Mechanisms of caesium uptake by plants

Caesium (Cs) is a Group I alkali metal with chemical properties similar to potassium (K). It is present in solution as the monovalent cation Cs+. Concentrations of the stable caesium isotope Cs-133 in soils occur up to 25 mu g g(-1) dry soil. This corresponds to low micromolar Cs+ concentrations in soil solutions. There is no known role for Cs in plant nutrition, but excessive Cs can be toxic to plants. Studies of the mechanism of Cs+ uptake are important for understanding the implications arising from releases of radioisotopes of Cs, which are produced in nuclear reactors and thermonuclear explosions. Two radioisotopes of Cs (Cs-134 and Cs-137) are of environmental concern owing to their relatively long half-lives, emissions of beta and gamma radiation during decay and rapid incorporation into biological systems. The soil concentrations of these radioisotopes are six orders of magnitude loner than those of Cs-133. Early physiological studies demonstrated that K+ and Cs+ competed for influx to excised roots, suggesting that the influx of these cations to root cells is mediated by the same molecular mechanism(s). The molecular identity and/or electrophysiological signature of many K+ transporters expressed in the plasma membrane of root cells have been described. The inward-rectifying K+ (KIR), outward-rectifying K+ (KOR) and voltage-insensitive cation (VIC) channels are all permeable to Cs+ and, by analogy with their bacterial counterparts, it is likely that 'high-affinity' K+/H+ symporters (tentatively ascribed here to KUP genes) also transport Cs+. By modelling cation fluxes through these transporters into a stereotypical root cell, it can be predicted that VIC channels mediate most (30-90%) of the Cs+ influx under physiological conditions and that the KUP transporters mediate the bulk of the remainder. Cation influx through KIR channels is likely to be blocked by extracellular Cs+ under typical ionic conditions in the soil. Further simulations suggest that the combined Cs+ influxes through VIC channels and KUP transporters can produce the characteristic 'dual isotherm' relationship between Cs+ influx to escised roots and external Cs+ concentrations below 209 mu M. Thus, molecular targets for modulating Cs+ influx to root cells have been identified. This information can be used to direct future genetic modification of plants, allowing them to accumulate more, or less, Cs and thereby to remediate contaminated sites.