The cellulose-xyloglucan framework functions as the load-bearing structure of the cell wall and constrains cell shape in plants. Xyloglucan cross-links which underpin the framework structure can be modified by transferases and hydrolases encoded by xyloglucan-related protein (XRP) family genes. These enzymes are considered to play critical roles in the construction and restructuring of the three-dimensional structure of the plant cell wall. Although analyses of their protein structures and gene-expression profiles for individual members of XRPs have disclosed their potentially divergent roles in plants, the biochemical reactions catalyzed by individual XRPs and their biochemical implications remain to be clarified. This review focuses on the XRP-catalyzed chemical processes occurring in the apoplast and considers the biochemical steps involved in the construction and restructuring of the cellulose-xyloglucan framework, an ensemble of chemical reactions that are more complicated than commonly supposed.
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