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Recently, a new member of the ABC transporter superfamily of Arabidopsis, AtMRP5, was identified and characterized. In the present work, we found that AtMRP5 can bind specifically to sulfonurea when it is expressed in HEK293 cells. We also present evidence for a new role of AtMRP5 in the salt stress response of Arabidopsis. We used reverse genetics to identify an Arabidopsis mutant (atmrp5-2) in which the AtMRP5 gene was disrupted by transferred DNA insertion. In root-bending assays using Murashige and Skoog medium supplemented with 100 mM NaC1, root growth of atmrp5-2 was substantially inhibited in contrast to the almost normal growth of wild-type seedlings. This hypersensitive response of the atmrp5-2 mutant was not observed during mannitol treatment. The root growth of the wild-type plant grown in Murashige and Skoog medium supplemented with the MRP inhibitor glibenclamide and NaC1 was inhibited to a very similar extent as the root growth of atmrp5-2 grown in NaCl alone. The Na[sup +]-dependent reduction of root growth of the wild-type plant in the presence of glibenclamide was partially restored by diazoxide, a known K[sup +] channel opener that reverses the inhibitory effects of sulfonylureas in animal cells. Moreover, the atmrp5-2 mutant was defective in [sup 86]Rb[sup +] uptake. When seedlings were treated with 100 mM NaCl, atmrp5-2 seedlings accumulated less K[sup +] and more Na[sup +] than those of the wild type. These observations suggest that AtMRP5 is a putative sulfonylurea receptor that is involved in K[sup +] homeostasis and, thus, also participates in the NaCl stress response. [ABSTRACT FROM AUTHOR]