Xylan acetylation

Abstract: Two rat monoclonal antibodies have been generated to plant cell wall (1→4)-β-D-xylans using a penta-1,4-xylanoside-containing neoglycoprotein as an immunogen. The monoclonal antibodies, designated LM10 and LM11, have different specificities to xylans in relation to the substitution of the xylan backbone as indicated by immunodot assays and competitive-inhibition ELISAs. LM10 is specific to unsubstituted or low-substituted xylans, whereas LM11 binds to wheat arabinoxylan in addition to unsubstituted xylans. Immunocytochemical analyses indicated the presence of both epitopes in secondary cell walls of xylem but differences in occurrence in other cell types.

Abstract: Previous studies using co-expression analysis have identified a large number of genes likely to be involved in secondary cell-wall formation. However, the function of very few of these genes is known. We have studied the cell-wall phenotype of irx7, irx8 and irx9, three previously described irregular xylem (irx) mutants, and irx14 and parvus-3, which we now show also to be secondary cell-wall mutants. All five mutants, which have mutations in genes encoding putative glycosyltransferases, exhibited large decreases in xylan. In addition, all five mutants were found to have the same specific defect in xylan structure, retaining MeGlcUA but lacking GlcUA side branches. Polysaccharide analysis by carbohydrate gel electrophoresis (PACE) was used to determine the xylan structure in Arabidopsis, and revealed that side branches are added to approximately one in every eight xylose residues. Interestingly, this ratio is constant in all the lines analysed despite the wide variation in xylan content and the absence of GlcUA branches. Xylanase digestion of xylan from wild-type plants released a short oligosaccharide sequence at the reducing end of the xylan chain. MALDI-TOF MS analysis indicated that this sequence of sugars was absent in xylan from irx7, irx8 and parvus-3 mutants, but was present in irx9 and irx14. This is consistent with previous NMR analysis of xylan from irx7, irx8 and irx9, and suggests that PARVUS may be involved in the synthesis of a xylan primer whereas IRX14 may be required to synthesize the xylan backbone. This hypothesis is supported by assays showing that irx9 and irx14 are both defective in incorporation of radiolabel from UDP (14)C-xylose. This study has important implications for both our understanding of xylan biosynthesis and the functional analysis of cell-wall biosynthesis genes.

Abstract: Xylan, the major hemicellulosic polysaccharide in Arabidopsis secondary cell walls, requires a number of glycosyltransferases (GT) to catalyse formation of the various glycosidic linkages found in the polymer. In this study, we characterized IRX10 and IRX10-like (IRX10-L), two highly homologous genes encoding members of the glycosyltransferase family 47 (GT47). T-DNA insertions in IRX10 gave a mild irregular xylem (irx) phenotype consistent with a minor defect in secondary cell-wall synthesis, whereas plants containing mutations in IRX10-L showed no change. However, irx10 irx10-L double mutant plants showed a much more severe irx and whole-plant phenotype, suggesting considerable functional redundancy between these two genes. Detailed biochemical analysis of the irx10 irx10-L double mutant showed a large reduction of xylan in the secondary cell walls, consistent with a specific defect in xylan biosynthesis. Furthermore, the irx10 irx10-L mutant retains the unique oligosaccharide found at the reducing end of Arabidopsis xylan, but shows a severe reduction in beta(1,4) xylosyltransferase activity. These characteristics are similar to those of irx9 and irx14, mutants that are believed to be defective in xylan chain elongation, and suggests that IRX10 and IRX10-L also play a role in elongation of the xylan backbone.