The Bax inhibitor GmBI-1 interacts with a Nod factor receptor and plays a dual role in the legume-rhizobia symbiosis

The gene networks surrounding Nod factor receptors that govern the symbiotic process between legumes and rhizobia remain largely unexplored. Here, we identify 13 novel GmNFR1a-associated proteins by yeast two-hybrid screening, and describe a potential interacting protein, GmBI-1a. GmBI-1a had the highest positive correlation with GmNFR1a in a co-expression network analysis, and its expression at the mRNA level in roots was enhanced by rhizobial infection. Moreover, GmBI-1a-GmNFR1a interaction was shown to occur in vitro and in vivo. The GmBI-1a protein was localized to multiple subcellular locations, including the endoplasmic reticulum and plasma membrane. Overexpression of GmBI-1a increased the nodule number in transgenic hairy roots or transgenic soybean, whereas down-regulation of GmBI-1a transcripts by RNA interference reduced the nodule number. In addition, the nodules in GmBI-1a-overexpressing plants became smaller in size and infected area with reduced nitrogenase activity. In GmBI-1a-overexpressing transgenic soybean, the elevated GmBI-1a also promoted plant growth and suppressed the expression of defense signaling-related genes. Infection thread analysis of GmBI-1a-overexpressing plants showed that GmBI-1a promoted rhizobial infection. Collectively, our findings support a GmNFR1a-associated protein in the Nod factor signaling pathway and shed new light on the regulatory mechanism of GmNFR1a in rhizobial symbiosis.