Abstract: The tree peony(Paeonia Sect.Moutan), native to China, is famous for its ornamental value, medical use and edible oil production; an effective tissue culture system is an urgent requirement for its breeding and propagation. Tissue culture of tree peony has been extensively studied during the past half-century, and the main advances include the following 3 aspects:1) Organ and tissue regeneration culture:there are 3 ways for regeneration including organogenesis, somatic embryogenesis and in vitro shoot proliferation. (1) Tree peony organogenesis includes callus culture and meristematic nodules culture. Vegetative and reproductive organs can both be used as explants to induce callus, but it is very difficult to induce shoots from callus, and a complete plant is still not obtained. With the yellowing stem segments and thin-layer petiole as explants, effective meristematic nodules culture systems have been developed for P. rockii and P. itoh by selecting plant growth regulators and improving culture medium. Adventitious buds and thallus were induced from the meristematic nodules; however, a complete plant is not obtained. In addition, the meristematic nodules of tree peony have been found to contain abundant peoniflorin and paeonol which indicate its value in pharmaceutical ingredients production. (2) Somatic embryogenesis:tree peony somatic embryogenesis includes direct and indirect ways. Somatic embryos have been induced in many tree peony cultivars by selecting genotype, culture medium and embryo development stage; however, the somatic embryo has been found difficult for germination and plant regeneration. Tree peony petioles have been used as explants to induce callus though indirect organogenesis, and the somatic embryo have been induced from callus with however extremely low induction rate. (3) In vitro shoot proliferation:in vitro shoot proliferation of tree peony has been studied for nearly 30 years. Effective initiation, multiplication, rooting and acclimatization system have been established; however, the protocol is unviable for commercial use due to the dormancy of the rooted shoots, the high cost of the two-step rooting protocol and the poor survival rate during acclimatization. 2) Ovule and embryo culture:Using mature ovule (60 days after flowering) and mature embryo (90 days after flowering) as explants, ovule and embryo culture systems have been developed for P. rockii and P. ostii by selecting plant growth regulators, and complete plants have been obtained which however died during acclimatization. 3) Anther (pollen) culture:Pollen at the first stage of division is the best explant for anther (pollen) culture. In tissue culture, the pollen embryos in different development states vary in organ differentiating capacity. The clustered pollen embryo can not produce shoots or roots, while the pollen embryo developing independently can produce roots, however without shoots produced. In conclusion, tissue culture of tree peony is still in the primary stage of applied basic research, the future technology research should focus on browning, the difficulties in indirect organogenesis, and the rooting and acclimatization problems to optimize the protocol. Meanwhile studies on the physiological and the molecular mechanism of the browning, organ differentiation, apical shoot dormancy, etc. is essential, in order to provide a theoretical basis for the establishment of tree peony tissue culture system.

Key words: tree peony, tissue culture, organogenesis, somatic embryo, in vitro shoot proliferation, ovule culture, embryo culture, anther (pollen) culture