REGULATION OF CYTOKININ CONTENT IN PLANT-CELLS

Cytokinin levels in plant cells are dependent on cytokinin biosynthesi s and/or uptake from extracellular sources, metabolic interconversions , inactivation and degradation. Cytokinin conversion to compounds diff ering in polarity seems to be decisive for their entrapment within the cell and intracellular compartmentation, which affects their metaboli c stability. Increase in cytokinin levels, resulting either from their uptake or intracellular biosynthesis, may promote further autoinducti ve accumulation of cytokinins which may function in the induction of c ytokinin-initiated physiological processes. Accumulated cytokinins are capable of inducing cytokinin oxidase which consequently decreases cy tokinin levels. This seems to be the mechanism of re-establishment and maintenance of cytokinin homeostasis required fbr further development of physiological events induced by transient cytokinin accumulation. Auxin may influence cytokinin levels by down regulation of cytokinin b iosynthesis and/or by promotion of cytokinin degradation. A model of t he regulation of cytokinin levels in plant cells based on these phenom ena is presented and its physiological role(s) is discussed.