ATP binding cassette proteins ABCG37 and ABCG33 function as potassium-independent cesium uptake carriers in Arabidopsis roots.

Radiocesium accumulated in the soil by nuclear accidents is a major environmental concern. The transport process of cesium (Cs) is tightly linked to the indispensable plant nutrient potassium (K) as they both belong to the group I alkali metals with similar chemical properties. Most of the transporters that had been characterized to date as Cs transporters are directly or indirectly linked to K. Using a combinatorial approach of physiology, genetics, cell biology, and root uptake assay, here we identified two ATP-binding cassette (ABC) proteins, ABCG37 and ABCG33, as facilitators of Cs influx. A gain-of-function mutant of ABCG37 (abcg37-1) showed increased sensitivity to Cs-induced root growth inhibition, while the double knockout mutant of ABCG33 and ABCG37 (abcg33-1abcg37-2) showed resistance, whereas the single loss-of-function mutants of ABCG33 and ABCG37 did not show any alteration in Cs response. In planta short-term radioactive Cs-uptake assay along with growth and uptake assays in a heterologous system confirmed ABCG33 and ABCG37 as Cs-uptake carriers. Potassium response and content were unaffected in the double-mutant background and yeast cells lacking potassium-uptake carriers transformed with ABCG33 and ABCG37 failed to grow in the absence of K, confirming that Cs uptake by ABCG33 and ABCG37 is independent of K. Collectively, this work identified two ABC proteins as new Cs-influx carriers that act redundantly and independent of the K-uptake pathway.