Abstract
Flower color is an important characteristic that determines the commercial value of ornamental plants. Gentian flowers occur in a limited range of colors because this species is not widely cultivated as a cut flower. Gentiana lutea L. var. aurantiaca (abbr, aurantiaca) is characterized by its orange flowers, but the specific pigments responsible for this coloration are unknown. We therefore investigated the carotenoid and flavonoid composition of petals during flower development in the orange-flowered gentian variety of aurantiaca and the yellow-flowered variety of G. lutea L. var. lutea (abbr, lutea). We observed minor varietal differences in the concentration of carotenoids at the early and final stages, but only aurantiaca petals accumulated pelargonidin glycosides, whereas these compounds were not found in lutea petals. We cloned and sequenced the anthocyanin biosynthetic gene fragments from petals, and analyzed the expression of these genes in the petals of both varieties to determine the molecular mechanisms responsible for the differences in petal color. Comparisons of deduced amino acid sequences encoded by the isolated anthocyanin cDNA fragments indicated that chalcone synthase (CHS), chalcone isomerase (CHI), anthocyanidin synthase 1 (ANS1) and ANS2 are identical in both aurantiaca and lutea varieties whereas minor amino acid differences of the deduced flavonone 3-hydroxylase (F3H) and dihydroflavonol 4-reductase (DFR) between both varieties were observed. The aurantiaca petals expressed substantially higher levels of transcripts representing CHS, F3H, DFR, ANS and UDP-glucose:flavonoid-3-O-glucosyltransferase genes, compared to lutea petals. Pelargonidin glycoside synthesis in aurantiaca petals therefore appears to reflect the higher steady-state levels of pelargonidin synthesis transcripts. Moreover, possible changes in the substrate specificity of DFR enzymes may represent additional mechanisms for producing red pelargonidin glycosides in petals of aurantiaca. Our report describing the exclusive accumulation of pelargonidin glycosides in aurantiaca petals may facilitate the modification of gentian flower color by the production of red anthocyanins.
Highlights
Flower pigmentation reflects the accumulation of secondary metabolites such as anthocyanins, carotenoids and betalains within petal epidermal cells [1]
The genus Gentiana comprises more than 400 species, including G. triflora with blue flowers resulting from the accumulation of minor cyanidin and major delphinidin derivatives (602 μg/g fresh weight delphinidin) in petals [6,8,9], and G. scabra with pink flowers with exclusive accumulation of gentiocyanins anthocyanins in petals [7,8], both of which have similar amounts of gentiocyanins but do not accumulate carotenoids in petals [8]
As compared to the lutea petals, the aurantiaca petals expressed similar or lower levels of flavonoid 3'-hydroxlase (F3'H) and F3'5'H mRNA, encoding enzymes involved in cyanidin and delphinidin biosynthesis, respectively (Figs 4, 7G and 7H). These results suggest that the presence versus absence of pelargonidin-derived anthocyanin pigments in the orangeflowered aurantiaca and yellow-flowered lutea petals may be predominantly due to the expression differences of pelargonidin-derived anthocyanin pathway genes, but they do not rule out the possibility that dihydroflavonol 4-reductase (DFR) enzymes from the yellow-flowered lutea petals are nonfunctional since there are minor differences in the deduced amino acid sequences encoded by the isolated DFR compared the key anthocyanin biosynthetic gene fragments (cDNAs) between aurantiaca and lutea varieties (Fig 6)
Summary
Flower pigmentation reflects the accumulation of secondary metabolites such as anthocyanins, carotenoids and betalains within petal epidermal cells [1]. Carotenoids and anthocyanins are the major pigments in petals, and are associated with the yellow/orange and red/blue color ranges, respectively [2,3]. The blue flowers of G. triflora (hereafter abbreviated to triflora) predominantly reflect the accumulation of gentiodelphin, a unique polyacylated delphinidin-type anthocyanin [6]. Pink-flowered gentian plants were bred from spontaneous mutations of blue-flowered varieties, reflecting the insertion of transposable elements in the flavonoid 3',5'-hydroxylase (F3'5'H) gene [8]. The characteristic yellow petal color of G. lutea reflects the accumulation of carotenoids such as β-carotene, antheraxanthin, α-cryptoxanthin, β-cryptoxanthin, neoxanthin, violaxanthin, and predominantly lutein [10,11]. G. lutea is a medicinal plant rich in pharmacologically relevant groups of compounds, such as iridoids, secoiridoids and xanthones [12]
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