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JXB Advance Access published online on September 14, 2007
Journal of Experimental Botany, doi:10.1093/jxb/erm157
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© The Author [2007]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

SPECIAL ISSUE PAPER

Regulation of cytokinin biosynthesis, compartmentalization and translocation

Naoya Hirose1 *, Kentaro Takei1, Takeshi Kuroha1, Tomoe Kamada-Nobusada1, Hiroaki Hayashi2 {dagger} and Hitoshi Sakakibara1,{ddagger}

1RIKEN Plant Science Center, Suehiro 1-7-22, Tsurumi, Yokohama 230-0045, Japan
2Graduate School of Agricultural and Life Sciences, The University of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan

{ddagger} To whom correspondence should be addressed. E-mail: sakaki{at}riken.jp

Cytokinins, a group of mobile phytohormones, play an important role in plant growth and development, and their activity is finely controlled by environmental factors in the control of morphogenic and metabolic adaptations. Inorganic nitrogen sources, such as nitrate, are a major factor regulating gene expression of adenosine phosphate-isopentenyltransferase (IPT), a key enzyme of cytokinin biosynthesis. Modulation of IPT and macronutrient transporter gene expression in response to nitrate, sulphate and phosphate, and cytokinin-dependent repression of the transporter genes suggest that cytokinins play a critical role in balancing acquisition and distribution of macronutrients. Biased distribution of trans-zeatin (tZ)-type cytokinins in xylem and N6-({Delta}2-isopentenyl)adenine (iP)-type cytokinins in phloem saps suggest that, in addition to acting as local signals, cytokinins communicate acropetal and systemic long-distance signals, and that structural side chain variations mediate different biological messages. The compartmentalization of tZ- and iP-type cytokinins implies the involvement of a selective transport system. Recent studies have raised the possibility of subsets of the purine permease family as a transporter of cytokinin nucleobases and equilibrative nucleoside transporters (ENT) for cytokinin nucleosides. These biochemical and transgenic data suggest that AtENT6, an Arabidopsis ENT, could also participate in cytokinin nucleoside transport with a preference for iP riboside in vascular tissue.

Key words: Cytokinin, nitrogen, phloem, transporter, trans-zeatin, xylem

* Present address: Plant Genome Center, Tsukuba, Ibaraki 305-0856, Japan.

{dagger} Present address: 694 Futago, Aki, Kunisaki, Oita 873-0356, Japan.

Received 24 April 2007; Revised 6 June 2007 Accepted 14 June 2007


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