Update on Cytokinins Cytokinins. New Insights into a Classic Phytohormone

Cytokinins were discovered in the search for factors that promoted division of plant cells in culture. Naturally occurring cytokinins are N-substituted adenine derivatives that generally contain an isoprenoid derivative side chain. These hormones influence numerous aspects of plant development and physiology, including seed germination, de-etiolation, chloroplast differentiation, apical dominance, plantpathogen interactions, flower and fruit development, and leaf senescence. These processes are also influenced by various other stimuli (e.g. light and other phytohormones), and the physiological and developmental outcomes reflect a highly integrated response to these multiple stimuli. For example, the classical reports of Skoog and Miller (1957) revealed that undifferentiated callus cultures would form into roots or shoots depending on the relative amount of cytokinins and auxin in the medium; the ratio rather than the absolute amount of these two hormones is critical. A balanced ratio keeps the cells in an undifferentiated state, while high cytokinin to auxin ratios promote shoot and low ratios promote root development. In the last decade, genetic and molecular analysis of mutants has provided valuable insights into the molecular mechanisms underlying the action of other phytohormones. Cytokinin has lagged behind in such studies, but several exciting recent reports show that this is now changing. This review focuses on the dramatic recent progress in understanding cytokinin metabolism, perception, and signaling. The analysis of all three aspects is required to understand the different effects of cytokinin on plant physiology and its role as a developmental signal.