Abstract: Considerable progress has been made on the phytochemical basis of flower color formation on tree peony（Paeonia suffruticosa Andrews）. However，due to lack of genetic transformation system，function of the genes related to flower color formation has to be characterized using other model plant systems. We utilized virus induced gene silencing（VIGS）for establishing the functional characterizing technique to study UDP-glucose：flavonoid glycosyltransferases（UFGTs）genes in tree peony. The fragments with 413 bp and 418 bp length with a putatively conserved domain of PsUF3GT and PsUF5GT were selected for constructing VIGS recombinant vectors，respectively. Nine treatments with two factors and three levels were conducted to test the gene silencing effects. The results showed that the expression level of PsUF3GT after infiltrating petals of blooming for 15 min，or the expression level of PsUF5GT after infiltrating petals of bud stage for 10 min，were reduced 65.0% and 85.0% as compared with that of control petals of blooming or bud stage infiltrated using empty vectors for 10 min or 15 min，respectively. Meanwhile，total anthocyanins content in petal blotch were also decreased 24.2% and 28.2% after treatment of flowers at blooming or bud stage by infiltration for 15 min or 10 min，respectively，in which，cyanidin 3-O-glucoside reduced 92.2% in petals at blooming stage infiltrated for 15 min and cyanidin 3,5-O-diglucoside reduced 54.9 % in petals at bud staged infiltrated for 10 min. Therefore，an optimal VIGS system has been established in tree peony，namely，flowers at blooming or bud stage were subjected to vacuum infiltration for 10–15 min，then kept in dark for 1 d at 8 ℃ with relative 60% humidity，afterwards，kept at 23–25 ℃ with normal light and relative 60% humidity for 3 d，then，treated samples could be used for further functional characterization of the silenced genes. Our VIGS method would be beneficial to functional characterization of the genes related to flower color formation，and study the molecular mechanism of flower color variation.

Key words: tree peony, flower color, VIGS, flavonoid, UDP-glucose, flavonoid glycosyltransferases （UFGTs）, functional characterization