Nishikubo N, Takahashi J, Roos AA, Derba-Maceluch M, Piens K, Brumer H, Teeri TT, Stålbrand H, Mellerowicz EJ
XET-mediated xyloglucan rearrangements in developing wood of hybrid aspen (Populus tremula x tremuloides)
Plant Physiology: 2011 155:399-413

Abstract
Xyloglucan endo-transglycosylases (XETs) encoded by XTH genesmodify the xyloglucan-cellulose framework of plant cell walls,thereby regulating their expansion and strength. To evaluatethe importance of XET in wood development, we studied xyloglucandynamics and XTH gene expression in developing wood, and modifiedXET activity in aspen by overexpressing PtxtXET16-34. We showthat developmental modifications during xylem differentiationinclude changes from loosely to tightly bound forms of xyloglucanand increases in the abundance of fucosylated xyloglucan epitoperecognized by the CCRC-M1-antibody. We found that at least16 Populus XTH genes, all likely encoding XETs, are expressedin developing wood. Five genes were highly and ubiquitouslyexpressed, whereas PtxtXET16-34 was expressed more weakly, butspecifically in developing wood. Transgenic upregulation ofXET activity induced changes in cell wall xyloglucan, but itseffects were dependent on developmental stage. For instance,XET overexpression increased abundance of the CCRC-M1 epitopein cambial cells and xylem cells in early stages of differentiation,but not in mature xylem. Correspondingly, an increase in tightlybound xyloglucan content was observed in primary-walled xylem,but a decrease in secondary-walled xylem. Thus, in young xylemcells, XET activity limits xyloglucan incorporation into thetightly bound wall network, but removes it from cell walls inolder cells. XET overexpression promoted vessel element growth,but not fiber expansion. We suggest that the amount of nascentxyloglucan relative to XET is an important determinant of whetherXET strengthens or loosens the cell wall.

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