Enzymatic treatments reveal differential capacities for xylan recognition and degradation in primary and secondary plant cell walls

The capacity of four xylan-directed probes (carbohydrate-binding modules CfCBM2b-1-2 and CjCBM15; monoclonal antibodies LM10 and LM11) to recognize xylan polysaccharides in primary and secondary cell walls of tobacco stem sections has been determined. Enzymatic removal of pectic homogalacturonan revealed differential recognition of xylans in restricted regions of cortical primary cell walls. Monoclonal antibody binding to these exposed xylans was more sensitive to xylanase action than carbohydrate-binding module (CBM) binding. In contrast, the recognition of xylans by CBMs in secondary cell walls of the same organ was more sensitive to xylanase action than the recognition of xylans by the monoclonal antibodies. A methodology was developed to quantify indirect immunofluorescence intensities, and to evaluate xylanase impacts. The four xylan probes were also used to detect xylan populations in chromatographic separations of solubilized cell wall materials from tobacco stems. Altogether, these observations reveal the heterogeneity of the xylans in plant cell walls. They indicate that although CBM and antibody probes can exhibit similar specificities against solubilized polymers, they can have differential capacities for xylan recognition in muro, and that the access of molecular probes and enzymes to xylan epitopes/ligands also varies between primary and secondary cell walls that are present in the same organ.