Abstract: We isolated the AP3/DEF-homologue from the onion variety‘RUPI’using RACE（rapid amplification of cDNA ends），and characterized its expression patterns in vegetative and floral organs using semi-quantitative RT-PCR and quantitative real-time PCR. Phylogenetic analysis indicates that our onion gene，denoted AcDEF，belongs to the AP3/ DEF subfamily of the B-function gene family and has sequence characteristics similar to other monocot paleoAP3 genes. The cDNA sequence is 1 014 bp long，includes both 5′ and 3′ untranslated regions，a poly（A）tail，and an open reading frame encoding a protein with a predicted length of 224 amino acids. Expression analyses indicate that AcDEF is specifically expressed in flower with no detectable signal in vegetative organs，such as roots，cauloids，leaves and bulbs. In addition，expression is tissue-specific in floral organs，with the highest level in the petals and stamens，moderate level in scapes and membranous sheath. AcDEF is expressed at very low levels in the carpels. Accumulation of AcDEF transcripts is dynamically changed and associated with the flower-bud formation and development. Expression of AcDEF is strongly detected during the initiation and early development of petals and stamens，but expression levels in these organs are somewhat reduced at later developmental stages. There also appears to be a gradual and obvious decrease in AcDEF expression during the different developmental stages of scapes，membranous sheath and carpels，such that it is almost absent from the membranous sheath and carpels in full-opening flowers.

Key words: onion, Allium cepa L., flower development, B-function MADS-box genes, AcDEF gene