Potassium homeostasis and signalling: from the whole plant to the subcellular level.

Potassium is an essential macronutrient required for plant growth and development. Over the recent decade, an important signalling role of K has emerged. Here, we discuss some aspects of such signalling at the various levels of plant functional organisation. The topic covered include: (1) mechanisms of long-distant K transport in the xylem and phloem and the molecular identity and regulation of K loading and unloading into plant vasculature; (2) essentiality and physiological roles of K cycling between shoots and roots; (3) plant sensing and signalling of low K; (4) maintenance of K homeostasis at the cellular level; (5) stress-induced modulation of cytosolic K as a signal in plant adaptive responses to hostile environment; (6) stress-specific K "signatures" and mechanisms of their decoding by regulation of purine metabolism and H-ATPase activity; (7) cytosolic K loss as a metabolic switch and a regulator of autophagy; and (8) vacuolar K transport and sensing.