Formation of 6´-deoxychalcones in Asteraceae species

The ability to form 6`-deoxychalcones in the plant kingdom is mainly restricted to the Leguminosae and the Asteraceae, where they fulfil divergent physiological functions. In Asteraceae they serve for pollinator attraction either as yellow pigments or as UV honey guides. In Leguminosae, 6`deoxychalcones are intermediates for the formation of the rare 5-deoxyflavonoids which act as phytoalexins and phytoestrogens. So far, formation of 6`-deoxychalcones was only studied in Leguminosae. Here it was shown that a chalcone reductase (CHR) in co- action with chalcone synthase catalyzes the formation of the rare 6`-deoxychalcones. It can be assumed that formation in the Asteraceae follows a comparable scheme but with some distinctions as the abundant CHR sequences from Leguminous species did not allow the isolation of a CHR from Asteraceae so far. This project will study the unknown key step in the biosynthesis of 6`-deoxychalcones in the Asteraceae family. A partial cDNA clone of a putative chalcone reductase (CHR) was recently isolated from yellow flowers of Dahlia variabilis. Comparison of gene expression in differently coloured dahlia flowers showed a specifically high expression in petals accumulating anthochlors as yellow flower pigments. Based on the novel sequence, the project will isolate full-size cDNA clones from different Asteraceae species and confirm their identity as CHRs by functional expression in E. coli. The corresponding genes including 5`-flanking regulatory regions and the encoded recombinant enzymes will be characterized in detail. In addition, the novel clones will be used for the construction of vectors harbouring different gene combinations for engineering the anthochlor pathway in planta and evaluating the impact and interaction of the different genes such as chalcone 3-hydroxylase, aurone synthase and various glucosyltransferases in further projects. The identification of the first CHR gene will also allow to close the gap in our knowledge on the formation of the establishment of flower colours in dahlia. Whereas the occurrence of red, black and white varieties is well understood, the formation of yellow dahlia flowers still remains a puzzle. In the postulated pathway, currently CHR is the missing link, because the other steps (chalcone formation, chalcone hydroxylation and aurone formation) were established only recently. CHR is the postulated switch establishing anthochlor pigment formation because all further steps are present in petals regardless of the colour type. The first isolation and characterization of the missing link in the biosynthesis of the anthochlors in the Asteraceae family will enable future studies on the mechanism on the formation of honey guides and spectacular ornamental patterns.