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Published on February 22, 2005; 10.1104/pp.104.057174

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Received November 29, 2004
Returned for revision January 3, 2005
Accepted January 5, 2005

Topolins and Hydroxylated Thidiazuron Derivatives Are Substrates of Cytokinin O-Glucosyltransferase with Position Specificity Related to Receptor Recognition

Machteld C. Mok *, Ruth C. Martin , Petre I. Dobrev , Radomira Vanková , P. Shing Ho , Keiko Yonekura-Sakakibara , Hitoshi Sakakibara , and David W.S. Mok

Department of Horticulture and Center for Gene Research and Biotechnology, Oregon State University, Corvallis, Oregon 97331-7304
Institute of Experimental Botany, Academy of Sciences of the Czech Republic, 165 02 Prague 6, Czech Republic
Department of Biochemistry and Biophysics, Oregon State University, Corvallis, Oregon 97331-7304
Plant Science Center, RIKEN, The Institute of Physical and Chemical Research, 1-7-22, Suehiro, Tsurumi, Yokohama 230-0045, Japan

* Corresponding author; email: mokm{at}science.oregonstate.edu.

Glucosides of trans-zeatin occur widely in plant tissues, formed either by O-glucosylation of the hydroxylated side chain or N-glucosylation of the purine ring structure. O-Glucosylation is stereo-specific: the O-glucosyltransferase encoded by the Phaseolus lunatus ZOG1 gene has high affinity for trans-zeatin as the substrate, whereas the enzyme encoded by the maize (Zea mays) cisZOG1 gene prefers cis-zeatin. Here we show that hydroxylated derivatives of benzyladenine (topolins) are also substrates of ZOG1 and cisZOG1. The m-OH and o-OH derivatives are the preferred substrate of ZOG1 and cisZOG1, respectively. Among the hydroxylated derivatives of thidiazuron tested, the only enzyme/substrate combination resulting in conversion was cisZOG1/(o-OH) thidiazuron. The abilities of these cytokinins to serve as substrates to the glucosyltransferases were in a large part correlated with their biological activities in the P. lunatus callus bioassay, indicating that there may be similarities between cytokinin-binding sites on the enzymes and cytokinin receptors. Further support for this interpretation is provided by cytokinin recognition studies involving the Arabidopsis (Arabidopsis thaliana) CRE1/WOL/AHK4 and maize ZmHK1 receptors. The AHK4 receptor responded to trans-zeatin and m-topolin, while the ZmHK1 receptor responded also to cis-zeatin and o-topolin. Three-dimensional molecular models of the substrates were applied to explain the results.




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