A maize cytokinin gene encoding an O -glucosyltransferase specific to cis -zeatin

Abstract

Zeatin is a naturally occurring cytokinin. Biosynthesis and metabolism studies of zeatin have been directed mostly at the trans isomer, although cis-zeatin and its riboside occur as major components in some plant species. It is not known whether parallel regulatory pathways exist for the two isomers. Based on the sequence of the gene ZOG1 encoding a trans-zeatin O-glucosyltransferase from Phaseolus (EC 2.4.1.203), a cis-zeatin-specific Oglucosyltransferase was isolated from maize. This gene, cisZOG1, contains an ORF of 1,401 nucleotides encoding a protein of 51.1 kDa with 41% identity to the Phaseolus ZOG1 protein. Unexpectedly, the maize enzyme recognizes as substrates cis-zeatin and UDP-glucose but not cis-ribosylzeatin, trans-zeatin, or trans-ribosylzeatin. This finding indicates the existence of cis-specific regulatory elements in plants and suggests that cis-zeatin and derivatives may be more important in cytokinin homeostasis than currently recognized. C ytokinins are essential hormones for plant growth. In addition to promoting cell division and differentiation in tissue culture, cytokinins regulate a host of developmental events in whole plants such as bud formation, leaf expansion, delay of senescence, promotion of seed germination, and chloroplast formation (reviewed in ref. 1). Naturally occurring cytokinins are adenine derivatives with a side chain at the N 6 -position; cytokinins with a hydroxylated isoprenoid side chain such as trans-zeatin, first identified in maize (2), are major constituents in plants. Studies on the biosynthesis and metabolism of cytokinins have centered on trans-zeatin, although cis-zeatin and its derivatives have been isolated from a number of species including potato (3, 4), Mercurialis (5), hops (6), rice (7-9), wheat (10), oats (10), and chickpeas (11). This emphasis on the trans isomers may be attributed to their higher biological activity.Two pathways for cytokinin biosynthesis have been proposed. The first is the direct pathway, involving formation of N 6 -isopentenyladenosine monophosphate from AMP and dimethylallyl pyrophosphate, followed by hydroxylation of the side chain to form trans-zeatin-type cytokinins. In support of this pathway, isopentenyltransferase activity in tobacco (12) and hydroxylation activity in cauliflower (13) have been reported. The discovery of the Agrobacterium ipt gene (14, 15) lends support to the existence of a similar biosynthetic pathway in plants. However, plant genes encoding such enzymes have not been identified, and indirect evidence suggests the existence of a biosynthetic pathway for zeatin without isopentenyl intermediates (16, 17). The second, indirect pathway involves release of cytokinins by turnover of tRNAs containing cis-zeatin. Because cis-zeatin is much less active in bioassays, conversion to the trans isomer by enzymes such as the cis-trans isomerase of zeatin (18) Here we report the isolation and characterization of a gene encoding a cis-zeatin specific O-glucosyltransferase from maize. The finding of a gene and enzyme sp...

N-Glucosylation of Cytokinins by Glycosyltransferases of Arabidopsis thaliana

“…In contrast to earlier studies on cytokinin O-glycosyltransferases from other plant species (27,28), recombinant UGTs from Arabidopsis recognized both trans-and cis-zeatin in vitro. Fig.…”

“…Although O-glucosylation and N-glucosylation of cytokinins have been described and reviewed, only O-glucosylation has been investigated in depth because of the availability of genes encoding the enzymes and biochemical analyses of their recombinant products (3). Genes encoding these enzymes from several plant species have been identified by Mok and coworkers (24,25,27,28), including ZOG1 from Phaseolus lunatus, ZOX1 from Phaseolus vulgaris, and cis-ZOG1 and cis-ZOG2 from maize. In contrast, nothing has been known of the N-glucosyltransferases of cytokinins, except early biochemical studies on partially purified enzyme preparations from a range of different plant species (29,30,35).…”

“…Conjugation of cytokinins also occurs and can involve Oxylosylation, O-glucosylation, and N-glucosylation (23)(24)(25)(26)(27)(28)(29)(30). Although all of these cytokinin glycosides are known to be inactive, their role in homeostasis is uncertain (3,13).…”

Cytokinins - recent news and views of evolutionally old molecules

“…The strict substrate specificity of the enzymes suggests that O-glycosylation is a precisely regulated process. This is supported by identification of a maize gene encoding a cis-zeatin O-glucosyltransferase (cisZOG) that does not recognise either trans-zeatin or dihydrozeatin, and uses UPDG as the only sugar donor (Martin et al 2001). Besides trans-zeatin and cis-zeatin, ZOG and cisZOG also recognise topolins and hydroxylated phenylureas as substrates, and the substrate specificities correlate with CK recognition by the Arabidopsis thaliana and maize CK receptors (Mok et al 2005).…”

Euglena gracilis(Euglenophyceae) produces abscisic acid and cytokinins and responds to their exogenous application singly and in combination with other growth regulators

“…Isoprenoid CKs are divided into four groups: trans-zeatin (tZ), cis-zeatin (cZ), dihydrozeatin (DHZ) and isopentenyladenine (iP), with aromatic CKs consisting of benzylaminopurine (BAP) and topolins (T) (Stirk & Van Staden, 2010;Spichal, 2012). In addition to stimulating cell division and differentiation in vascular plants, CKs also regulate shoot and root differentiation, growth of lateral buds and leaf expansion, floral transition, chloroplast development, seed germination, nutrient mobilization, biomass distribution and delay of senescence (Mok, 1994;Martin et al, 2001;Burkle et al, 2003;Tarakhovskaya et al, 2007). Endogenous CKs have also been described in numerous algal taxa.…”