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

NH 4 + is important for the growth and the yield production of paddy-soil grown rice. However, the mechanism of NH 4 + signaling is largely unknown. Previously, we performed a microarray analysis to examine genomic responses and signaling pathways in rice roots exposed to NH 4 + , and interestingly found that transcription of a large number of genes including kinase, phosphatase, and transcription factors were sensitive to NH 4 + . In this study, we further analyzed transcription factors respond to NH 4 + , and identified an NH 4 + -repressed bHLH transcription factor involved in NH 4 + 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 NH 4 + contents in transgenic rice roots, respectively. NH 4 + -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, NH 4 + 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 NH 4 + uptake. Taken together, these results provide a new regulatory path by which bHLH regulates NH 4 + uptake in rice.