Biochemical formation of anthocyanins in silk tissue of Zea mays

Two types of anthocyanins are formed in tissues of Zea mays (maize), the common 3-hydroxyanthocyanins and the rare 3-deoxyanthocyanins. Their formation is generally controlled by independent regulatory genes. Whereas the formation of 3-hydroxyanthocyanins is well established, the enzymes involved in the biosynthesis of the 3-deoxyanthocyanins remained unclear for a long time. In order to elucidate the key reaction involved, the reduction of flavanones to flavan 4-ols, dihydroflavonol 4-reductase (DFR) and flavanone 4-reductase (FNR) activities were investigated using enzyme preparations from silks of different maize lines as well as from transgenic petunia and yeast, respectively, expressing the A1 gene of maize. This gene is known to encode DFR. DFR and FNR activities could not be separated by biochemical investigations and the use of different enzyme inhibitors. In maize genotypes with recessive alleles (a1a1) of the A1 gene, neither DFR nor FNR activity was present. Moreover, heterologous expression of A1 in petunia and yeast has shown that the gene encodes for a reductase, which is able to convert both dihydroflavonols (DFR activity) and flavanones (FNR activity) to flavan 3,4-diols and flavan 4-ols, respectively. Thus, DFR and FNR activities are provided by one and the same enzyme with a broad substrate acceptance, which even includes the 5-deoxydihydroflavonols. Further studies suggested that the anthocyanin type formed in the silks essentially depends on the strength of flavanone 3-hydroxylase activity.