O-glucosylation of cis-zeatin in maize. Characterization of genes, enzymes, and endogenous cytokinins.
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, K(m) values of 46 and 96 microM, respectively, for cis-zeatin and a pH optimum of 7.5. Other cytokinins, including N(6)-(delta(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.
Veach
Yeonjin K
YK
Department of Horticulture and Center for Gene Research and Biotechnology, Oregon State University, Corvallis, Oregon 97331-7304, USA.
Martin
Ruth C
RC
Mok
David W S
DW
Malbeck
Jiri
J
Vankova
Radomira
R
Mok
Machteld C
MC
D000595:Amino Acid Sequence; D003583:Cytokinins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D015971:Gene Expression Regulation, Enzymologic; D018506:Gene Expression Regulation, Plant; D005964:Glucosyltransferases / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D006031:Glycosylation; D007527:Isoenzymes / Q000235:genetics / Q000378:metabolism; D008969:Molecular Sequence Data; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism; D018517:Plant Roots / Q000235:genetics / Q000378:metabolism; D011499:Protein Processing, Post-Translational; D012639:Seeds / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D003313:Zea mays / Q000235:genetics / Q000378:metabolism*; D015026:Zeatin / Q000378:metabolism*
D003583:Cytokinins; D007527:Isoenzymes; D010940:Plant Proteins; D015026:Zeatin; D005964:Glucosyltransferases; C117387:UDPglucose zeatin O-glucosyltransferase, plant