Biophysical and biochemical control of cell expansion in roots and leaves

Abstract

Abstract The endogenous regulation of the extension of cells and tissues appears to be dominated by cell wail mechanics. By simultaneously measuring growth rate at near-single cell resolution and single cell turgor pressure we have begun to chart the behaviour of an expanding cell as it passes from the meristem to the mature region of the roots of maize, wheat, barley, tomato, and Pinus pinaster. A key feature is that turgor pressure is unchanged throughout the development of the cell. Two phases can be distinguished within the expanding zone, (i) An acceleration phase in which the cell walls loosen. The cells of this phase can rapidly alter their wall mechanics following water stress and have the most powerful capacity for osmotic adjustment. This phase is associated with a peak in the activity of the potential cell-wall-loosening enzyme XET. (ii) A decelerating phase in which the walls stiffen. This phase can be initiated prematurely by a number of external factors such as NaCI, high osmotic pressure, galactose and, possibly, soil impedance. The corresponding behaviour of examples of leaf (Lolium, wheat and barley) and stem (Sinapis) growth are discussed. Finally the presence of, and roles for, high concentrations of solutes in the walls of expanding cells are discussed. In this context motor cells that are responsible for reversible movement (e.g. those of Phaseolus pulvini) are compared and contrasted with those undergoing irreversible growth. In the pulvini, movement is due to changes in turgor pressure brought about by changes in the apoplast osmotic pressure.