Aquaporins. A molecular entry into plant water relations.

Water relations are obviously crucial to the physiology of terrestrial plants, but 25 years ago most plant biologists viewed this area of research as somewhat of an oddity. Dominated by equations and unusual physical theories about liquids flowing in pipes, water relations seemed to be a field with little seeming need for the burgeoning concepts and approaches offered by molecular and cellular biology. The discovery of aquaporins united these two “cultures” of biophysicists and molecular geneticists and piqued the interest of cell and molecular biologists in plant water relations. Early (and present day) plant biophysicists, exploiting sophisticated physical theories, formulated the general and unifying concept that water flow through plant cells and tissues can be understood as the product of motive force and conductance (3, 16). In plants, under natural conditions, water uptake by the roots and loss from the leaves are driven by ever changing forces, and plants keep a proper water balance by continuously adjusting the water conductance of their tissues. The concept of water potential unifies the description of these forces, whether hydrostatic, osmotic, matrix-derived, or gravitational in nature (3, 16). However, the in situ measurement of these forces is subject to pitfalls, as exemplified by current controversies over the mechanisms of the ascent of xylem sap. Water potential gradients can also be experimentally manipulated and in these studies the conductance of plant cells and tissues has long been viewed as a black box (16). How do cells regulate their conductance in this black box?