MECHANISMS OF POTASSIUM ABSORPTION BY HIGHER-PLANT ROOTS

Potassium, as a plant macronutrient, is accumulated in plant cells fro m relatively dilute soil solutions and is indispensable for many vital processes. Studies characterising potassium uptake by roots stretch b ack over many decades, However, it is only with the introduction of mo dem electrophysiological and molecular techniques that investigations have been possible at a molecular level. Such approaches have confirme d the existence of discrete high and low affinity uptake systems at th e root plasma membrane and have greatly enhanced our understanding of the underlying molecular nature of these uptake systems. High affinity K+ uptake from micromolar external K+ levels is coupled to H+ transpo rt as demonstrated independently by patch clamping of single root prot oplasts and by studying the transport system after expression in Xenop us oocytes. The measured coupling ratio between the two ions is 1:1 an d is sufficient to account for an accumulation ratio in excess of 10(6 ), a value which encompasses experimental observations on K+ accumulat ion. Low affinity K+ uptake activates at relatively high external K+ l evels in the millimolar range and is 'passive' i.e. down the electroch emical gradient for potassium. In two higher plant species single cell inward potassium currents have been identified which are associated w ith low affinity potassium uptake. Furthermore, specific ion channels which underlie these potassium influxes and form a major constituent o f the low affinity potassium uptake pathway have been identified and c haracterised.