Résumé :
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The main macroscopic view of the mechanical properties of the elongating plant cell wall (the Lockhart model) embraces the notions of yield threshold (Y) and extensibility (Phi). The model would be especially powerful if Y and Phi were parameters. Often, however, they behave more like variables that tend to maintain elongation rate constant despite changes in turgor. To understand the variable nature of Y and Phi requires the macroscopic model to be augmented with a molecular one. This paper describes a molecular model of the expanding wall that mimics the variable nature of Y and Phi. The main postulates are (1) that there are two functionally disparate populations of hemicellulose molecules that tie the cellulose microfibrils of the wall together: those that are taut and load-bearing, and those that are slack and not load-bearing; and (2) that there are enzymes that cleave or loosen the load-bearing molecules. It is supposed that during the stretching of the wall slack molecules become taut and are recruited to the load-bearing contingent, thereby stiffening the wall. This stiffening is undone by enzymes that loosen load-bearing molecules. The net result is that changes in turgor alter the distribution of the hemicellulose molecules between the slack and taut populations without necessarily altering the rates of recruitment from one to the other, except transiently. The expansion rate depends on the frequency at which load-bearing molecules are loosened multiplied by the amount of expansion that occurs before the next load-bearing molecule is recruited. Effects of changing water status on expansion rate may be mediated not necessarily through turgor, but possibly through the hydration of the wall, which may shrink at low water potential, thereby inactivating enzyme molecules by restricting their freedom of movement.
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