Biosynthesis of phloridzin in apple (Malus domestica Borkh.)

In contrast to the ubiquitously present flavonoids, dihydrochalcones seem to be restricted to approx. 30 plant families. In apple (Malus domestica Borkh.), the dihydrochalcone phloridzin (phloretin 2′-O-glucoside) is dominant representing more than 90% of the soluble phenolics in the leaves. Dihydrochalcones are biochemically related to flavonoids, but the knowledge of their biosynthesis is still limited. We investigated three biosynthetic steps leading to phloridzin formation in apple: (i) NADPH-dependent formation of 4-hydroxydihydrocinnamoyl-CoA from p-coumaroyl-CoA, (ii) phloretin formation by chalcone synthase (CHS) and (iii) glucosylation of phloretin in position 2′. The formation of 4-hydroxydihydrocinnamoyl-CoA does not seem to be apple specific, although not all plants tested were able to catalyze the reaction. Four CHS cDNAs were identified as showing sequence differences at the amino acid level. The recombinant isoenzymes catalyzed both the formation of naringenin chalcone and phloretin to a comparable extent. This underpins the hypothesis that the dihydrochalcone core structure is formed by the common CHS. The absence of phloridzin in the closely related pear (Pyrus communis L.) is based on the lack of ability to catalyze the first step in the phloridzin biosynthesis.