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

Understanding molecular mechanisms which underlie transport of cesium (Cs+) in plants is important to limit entry of its radioisotopes from contaminated area to 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. On the 13 KUP identified in Arabidopsis 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 mutants, we proposed that KUP9 disruption could impair Cs+ efflux. By contrast, we did not measure significant impairment of K+ status in kup9 mutants suggesting that KUP9 disruption does not alter substantially K+ transport in experimental conditions used here. Putative primary role of KUP9 in plants is further discussed.