Re-constructing our models of cellulose and primary cell wall assembly

• Cellulose interactions with the matrix depend on microfibril shape.
• Physical data point to a microfibril made of 18–24 chains, not 36 chains.
• Recent structures for cellulose synthases yield insights into glucan polymerization.
• Revised roles for xyloglucan and pectin in wall structure are emerging.
• Direct contact between cellulose microfibrils may control wall extensibility.

The cellulose microfibril has more subtlety than is commonly recognized. Details of its structure may influence how matrix polysaccharides interact with its distinctive hydrophobic and hydrophilic surfaces to form a strong yet extensible structure. Recent advances in this field include the first structures of bacterial and plant cellulose synthases and revised estimates of microfibril structure, reduced from 36 to 18 chains. New results also indicate that cellulose interactions with xyloglucan are more limited than commonly believed, whereas pectin–cellulose interactions are more prevalent. Computational results indicate that xyloglucan binds tightest to the hydrophobic surface of cellulose microfibrils. Wall extensibility may be controlled at limited regions (‘biomechanical hotspots’) where cellulose–cellulose contacts are made, potentially mediated by trace amounts of xyloglucan.

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