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+. Conc entrations 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 exces sive Cs can be toxic to plants. Studies of the mechanism of Cs+ uptake are important for understanding the implications arising from releases of radio isotopes of Cs, which are produced in nuclear reactors and thermonuclear ex plosions. Two radioisotopes of Cs (Cs-134 and Cs-137) are of environmental concern owing to their relatively long half-lives, emissions of beta and ga mma 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 des cribed. The inward-rectifying K+ (KIR), outward-rectifying K+ (KOR) and vol tage-insensitive cation (VIC) channels are all permeable to Cs+ and, by ana logy 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 stereotypica l root cell, it can be predicted that VIC channels mediate most (30-90%) of the Cs+ influx under physiological conditions and that the KUP transporter s 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 throu gh VIC channels and KUP transporters can produce the characteristic 'dual i sotherm' relationship between Cs+ influx to escised roots and external Csconcentrations below 209 mu M. Thus, molecular targets for modulating Cs+ i nflux to root cells have been identified. This information can be used to d irect future genetic modification of plants, allowing them to accumulate mo re, or less, Cs and thereby to remediate contaminated sites.