OPINION PAPER How do cell walls regulate plant growth?

The cell wall of growing plant tissues has frequently been interpreted in terms of inextensible cellulose microfibrils ‘tethered ’ by hemicellulose polymers at-tached to the microfibril surface by hydrogen bonds, with growth occurring when tethers are broken or ‘peeled ’ off the microfibril surface by expansins. This has sometimes been described as the ‘sticky network’ model. In this paper, a number of theoretical difficulties with this model, and discrepancies between predicted behaviour and observations by a number of research-ers, are noted. (i) Predictions of cell wall moduli, based upon the sticky network model, suggest that the cell wall should be much weaker than is observed. (ii) The maximum hydrogen bond energy between tethers and microfibrils is less than the work done in expansion and therefore breakage of such hydrogen bonds is unlikely to limit growth. (iii) Composites of bacterial cellulose with xyloglucan are weaker than pellicles of pure cellu-lose so that it seems unlikely that hemicelluloses bind themicrofibrils together. (iv) Calcium chelators promote creep of plant material in a similar way to expansins. (v) Reduced relative ‘permittivities ’ inhibit the contrac-tion of cell wall material when an applied stress is decreased. Revisions of the sticky network model that might address these issues are considered, as are alternatives including a model of cell wall biophysics in which cell wall polymers act as ‘scaffolds ’ to regulate the space available for microfibril movement. Experi-ments that support the latter hypothesis, by demon-strating that reducing cell wall free volume decreases extensibility, are briefly described. Key words: Cell wall, expansins, plant growth, rheology