Disruption of AtHAK/KT/KUP9 enhances plant cesium accumulation under low potassium supply.

Understanding the molecular mechanisms that underlie cesium (Cs ) transport in plants is important to limit the entry of its radioisotopes from contaminated areas into the food chain. The potentially toxic element Cs , which is not involved in any biological process, is chemically closed to the macronutrient potassium (K ). Among the multiple K carriers, the high-affinity K transporters family HAK/KT/KUP is thought to be relevant in mediating opportunistic Cs transport. Of the 13 KUP identified in A. thaliana, only HAK5, the major contributor to root K acquisition under low K supply, has been functionally demonstrated to be involved in Cs uptake in planta. In the present study, we showed that accumulation of Cs increased by up to 30% in two A. thaliana mutant lines lacking KUP9 and grown under low K supply. Since further experiments revealed that Cs release from contaminated plants to the external medium is proportionally lower in the two kup9 mutant alleles, we proposed that KUP9 disruption could impair Cs efflux. By contrast, K status in kup9 mutants is not affected, suggesting that KUP9 disruption does not alter substantially K transport in experimental conditions used. The putative primary role of KUP9 in plants is further discussed.