Regulation of source/sink relations by cytokinins

Many physiological effects of cytokinins are well established and are known to be involved in various aspects of the plant life cycle. In contrast, little is known about how these effects are evoked at the molecular level. Since cytokinins have been shown to play a major role in the regulation of various processes associated with active growth and thus an enhanced demand for carbohydrates, a link to the regulation of assimilate partitioning has been suggested. This review discusses the current knowledge of the role of cytokinins in the regulation of source-sink relations, based on the finding of the co-ordinated cytokinin induction of an extracellular invertase and a hexose transporter. The induction of these key enzymes of an apoplastic unloading mechanism may be one important molecular prerequisite for different cytokinin-mediated effects.