Transgenic Plant Journal ©2007 Global Science Books Molecular Breeding of Novel Yellow Flowers by Engineering the Aurone Biosynthetic Pathway Eiichiro Ono1* • Toru Nakayama2 1 Institute for Health Care Science, Suntory Ltd., Suntory Research Center, 1-1-1 Wakayamadai, Shimamoto, Mishima, Osaka 618-8503, Japan 2 Department of Biomolecular Engineering, Graduate School of Engineering, Tohoku University, 6-6-11 Aoba, Sendai, Miyagi 980-8579, Japan Corresponding author : *
[email protected] ABSTRACT Aurone flavonoids confer a bright yellow color to flowers, such as snapdragon (Antirrhinum majus). A. majus aureusidin synthase (AmAS1), a polyphenol oxidase, was identified as the key enzyme catalyzing the oxidative formation of aurones from chalcones. To date, all known PPOs have been found to be localized in plastids, whereas flavonoid biosynthesis is thought to take place on the cytoplasmic surface of the endoplasmic reticulum. Interestingly, AmAS1 is transported to the vacuole lumen, but not to the plastid, via ER-to-Golgi trafficking. A sequence-specific vacuolar sorting determinant is encoded in the 53-residue N-terminal sequence of the precursor, demonstrating the first example of the biosynthesis of a flavonoid skeleton in vacuoles. Transgenic flowers overexpressing AmAS1, however, failed to produce aurones. The identification of A. majus chalcone 4'-O-glucosyltransferase (UGT88D3) showed that the glucosylation of chalcone by cytosolic UGT88D3 followed by oxidative cyclization by vacuolar aureusidin synthase is the biochemical basis of the formation of aurone 6-O-glucosides in vivo. Co-expression of the UGT88D3 and AmAS1 genes was sufficient for the accumulation of aurone 6-O-glucoside in transgenic flowers, suggesting that glucosylation facilitates vacuolar transport of chalcones.