Basic helix-loop-helix (bHLH) transcriptional activator regulates ammonium uptake in rice

NH4+ is important for the growth and the yield production of paddy-soil grown rice. However, the mechanism of NH4+ signaling is largely unknown. Previously, we performed a microarray analysis to examine genomic responses and signaling pathways in rice roots exposed to NH4+, and interestingly found that transcription of a large number of genes including kinase, phosphatase, and transcription factors were sensitive to NH4+. In this study, we further analyzed transcription factors respond to NH4+, and identified an NH4+-repressed bHLH transcription factor involved in NH4+ acquisition process. bHLH is a nuclear protein and has transcription-activation activity in yeast and plants. RNAi-mediated suppression of bHLH and bHLH overexpression (bHLH OX) resulted in the accumulation of lower and higher NH4+ contents in transgenic rice roots, respectively. NH4+-mediated induction of AMT1;1 and AMT1;2 is lower in bHLH RNAi, but higher in bHLH OX as compared to in wild-type plant roots. However, bHLH does not directly activate AMT1;1 and AMT1;2 transcriptions. IDD10 expression is not altered in bHLH RNAi plant roots, and bHLH maintained similar expression level in idd10 mutant roots compared to in wild-type plant roots. Furthermore, NH4+ contents in idd10;bHLH RNAi double mutant roots were much lower than in each single mutant and wild-type plant roots, indicating that IDD10 and bHLH independently regulates NH4+ uptake. Taken together, these results provide a new regulatory path by which bHLH regulates NH4+ uptake in rice.