K+ nutrition and Na+ toxicity: The basis of cellular K+/Na+ ratios

The capacity of plants to maintain a high cytosolic K+/Na+ ratio is likely to be one of the key determinants of plant salt tolerance. Important progre ss has been made in recent years regarding the identification and character ization of genes and transporters that contribute to the cytosolic K+/Na+ r atio. For K+ uptake, K+ efflux and K+ translocation to the shoot, genes hav e been isolated that encode K+ uptake and KC release ion channels and K+ ca rriers that are coupled to either a H+ or Na+ gradient. Although the pictur e is less clear for the movement of Na+, one pathway, in the form of non-se lective ion channels, is likely to play a role in Na+ uptake, whereas Na+ e fflux and compartmentation are likely to be mediated by H+-coupled antiport . In addition, several proteins have been characterized that play prominent roles in the regulation of K+ and/or Na+ fluxes. In this Botanical Briefin g we will discuss the functions and interactions of these genes and transpo rters in the broader context of K+ nutrition and Na+ toxicity. (C) 1999 Ann als of Botany Company.