Regulation of Na(+) and K(+) homeostasis in plants: towards improved salt stress tolerance in crop plants

Soil salinity is a major abiotic stress that results in considerable crop yield losses worldwide. However, some plant genotypes show a high tolerance to soil salinity, as they manage to maintain a high K(+)/Na(+) ratio in the cytosol, in contrast to salt stress susceptible genotypes. Although, different plant genotypes show different salt tolerance mechanisms, they all rely on the regulation and function of K(+) and Na(+) transporters and H(+) pumps, which generate the driving force for K(+) and Na(+) transport. In this review we will introduce salt stress responses in plants and summarize the current knowledge about the most important ion transporters that facilitate intra- and intercellular K(+) and Na(+) homeostasis in these organisms. We will describe and discuss the regulation and function of the H(+)-ATPases, H(+)-PPases, SOS1, HKTs, and NHXs, including the specific tissues where they work and their response to salt stress.