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 progress has been made in recent years regarding the identification and characterization of genes and transporters that contribute to the cytosolic K+/Na+ ratio. For K+ uptake, K+ efflux and K+ translocation to the shoot, genes have been isolated that encode K+ uptake and KC release ion channels and K+ carriers that are coupled to either a H+ or Na+ gradient. Although the picture is less clear for the movement of Na+, one pathway, in the form of non-selective ion channels, is likely to play a role in Na+ uptake, whereas Na+ efflux 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 Briefing we will discuss the functions and interactions of these genes and transporters in the broader context of K+ nutrition and Na+ toxicity. (C) 1999 Annals of Botany Company.