Pea Xyloglucan and Cellulose

Abstract

A macromolecular complex composed of xyloglucan and cellulose was obtained from elongating regions of etiolated pea stems. Xyloglucan could be solubilized by extraction of this complex with 24% KOH-0.1% NaBH4 or by extended treatment with endo-1,4-.beta.-glucanase. The polysacchardie was homogeneous by ultracentrifugal analysis and gel filtration on Sepharose CL-6B, MW 330,000. The structure of pea xyloglucan was examined by fragmentation analysis of enzymic hydrolysates, methylation analysis and precipitation tests with fucose- or galactose-binding lectins. The polysaccharide was composed of equal amounts of 2 subunits, a nonasaccharide (glucose/xylose/galactose/fucose, 4:3:1:1) and a heptasaccharide (glucose/xylose, 4:3), which appeared to be distributed at random, but primarily in alternating sequence. The xyloglucan:cellulose complex was examined by light microscopy using iodine staining, by radioautography after labeling with [3H]fucose, by fluorescence microscopy using a fluorescein-lectin (fucose-binding) as probe and by EM after shadowing. The techniques all demonstrated that the macromolecule was present in files of cell shapes, referred to here as cell-wall ghosts, in which xyloglucan was localized both on and between the cellulose microfibrils. Since the average chain length of pea xyloglucan was may times the diameter of cellulose microfibrils, it could introduce cross-links by binding to adjacent fibrils and thereby contribute rigidity to the wall.