GmBEHL1, a BES1/BZR1 family protein, negatively regulates soybean nodulation

Brassinosteroids (BRs) play an essential role in plant growth, and BRI1-EMS suppressor 1 (BES1)/brassinazole-resistant 1 (BZR1) family transcription factors integrate a variety of plant signaling pathways. Despite the fact that BRs inhibit nodulation in leguminous plants, how BRs modulate rhizobia-host interactions and nodule morphogenesis is unknown. Here, we show that GmBEHL1, a soybean homolog of Arabidopsis BES1/BZR1 homolog 1 (BEH1), is an interacting partner of Nodule Number Control 1, a transcriptional repressor that mediates soybean nodulation. GmBEHL1 was highly expressed at the basal parts of emerging nodules, and its expression gradually expanded during nodule maturation. The overexpression and downregulation of GmBEHL1 inhibited and enhanced the number of nodules, respectively, in soybean. Intriguingly, alterations in GmBEHL1 expression repressed the expression of genes in the BR biosynthesis pathway, including homologs of Arabidopsis Constitutive Photomorphogenesis and Dwarf and Dwarf 4. We also detected an interaction between GmBEHL1 and GmBIN2, a putative BR-insensitive 2 (BIN2) homolog, in soybean. Moreover, BR treatment reduced the number, but increased the size, of soybean nodules. Our results reveal GmBEHL1 to be a potent gene that integrates BR signaling with nodulation signaling pathways to regulate symbiotic nodulation.