Two soybean bHLH factors regulate response to iron deficiency

Iron is an indispensable micronutrient for plant growth and development. Limited bioavailability of Fe in the soil leads to iron deficiency chlorosis in plants and yield loss. In this study, two soybean basic helix-loop-helix transcription factors, GmbHLH57 and GmbHLH300, were identified in response to Fe-deficiency. Both transcription factors are expressed in roots and nodules, and are induced by Fe deficiency; these patterns were confirmed in transgenic hairy roots expressing constructs of the endogenous promoters fused to a GUS reporter gene. Bimolecular fluorescence complementation, yeast two-hybrid and coimmunoprecipitation (co-IP) assays indicated a physical interaction between GmbHLH57 and GmbHLH300. Studies on transgenic soybeans overexpressing GmbHLH57 and GmbHLH300 revealed that overexpression of each transcription factor, alone, results in no change of the responses to Fe deficiency, whereas overexpression of both transcription factors upregulated the downstream Fe uptake genes and increased the Fe content in these transgenic plants. Compared to wild type, these double overexpression transgenic plants were more tolerant to Fe deficiency. Taken together, our findings establish that GmbHLH57 and GmbHLH300 are important transcription factors involved in Fe homeostasis in soybean.