Hormonal regulation of growth in cultured plant cells.

Abstract

The group of naturally occurring plant hormones known as the cytokinins are defined by their ability to stimulate cell division in mature, differentiated, mitotically inactive cells in tissue culture. Evidence from the literature suggests that cytokinins play a specific role in regulating the progress of a plant cell through its division cycle;. the hormone appears to trigger the transition from G2 to mitosis. However the cytokinins are capable of evoking an array of physiological and developmental responses (many of which do not involve cell division) from different plant tissues and organs. One biochemical effect of the cytokinins is a dramatic and rapid stimulation of polyribosome formation in cultured soybean, cells which require these hormones for growth. Stationary-phase soybean cells, transferred to a medium containing a cytokinin, double in cell number within 36 hr, but when transferred to a medium of the same composition lacking a cytokinin, they do not grow., In vivo labeling with [35S] methionine and slab-gel electrophoresis demonstrated that cytokinin brings about qualitative changes in the spectrum of proteins synthesized by soybean cells which precede hormone-induced cell division. We have shown that cytokinin-induced polyribosome formation is the result of an effect of the hormone on protein synthesis at the translational level. We propose that, in the absence of the hormone, certain genes are transcribed but their messengers are not translated. These include mRNA's for specific cell division proteins. Cytokinins act as permissive factors, allowing cells to complete a genetically programmed sequence of events which was initiated by other factors.