| Abstract
| - 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.
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