GmNMH7, a MADS-box transcription factor, inhibits root development and nodulation of soybean (Glycine max [L.] Merr.)

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As an important food crop and oil crop, soybean (Glycine max [L.] Merr.) is capable of nitrogen-fixing by root nodule. Previous studies showed that GmNMH7, a transcription factor of MADS-box family, is associated with nodule development, but its specific function remained unknown. In this study, we found that GmNMH7 was specifically expressed in root and nodule and the expression pattern of GmNMH7 was similar to several genes involved in early development of nodule (GmENOD40-1, GmENOD40-2, GmNFR1a, GmNFR5a, and GmNIN) after rhizobia inoculation. The earlier expression peak of GmNMH7 compared to the other genes (GmENOD40-1, GmENOD40-2, GmNFR1a, GmNFR5a, and GmNIN) indicated that the gene is related to the nod factor (NF) signaling pathway and functions at the early development of nodule. Over-expression of GmNMH7 in hairy roots significantly reduced the nodule number and the root length. In the transgenic hairy roots, over-expression of GmNMH7 significantly down-regulated the expression levels of GmENOD40-1, GmENOD40-2, and GmNFR5α. Moreover, the expression of GmNMH7 could respond to abscisic acid (ABA) and gibberellin (GA3) treatment in the root of Zigongdongdou seedlings. Over-expressing GmNMH7 gene reduced the content of ABA, and increased the content of GA3 in the positive transgenic hairy roots. Therefore, we concluded that GmNMH7 might participate in the NF signaling pathway and negatively regulate nodulation probably through regulating the content of GA3.