Potassium homeostasis and membrane transport

Cytosolic K+ activity in plant cells is about 80 mM and is maintained durin g moderate K+-deprivation. It decreases to much lower values only in extrem e K+-deficiency. In contrast, the vacuolar K+ concentration responds direct ly to the K+ supply and can fall to very low values in severely K+-deprrved cells. However, there is good evidence for an upper limit for vacuolar Kconcen tration which is different in roots and leaves. Understanding of the molecular basis of active and passive K+ transport in plants has increased enormously in recent years but the role of individual transporters in upta ke has still to be fully resolved, as has their regulation in relation to t he maintenance of cytosolic and vacuolar K+ concentrations. In particular, the inverse relationship between the rate of K+ uptake and internal K+ conc entration that was established over 25 years ago has still not been credibl y explained at the molecular level.