Xyloglucan Endo-transglycosylase (XET) Functions in Gelatinous Layers of Tension Wood Fibers in Poplar—A Glimpse into the Mechanism of the Balancing Act of Trees

Abstract

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.

Maturation Stress Generation in Poplar Tension Wood Studied by Synchrotron Radiation Microdiffraction

“…The specific organization of the G-layer suggests a tensile force induced in the microfibrils during the maturation process. Different hypotheses have been proposed to explain this mechanism, such as the contraction of amorphous zones within the cellulose microfibrils (Yamamoto, 2004), the action of xyloglucans during the formation of microfibril aggregates (Nishikubo et al, 2007;, and the effect of changes in moisture content stimulated by pectin-like substances (Bowling and Vaughn, 2008). A recent work (Goswami et al, 2008) argued an alternative model, initially proposed by Mü nch (1938), which proposed that the maturation stress originates in the swelling of the G-layer during cell maturation and is transmitted to the adjacent secondary layers, where the larger MFAs allow an efficient conversion of lateral stress into axial tensile stress.…”

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

“…For comparison with previous studies, XET activity was visualized in Col-0 and xth31-1/ xth32-1 roots by monitoring the incorporation of the xyloglucan oligosaccharide-sulforhodamine conjugate XXXG-SR into the cell wall by confocal fluorescence microscopy. In this stopped assay, XXXG-SR functions as an alternate glycosyl acceptor for enzymes with XET activity, thus forming covalent xyloglucan-XXXG-SR conjugates at sites where the polysaccharide and enzyme are colocalized (Vissenberg et al, 2000;Nishikubo et al, 2007). As shown in Figure 3, the localization of XET activity was essentially identical in wild-type (Fig.…”

“…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).…”

“…The in situ detection of XET activity was based on the method described by Vissenberg et al (2000) with minor modifications. Hypocotyls from 5-d-old plants and roots from 6-d-old plants were collected in 25 mM MES buffer and subsequently incubated with 6.5 mM XXXG-SR (synthesized as described by Nishikubo et al [2007]) in 25 mM MES buffer, pH 5.5, in the dark at room temperature for 2 h. Hypocotyls or roots were intensively washed in ethanol:formic acid:water (15:1:4, v/v) for 10 min and subsequently washed with 5% formic acid for 3 h to remove unincorporated XXXG-SR. The tissues were then examined by confocal laser scanning microscopy (Leica Microsystems) with a 561-nm laser, and the signal was detected between 566 and 650 nm.…”

Photomorphogenesis in Dracaena draco

“…3. Hormones, especially auxin, rather than physical force (compression and tension), play a role in RW appearance (Brown 1974;Timell 1986;Jaffe et al 2002;Nishikubo et al 2007;Brereton et al 2012). …”