title: Topolins and hydroxylated thidiazuron derivatives are substrates of cytokinin O-glucosyltransferase with position specificity related to receptor recognition.
abstract: 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.
authors.affiliation: Department of Horticulture and Center for Gene Research and Biotechnology, Oregon State University, Corvallis, Oregon 97331-7304, USA. mokm@science.oregonstate.edu
authors.forename: Machteld C
authors.initials: MC
authors.lastname: Mok
authors.forename: Ruth C
authors.initials: RC
authors.lastname: Martin
authors.forename: Petre I
authors.initials: PI
authors.lastname: Dobrev
authors.forename: Radomira
authors.initials: R
authors.lastname: Vanková
authors.forename: P Shing
authors.initials: PS
authors.lastname: Ho
authors.forename: Keiko
authors.initials: K
authors.lastname: Yonekura-Sakakibara
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Sakakibara
authors.forename: David W S
authors.initials: DW
authors.lastname: Mok
mesh_terms: D000225:Adenine / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism; D003583:Cytokinins / Q000378:metabolism*; D004926:Escherichia coli; D015870:Gene Expression; D005964:Glucosyltransferases / Q000378:metabolism*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D015394:Molecular Structure; D030781:Organisms, Genetically Modified; D027805:Phaseolus / Q000201:enzymology; D010671:Phenylurea Compounds / Q000737:chemistry / Q000378:metabolism*; D010940:Plant Proteins / Q000378:metabolism*; D011956:Receptors, Cell Surface / Q000378:metabolism; D013379:Substrate Specificity; D013830:Thiadiazoles / Q000737:chemistry / Q000378:metabolism*; D014026:Nicotiana; D015026:Zeatin / Q000378:metabolism
chemical_list: D003583:Cytokinins; D010671:Phenylurea Compounds; D010940:Plant Proteins; D011956:Receptors, Cell Surface; D013830:Thiadiazoles; C016785:thidiazuron; D015026:Zeatin; D005964:Glucosyltransferases; C117387:UDPglucose zeatin O-glucosyltransferase, plant; D000225:Adenine
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