Plant Cell Wall Hydration and Plant Physiology: An Exploration of the Consequences of Direct Effects of Water Deficit on the Plant Cell Wall.

The extensibility of synthetic polymers is routinely modulated by the addition of lower molecular weight spacing molecules known as plasticizers, and there is some evidence that water may have similar effects on plant cell walls. Furthermore, it appears that changes in wall hydration could affect wall behavior to a degree that seems likely to have physiological consequences at water potentials that many plants would experience under field conditions. Osmotica large enough to be excluded from plant cell walls and bacterial cellulose composites with other cell wall polysaccharides were used to alter their water content and to demonstrate that the relationship between water potential and degree of hydration of these materials is affected by their composition. Additionally, it was found that expansins facilitate rehydration of bacterial cellulose and cellulose composites and cause swelling of plant cell wall fragments in suspension and that these responses are also affected by polysaccharide composition. Given these observations, it seems probable that plant environmental responses include measures to regulate cell wall water content or mitigate the consequences of changes in wall hydration and that it may be possible to exploit such mechanisms to improve crop resilience.

Cell wall water content has a direct effect on extensibility in growing hypocotyls of sunflower (Helianthus annuus L.).

It has been proposed that spacing between cellulose microfibrils within plant cell walls may be an important determinant of their mechanical properties. A consequence of this hypothesis is that the water content of cell walls may alter their extensibility and that low water potentials may directly reduce growth rates by reducing cell wall spacing. This paper describes a number of experiments in which the water potential of frozen and thawed growing hypocotyls of sunflower (Helianthus annuus L.) were altered using solutions of high molecular weight polyethylene glycol (PEG) or Dextran while their extension under constant stress was monitored using a creep extensiometer (frozen and thawed tissue was used to avoid confounding effects of turgor or active responses to the treatments). Clear reductions in extensibility were observed using both PEG and Dextran, with effects observed in hypocotyl segments treated with PEG 35 000 solutions with osmotic pressures of > or =0.21 MPa suggesting that the relatively mild stresses required to reduce water potentials of plants in vivo by 0.21 MPa may be sufficient to reduce growth rates via a direct effect on wall extensibility. It is noted, therefore, that the water binding capacity of plant cell walls may be of ecophysiological importance. Measurements of cell walls of sunflower hypocotyls using scanning electron microscopy confirmed that treatment of hypocotyls with PEG solutions reduced wall thickness, supporting the hypothesis that the spatial constraint of movement of cellulose microfibrils affects the mechanical properties of the cell wall.

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 attached 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 researchers, 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 cellulose so that it seems unlikely that hemicelluloses bind the microfibrils together. (iv) Calcium chelators promote creep of plant material in a similar way to expansins. (v) Reduced relative 'permittivities' inhibit the contraction 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. Experiments that support the latter hypothesis, by demonstrating that reducing cell wall free volume decreases extensibility, are briefly described.