Comparative Analysis of Polyphenolic Content and Gene Expression Patterns of Key Flavonoid Biosynthetic Genes in Fragaria Spp. Color Mutants

Strawberries are a rich source of polyphenols which contribute to berry color and flavor and have been shown to lower the risk of chronic diseases such as cancer and cardiovascular disease when consumed. While there is a considerable body of work on the polyphenolic composition of commercial strawberry (Fragaria x ananassa Duch.), less information is available concerning polyphenols in Fragaria vesca, or Alpine strawberry, now considered a model system for the Rosaceae family. To provide insight into metabolic flux through the complex, branched flavonoid biosynthetic pathway, the identity and quantity of the major phenolic-derived anthocyanins, flavonols, flavan-3-ols, hydroxycinnamic acids, and ellagic acid (EA)-derived compounds, and expression patterns of key flavonoid biosynthetic genes, of red cultivars versus white mutants of F. vesca and F. x ananassa were compared. Polyphenolic compounds of ripe berries were identified and quantified in six cultivars of F. vesca , 5 white and 1 red, and two cultivars of F. x ananassa, 1 white and 1 red,  by high performance liquid chromatography coupled with electrospray ionization mass spectrometry in both positive and negative ion modes. Six anthocyanins were identified, though not in all genotypes. There was more pelargonidin-3-glucoside than cyanidin-3-glucoside in red F. x ananassa, while the opposite was true for red F. vesca. The white genotypes of both species had very low anthocyanin levels. Twenty-one other phenolic-derived compounds were identified. Among the flavonols, quercetin-3-glucoronide and kaempferol-3-glucoside were the most abundant in both F. vesca and F. x ananassa, with more in the latter. Flavan-3-ols were present in dimeric forms of catechin and epicatechin with consistently high amounts in the all of the white cultivars of both species. Total hydroxycinnamic acids were generally the least abundant across species and genotypes. Total content of free EA and its conjugated forms, EA pentoside, methyl EA pentoside, and EA hexoside, were generally higher in white versus red F. vesca, and red versus white F. x ananassa. Galloyl bis HHDP hexose and HHDP galloyl hexoside were the most common ellagitannins in both species though no clear species or genotype differences were evident.  Transcriptional analyses of the flavonoid biosynthetic genes revealed correlations with the final concentrations of polyphenolic groups in red and white F. vesca, especially the differential expression patterns of dihydroflavonol-4-reductase, anthocyanidin synthase, and UDP-glucose-flavonoid-3-O-glucosyltransferase genes. These results will be discussed in terms of final metabolite pools.