The rice bean (Vigna umbellata) root apex specifically secretes citrate through expression activation of Vigna umbellata Multidrug and Toxic Compound Extrusion 1 (VuMATE1) under aluminum (Al(3+) ) stress. However, the underlying mechanisms regulating VuMATE1 expression remain unknown. We isolated and characterized a gene encoding Sensitive to Proton Rhizotoxicity1 (STOP1)-like protein, VuSTOP1, from rice bean. The role of VuSTOP1 in regulating VuMATE1 expression was investigated using the yeast one-hybrid assay. We characterized the function of VuSTOP1 in Al(3) (+) - and H(+) -tolerance using in planta complementation assays. We demonstrated that VuSTOP1 has transactivation potential. We found that VuSTOP1 expression is inducible by Al(3+) and H(+) stress. However, although VuSTOP1 binds to the promoter of VuMATE1, the inconsistent tissue localization patterns of VuSTOP1 and VuMATE1 preclude VuSTOP1 as the major factor regulating VuMATE1 expression. In addition, when a protein translation inhibitor increased expression of VuSTOP1, VuMATE1 expression was inhibited. In planta complementation assay demonstrated that VuSTOP1 could fully restore expression of genes involved in H(+) tolerance, but could only partially restore expression of AtMATE. We conclude that VuSTOP1 plays a major role in H(+) tolerance, but only a minor role in Al(3+) tolerance. The differential transcriptional regulation of VuSTOP1 and VuMATE1 reveals a complex regulatory system controlling VuMATE1 expression.
Keywords: Sensitive to Proton Rhizotoxicity1 (STOP1); aluminum (Al) toxicity; citrate secretion; proton toxicity; rice bean (Vigna umbellata); transcriptional regulation.
© 2015 The Authors. New Phytologist © 2015 New Phytologist Trust.