Molecular insights into the structure and function of plant K+ transport mechanisms

Our understanding of plant potassium transport has increased in the past de cade through the application of molecular biological techniques. In this re view, recent work on inward and outward rectifying K+ channels as well as h igh affinity K+ transporters is described. Through the work on inward recti fying K+ channels, we now have precise details on how the structure of thes e proteins determines functional characteristics such as ion conduction, pH sensitivity, selectivity and voltage sensing. The physiological function o f inward rectifying K+ channels in plants has been clarified through the an alysis of expression patterns and mutational analysis. Two classes of outwa rd rectifying K+ channels have now been cloned from plants and their initia l characterisation is reviewed. The physiological role of one class of outw ard rectifying K+ channel has been demonstrated to be involved in long dist ance transport of K+ from roots to shoots. The molecular structure and func tion of two classes of energised K+ transporters are also reviewed. The fir st class is energised by Na+ and shares structural similarities with K+ tra nsport mechanisms in bacteria and fungi. Structure-function studies suggest that it should be possible to increase the K+ and Na+ selectivity of these transporters, which will enhance the salt tolerance of higher plants. The second class of K+ transporter is comprised of a large gene family and appe ars to have a dual affinity for K+. A suite of molecular techniques, includ ing gene cloning, oocyte expression, RNA localisation and gene inactivation , is now being used to fully characterise the biophysical and physiological function of plants K+ transport mechanisms. (C) 2000 Elsevier Science B.V. All rights reserved.