Plant Aquaporins and Cell Elongation

Abstract

There exists an increasing number of reports which show that the gene transcript, and in some cases also protein level, of particular aquaporin (AQP) isoforms is higher in growing than in nongrowing plant tissues. This suggests that AQPs play a role in the process of cell expansion. The most likely role of AQPs is that of facilitating water inflow into cells as they expand to a multiple of their original volume. The question is whether this is the major role which AQPs play in expanding cells and whether expanding cells actually need AQPs given the rate at which they expand and the hydraulic conductivity (Lp) of their membranes. These questions are addressed in this chapter by using a combination of molecular (AQP), biophysical (Lp, driving forces and water potential difference), anatomical (apoplastic barriers) and physiological (cell dimensions and relative growth rates) data for growing plant tissues and cells. The focus of analyses is on growing root and leaf tissues and on plasma membrane intrinsic (PIPs) and tonoplast intrinsic proteins (TIPs). It is concluded that a high expression of AQPs and a high Lp in growing plant cells are required more for facilitating water transport at significant (and high) rates through cells and tissues rather than for facilitating water transport into cells to sustain the (comparatively smaller) water uptake rates required for the volume expansion of these cells.