Ion homeostasis: plants feel better with proper control

Abstract

Plant growth and the ecologically and agriculturally important processes of biomass accumulation and seed production are affected by numerous environmental factors, including air temperature, illumination, water availability and infestation with pathogens. Inorganic ions also represent an integral part of the environmental cocktail that affects plant growth. Of the best bio‐available alkali cations—Na+ and K+—plants have a strong preference for K+. K+, the most abundant cation in plant cells, is an essential macronutrient; by contrast, elevated concentrations of Na+ are generally toxic to plants. Therefore, it is not surprising that plants have evolved several strategies that enable them to keep the cytosolic concentration of Na+ low. Only a relatively small number of species can withstand saline environments, and plant researchers are keen to unravel the underlying mechanisms to improve the salt tolerance of crops (Yamaguchi & Blumwald, 2005). Despite the generally toxic effect of high concentrations of Na+, plants have transporters that allow the uptake of Na+ from the soil. Members of the HKT gene family of Na+ and Na+/K+ transporters seem to have a crucial role in controlling this process (Platten et al , 2006). Why do plants have such transporters, if Na+ uptake is so immensely harmful? Physiological studies from the early 1980s reported that at low concentrations Na+ can positively affect the growth of many species of plants. This indicates that, under these conditions, Na+acts as a nutrient rather than as a stress factor. Consequently, a model has been proposed claiming that Na+ can, to some extent, replace its ‘sibling’ K+ …