Cytokinin: From autoclaved DNA to two-component signaling

Abstract

Since its first identification in the 1950s as a regulator of cell division, cytokinin has been linked to many physiological processes in plants, spanning growth and development and various responses to the environment. Studies from the last two and one-half decades have revealed the pathways underlying the biosynthesis and metabolism of cytokinin and have elucidated the mechanisms of its perception and signaling, which reflects an ancient signaling system evolved from two-component elements in bacteria. Mutants in the genes encoding elements involved in these processes have helped refine our understanding of cytokinin functions in plants. Further, recent advances have provided insight into the mechanisms of intracellular and long-distance cytokinin transport and the identification of several proteins that operate downstream of cytokinin signaling. Here, we review these processes through a historical lens, providing an overview of cytokinin metabolism, transport, signaling, and functions in higher plants.

Figure 1.

Structures of various cytokinins. Cytokinins are adenine derivatives. The general structure of the adenosine backbone of cytokinin is indicated to the left. Various sides chains, shown on the right, can be attached at the N⁶ position of the adenine ring (indicated by an R*), leading to cytokinin species with differing biological activity. Note that BA and kinetin are not naturally occurring cytokinins.

Figure 2.

Generalized pathway for cytokinin biosynthesis and catabolism. Enzymes catalyzing each step are shown in blue. Inactive cytokinin ribosides are shown in red, and the active free base forms indicated in green. The source of the dimethylallyl pyrophosphate (DMPP) side chain of cytokinin is primarily from the methylerythritol phosphate (MEP) pathway, with minor contributions from mevalonic acid (MVA) pathway. The structure of the free base form of only zeatin is indicated; for the structures of iP and DHZ, see Fig. 1. Further, the conjugation and degradation of only trans-zeatin is indicated; both iP and DHZ type cytokinin are also subject to these processes to differing extents. The formation of cis-zeatin is not shown. See text for additional details.

Figure 3.

Canonical cytokinin signaling pathway. Cytokinin binds to the CHASE domain in the HK receptors and initiates a phosphorelay as indicated by the P. See text for additional details.

Figure 4.

Roles of cytokinins in plant growth and responses to the environment. See text for additional details.