Activities of chalcone synthase and UDPGal: flavonoid-3-o-glycosyltransferase in relation to anthocyanin synthesis in apple

Abstract

The activities of two enzymes, chalcone synthase (CS) and UDPGal:flavonoid-3-o-glycosyltransferase (UFGalT), and their relationships with anthocyanin synthesis in apple peel, were studied in 3 years of experiments. UFGalT activity was positively correlated with anthocyanin accumulation during fruit maturation in both ‘Delicious’ and ‘Ralls’ apples. Ethephon treatment at 110 days after full blossom (DAFB) increased UFGalT activity and also promoted anthocyanin formation. However, relatively high UFGalT activity was detected in control fruit at 90 DAFB although there was no anthocyanin in fruit peel. Also, when ethephon was applied at 70 DAFB, it induced high UFGalT activity and increased flavonoid accumulation without inducing anthocyanin formation at 90 DAFB. CS activity and flavonoid content were relatively high and constant from fruitlet to maturation stage. Fruit bagging suppressed accumulation of flavonoids and anthocyanins, but when the bags were removed at 110 DAFB, flavonoids and anthocyanins significantly increased by 130 DAFB. The data indicate that CS do not play a regulatory role in anthocyanin synthesis in apples exposed to light. UFGalT activity was correlated with anthocyanin synthesis during fruit maturation but it is not the only regulatory enzyme in this process since this enzyme was also active at 90 DAFB when no anthocyanin was present. We conclude that UFGalT probably catalyzes both quercetin glycoside and anthocyanin formations, and anthocyanin formation probably depends on cyanidin presence rather than on UFGalT activity. The controlling point in regulating anthocyanin synthesis is likely to be located at the steps from dihydroquercetin to the formation of cyanidin.