title: Overexpression of the UGT73C6 alters brassinosteroid glucoside formation in Arabidopsis thaliana.
abstract: BACKGROUND
  Brassinosteroids (BRs) are signaling molecules that play essential roles in the spatial regulation of plant growth and development. In contrast to other plant hormones BRs act locally, close to the sites of their synthesis, and thus homeostatic mechanisms must operate at the cellular level to equilibrate BR concentrations. Whilst it is recognized that levels of bioactive BRs are likely adjusted by controlling the relative rates of biosynthesis and by catabolism, few factors, which participate in these regulatory events, have as yet been identified. Previously we have shown that the UDP-glycosyltransferase UGT73C5 of Arabidopsis thaliana catalyzes 23-O-glucosylation of BRs and that glucosylation renders BRs inactive. This study identifies the closest homologue of UGT73C5, UGT73C6, as an enzyme that is also able to glucosylate BRs in planta.
  
  
  RESULTS
  In a candidate gene approach, in which homologues of UGT73C5 were screened for their potential to induce BR deficiency when over-expressed in plants, UGT73C6 was identified as an enzyme that can glucosylate the BRs CS and BL at their 23-O-positions in planta. GUS reporter analysis indicates that UGT73C6 shows over-lapping, but also distinct expression patterns with UGT73C5 and YFP reporter data suggests that at the cellular level, both UGTs localize to the cytoplasm and to the nucleus. A liquid chromatography high-resolution mass spectrometry method for BR metabolite analysis was developed and applied to determine the kinetics of formation and the catabolic fate of BR-23-O-glucosides in wild type and UGT73C5 and UGT73C6 over-expression lines. This approach identified novel BR catabolites, which are considered to be BR-malonylglucosides, and provided first evidence indicating that glucosylation protects BRs from cellular removal. The physiological significance of BR glucosylation, and the possible role of UGT73C6 as a regulatory factor in this process are discussed in light of the results presented.
  
  
  CONCLUSIONS
  The present study generates essential knowledge and molecular and biochemical tools, that will allow for the verification of a potential physiological role of UGT73C6 in BR glucosylation and will facilitate the investigation of the functional significance of BR glucoside formation in plants.
authors.affiliation: Max F, Perutz Laboratories, University of Vienna, Dr, Bohr-Gasse 9, 1030 Vienna, Austria.
authors.forename: Sigrid
authors.initials: S
authors.lastname: Husar
authors.forename: Franz
authors.initials: F
authors.lastname: Berthiller
authors.forename: Shozo
authors.initials: S
authors.lastname: Fujioka
authors.forename: Wilfried
authors.initials: W
authors.lastname: Rozhon
authors.forename: Mamoona
authors.initials: M
authors.lastname: Khan
authors.forename: Florian
authors.initials: F
authors.lastname: Kalaivanan
authors.forename: Luisa
authors.initials: L
authors.lastname: Elias
authors.forename: Gillian S
authors.initials: GS
authors.lastname: Higgins
authors.forename: Yi
authors.initials: Y
authors.lastname: Li
authors.forename: Rainer
authors.initials: R
authors.lastname: Schuhmacher
authors.forename: Rudolf
authors.initials: R
authors.lastname: Krska
authors.forename: Hideharu
authors.initials: H
authors.lastname: Seto
authors.forename: Fabian E
authors.initials: FE
authors.lastname: Vaistij
authors.forename: Dianna
authors.initials: D
authors.lastname: Bowles
authors.forename: Brigitte
authors.initials: B
authors.lastname: Poppenberger
mesh_terms: D017360:Arabidopsis / Q000201:enzymology / Q000235:genetics* / Q000254:growth & development / Q000378:metabolism; D029681:Arabidopsis Proteins / Q000235:genetics* / Q000378:metabolism*; D015870:Gene Expression; D005960:Glucosides / Q000096:biosynthesis*; D005964:Glucosyltransferases / Q000235:genetics* / Q000378:metabolism*; D010937:Plant Growth Regulators / Q000096:biosynthesis*; D021381:Protein Transport; D013256:Steroids / Q000096:biosynthesis*
chemical_list: D029681:Arabidopsis Proteins; D005960:Glucosides; D010937:Plant Growth Regulators; D013256:Steroids; D005964:Glucosyltransferases; C480036:UGT73C6 protein, Arabidopsis