Regulation of polyribosome formation and cell division in cultured soybean cells by cytokinin.

Cytokinin was shown to be required for cell division in cultured cells of Clycine max var. Sodifury. This cytokinin-induced mitotic activity was correlated with a high cellular content of polysomes. Within 24 h after transfer to a cytokinin-containing medium there was a 45-fold increase in the percentage of ribosomes bound as polyribosomes, as determined by sucrose density gradient centrifugation of extracted ribosomal material. Relatively high levels of polysomes and mitotic activity were maintained through the first 6 days of the culture period on cytokinin-containing medium. Thereafter, both cell division activity and the percentage of polyribosomes declined progressively with increasing time in culture. A comparatively small increase in polyribosomes occurred within 24 h of transfer to medium lacking cytokinins, followed by the progressive decline of the level of polyribosomes. The time course of cytokinininduced polyribosome formation was determined by treating cells with cytokinin after they had been cultured for 24 h on a medium lacking cytokinin. Under these conditions there was a rapid increase in polyribosomes over the next 3 h with no detectable lag period, and near maximal levels of polyribosomes after 6 h of treatment. The initial stimulation of polyribosome formation by cytokinin was not blocked by actinomycin D. Cytokinin was shown to have a comparatively small effect on the ribonuclease activity in extracts of these cells.