Abstract: A fatty acid desaturase2（FAD2）gene，designated as PoFAD2，was isolated from the ovule of Paeonia ostii T. Hong & J. X. Zhang using reverse transcription PCR and RACE technology. The full-length cDNA was 1 695 bp with an open reading frame of 1 152 bp，encoding a putative protein of 383 amino acids. The phylogenetic tree analysis indicated that PoFAD2 had the highest similarity with FAD2 from P. lactiflora. In addition，the amino acid sequences encoded by PoFAD2 contained three highly conservative histidine clusters，four transmembrane domains and a specific hit of Delta12-FADs，which peculiarly belong to the family of fatty acids desaturase. Real-time quantitative PCR showed that PoFAD2 was differentially expressed in various tissues of P. ostii. It was barely expressed in stems，sepals，stamens or petals，weakly present in leaves（about one seventieth of ovules），moderately in buds and pistils（aboutone tenth of ovules），and extremely highly in ovules. This gene was therefore considered as seed specific. During seed development，PoFAD2 transcript in ovules rapidly increased and reached the highest level at 75 day after florescence，followed by a sharp decline and gradual recovery.

Key words: Paeonia ostii, PoFAD2, molecular cloning, expression analysis, seed