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Plant cell walls constitute the bulk of the earth renewable source of energy and are a component in the diet of humans and herbivores. l-Arabinofuranosyl (Araf) residues are a quantifiably important constituent of these walls. Plants use uridine diphosphate (UDP)-l-arabinofuranose (UDP-Araf) to donate Araf residues in the biosynthesis of Araf-containing polysaccharides, proteoglycans, and glycoproteins. However, little is known about the formation of UDP-Araf. We now describe the purification and partial characterization of a rice UDP-arabinopyranose mutase (UAM) that catalyzes the formation of UDP-Araf from UDP-arabinopyranose (UDP-Arap). The reaction is reversible and at thermodynamic equilibrium the pyranose form is favored over the furanose form (90 : 10). Three related proteins that are encoded by rice gene loci Os03g40270, Os04g56520, and Os07g41360 were identified from partial amino acid sequences of UAM. These proteins have >80% sequence identity with polypeptides that are reversibly glycosylated in the presence of UDP-sugars. The rice mutase and two functionally active recombinant mutases were shown to be reversibly glycosylated in the presence of UDP-Glc. The cofactor, flavin-adenine-dinucleotide (FAD), is required for the catalytic activity of UDP-galactose mutases of prokaryotes, fungi, and protozoa. The plant mutases, which do not require a cofactor, must therefore have a different catalytic mechanism. Putative UAM-encoding genes are present in the green algae Chlamydomonas reinhardtii, the moss Physcomitrella patens, the gymnosperm Pinus taeda (loblolly pine), and in numerous dicots and monocots, indicating that UAMs are widespread in green plants.

Tension wood is a specialized tissue of deciduous trees that functions in bending woody stems to optimize their position in space. Tension wood fibers that develop on one side of the stem have an increased potency to shrink compared with fibers on the opposite side, thus creating a bending moment. It is believed that the gelatinous (G) cell wall layer containing almost pure cellulose of tension wood fibers is pivotal to their shrinking. By analyzing saccharide composition and linkage in isolated G-layers of poplar, we found that they contain some matrix components in addition to cellulose, of which xyloglucan is the most abundant. Xyloglucan, xyloglucan endo-transglycosylase (XET) activity and xyloglucan endo-transglycosylase/hydrolase (XTH) gene products were detected in developing G-layers by labeling using CCRC-M1 monoclonal antibody, in situ incorporation of XXXG-SR and the polyclonal antibody to poplar PttXET16-34, respectively, indicating that xyloglucan is incorporated into the G-layer during its development. Moreover, several XTH transcripts were altered and were generally up-regulated in developing tension wood compared with normal wood. In mature G-fibers, XTH gene products were detected in the G-layers while the XET activity was evident in the adjacent S(2) wall layer. We propose that XET activity is essential for G-fiber shrinking by repairing xyloglucan cross-links between G- and S(2)-layers and thus maintaining their contact. Surprisingly, XTH gene products and XET activity persisted in mature G-fibers for several years, suggesting that the enzyme functions after cell death repairing the cross-links as they are being broken during the shrinking process.

A revised system of abbreviated names is proposed for xyloglucan‐derived oligosaccharides. Each (1→4)‐linked β‐d‐glucosyl residue (and the reducing terminal d‐glucose moiety) of the backbone is given a one‐letter code according to its substituents. The name of the oligosaccharide consists of these code letters listed in sequence from non‐reducing to reducing terminus of the backbone.