The impact of arbuscular mycorrhizal fungi on radiocaesium uptake by plants

Radiocaesium (Cs) contamination of agricultural, semi-natural and natural areas is a worldwide problem that has arisen from human activities, such as intentional and unintentional discharges from the nuclear industry. The contamination of soils by Cs is of serious concern because of the long half-lives of the radionuclides (Cs = 2yr, Cs = 30yr) and the emission of harmful β and γ radiation during decay. Radiocaesium is rapidly incorporated into biological systems and enters the food chain through vegetation. Plants acquire Cs from the soil solution. Because of the chemical similarity of Cs and potassium (K), root uptake mechanisms cannot differentiate between these elements easily. Several types of transport proteins contribute to Cs uptake by roots and the delivery of Cs to the shoot. These include the inward-rectifying K channels (KIRC), outward-rectifying K channels (KORC), voltage-insensitive cation channels (VICC) and highaffinity K/H symporters (KUP). White and Broadley (2000) predicted by modeling Cs fluxes through these transporters that, in K-replete plants, VICC mediate most of the Cs uptake (Fig. 1A) and KUP contribute the remainder. They also proposed that KORC facilitate Cs movement to the shoot. It was subsequently observed that the complement of Cs transporters differs between K-replete and K-starved plants (Hampton et al., 2004). When plants are starved of K, the expression of genes encoding KUP increases, and KUP then dominate plant Cs uptake (Fig. 1B).