Tansley Review No. 113 Mechanisms of caesium uptake by plants

Abstract

Summary 241

I. INTRODUCTION: CAESIUM IN THE ENVIRONMENT 242

II. UPTAKE OF CAESIUM BY PLANT ROOTS 243

1. Evidence for multiple mechanisms of Cs⁺uptake by plant roots 243

2. Caesium uptake is affected by the presence of other cations 244

3. Caesium inhibits the uptake of other cations 244

III. MOLECULAR MECHANISMS CATALYSING CAESIUM UPTAKE 245

1. ‘High-affinity’ transport mechanisms 245

2. Inward-rectifying potassium (KIR) channels 245

3. Outward-rectifying potassium (KOR) channels 248

4. Voltage-insensitive cation (VIC) channels 249

5. Ca²⁺-permeable channels 249

IV. MODELLING CAESIUM INFLUX TO ROOT CELLS 249

1. Predicted Cs⁺influx through high-affinity mechanisms 250

2. Predicted Cs⁺influx through cation channels 250

3. Predicted dependence of Cs⁺influx on [Cs⁺]_ext 252

V. PERSPECTIVE 253

Acknowledgements 254

References 254

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 ¹³³Cs in soils occur up to 25 μg 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 (¹³⁴Cs and ¹³⁷Cs) are of environmental concern owing to their relatively long half-lives, emissions of β and γ radiation during decay and rapid incorporation into biological systems. The soil concentrations of these radioisotopes are six orders of magnitude lower than those of ¹³³Cs. 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 excised roots and external Cs⁺ concentrations below 200 μM. 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.