Mediation of sterol‐induced calmodulin synthesis in Phaseolus vulgaris roots by Ca 2+ and its possible relationship to plant growth regulators

Vitamin D 3 and stigmasterol have been previously shown to stimulate growth, Ca 2+ fluxes and calmodulin synthesis in Phaseolus vulgaris roots. In this study, these sterols (10 −9 M ) were shown to accelerate the incorporation of [ 3 H]‐thymidine into DNA in Phaseolus vulgaris (L. cv. Contraancha) root apices, similarly to a mixture of the mitogenic plant growth factors 2,4‐dichlorophenoxyacetic acid and kinetin (4.6 μ M each). The effects of stigmasterol were blocked by flufenazine, a calmodulin antagonist. Analogously to stigmasterol, the plant hormones stimulated calmodulin synthesis as shown by double labeling of root proteins with [ 14 C]‐leucine and [ 3 H]‐leucine, respectively, followed by their separation on sodium dodecyl sulfate‐po‐lyacrylamide gels and a calmodulin affinity column, immunoblot analysis and cyclic AMP phosphodiesterase activation assays. The stimulation of root calmodulin formation by stigmasterol was abolished in the absence of Ca 2+ in the incubation medium and was mimicked by the Ca 2+ ionophore A–23187. The results suggest that the sterols, like plant mitogenic hormones, promote DNA synthesis, and that these compounds stimulate calmodulin synthesis as a consequence of their mitogenic activity. Ca2+ appears to mediate the action of the sterols.