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...

Cellulose synthesis in plants is believed to be carried out by the plasma membrane-associated rosette structure which can be observed by electron microscopy. Despite decade-long speculation, it had not been demonstrated whether the rosette is the site of catalytic activity of cellulose synthesis. To determine the relationship between this structure and cellulose synthesis, we successfully isolated detergent-insoluble rosettes from the plasma membrane of bean epicotyls. However, the purified rosettes did not possess cellulose synthesis activity in vitro. Conversely, detergent-soluble granular particles of approximately 9.5-10 nm diameter were also isolated and exhibited UDP-glucose binding activity and possessed beta-1,4-glucan (cellulose) synthesis activity in vitro. The particle, referred to as the catalytic unit of cellulose synthesis, was enriched with a 78 kDa polypeptide which was verified as sucrose synthase like by mass spectrometry and immunoblotting. The catalytic units were able to bind to the rosettes and retained the cellulose synthesis activity in the presence of UDP-glucose or sucrose plus UDP when supplemented with magnesium. The incorporation of the catalytic unit into the rosette structure was confirmed by immunogold labeling with anti-sucrose synthase antibodies under an electron microscope. Our results suggest that the plasma membrane-associated rosette anchors the catalytic unit of cellulose synthesis to form the functional cellulose synthesis machinery.

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In vitro experiments show that ␥ -tubulin is detectable on the surface of isolated plastids and nuclei of D. hirsuta , and microtubules can be repolymerized from the isolated plastids. ␥ -Tubulin localization patterns on plastid and nuclear surfaces are not affected by the destruction of microtubules by oryzalin. We conclude that ␥ -tubulin is a highly conserved protein associated with microtubule nucleation in basal land plants and that it has a cell cycle-dependent distribution essential for the orderly succession of microtubule arrays.

An active glycoprotein fraction containing 58 % protein was isolated from Aloe vera gel by precipitation with 55 % ammonium sulfate followed by gel permeation using DEAE-Sephacel A-25, Sepharose 6B and Sephadex G-50 columns in a yield of 3 x 10 -3 %. The glycoprotein fraction showed a single band corresponding to a subunit of verectin at the same position when stained with both Coomassie brilliant blue and periodic acid-Schiff reagents on 18 % SDS-PAGE. The molecular weight (14 kDa) was confirmed by Sephadex G-50 column chromatography. The glycoprotein fraction showed a radical scavenging activity against superoxide anion generated by the xanthine-xanthine oxidase system as well as inhibition of cyclooxygenase-2 and reduction of thromboxane A 2 synthase level in vitro.

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