To provide an insight into stress defense mechanism of birchleaf pear,a choline monooxygenase gene (PbCMO) was cloned via reverse transcriptase PCR,the rapidamplification of cDNA ends (RACE) and long PCR methods.At the same time,PbCMO sequence characteristics were analyzed by the bioinformatics software.Furthermore,semiquantitative PCR with crossintron primers was adopted to study the expression patterns of PbCMO gene under abiotic stresses.The results showed that:(1)PbCMO cDNA sequence contained a 1 227 bp length coding region which encoded 408 amino acid residues.PbCMO genomic DNA sequence length was 2 928 bp which consisted of 10 exons and 9 introns.Predicted isoelectric point and relative molecular mass of PbCMO were 6.19 and 46.27 kD,respectively.PbCMO encoded polypeptide included Riesketype ironsulfur ［2Fe2S］ clusterbinding region and the conserved residues of the mononuclear Febinding motif.Homology analysis showed that the deduced PbCMO protein was highly similar with medlar CMO protein (71%).Phylogenetic analysis also indicated that PbCMO was more related to medlar CMO.(2)PbCMO gene expression was inducible and its transcription abundances climbed up quickly after 100 mmol/L NaCl,10% (W/V) PEG6000,180 mmol/L mannitol or 20 μmol/L ABA treatment.It is showed that PbCMO expression was response to salt,drought,osmotic and ABA stress.Conclusively,PbCMO was involved in the defense mechanism against abiotic stress in birchleaf pear.