Effect of brassinosteroids on ammonium uptake via regulation of ammonium transporter and N-metabolism genes in Arabidopsis

Abstract

Several studies have been performed to elucidate the role of brassinosteroids (BRs) in plant growth and development. However, information on the role of BR signaling in nutrient uptake is limited. This study explores the relationship between BRs and ammonium transporter 1 (AMT1) expression in Arabidopsis roots. We found that BR treatment reduced the expression of AMT1 genes and that a BR receptor BRI1 mutant bri1-5 reversed its BR-repressed expression. Furthermore, the BR signaling transcription factor, BES1, regulates AMT1 expression in roots. NH4 +-mediated repression of AMT1;1, AMT1;2, and AMT1;3 was suppressed in a gain-of-function BES1 mutant (bes1-D). This mutant was more sensitive to methyl-ammonium and contained a higher ammonium content compared to wild-type plants. However, BES1 failed to bind E-box elements present in the promoter region of the AMT1 genes. Furthermore, NH4 +-mediated glutamine synthetase (GS) and glutamine oxoglutarate aminotransferase (GOGAT) gene expressions were partially inhibited, and GS activity was slightly lower in the bes1-D mutant relative to that observed in wild-type En2 roots. NH4 +-mediated AMT1 suppressions are known to be caused by N-metabolites rather than NH4 + itself, and glutamine application inhibited AMT1 expression in both En2 and bes1-D indicating that BES1 activation inhibited NH4 +-mediated GS/GOGAT induction, which might in turn inhibit AMT1 repression. In conclusion, the present study demonstrates that BR regulated nitrogen uptake and assimilation via the BR signaling pathway.