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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.

Xyloglucan is a key polymer in the walls of growing plant cells. Using split pea stem segments and stem segments from which the epidermis had been peeled off, we demonstrate that the integration of xyloglucan mediated by the action of wall-bound xyloglucan endotransglycosylase suppressed cell elongation, whereas that of its fragment oligosaccharide accelerated it. Whole xyloglucan was incorporated into the cell wall and induced the rearrangement of cortical microtubules from transverse to longitudinal; in contrast, the oligosaccharide solubilized xyloglucan from the cell wall and maintained the microtubules in a transverse orientation. This paper proposes that xyloglucan metabolism controls the elongation of plant cells.X yloglucan, which occurs widely in the primary walls of higher plants, possesses a 1,4-␤-glucan backbone with 1,6-␣-xylosyl residues along the backbone. Because the 1,4-␤-glucan backbone can bind specifically to cellulose microfibrils by hydrogen bonds (1), the xyloglucan probably contributes to the rigidity of the cell wall by cross-linking adjacent microfibrils (2). In fact, microfibrils seem to be coated with xyloglucan, which is located both on and between microfibrils throughout cell elongation (3). Masking xyloglucans in cell walls should prevent xyloglucan metabolism; in agreement with this prediction, an antibody specific to xyloglucan prevented an auxin-induced decrease in molecular size of xyloglucan and inhibited indole-3-acetic acid (IAA)-induced cell elongation in azuki hypocotyl segments (4). Xyloglucan endotransglycosylase (XET) has been proposed to participate in the dynamic changes of xyloglucan cross-linking (5, 6), but there is little direct evidence for this hypothesis. The question at issue concerns the structural function of xyloglucan, namely whether xyloglucan contributes to the extensibility of the wall by cross-linking adjacent microfibrils.Fucose-containing xyloglucan oligosaccharides have been shown to inhibit auxin-induced elongation of pea stems (7,8). Their inhibitory activity is approximately maximal at a concentration of 10 Ϫ8 to 10 Ϫ9 M. In the absence of auxin, a high concentration (Ͼ10 Ϫ8 M) of xyloglucan oligosaccharide did not inhibit but slightly promoted cell elongation in pea stem segments (9). Thus, the oligosaccharides may provide either negative or positive feedback control during cell elongation. However, the positive feedback reaction has not been reproducibly observed in pea stems (T.T. and T.H., unpublished results) and also was not observed in maize primary roots (10). In the present communication, we examine whether xyloglucan oligosaccharides control the elongation growth of plant cells.In cylindrical plant organs such as roots or stems, cells expand anisotropically, with the direction of most rapid growth parallel to the long axis of the organ, leading to elongation of the organ. It has been known for many years that the direction of elongation is perpendicular to the direction of net orientation of cellulose microfibrils (11). Therefore, paral...

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Vegetative desiccation tolerance is common in bryophytes, although this character has been lost in most vascular plants. The moss Physcomitrella patens survives complete desiccation if treated with abscisic acid (ABA). Group A protein phosphatases type 2C (PP2C) are negative regulators of abscisic acid signalling. Here we show that the elimination of Group A PP2C is sufficient to ensure P. patens survival to full desiccation, without ABA treatment, although its growth is severely hindered. Microarray analysis shows that the Group A PP2C-regulated genes exclusively overlap with genes exhibiting a high level of ABA induction. Group A PP2C disruption weakly affects ABA-activated kinase activity, indicating Group A PP2C action downstream of these kinases in the moss. We propose that Group A PP2C emerged in land plants to repress desiccation tolerance mechanisms, possibly facilitating plants propagation on land, whereas ABA releases the intrinsic desiccation tolerance from Group A PP2C regulation.