent betacyanin synthesis with an order of inhibitory potency of pimozide = trifluoperazine > penfluridol > chlorpromazine > haloperidol > tetracaine. The effective concentration range (IC₅₀) was from 0.07 to 0.5 millimolar.

Inhibition of betacyanin synthesis by 0.15 millimolar trifluoperazine and of soybean callus growth by 2 millimolar tetracaine were both reversible.