Molecular mechanisms involved in plant adaptation to low K+ availability

Potassium is a major inorganic constituent of the living cell and the most abundant cation in the cytosol. It plays a role in various functions at the cell level, such as electrical neutralization of anionic charges, protein synthesis, long- and short-term control of membrane polarization, and regulation of the osmotic potential. Through the latter function, K is involved at the whole-plant level in osmotically driven functions such as cell movements, regulation of stomatal aperture, or phloem transport. Thus, plant growth and development require that large amounts of K are taken up from the soil and translocated to the various organs. In most ecosystems, however, soil K availability is low and fluctuating, so plants have developed strategies to take up K more efficiently and preserve vital functions and growth when K availability is becoming limited. These strategies include increased capacity for high-affinity K uptake from the soil, K redistribution between the cytosolic and vacuolar pools, ensuring cytosolic homeostasis, and modification of root system development and architecture. Our knowledge about the mechanisms and signalling cascades involved in these different adaptive responses has been rapidly growing during the last decade, revealing a highly complex network of interacting processes. This review is focused on the different physiological responses induced by K deprivation, their underlying molecular events, and the present knowledge and hypotheses regarding the mechanisms responsible for K sensing and signalling.