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...

106), we expressed Aspergillus xyloglucanase constitutively in Populus alba. The expression increased the length of stem even in the presence of sucrose. Increased stem growth was accompanied by a decrease in Young's elastic modulus in the growth zone but an increased elasticity in mature tissue. The increased internode length corresponded to an increase in cellulose content as well as speci¢c gravity, showing that the removal of xyloglucan might cause an increase in cellulose density in the secondary xylem.

SummaryTo study the role of cellulose and cellulase in plant growth, we expressed poplar cellulase (PaPopCel1) constitutively in Arabidopsis thaliana. Expression increased the size of the rosettes due to increased cell size. The change in growth was accompanied by changes in biomechanical properties due to cell wall structure indicative of decrease in xyloglucan cross-linked with cellulose micro®brils by chemical analysis and nuclear magnetic resonance (NMR) spectra. The result supports the concept that the paracrystalline sites of cellulose micro®brils are attacked by poplar cellulase to loosen xyloglucan intercalation and this irreversible wall modi®cation promotes the enlargement of plant cells.

To clarify effects of lignin on the thermal-softening properties of water-swollen wood, dynamic viscoelasticities of water-swollen wood were measured in the temperature range from 5 to 100. The results obtained were as follows. 1) Delignified wood and moso bamboo did not show thermal softening around 60~80 which was found for untreated wood and moso bamboo. From this result, it was found that the thermal softening of lingo-cellulosic materials around 60~80 were attributable to lignin. Decrease in storage elastic moduli (E') of wood and moso bamboo from 20 to 100 remarkably decreased with decreasing in their lignin contents. From this result, it can be concluded that the degree of thermal softening around 60~80 largely depends on lignin contents. 2) The thermal-softening temperatures were different among many kind of wood species (softwood, Japanese hardwood, Tropical hardwood) and between an untreated katsura (Cercidiphyllum japonicum) sample and a sample slightly delignified. Consequently, it is deduced that the thermal-softening behaviors of water-swollen wood were largely affected by cross-linking of lignin. 3) The thermal softening behaviors of katsura were quite different among specimens experienced cooling with different rates. This suggests that the thermal-softening behaviors of water-swollen wood largely depend on the conformation of lignin.

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The fluidity of wood remarkably increases during moisture changes. This phenomenon is termed mechano-sorptive creep. The mechanism of mechano-sorptive creep has been studied, including a previous report by our group. Here, creep tests in bending were carried out for wood during and immediately after adsorption of moisture and after a long moisture conditioning. The effects of the rate of moisture adsorption on creep were also examined. The results and conclusions are as follows: (I) Greater creep occurred immediately after the adsorption process as compared with that after a long moisture conditioning, whereas much greater creep occurred during the same adsorption process, similar to the case of drying. Therefore, during the changes in moisture, not only destabilization but also stabilization should occur simultaneously, so that the wood during the changing process is in a remarkably unstable state. (II) Smaller creep occurred immediately after a slower adsorption as compared with that immediately after a more rapid adsorption. This suggests that wood is more stabilized during a slower versus a more rapid adsorption process. However, difference in creep between the final stages of the slower and the more rapid adsorption process was scarcely found. This is considered to result from the difference in degree of stabilization caused by the different duration of both adsorptions. In other words, this result is only an outward appearance. (III) Greater creeps were recognized during larger changes in moisture content (Δu) during the adsorption processes corresponding to the drying process. Therefore, mechano-sorptive creep depends not only on Δu but also on the range of relative humidity (RH). Larger stabilization was found during the changing process of larger Δu.

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