Many physiological effects of cytokinins are well established and are known
to be involved in various aspects of the plant life cycle. In contrast, li
ttle is known about how these effects are evoked at the molecular level. Si
nce cytokinins have been shown to play a major role in the regulation of va
rious processes associated with active growth and thus an enhanced demand f
or carbohydrates, a link to the regulation of assimilate partitioning has b
een suggested. This review discusses the current knowledge of the role of c
ytokinins 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 un
loading mechanism may be one important molecular prerequisite for different
cytokinin-mediated effects.