Cytokinin signaling in Arabidopsis.

Cytokinins have been implicated in many developmental processes and environmental responses of plants, including leaf senescence, apical dominance, chloroplast development, anthocyanin production, and the regulation of cell division and sink/source relationships. They were identified in the pioneering work of Skoog and Miller (1957) by their ability to promote cell division in cultured cells in combination with another phytohormone, auxin. Naturally occurring cytokinins are N 6 -substituted adenine derivatives (reviewed by Mok and Mok, 2001). There are multiple forms of cytokinin that differ in the structure of their side chain at the N 6 position, and each of these can occur in the free base, riboside, or ribotide form. Cytokinins can be modified by the conjugation of Glc to nitrogen at various positions of the adenine ring. They also can be modified by the conjugation of Glc and, to a lesser extent, Xyl to the hydroxyl group of the N 6 side chain. These modifications generally inactivate the cytokinin and in some cases are reversible. A subset of cytokinins can be degraded irreversibly by cleavage of the N 6 side chain by the enzyme cytokinin oxidase. The existence of pathways for the degradation and conjugation of cytokinins suggests that the level of this signaling compound is tightly regulated. Several of the enzymes encoding the proteins that catalyze these metabolic reactions have been cloned (Houba-Hérin et al., 1999; Martin et al., 1999a, 1999b, 2001; Morris et al., 1999), as have the genes encoding a key enzyme in cytokinin biosynthesis, isopentyl transferase (Kakimoto, 2001; Takei et al., 2001). These advances in the study of cytokinin biosynthesis and metabolism are important in understanding the role of cytokinins in plant growth and development and have been reviewed elsewhere (Haberer and Kieber, 2001; Mok and Mok, 2001). This review will focus on early events in cytokinin signaling and highlight some likely future directions in this field. A number of recent findings have begun to shed light on the molecular basis of cytokinin signaling. These breakthroughs have come mainly from molecular, genetic, and biochemical studies in Arabidopsis thaliana and include the identification of cytokinin primary response genes, genes encoding cytokinin receptors, and a skeletal pathway of elements that mediate signaling between these components. The proposed cytokinin signal transduction pathway is a phosphorelay pathway similar to bacterial two-component response systems, so we will begin with a brief outline of the relevant details of two-component signaling in prokaryotic and fungal systems.