Modern biotechnology has developed powerful tools for genetic engineering and flower colours are an excellent object to study possibilities and limitations of engineering strategies. Osteospermum hybrida became a popular ornamental plant within the last 20 years. Many cultivars display rose to lilac flower colours mainly based on delphinidin-derived anthocyanins. The predominant synthesis of delphinidin derivatives is referred to a strong endogenous flavonoid 3',5'-hydroxylase (F3'5'H) activity. Furthermore, since dihydroflavonol 4-reductase (DFR) of Osteospermum does not convert dihydrokaempferol (DHK) to leucopelargonidin, synthesis of pelargonidin-based anthocyanins is naturally not realised. In order to redirect anthocyanin biosynthesis in Osteospermum towards pelargonidin derivatives, we introduced cDNAs coding for DFRs which efficiently convert DHK to LPg. But neither the expression of Gerbera hybrida DFR nor of Fragaria x ananassa DFR - the latter is characterised by an unusual high substrate preference for DHK - altered anthocyanin composition in flowers of transgenic plants. However, chemical inhibition of F3'5'H activity in ray florets of dfr transgenic plants resulted in the accumulation of pelargonidin derivatives. Accordingly, retransformation of a transgenic plant expressing Gerbera DFR with a construct for RNAi-mediated suppression of F3'5'H activity resulted in double transgenic plants accumulating predominantly pelargonidin derivatives in flowers.