Control of k⁺ homeostasis: an essential stress tolerance mechanism in plants

Soil salinity is one of the most critical environmental stressors that reduces crop yields worldwide and affects wild plants distribution in nature. Climate change is increasing the salinity of irrigated cropland and natural saline habitats of high ecological value, highlighting the interest in elucidating salt stress tolerance mechanisms in crops and wild plants. One of the well-known adverse effects of salt is the interference of toxic Na+ ions with K+ uptake and homeostasis, as both cations compete for the same binding sites and transport proteins. Therefore, an increase in substrate salinity is usually accompanied by a reduction of K+ concentrations in the plant organs, as it has been observed in many species, both salt sensitive and tolerant. However, in other plants, K+ contents are maintained or even increase with increasing Na+ concentrations; for example, in some species, K+ transport to the leaves is activated at high external salinity to counteract the toxic Na+ effects. This review will present several examples of these mechanisms and their relevance for stress tolerance, based primarily on our group's work during the last 20 years.