METABOLISM OF XYLOGLUCAN GENERATES XYLOSE-DEFICIENT OLIGOSACCHARIDE SUBUNITS OF THIS POLYSACCHARIDE IN ETIOLATED PEAS

Oligosaccharide subunits of xyloglucan were isolated from the stems an d roots of etiolated pea plants and structurally characterized. The tw o most abundant subunits of pea xyloglucan are the well-known nonasacc haride, XXFG, and heptasaccharide, XXXG. In addition, significant amou nts of oligosaccharides that have not previously been reported to be s ubunits of pea xyloglucan were detected, including a decasaccharide, X LFG, two octasaccharides, XLXG and XXLG, a pentasaccharide, XXG, and a trisaccharide, XG. Several novel oligosaccharide subunits, including the octasaccharide, GXFG, and the hexasaccharide, GXXG, were also foun d. Xyloglucan oligosaccharides generated by treatment of intact pea st em cell wails were compared to oligosaccharides generated by endogluca nase treatment of xyloglucan polysaccharides obtained by subsequent al kali extraction of the same cell walls. The results suggest that the x yloglucan in etiolated pea stems is distributed between at least two d omains, one of which is distinguished by its enzyme accessibility. We further hypothesize that the chemical modification of a xyloglucan dur ing cell-wall maturation depends on its physical environment (i.e., th e domain in which it resides). For example, only the endoglucanase-rel eased material, representing the enzyme-accessible xyloglucan domain, contains significant amounts of the two unusual oligosaccharide subuni ts, GXXG and GXFG, both of which have a nonreducing terminal glucosyl residue. This structure may be generated during cell-wall maturation b y the sequential action of an endolytic enzyme (such as xyloglucan end otransglycosylase or endoglucanase) and an alpha-xylosidase.