Submitted to: American Journal of Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/13/2008
Publication Date: 5/21/2008
Citation: Bowling, A.J. and Vaughn, K.C. 2008. Immunocytochemical characterization of tension wood: gelatinous fibers contain more than just cellulose. American Journal of Botany 95(6):655-663.
Interpretive Summary: Gelatinous fibers are the specialized type of wood cell that is responsible for the righting of leaning tree trunks and for holding heavy tree branches horizontally against the force of gravity. Scientists in the Southern Weed Science Research Unit had previously discovered that gelatinous fibers are also responsible for the twining of stems and the coiling of tendrils in the noxious weed redvine. Although we had characterized the gelatinous fibers in vines, little is known about the more widespread occurrence of these fibers in trees or whether they are even similar in composition. Therefore, we examined gelatinous fibers of the sweetgum tree using microscopy and immunocytochemical techniques. These studies showed that the gelatinous fibers of sweetgum were compositionally very similar to those of vines, containing mucilaginous pectins that swell and contract with varying moisture content to create a tension. Because the tendrils of vines have the same composition and structure as tree trunks and branches, it is very likely that it is this composition that causes the curling of tendrils and the righting of trees.
Technical Abstract: Gelatinous fibers (G-fibers) are the active component of tension wood. G-fibers are unlike traditional fiber cells in that they possess a thick gelatinous layer (G-layer) located next to the plasmalemma which is devoid of lignin. This G-layer has generally been presumed to be a crystalline cellulose layer which contracts following deposition and exerts tension on other layers sufficient to right a leaning tree trunk. However, other studies have questioned this notion of a nearly pure, mega-crystalline cellulose layer. In this report, immunocytochemical techniques were used to investigate the polysaccharide composition of G-fibers in Sweetgum tension wood. Surprisingly, antibodies that recognize arabinogalactan proteins and RG I- type pectin molecules were found to bind to the gelatinous layer. Because AGPs and pectic mucilages are found in tissues where swelling reactions occur, these polymers may be the source of the contractile forces which act on the cellulose microfibrils to provide the tension force necessary to bend the tree trunk.