Suppression and acceleration of cell elongation by integration of xyloglucans in pea stem segments

Abstract

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...

Group III-AXTHGenes of Arabidopsis Encode Predominant Xyloglucan Endohydrolases That Are Dispensable for Normal Growth

“…Although the molecular mechanisms of cell wall extension are largely unknown, a key tenet is transient wall loosening effected by the cleavage and religation of matrix xyloglucan by XET (EC 2.4.1.207). Indeed, since the discovery of XET activity in the early 1990s, a strong focus on xyloglucan transglycosylation in the context of wall extension and remodeling continues to be sustained (Takeda et al, 2002;Nishikubo et al, 2007Nishikubo et al, , 2011Baba et al, 2009;Hernández-Nistal et al, 2010;Lee et al, 2010;Miedes et al, 2010Miedes et al, , 2011Opazo et al, 2010;Stratilová et al, 2010).…”

Lack of α-Xylosidase Activity in Arabidopsis Alters Xyloglucan Composition and Results in Growth Defects

“…Xyloglucan oligosaccharides can increase the elongation rate of pea segments, especially if they have been treated with auxin (McDougall and Fry, 1990;Takeda et al, 2002). This is most likely caused by XET activities that employ oligosaccharides as acceptors of transglycosylation, breaking down xyloglucan chains and weakening the wall (Fry et al, 1993;Takeda et al, 2002).…”

“…This is most likely caused by XET activities that employ oligosaccharides as acceptors of transglycosylation, breaking down xyloglucan chains and weakening the wall (Fry et al, 1993;Takeda et al, 2002). The elongation defect in Atxyl1 lines cannot be directly explained by this mechanism, although it is possible that excessive weakening triggers a strengthening response.…”

“…An alternative explanation is suggested by the observation that polymeric xyloglucan added to pea segments reduces their elongation, presumably by participating in endotransglycosylation reactions and therefore increasing the number of connections between microfibrils (Takeda et al, 2002). A similar mechanism could explain the reduced root elongation caused by the addition of two purified XTHs with XET activity (Maris et al, 2009).…”

“…4B). Addition of polymeric xyloglucan to growing pea segments causes a faster reorientation of microtubules from transversal to longitudinal orientation (Takeda et al, 2002). It would be interesting to investigate if lack of a-xylosidase activity, by increasing microfibril connections, has a similar effect.…”