Journal
PLANT JOURNAL
Volume 95, Issue 4, Pages 743-757Publisher
WILEY
DOI: 10.1111/tpj.13984
Keywords
oxalyl-sugars; vitamin C; cell-wall modification; oxalic acid; apoplast; cell-suspension culture; oxalyl-threonate; spinach; Arabidopsis
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Funding
- UK Biotechnology and Biological Sciences Research Council (BBSRC)
- Vitacress Salads Ltd, St Mary Bourne, Hampshire, UK [BB/I015531/1]
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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-C-14]OxT was incubated with living spinach (Spinacia oleracea) and Arabidopsis cell-suspension cultures in the presence or absence of proposed acceptor substrates (carbohydrates). In addition, [C-14]OxT and [C-14]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, [C-14] oxalylsugars were formed, in competition with OxT hydrolysis. Preferred acceptor substrates were carbohydrates possessing primary alcohols e. g. glucose. A model transacylation product, [C-14] oxalyl-glucose, was relatively stable in vivo (half-life > 24 h), whereas [C-14] OxT underwent rapid turnover (half-life similar to 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. We conclude that plants possess apoplastic acyltransferase (oxalyltransferase) activity that transfers oxalyl groups from ascorbate catabolites to carbohydrates, forming relatively long-lived Ooxalyl-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.
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