title: Extracellular cross-linking of maize arabinoxylans by oxidation of feruloyl esters to form oligoferuloyl esters and ether-like bonds.
abstract: Primary cell walls of grasses and cereals contain arabinoxylans with esterified ferulate side chains, which are proposed to cross-link the polysaccharides during maturation by undergoing oxidative coupling. However, the mechanisms and control of arabinoxylan cross-linking in vivo are unclear. Non-lignifying maize (Zea mays L.) cell cultures were incubated with l-[1-(3)H]arabinose or (E)-[U-(14)C]cinnamate (radiolabelling the pentosyl and feruloyl groups of endogenous arabinoxylans, respectively), or with exogenous feruloyl-[(3)H]arabinoxylans. The cross-linking rate of soluble extracellular arabinoxylans, monitored on Sepharose CL-2B, peaked suddenly and transiently, typically at approximately 9 days after subculture. This peak was not associated with appreciable changes in peroxidase activity, and was probably governed by fluctuations in H(2)O(2) and/or inhibitors. De-esterified arabinoxylans failed to cross-link, supporting a role for the feruloyl ester groups. The cross-links were stable in vivo. Some of them also withstood mild alkaline conditions, indicating that they were not (only) based on ester bonds; however, most were cleaved by 6 m NaOH, which is a property of p-hydroxybenzyl-sugar ether bonds. Cross-linking of [(14)C]feruloyl-arabinoxylans also occurred in vitro, in the presence of endogenous peroxidases plus exogenous H(2)O(2). During cross-linking, the feruloyl groups were oxidized, as shown by ultraviolet spectra and thin-layer chromatography. Esterified diferulates were minor oxidation products; major products were: (i) esterified oligoferulates, released by treatment with mild alkali; and (ii) phenolic components attached to polysaccharides via relatively alkali-stable (ether-like) bonds. Thus, feruloyl esters participate in polysaccharide cross-linking, but mainly by oligomerization rather than by dimerization. We propose that, after the oxidative coupling, strong p-hydroxybenzyl-polysaccharide ether bonds are formed via quinone-methide intermediates.
authors.affiliation: Institute of Molecular Plant Sciences, The University of Edinburgh, UK.
authors.forename: Sally J
authors.initials: SJ
authors.lastname: Burr
authors.forename: Stephen C
authors.initials: SC
authors.lastname: Fry
mesh_terms: D002478:Cells, Cultured; D003432:Cross-Linking Reagents; D004952:Esters / Q000737:chemistry*; D010084:Oxidation-Reduction; D055503:Protein Multimerization; D014990:Xylans / Q000737:chemistry*; D003313:Zea mays / Q000737:chemistry*
chemical_list: D003432:Cross-Linking Reagents; D004952:Esters; D014990:Xylans; C085118:arabinoxylan