A cDNA library of Astragalus sinicus L. genes specifically expressed in infected roots by Mesorhizobium huakuii 7653R was generated by using a PCR-based suppressive subtractive hybridization (SSH) technique with two mRNA populations of infected and uninfected roots.A total number of approximately 527 SSH cDNA clones were obtained,including 341 up-regulated gene clones and 186 down-regulated gene clones.The full-length cDNA sequence of up-regulated AsB6 gene was obtained by RACE and analyzed through Blasting,GenBank,GenBank EST and Tiger Porcine EST.AsB6 gene showed 66% similarity with a-etoacyl-ketoacyl synthase and SAM dependent carboxyl methyltransferase in Medicago truncatula.The temporal and spatial expression pattern of AsB6 was estimated using quantitative fluorescence real-time RT-PCR analysis.It was found that its expression level,induced by M.huakuii 7653R,in infected roots and nodule was significantly enhanced,and it reached the peak level during the early period of nodulation.The results suggested the symbiotic function of AsB6 gene might involved in the rhizobium infection,nodulation and maintenance of nodule metabolism.