Mesorhizobium loti possesses a symbiosis island (ICEMlSym[R7A]) that confers upon the bacterium the ability to form a symbiotic association with legumes of the genus Lotus. Nodulation (nod, nol and noe) genes located on the ICEMlSym[R7A] encode enzymes that are responsible for the production of a species-specific signaling molecule, named Nod factor. Perception of Nod factors by plant receptors triggers several plant responses and facilitates bacterial invasion, leading to the formation of root nodules. The studies in this thesis aimed to examine the impact of various structural components of the M. loti Nod factor on host specificity and recognition within Lotus species. The minimal gene requirement for eliciting nodule development on Lotus plants was also determined. The M. loti strain R7A Nod factor has a backbone of five N-acetyl-D-glucosamine (GlcNAc) residues. The non-reducing terminal GlcNAc residue carries an acyl chain of either a vaccenic acid (C[18:1]) or palmitic acid (C[16:0]), a carbamoyl group and a methyl group, while an acetylfucose is present at the reducing terminus. Analysis of loss-of-function [Delta]nodZ and [Delta]nolL mutants showed that the acetylfucose at the reducing terminus was required for efficient nodulation of Lotus species, especially during the initiation of infection threads and for induction of symbiotic gene, NIN. Upon inoculation with R7A[Delta]nodZ, nodulation of Lotus corniculatus and L. filicaulis was significantly delayed and reduced, while only a delay in the onset of nodulation was observed with L. japonicus. Interestingly, nodulation of L. burttii induced by R7A[Delta]nodZ was as efficient as that induced by R7A. Hence, the absolute requirement for the acetylfucose during nodulation was host-dependent. In planta complementation and domain swap experiments using transgenic L. japonicus nfr1 and nfr5 mutants were employed to investigate the role of the reducing terminal acetylfucose in the perception of Nod factor. Nodulation of complemented L. japonicus nfr1 and nfr5 mutants inoculated with R7A[Delta]nodZ was poor, whereas similar plants inoculated with R7A nodulated well. This suggests that the in planta complementation was inefficient and as a result accentuated the effect of the acetylfucose on the Nod factor recognition. The responses of recombinant inbred lines (RILs) derived from a cross between L. filicaulis and L. japonicus to inoculation with strain R7A[Delta]nodZ suggested that at least two genetic loci on chromosome 4, in addition to the Nfr1 and Nfr5 genes, contribute to Nod factor perception and in particular the host-specific recognition of the acetylfucose, This suggests the involvement of multiple receptors or a receptor with multiple components in the perception of Nod factors. A gain-of-function study demonstrated that the presence of nodulation genes alone in nonsymbiotic mesorhizobia was sufficient to induce nodulation and bacteroid formation on Lotus plants, indicating that no other ICEMlSym[R7A] genes were required for infection thread formation or bacterial release. Nodulation assays of four Lotus species indicated host-specific requirements for nodulation genes. The presence of the nodA, nodC, nodD1, nodD2, nodZ, noeL and nolK genes was sufficient to permit nodulation of L. burttii, but was insufficient to induce nodulation of L. japonicus, L. corniculatus and L. filicaulis. The importance of the carbamoyl and methyl groups, and the influence of Nod factor concentration during nodulation were also implicated in this study. A model for the Nod factor perception in Lotus was proposed.