The Role of Aquaporins in Root Water Uptake

Received: 24 January 2002 Returned for revision: 16 April 2002 Accepted: 6 June 2002 Published electronically: 5 August 2002The capacity of roots to take up water is determined in part by the resistance of living tissues to radial waterflow. Both the apoplastic and cell-to-cell paths mediate water transport in these tissues but the contribution ofcell membranes to the latter path has long been difficult to estimate. Aquaporins are water channel proteins thatare expressed in various membrane compartments of plant cells, including the plasma and vacuolar membranes.Plant aquaporins are encoded by a large multigene family, with 35 members in Arabidopsis thaliana, and manyof these aquaporins show a cell-specific expression pattern in the root. Mercury acts as an efficient blocker ofmost aquaporins and has been used to demonstrate the significant contribution of water channels to overall rootwater transport. Aquaporin-rich membranes may be needed to facilitate intense water flow across root tissuesand may represent critical points where an efficient and spatially restricted control of water uptake can beexerted. Roots, in particular, show a remarkable capacity to alter their water permeability over the short term(i.e. in a few hours to less than 2–3 d) in response to many stimuli, such as day/night cycles, nutrient deficiencyor stress. Recent data suggest that these rapid changes can be mostly accounted for by changes in cell membranepermeability and are mediated by aquaporins. Although the processes that allow perception of environmentalchanges by root cells and subsequent aquaporin regulation are nearly unknown, the study of root aquaporins pro-vides an interesting model to understand the regulation of water transport in plants and sheds light on the basicmechanisms of water uptake by roots. a 2002 Annals of Botany CompanyKey words: Hydraulic conductivity, membrane, MIP, stress, water relation, water channel.