Abstract
Caesium (Cs) is not required by plants and rarely reaches toxic concentrations in the natural environment. However, two harmful, long-lived, radioisotopes of Cs (Cs-134 and Cs-137) are produced anthropogenically. These radioisotopes enter the terrestrial food chain through plants and thereby impact on human health and commerce. Since radiocaesium enters plants across the plasma membranes of root cells, reducing radiocaesium influx to root cells is expected to reduce its concentration in edible tissues. Theoretical models indicate that, in K-replete plants, most Cs (30-90%) enters root cells through voltage-independent cation channels (VICCs), with K+/H+ symporters (KUPs) contributing the remainder. This conclusion is consistent with the pharmacology of Cs+ influx to K-replete plants, which is identical to that of VICCs, and the phenotypes of Arabidopsis mutants lacking particular VICCs, such as cyclic nucleotide gated channels (CNGCs), which have lower shoot Cs concentrations than wild-type plants. During K-starvation, the expression of genes encoding KUPs, such as AtHAK5, increases, resulting in increased Cs+ uptake and an increased contribution of KUPs to total Cs+ uptake, as witnessed by changes in the pharmacology of Cs+ influx to roots and the phenotype of Arabidopsis mutants lacking AtHAK5, which accumulate less Cs than wild-type plants. Unfortunately, the absence of CNGCs and KUPs has pleiotropic effects on plant growth, and manipulation of their cationic selectivity may be required to develop crop genotypes with reduced radiocaesium accumulation.