Xyloglucan: The molecular muscle of trees
Annals of Botany: 2008 102:659-665
Background: Tension wood evolved in woody angiosperms to allow stems withsecondary thickening to bend and thus maintain an optimal orientation.Stem bending is the result of longitudinal tensile stress thatdevelops in tension wood tissues. In many species, a specializedsecondary cell wall layer, the so-called gelatinous (G)-layer,develops, containing longitudinally orientated crystalline cellulosefibrils; these have been recently shown to generate the tensilestress by an unknown mechanism. The cellulose fibrils cannot,however, work in isolation. Both coherence between the fibrilsand adherence of the G-layer to the adjacent cell wall layersare required to transfer the tensile stresses of the cellulosefibrils to the tissue. Previous work had not identified hemicelluloseswithin the G-layer.
Recent Progress: Sugar composition and polysaccharide linkage analyses of pureG-layers isolated by sonication have recently identified xyloglucanas the main non-cellulosic component of the G-layer. Xyloglucanhas been detected by immunolabelling with the CCRC-M1 monoclonalantibody and by in-situ activity assays using XXXGâ€“sulforhodaminesubstrate in the developing G-layers but not in the mature ones.However, xyloglucan endotransglucosylase/hydrolase (XTH) proteinspersist in the G-layer for several years and the correspondingxyloglucan endotransglucosylase (XET) activity (EC 2Â·4Â·1Â·207)occurs in the adjacent layers. Correspondingly, several XTH-encodingtranscripts were found to be up-regulated in developing tensionwood compared with normal wood.Scope: We propose that, during cellulose crystallization, a part ofthe xyloglucan is trapped inside the crystal, inducing longitudinaltensile stress within it; another part of it is accessible andpresent between the G-layer and the outer wall layers. XET activitythat occurs persistently in the G-fibres maintains coherencebetween the G-layer and the adjacent secondary wall layers.It is postulated that these activities are essential for generationof tensile stress during fibre maturation in tension wood.
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