A newly-evolved chimeric lysin motif receptor-like kinase in Medicago truncatula spp. tricycla R108 extends its Rhizobia symbiotic partnership

O_LIRhizobial lipochitooligosaccharidic Nod factors (NFs), specified by nod genes, are the primary determinants of host specificity in the legume-Rhizobia symbiosis. C_LIO_LIWe examined the nodulation ability of Medicago truncatula cv Jemalong A17 and M. truncatula ssp. tricycla R108 with the Sinorhizobium meliloti nodF/nodL mutant, which produces modified NFs. We then applied genetic and functional approaches to study the genetic basis and mechanism of nodulation of R108 by this mutant. C_LIO_LIWe show that the nodF/nodL mutant can nodulate R108 but not A17. Using genomics and reverse genetics, we identified a newly-evolved, chimeric LysM receptor-like kinase gene in R108, LYK2bis, which is responsible for the phenotype and can allow A17 to gain nodulation with the nodF/nodL mutant. We found that LYK2bis is involved in nodulation by mutants producing non-O-acetylated NFs and interacts with the key receptor protein NFP. Many, but not all natural S. meliloti and S. medicae strains tested require LYK2bis for efficient nodulation of R108. C_LIO_LIOur findings reveal that a newly-evolved gene in R108, LYK2bis, extends nodulation specificity to mutants producing non-O-acetylated NFs and is important for nodulation by many natural Sinorhizobia. Evolution of this gene may present an adaptive advantage to allow nodulation by a greater variety of strains. C_LI