Abstract:Chalcone synthase-A (CHSA) is a key enzyme in the biosynthesis of all classes of flovonoids, and variation of its expression might affect the colour of flowers. CHSA gene was cloned from flower petals just coming into bloom and inserted in procaryotic expression vector and eucaryotic vector which contains CaMV 35S promoter in a sense-orientation. CHSA gene was highly expressed in procaryotic expression system. Petunia (Petunia hybrida) leaf discs were transformed by Agrobacterium tumefaciences LBA4404. Not only flower colours of the resulted transgenic plant were altered, but the transgenic plants became malesterile. Northern blotting assays showed that the transcription of both transgene and endogenous genes were suppressed in the flower petals of transgenic plant.