Polyribosome Formation in Relation to Cytokinin-induced Cell Division in Suspension Cultures of Glycine max [L.] Merr.

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Abstract
The relationship between cell proliferation and protein synthetic capacity in a cytokinin-requiring strain of cultured soybean cells (G. max [L.] Merr. cv. Sodifuri, of cotyledonary origin) in suspension culture was investigated. When transferred to a defined medium lacking cytokinin, very little cell division or cell enlargement took place over the course of a 6 day culture period. Cells transferred to medium of the same composition, but containing 0.5 .mu.M zeatin, exhibited rapid initial growth, with maximum mitotic activity occurring after 24 h in culture, and a doubling of the cell population within the first 36 h of the culture period. The polyribosomal RNA content of the cells decreased over the course of the first 24 h of the growth cycle while the polyribosome to monoribosome (P/M) ratio increased. The increase in the P/M ratio was greater in the cytokinin-treated cells. This apparent relationship between cytokinin-induced cell proliferation and polyribosome formation was examined further. Polyribosome formation was stimulated when zeatin was added directly to cell populations which had been cultured for 24 h in medium lacking a cytokinin. Transfer to fresh medium alone also stimulated polyribosome formation, whether this medium contained a cytokinin or not. The magnitude of transfer-induced polyribosome formation depended upon the initial cell density (number of cells/ml of medium). Regardless of the initial cell density and independent of the P/M ratios attained, the cytokinin-treated cell populations divided while the cytokinin-deprived cell populations did not. In vivo labeling with [35S]methionine and slab gel electrophoretic separation of sodium dodecyl sulfate derivatives of the labeled polypeptides demonstrated qualitative changes in the spectrum of proteins synthesized by the cytokinin-treated cells. These qualitative changes were independent of the cell density (and hence, independent of the P/M ratio) but they preceded cytokinin-induced cell division.