Regulation of shoot/root ratio by cytokinins from roots inUrtica dioica: Opinion

According to current knowledge, cytokinins are predominantly root-born phytohormones which are transported into the shoot by the transpiration stream. In the hormone message concept they are considered the root signals, which mediate the flux of the photosynthates to the various sinks of the plant. In this review, experiments are assessed, in which changes of the shoot to root ratio of biomass, caused by different levels of nitrogen supply to a model plant,Urtica dioica, could be traced to the natural cytokinin relations of the plant. Disturbance of the internal cytokinin balance of the plant resulted in a disproportionate distribution of the assimilates in favour of the cytokinin-enriched shoot. Inspite of some shortcomings of the hormone message concept, the presented work corroborates the significance of root-sourced cytokinins in the regulation of biomass partitioning between shoot and root.