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

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 (GmNFR1α and GmNFR1β) as putative Nod factor receptor components in soybean. Both mutants contained frameshift mutations in GmNFR1α that would yield protein truncations. Both mutants contained a seemingly functional GmNFR1β homeologue, characterized by a 374-bp deletion in intron 6 and 20-100 times lower transcript levels than GmNFR1α, yet both mutants were unable to form nodules. Mutations in GmNFR1β within other genotypes had no defects in nodulation, showing that GmNFR1β was redundant. Transgenic overexpression of GmNFR1α, but not of GmNFR1β, 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. GmNFR1α overexpressing roots also formed nodules in nodulation-restrictive acid soil (pH 4.7). Our results show that: (i) NFR1α 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 GmNFR1α.