of Plant Research J
J Plant Res (2003) 116:221–231 Digital Object Identiﬁer (DOI) 10.1007/s10265-003-0094-6
© The Botanical Society of Japan and Springer-Verlag Tokyo 2003
Takashi Aoyama • Atsuhiro Oka
Cytokinin signal transduction in plant cells
Received: February 4, 2003 / Accepted: March 8, 2003 / Published online: April 17, 2003
Abstract Cytokinins regulate various events in plant development according to the intrinsic developmental program and in response to environmental stimuli. Recent genetic and molecular biological studies have revealed the framework of the intracellular signal transduction pathway from cytokinin perception to transcriptional regulation of primary cytokinin-responsive genes in Arabidopsis thaliana. Membrane-bound histidine kinases, including CRE1/ AHK4, AHK2, and AHK3, perceive cytokinins. The signal is then transferred via histidine-containing phosphotransfer factors, AHPs, to transcription-factor-type response regulators, such as ARR1, which execute the signal-dependent transactivation of primary cytokinin-responsive genes, including those for other types of response regulator. Simply stated, the cytokinin signal is mediated by the His-Asp phosphorelay, which was originally found in bacterial twocomponent regulatory systems. However, many details, especially those that are essential for elucidating the regulatory mechanisms underlying complicated cytokinin responses, remain unknown. Key words His-Asp phosphorelay · Intracellular signal transduction · Phytohormone · Signal perception · Transcriptional regulation · Two-component regulatory system
Plants develop in response to both their intrinsic development program and environmental stimuli. Cytokinins play regulatory roles in plant development, including chloroplast development, leaf senescence, lateral shoot development, and vascular differentiation, by signaling cell proliferation and differentiation (for a review, see Mok and Mok 2001).
T. Aoyama (*) · A. Oka Laboratory of Molecular Biology, Institute for Chemical Research, Kyoto University, Gokasho, Uji , Kyoto 611-0011, Japan Tel. +81-774-383263; Fax +81-774-383259 e-mail: email@example.com
Given the importance of cytokinins in plant biology, the study of the molecular basis of cytokinin responses has started only recently. Several years ago, the mechanism of cytokinin signal transduction was totally unknown, unlike those of other phytohormones. Arabidopsis genes involved in responses to auxin, ethylene, and abscisic acid have long been identiﬁed, which is not the case for cytokinins (Bowler and Chua 1994). In 1996, some insight into this frustrating situation resulted from the identiﬁcation of the Arabidopsis thaliana CKI1 gene. This gene encodes a protein with the typical primary structure of bacterial sensor histidine kinases, and its overexpression results in cytokinin-independent shoot formation in Arabidopsis calli (Kakimoto 1996). Therefore, its gene product was thought to be a cytokinin receptor, and the cytokinin signal was thought to be mediated by a histidyl to aspartyl or aspartyl to histidyl (HisAsp) phosphorelay, a signal transduction mechanism typically found in bacterial adaptive responses. This provided an important clue to the molecular mechanism of cytokinin signal transduction, but the involvement of CKI1 in sensing cytokinin remained unclear. The ﬁrst year of the twenty-ﬁrst century was an important year in the study of cytokinins, when several laboratories reported compelling ﬁndings. First, an authentic cytokinin receptor, CRE1/AHK4, was identiﬁed. This also proved to be a sensor histidine kinase, which initiated the His-Asp phosphorelay in a cytokinin-dependent manner in yeast and bacterial cells (Inoue et al. 2001; Suzuki et al. 2001b; Ueguchi et al. 2001b) and interacted physically with cytokinins...