Signalling by potassium: another second messenger to add to the list?

Cytosolic potassium homeostasis and the ability of various tissues to retain potassium under stress have emerged as important for salinity tolerance in plants, but recent evidence suggests that stress-induced K+ efflux may be equally important in mediating growth and development under hostile conditions. Here, the evidence is assessed, and the already-proposed concept of potassium efflux being a switch between metabolic and defence responses is developed. A new model is put forward which suggests signalling roles for cytosolic K+ changes, alongside wellknown cytosolic Ca2+ and ROS 'signatures'.