Oxalyltransferase, a plant cell-wall acyltransferase activity, transfers oxalate groups from ascorbate metabolites to carbohydrates

Rebecca A Dewhirst, Stephen C Fry

Research output: Contribution to journalArticlepeer-review

Abstract

In the plant apoplast, ascorbate is oxidised, via dehydroascorbic acid, to O-oxalyl esters [oxalyl-L-threonate (OxT) and cyclic oxalyl-L-threonate (cOxT)]. We tested whether OxT and cOxT can donate the oxalyl group in transacylation reactions to form oxalyl-polysaccharides, potentially modifying the cell wall. [oxalyl-14C]OxT was incubated with living spinach (Spinacia oleracea) and Arabidopsis cell-suspension cultures in the presence and absence of proposed acceptor substrates (carbohydrates). In addition, [14C]OxT and [14C]cOxT were incubated in vitro with cell-wall enzyme preparations plus proposed acceptor substrates. Radioactive products were monitored electrophoretically. Oxalyltransferase activity was detected. Living cells incorporated oxalate groups from OxT into cell-wall polymers via ester bonds. When sugars were added, [14C]oxalyl-sugars were formed, in competition with OxT hydrolysis. Preferred acceptor substrates were carbohydrates possessing primary alcohols e.g. glucose. A model transacylation product, [14C]oxalyl-glucose, was relatively stable in vivo (half-life >24 h), whereas [14C]OxT underwent rapid turnover (half-life ~6 h). Ionically wall-bound enzymes catalysed similar transacylation reactions in vitro with OxT or cOxT as oxalyl donor substrates and any of a range of sugars or hemicelluloses as acceptor substrates. Glucosamine was O-oxalylated, not N-oxalylated. Plants possess apoplastic acyltransferase (oxalyltransferase) activity that transfers oxalyl groups from ascorbate catabolites to carbohydrates, forming relatively long-lived O-oxalyl-carbohydrates. The findings increase the range of known metabolites whose accumulation in vivo indicates vitamin C catabolism. Possible signalling roles of the resulting oxalyl-sugars can now be investigated, as can the potential ability of polysaccharide oxalylation to modify the wall's physical properties. 

Original languageEnglish
Pages (from-to)743–757
Number of pages15
JournalThe Plant Journal
Volume95
Early online date8 Jun 2018
DOIs
Publication statusPublished - 2 Aug 2018

Keywords

  • Arabidopsis
  • apoplast
  • cell-suspension culture
  • cell-wall modification
  • oxalic acid
  • oxalyl-sugars
  • oxalyl-threonate
  • spinach
  • vitamin C

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