The Role of Hormones in Wall Loosening and Plant Growth

Abstract

The rate at which any plant cell enlarges is determined by the product of two cellular parameters; the wall extensibility (m) and the effective turgor (�*p - Y), where �*p is the turgor pressure and Y is the wall yield threshold. When hormones modulate the rate of cell enlargement, they do so by altering one or both of these parameters. To determine whether the affected parameter is m, one must be able to measure it. There are four methods for assessing m; the Instron technique; stress relaxation; turgor relaxation; and the growth rate v. turgor method. Each has advantages and disadvantages. Ideally, at least two methods should be used and the results should agree, at least qualitatively. In every case so far examined, wherever auxin promotes cell elongation, it also increases m. Gibberellins and cytokinins, on the other hand, sometimes increase m and sometimes do not. Abscisic acid, in the few cases tested, decreases m, while the effects of ethylene are mixed. A change in m can occur only if the cell exports (or takes up) a wall loosening factor (WLF) in response to the hormone. In some cases the WLF must be, at least in part, protons, but in other cases it is clear that it is something different. In dicotyledonous stems it could even be the uptake of Ca�+ from the walls. Turgor can be influenced by hormones but, with the exception of gibberellins, the responses appear to be secondary, induced by the growth processes rather than by the hormone. However, the ability of cells to take up osmotic solutes and maintain �*p may be the most important factor, in nature, modulating the growth rate of plants on an hour-to-hour basis.