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Plant Physiol, March 2003, Vol. 131, pp. 1374-1380
O-Glucosylation of cis-Zeatin in Maize.
Characterization of Genes, Enzymes, and Endogenous
Cytokinins1
Yeonjin K.
Veach,
Ruth C.
Martin,
David W.S.
Mok,
Jiri
Malbeck,
Radomira
Vankova, and
Machteld C.
Mok*
Department of Horticulture and Center for Gene Research and
Biotechnology, Oregon State University, Corvallis, Oregon 97331-7304
(Y.K.V., R.C.M., D.W.S.M., M.C.M.); and Institute of Experimental
Botany, Academy of Sciences of the Czech Republic, Rozvojová 135, 165 02 Prague 6, Czech Republic (J.M., R.V.)
trans-Zeatin is a major and ubiquitous cytokinin in higher
plants. cis-Zeatin has traditionally been viewed as an adjunct with low
activity and rare occurrence. Recent reports of cis-zeatin and its
derivatives as the predominant cytokinin components in some plant
tissues may call for a different perspective on cis-isomers. The
existence of a maize (Zea mays) gene
(cisZOG1) encoding an O-glucosyltransferase
specific to cis-zeatin (R.C. Martin, M.C. Mok, J.E. Habben, D.W.S. Mok
[2001] Proc Natl Acad Sci USA 98: 5922-5926) lends further support
to this view. Results described here include the isolation of a second
maize cisZOG gene, differential expression of
cisZOG1 and cisZOG2, and identification of
substantial amounts of cis-isomers in maize tissues. The open reading
frame of cisZOG2 has 98.3% identity to cisZOG1
at the nucleotide level and 97.8% at the amino acid level. The
upstream regions contain common and unique segments. The recombinant
enzymes have similar properties, Km values
of 46 and 96 µM, respectively, for cis-zeatin and a pH
optimum of 7.5. Other cytokinins, including
N6-( 2-isopentenyl)adenine, trans-zeatin,
benzyladenine, kinetin, and thidiazuron inhibited the reaction.
Expression of cisZOG1 was high in maize roots and kernels,
whereas cisZOG2 expression was high in roots but low in
kernels. cis-Zeatin, cis-zeatin riboside, and their
O-glucosides were detected in all maize tissues, with immature kernels containing very high levels of the
O-glucoside of cis-zeatin riboside. The results are a
clear indication that O-glucosylation of cis-zeatin is a
natural metabolic process in maize. Whether cis-zeatin serves as a
precursor to the active trans-isomer or has any other unique function
remains to be demonstrated.
1
This work was supported by the National Science
Foundation (grant nos. IBN-9981974 and IBN-0086731), by the U.S.
Department of Agriculture National Research Initiative Competitive
Grants Program (grant no. 01-02015), by the Nucleic Acids and Proteins Core Facility of the Oregon State Environmental Health Sciences Center
(grant no. 01-02015), by Pioneer Hi-Bred International, and by the
Czech Ministry of Education, Youth and Sports (grant no. Kontakt ME
406). This is paper no. 11,892 of the Oregon Agricultural Experiment Station.
*
Corresponding author; e-mail
mokm{at}science.oregonstate.edu; fax 541-737-3479.
© 2003 American Society of Plant Biologists
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