Ion homeostasis: plants feel better with proper control.

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+ in carrying out crucial cellular functions. However, a transporter for Na+ uptake at low external concentrations of Na+ was not known in plants. In a recent article published in The EMBO Journal, Tomoaki Horie and co-workers from three laboratories in San Diego (USA), Pohang (Korea) and Tsukuba ( Japan) report their findings on mutant rice plants that lack a functional Na+ transporter called OsHKT2;1 (Horie et al, 2007; Fig 1A). In such lines the OsHKT2;1 gene was disrupted through the insertion of a transposable element—Tos17— which undergoes local transposition events during tissue culture but remains non-mobile in regenerated rice plants. Under standard growth conditions, the knockout plants were indistinguishable from their wild-type counterparts; however, importantly, oshkt2;1 mutants grew to much smaller sizes than the control plants when challenged with low concentrations of Na+ under conditions of K+ starvation. They also accumulated considerably less Na+ in both shoots and roots. Thus, OsHKT2;1 functions in Na+ accumulation and favours plant biomass accumulation under such extreme conditions. These findings clearly show the nutritional aspect of Na+ and the involvement of a transporter in Na+ uptake. The rice genome contains nine HKT genes, two of which might be pseudogenes A Na OsHKT2;1 Na Na