Xyloglucan for Generating Tensile Stress to Bend Tree Stem

Abstract

In response to environmental variation, angiosperm trees bend their stems by forming tension wood, which consists of a cellulose-rich G (gelatinous)-layer in the walls of fiber cells and generates abnormal tensile stress in the secondary xylem. We produced transgenic poplar plants overexpressing several endoglycanases to reduce each specific polysaccharide in the cell wall, as the secondary xylem consists of primary and secondary wall layers. When placed horizontally, the basal regions of stems of transgenic poplars overexpressing xyloglucanase alone could not bend upward due to low strain in the tension side of the xylem. In the wild-type plants, xyloglucan was found in the inner surface of G-layers during multiple layering. In situ xyloglucan endotransglucosylase (XET) activity showed that the incorporation of whole xyloglucan, potentially for wall tightening, began at the inner surface layers S1 and S2 and was retained throughout G-layer development, while the incorporation of xyloglucan heptasaccharide (XXXG) for wall loosening occurred in the primary wall of the expanding zone. We propose that the xyloglucan network is reinforced by XET to form a further connection between wall-bound and secreted xyloglucans in order to withstand the tensile stress created within the cellulose G-layer microfibrils.

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).…”

Tree growth stress and related problems

“…Some have considered the role of non-crystalline polysaccharide (NCP) and the related protein, e.g., xyloglucan (XG) and xyloglucan-endotransglycosylase (XET), in G-layer [66,68,93,94], where the joint effect of XG and XET is hypothesized to induce the tensile-stress generation of G-layer. Others revived the mechanism proposed by Münch with a more sophisticated model [95], which was further integrated with the previous one involving various NCP in G-layer [69,70].…”

Cellulosic Fibers: Role of Matrix Polysaccharides in Structure and Function

“…To analyze the polysaccharides, which are especially strongly retained within cell wall, a special protocol was developed [83]. After removal of the extractable polysaccharides by chelators and concentrated alkali, the residual cell wall material was dissolved in solution of lithium chloride in N,N-dimethylacetamide and afterwards cellulose was precipitated by water.…”

“…In fibers with the G-layers, the major peak of matrix polysaccharide eluted between 100 and 400 kDa; its predominating component was pectic galactan. This galactan from flax fibers was characterized by various techniques, including 1 H and 13 C NMR and antibody binging [83]. The ratio between high and low molecular mass peaks on the elution profile depended on the proportion of the S-and G-layers within the fiber cell wall.…”

“…This nascent form of the pectic galactan can be collected from the tissue homogenization buffer and compared to the polymer strongly retained by cellulose microfibrils. The composition and structure of these polysaccharides together with tracing in pulse-chase experiments [68][69][70]83] permit to consider the entrapped by cellulose microfibrils galactan as a derivative of the nascent galactan. The comparison of these polymers revealed the following differences: the nascent polysaccharide elutes at gel-filtration as having higher molecular mass (in the 700-2000 kDa region) and has higher degree of branching and longer side chains, as compared to cell wall galactan [83].…”