Nishikubo N, Awano T, Banasiak A, Bourquin V, Ibatullin F, Funada R, Brumer H, Teeri TT, Hayashi T, Sundberg B, Mellerowicz EJ
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
Plant Cell Physiology: 2007 48:843-855

Abstract
Tension wood is a specialized tissue of deciduous trees thatfunctions in bending woody stems to optimize their positionin space. Tension wood fibers that develop on one side of thestem have an increased potency to shrink compared with fiberson the opposite side, thus creating a bending moment. It isbelieved that the gelatinous (G) cell wall layer containingalmost pure cellulose of tension wood fibers is pivotal to theirshrinking. By analyzing saccharide composition and linkage inisolated G-layers of poplar, we found that they contain somematrix components in addition to cellulose, of which xyloglucanis the most abundant. Xyloglucan, xyloglucan endo-transglycosylase(XET) activity and xyloglucan endo-transglycosylase/hydrolase(XTH) gene products were detected in developing G-layers bylabeling using CCRC-M1 monoclonal antibody, in situ incorporationof XXXG-SR and the polyclonal antibody to poplar PttXET16-34,respectively, indicating that xyloglucan is incorporated intothe G-layer during its development. Moreover, several XTH transcriptswere altered and were generally up-regulated in developing tensionwood compared with normal wood. In mature G-fibers, XTH geneproducts were detected in the G-layers while the XET activitywas evident in the adjacent S2 wall layer. We propose that XETactivity is essential for G-fiber shrinking by repairing xyloglucancross-links between G- and S2-layers and thus maintaining theircontact. Surprisingly, XTH gene products and XET activity persistedin mature G-fibers for several years, suggesting that the enzymefunctions after cell death repairing the cross-links as theyare being broken during the shrinking process.


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