The high affinity K+ transporter AtHAK5 plays a physiological role in planta at very low K+ concentrations and provides a caesium uptake pathway in Arabidopsis

Caesium (Cs⁺) is a potentially toxic mineral element that is released into the environment and taken up by plants. Although Cs⁺ is chemically similar to potassium (K⁺), and much is known about K⁺ transport mechanisms, it is not clear through which K⁺ transport mechanisms Cs⁺ is taken up by plant roots. In this study, the role of AtHAK5 in high affinity K⁺ and Cs⁺ uptake was characterized. It is demonstrated that AtHAK5 is localized to the plasma membrane under conditions of K⁺ deprivation, when it is expressed. Growth analysis showed that AtHAK5 plays a role during severe K⁺ deprivation. Under K⁺-deficient conditions in the presence of Cs⁺, Arabidopsis seedlings lacking AtHAK5 had increased inhibition of root growth and lower Cs⁺ accumulation, and significantly higher leaf chlorophyll concentrations than wild type. These data indicate that, in addition to transporting K⁺ in planta, AtHAK5 also transports Cs⁺. Further experiments showed that AtHAK5 mediated Cs⁺ uptake into yeast cells and that, although the K⁺ deficiency-induced expression of AtHAK5 was inhibited by low concentrations of NH₄⁺ in planta, Cs⁺ uptake by yeast was stimulated by low concentrations of NH₄⁺. Interestingly, the growth of the Arabidopsis atakt1-1 mutant was more sensitive to Cs⁺ than the wild type. This may be explained, in part, by increased expression of AtHAK5 in the atakt1-1 mutant. It is concluded that AtHAK5 is a root plasma membrane uptake mechanism for K⁺ and Cs⁺ under conditions of low K⁺ availability.