Abstract: G-type Barbarea vulgaris R.Br. which producesaponins is a rare insect resistant vegetable in Cruciferae. A beta-amyrin synthase gene which codes a key enzyme of saponin biosynthesis pathway was cloned from Barbarea vulgaris R.Br. using RT-PCR and RACE（Rapid Amplification of cDNA Ends）method，and named as Bv-beta-AS. The Bv-beta-AS contained a coding sequence of 2 289 bp（GenBank accession number：JQ172795），which encoded 762 amino acids. The gene of 4 107 bp from genomic DNA was further cloned，which contained 15 introns embedded in its coding sequences. The deduced protein of this gene contained the conserved characteristic motif of beta-amyrin synthase -DCTAE and QW. And the  high similarity of this gene with the beta-amyrin synthase from other plants indicated it was a homologue of beta-amyrin synthase. Multiple sequence alignments and phylogenetic tree analyses showed that Bv-beta-AS was clustered with Arabidopsis beta-amyrin synthase and showed the highly similarity（74%） in amino acid sequences. The expression of Bv-beta-AS under the diamondback moth（Plutella xylostella） attack was determined by real-time RT-PCR. The results showed that Bv-beta-AS gene expression was up-regulate within 12 h of diamondback moth attack，but decline tendency appeared after 12 hours. Both the sequences analysis results and the real-time RT-PCR data indicated that Bv-beta-AS may be an important candidate gene of saponin biosynthesis pathway. d

Key words: Barbarea vulgaris, Plutella xylostella, saponin, beta-amyrin synthase, cloning, real-time RT-PCR, insect resistant