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.