Abstract:
Heteroxylans are a group of polysaccharides that occur in the lignified secondary cell walls of all angiosperms. However, in non-lignified primary cell walls, heteroxylans have been reported in significant amounts from only those of the commelinid monocoytledons, where they occur as glucuronoarabinoxylans (GAXs). These are composed of a linear backbone of β-D-Xylp residues linked-(1→4), with many of the Xylp residues substituted with single α-L-Araf or α-D-GlcA (or its 4-O-methyl derivative) residues linked to the O-3 or O-2 positions of Xylp residues, respectively. The Xylp residues may also carry acetyl groups. Despite this, there have been few reports of labelling these non-lignified walls using xylan-specific antibodies. This may be because of interference from the xylan side chains. In this thesis, ethanol insoluble residues containing cells with non-lignified primary walls from five species of commelinid monocotyledons,representing five families were subjected to mild acid hydrolysis to remove the L-Araf residues, which are sensitive to acid hydrolysis. These ethanol-insoluble residues were fixed, embedded in resin and sectioned for immunofluorescence microscopy using the xylan-specific monoclonal antibodies LM 10 and LM 11. The sections were also treated with a solution of sodium carbonate to remove acetyl groups. None of the untreated primary walls was labelled with either antibody. However, after treatments with mild acid, all of the species examined showed labelling in the non-lignified primary walls, indicating the presence of xylans. In only one species, Tradescantia fluminensis,binding of the antibody was greater after treatment with sodium carbonate indicating the acetyl groups as well as Araf residues may prevent binding. The lignified walls of all species except one, were labelled with LM 10 and LM 11without acid hydrolysis. The walls of the exception, Ananas comosus were labelled after pre-treatment with acid. This shows that lignified secondary walls also contain xylans.