The gibberellic acid responsive GmbHLHm1 transcription factor influences nodule development, nitrogen fixation activity and shoot nitrogen content in soybean (Glycine max)

GmbHLHm1 is a basic Helix-Loop-Helix membrane (bHLHm1) DNA binding transcription factor localized to the symbiosome membrane and nucleus in soybean (Glycine max) nodules. Overexpression of GmbHLHm1 significantly increased nodule number and size, nitrogen fixation activity, and nitrogen delivery to the shoots. This contrasts with reduced nodule numbers per plant, nitrogen fixation activity and poor plant growth when silenced using RNAi. The promoter of GmbHLHm1 was found to be sensitive to exogenous GA supply, decreasing the level of GUS expression in transformed hairy roots in both nodules and roots and reducing native GmbHLHm1 expression in wild-type nodules. In summary, our study suggests that GmbHLHm1 positively regulates soybean nodulation and nitrogen fixation, and that GA can negatively regulate GmbHLHm1 expression in soybean nodules.