Small G protein Ropplays an important role as molecular switch during signal transduction in plant cells.A Rop coding gene LjRac1 in Lotus japonicus was cloned by RT-PCR.The gene sequence analysis was carried out by the bioinformation method firstly.Then the expression profile of LjRac1 was detected in different tissues by semi-quantitative RT-PCR and in inoculated roots on different stage of post-inoculation using real-time RT-PCR,respectively.Moreover,over-expression recombinant plasmid was constructed and hairy root transformation method was used to investigate the over-expression effect of LjRac1 mRNA on the early nodulation process.The study revealed that:(1)LjRac1 gene sequence analysis showed that the length of LjRac1 genewas 594 bp,which encoded a peptide with 197 amino acids.LjRac1 contained conserved domains of Rop GTP ase proteins.Homology analysis showed that the deduced amino acid sequence exhibited high similarity with Glycine max GmRac1 and Glycine soja GsRac1(94.42%).(2)LjRac1 was expressed in root,stem,leaf,nodule and flower of L.japonicus,and preferentially expressed in root and nodule.The expression level of LjRac1 was elevated in root 0.5 h after inoculation with Mesorhizobium loti,as compared with that in uninoculated control root.(3)The expression level of LjRac1 mRNA in over-expression transgenic plants was 14.3 times higher than that of the control plants,and the number of nodules in over-expression transgenic plants significantly increased compared with the control plants.The data showed that LjRac1 was the gene of enhancement expression by the induction of M.loti and over-expression LjRac1 could increase the number of nodules,which indicated that LjRac1 gene might be involved in the early stage of symbiosis nodulation and regulated nodule development in L.japonicus.