Nodulation factor receptor kinase 1 alpha controls nodule organ number in soybean (Glycine max L. Merr)

P>Two allelic non-nodulating mutants, nod49 and rj1, were characterized using map-based cloning and candidate gene approaches, and genetic complementation. From our results we propose two highly related lipo-oligochitin LysM-type receptor kinase genes (GmNFR1 alpha and GmNFR1 beta) as putative Nod factor receptor components in soybean. Both mutants contained frameshift mutations in GmNFR1 alpha that would yield protein truncations. Both mutants contained a seemingly functional GmNFR1 beta homeologue, characterized by a 374-bp deletion in intron 6 and 20-100 times lower transcript levels than GmNFR1 alpha, yet both mutants were unable to form nodules. Mutations in GmNFR1 beta within other genotypes had no defects in nodulation, showing that GmNFR1 beta was redundant. Transgenic overexpression of GmNFR1 alpha, but not of GmNFR1 beta, increased nodule number per plant, plant nitrogen content and the ability to form nodules with restrictive, ultra-low Bradyrhizobium japonicum titres in transgenic roots of both nod49 and rj1. GmNFR1 alpha overexpressing roots also formed nodules in nodulation-restrictive acid soil (pH 4.7). Our results show that: (i) NFR1 alpha expression controls nodule number in soybean, and (ii) acid soil tolerance for nodulation and suppression of nodulation deficiency at low titre can be achieved by overexpression of GmNFR1 alpha.