Flavonoids are one of the major secondary metabolites of plants. Flavonoids and their colored class, anthocyanins, are dominant constituents of flower color. Floral flavonoids are usually modified with one or more sugar moieties, and are localized in vacuoles (Tanaka et al. 2008). The glycosylation of flavonoids is catalyzed by family 1 glycosyltransferases of the 91 subfamilies of glycosyltransferases classified on the basis of sequence similarity, catalytic mechanisms, and the presence of conserved sequence motifs (Yonekura-Sakakibara 2009). Family 1 glycosyltransferases utilize specific UDPsugars, such as UDP-glucose, UDP-rhamnose, and UDPgalactose, as sugar donors. Glucosyltransferase (GT) that utilizes UDP-glucose is the most common in this family. Family 1 glycosyltransferases contain a carboxylterminal conserved sequence, the secondary product glycosyltransferase box of plants (Gachon et al. 2005), and are classified into many clusters depending on sequence homology. Glycosyltransferases from different plant species having the same function usually belong to the same clusters in the family.Glycosylation of flavonoids has been extensively studied in terms of its biochemistry and molecular biology. More recently, Arabidopsis flavonoid glycosyltransferases have been comprehensively analyzed (Yonekura-Sakakibara 2009). Flavonoid glycosylation, typically glucosylation, is catalyzed by position-specific glycosyltransferases. GT in the flavonoid biosynthetic pathway includes flavonoid 3-glucosyltransferase (3GT), flavonoid 5-glucosyltransferase (5GT), flavonoid 7-glucosyltransferase (7GT), and flavonoid 3Ј-glucosyltransferase (3ЈGT); 3GT and 5GT form separate clusters, and 7GT and 3ЈGT belong to the same cluster (FukuchiMizutani et al. 2003;Tanaka et al. 2008). Acyl-glucosedependent glycoside hydrolase-like anthocyanin GTs have been recently identified in carnations and delphiniums (Matsuba et al. 2010).Chalcones and aurones are yellow flavonoids. 4,2Ј,4Ј,6Ј-Tetrahydroxy chalcone (THC) imparts pale yellow color but is rapidly isomerized to colorless naringenin by the catalysis of chalcone isomerase (CHI) in vivo (Figure 1) Abstract 4,2Ј,4Ј,6Ј-Tetrahydroxychalcone (THC) 2Ј-glucoside that confers yellow color to the petals of carnation, cyclamen, and catharanthus is biosynthesized by the action of UDP-glucose-dependent THC 2Ј-glucosyltransferase (THC2ЈGT). We isolated 18 types of full-length cDNA encoding GT-like sequences from carnation petals. Expression of these cDNA in Escherichia coli identified five cDNAs encoding THC2ЈGT that were different from the previously isolated THC2ЈGT. We also isolated a cDNA encoding THC2ЈGT from both catharanthus and cyclamen. These THC2ЈGT cDNAs were introduced to petunia. Transgenic petunia that expressed three of the GTs produced THC 2Ј-glucoside, which indicated that they function as THC2ЈGT in vivo. These cDNAs could be useful molecular tools to yield yellow flower color, although the amount accumulated in the transgenic petals was too small to alter the flower color in this...