Shoot Root Interactions During Floral Transition - a Possible Role for Cytokinins

This review discusses the role of roots in the control of floral transition. Classical physiological understanding indicates that flowering and root initiation and/or elongation are usually antagonistic processes. In several species, root removal promotes flowering. High or low temperatures applied to the root system also influence flower initiation in some plants. These observations clearly implicate the root system as possessing at least partial control of flowering. In various species, the inhibitory effect of the root can be mimicked by cytokinins which are known to be produced by roots. A few studies revealed, however, that the effect of exogenous cytokinins is strongly dependent on other factors such as the applied concentration, the environmental conditions, and the time and site of application, and that promotion or inhibition can be observed. These findings indicate that there is a permissive range of cytokinin concentrations and that the endogenous status of the plant affects cytokinin action, which is not comparable in all plant organs. Studies with the long-day plant Sinapis alba and the short-day plant Xanthium strumarium suggested the existence of a shoot-to-root signal, which is under photoperiodic control and affects cytokinin synthesis in and/or release from the roots. As a result, cytokinin levels are altered in the plant; these changes, triggered by the inductive treatment, may be transient and are not necessarily similar in different plant organs. Remarkably, in all species investigated, increases in cytokinin levels, most usually in buds or phloem sap, have been detected during the floral transition, suggesting that cytokinins could be required. A role as a mitotic stimulus is possible, as indicated by the work with Sinapis.