Abstract
In the complex process of homeostasis of phytohormones cytokinins (CKs), O-glucosylation catalyzed by specific O-glucosyltransferases represents one of important mechanisms of their reversible inactivation. The CK O-glucosyltransferases belong to a highly divergent and polyphyletic multigene superfamily of glycosyltransferases, of which subfamily 1 containing UDP-glycosyltransferases (UGTs) is the largest in the plant kingdom. It contains recently discovered O and P subfamilies present in higher plant species but not in Arabidopsis thaliana. The cis-zeatin O-glucosyltransferase (cisZOG) genes belong to the O subfamily encoding a stereo-specific O-glucosylation of cis-zeatin-type CKs. We studied different homologous genes, their domains and motifs, and performed a phylogenetic reconstruction to elucidate the plant evolution of the cisZOG gene. We found that the cisZOG homologs do not form a clear separate clade, indicating that diversification of the cisZOG gene took place after the diversification of the main angiosperm families, probably within genera or closely related groups. We confirmed that the gene(s) from group O is(are) not present in A. thaliana and is(are) also missing in the family Brassicaceae. However, cisZOG or its metabolites are found among Brassicaceae clade, indicating that remaining genes from other groups (UGT73—group D and UGT85—group G) are able, at least in part, to substitute the function of group O lost during evolution. This study is the first detailed evolutionary evaluation of relationships among different plant ZOGs within angiosperms.
Highlights
Many aspects of plant growth and development are coordinated by plant hormones
Biosynthesis of phytohormones CKs in plants starts with transferring an isoprenoid moiety to an adenine present either in nucleotide form or bound to tRNA
CisZ-type CKs have been reported as essential components of some tRNAs in p lants[15], isoprenoid CKs generally occur as structural parts of certain tRNA species of all organisms from eubacteria to h umans[16]
Summary
Many aspects of plant growth and development are coordinated by plant hormones. Among them, cytokinins (CKs) constitute one of the major groups, playing a key role in the control of cell division and elongation as well as organogenesis and many other physiological processes in plants. In the complex process of CK homeostasis, N- and O-glucosylation represent important mechanisms of CK irreversible and reversible inactivation, respectively, catalyzed by specific N- and O-glucosyltransferases (for review see e.g. Refs.[1,2]) Products of these conjugation steps, CK N- and O-glucosides, have been frequently identified in plants[3], their roles in CK biology still remain somewhat unclear. The CK O-glucosyltransferases belong to the large enzyme superfamily of glycosyltransferases (GTs; EC 2.4.x.y)[4] These enzymes catalyze the transfer of sugar moieties from activated donor molecules to specific acceptor molecules, forming glycosidic bonds. A class of UGTs is defined by the presence of a C-terminal consensus sequence and can be found both in the plants and a nimals[5] The nomenclature of this polyphyletic multigene family has been recommended based on the evolutionary divergence[6]. Considering the abundance of cisZ-types in the plant k ingdom[17] and early calculations of tRNA turnover r ates[18], tRNA degradation does not, seem to be a sole pathway for cisZ formation in plants
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