Plant aquaporins: their molecular biology, biophysics and significance for plant water relations

Over the last decade, considerable advances have occurred in understanding the molecular biology and biophysics of water permeation across plant membranes and tissues. Spurred on by the rapid advances in cloning and functional characterization of a superfamily of major intrinsic proteins, some of which function as aquaporins, the biophysics of transport of water and small non-electrolytes across plant membranes is being re-examined based on the proposed function of these membrane-integral proteins in their native membranes. This review focuses on a number of issues that are central to an understanding of aquaporin function: (1) the need to be able to test for water-channel activity in native membranes; (2) the implications of the observed solute/water selectivity of aquaporins; (3) the putative functional roles of aquaporins at the cell, tissue and organ levels in plants; and (4) information that can be obtained from studies of the abundance, diversity and expression patterns of aquaporins. It is clear that to answer many of the critical questions that remain concerning aquaporin function, combined studies using appropriate molecular and biophysical techniques will be required.