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The rice bean ( Vigna umbellata) root apex specifically secretes citrate through expression activation of Vigna umbellata Multidrug and Toxic Compound Extrusion 1 ( Vu MATE1) under aluminum (Al3+) stress. However, the underlying mechanisms regulating Vu MATE1 expression remain unknown., We isolated and characterized a gene encoding Sensitive to Proton Rhizotoxicity1 ( STOP1)-like protein, Vu STOP1, from rice bean. The role of Vu STOP1 in regulating Vu MATE1 expression was investigated using the yeast one-hybrid assay. We characterized the function of Vu STOP1 in Al3 + - and H+-tolerance using in planta complementation assays., We demonstrated that Vu STOP1 has transactivation potential. We found that Vu STOP1 expression is inducible by Al3+ and H+ stress. However, although Vu STOP1 binds to the promoter of Vu MATE1, the inconsistent tissue localization patterns of Vu STOP1 and Vu MATE1 preclude Vu STOP1 as the major factor regulating Vu MATE1 expression. In addition, when a protein translation inhibitor increased expression of Vu STOP1, Vu MATE1 expression was inhibited. In planta complementation assay demonstrated that Vu STOP1 could fully restore expression of genes involved in H+ tolerance, but could only partially restore expression of At MATE., We conclude that Vu STOP1 plays a major role in H+ tolerance, but only a minor role in Al3+ tolerance. The differential transcriptional regulation of Vu STOP1 and Vu MATE1 reveals a complex regulatory system controlling Vu MATE1 expression. [ABSTRACT FROM AUTHOR]