Inactivation of duplicated nod factor receptor 5 (NFR5) genes in recessive loss-of-function non-nodulation mutants of allotetraploid soybean (Glycine max L. Merr.)

Abstract

Chemically induced non-nodulating nod139 and nn5 mutants of soybean (Glycine max) show no visible symptoms in response to rhizobial inoculation. Both exhibit recessive Mendelian inheritance suggesting loss of function. By allele determination and genetic complementation in nod139 and nn5, two highly related lipo-oligochitin LysM-type receptor kinase genes in Glycine max were cloned; they are presumed to be the critical nodulation-inducing (Nod) factor receptor similar to those of Lotus japonicus, pea and Medicago truncatula. These duplicated receptor genes were called GmNFR5alpha and GmNFR5beta. Nonsense mutations in GmNFR5alpha and GmNFR5beta were genetically complemented by both wild-type GmNFR5alpha and GmNFR5beta in transgenic roots, indicating that both genes are functional. Both genes lack introns. In cultivar Williams82 GmNFR5alpha is located in chromosome 11 and in tandem with GmLYK7 (a related LysM receptor kinase gene), while GmNFR5beta is in tandem with GmLYK4 in homologous chromosome 1, suggesting ancient synteny and regional segmental duplication. Both genes are wild type in G. soja CPI100070 and Harosoy63; however, a non-functional NFR5beta allele (NFR5beta*) was discovered in parental lines Bragg and Williams, which harbored an identical 1,407 bp retroelement-type insertion. This retroelement (GmRE-1) and related sequences are located in several soybean genome positions. Paradoxically, putatively unrelated soybean cultivars shared the same insertion, suggesting a smaller than anticipated genetic base in this crop. GmNFR5alpha but not GmNFR5beta* was expressed in inoculated and uninoculated tap and lateral root portions at about 10-25% of GmATS1 (ATP synthase subunit 1), but not in trifoliate leaves and shoot tips.