A New Classic of Cytokinin Research: Cytokinin-Deficient Arabidopsis Plants Provide New Insights into Cytokinin Biology

Cytokinins are a structurally diverse group of N 6 -substituted purine derivatives capable of inducing plant cell division. The discovery of cytokinins by Folke Skoog and colleagues in the 1950s initially focused on kinetin, a synthetic compound derived from autoclaved salmon sperm DNA (Miller et al., 1955). Miller, Skoog, and others subsequently found that cytokinins are naturally occurring plant hormones that are essential for normal plant development. In a classic series of experiments, Skoog and Miller (1957) showed that the ratio of cytokinin to auxin in nutrient media profoundly influences the morphogenesis of roots and shoots in plant tissue culture. Numerous plant developmental processes have been found to be influenced by cytokinin, including cell expansion, inhibition of leaf senescence, chloroplast development, mobilization of nutrients, and root and shoot branching (reviewed by Mok, 1994). Despite a wealth of knowledge on the effects of cytokinins on plant growth and development, a lack of specific mutants and specific inhibitors has retarded attempts to determine their in planta function(s). Rather, cytokinin function has been inferred from the effects of exogenous application or from increased accumulation of the hormone in transgenic plants expressing the bacterial cytokinin biosynthesis enzyme isopentenyl transferase. Because the artificial enhancement of hormone levels may cause artifactual, nonphysiological effects, hormone-deficient mutants are considered essential to determine in vivo function conclusively (Faure and Howell, 1999). In this issue of The Plant Cell , Werner et al. (pages 2532–2550) contribute significant insights into cytokinin function and metabolism with an in-depth study of cytokinindeficient transgenic Arabidopsis plants that overexpress members of the Arabidopsis cytokinin oxidase/dehydrogenase ( CKX ) gene family. CKX enzymes inactivate cytokinins irreversibly in a single enzymatic step. Cytokinin levels in plant tissues are determined by the rate (and location) of biosynthesis, the formation of inactive conjugates (mainly glycosides), and the rate of catabolism. Rapid turnover of cytokinin has been reported, and catabolism by CKX enzymes is believed to play a critical role in regulating cytokinin activity in vivo (Schmülling et al., 2003). The approach of Werner et al. overcomes the lack of cytokinin biosynthesis mutants and offers the opportunity for loss-of-function analysis. In addition, the authors present localization and expression analyses of different members of the CKX family in Arabidopsis (Figure 1).