Flower color, along with fragrance, floral shape and nectar reward, is important to the interaction between plants and pollinators; and preferences towards specific colors are exhibited by pollinators, whether they are birds, bees, butterflies or other insects. Commonly contributing to floral phenotypes are color combinations and patterning such as distinctive spots on the flower ‘lip' or pigment lines in the flower tube. These may provide more specific signals within the flower, e.g. acting as nectar guides to insects. In an extraordinary example of pollinator signaling, the flowers of the orchid genus Ophrys use color, scent and shape to mimic female bees, causing the male bee to attempt copulation, thus achieving pollination. The major pigments responsible for flower color are carotenoids, flavonoids and betalains. Although other pigment types such as chlorophylls, phenylphenalenones and quinochalcones can generate flower colors, their occurrence is rare. Carotenoids are lipid soluble, plastid-located terpenoids present in photosynthetic plants, algae and bacteria. For pigmentation of flowers (and fruits), carotenoids accumulate to high levels in specialized plastids called chromoplasts. These pigments participate in the harvesting and dissipation of light energy in chloroplasts, protecting the photosynthetic machinery from photo-oxidation.
[...] Flavonoids There are numerous examples of modification of flavonoid biosynthesis in flowers of transgenic plants. There are also several examples for grains, fruit and vegetable crops. The major methods are based on manipulation of pathway flux. The approaches to increasing, preventing or redirecting flux into or within the pathway have used up- or downregulation of the pathway using regulatory factors; introducing new biosynthetic activities; increasing specific endogenous biosynthetic activities; and abolishing branches of the pathway. The latter may cause substrate to accumulate or be directed into alternative biosynthetic branches. [...]
[...] For anthocyanin synthesis, it has been shown that major increases in transcript abundance for the biosynthetic genes precede pigment production, and that this is due to increased transcription of the genes. There are few studies for flowers pigmented by carotenoids or betalains. However, recent studies have shown that increases in transcript abundance occur for several carotenoid biosynthetic genes concomitant with flower coloration in daffodil. Similarly, transcript abundance for the betalain biosynthetic enzyme DOD correlates with pigmentation in flowers of P. [...]
[...] Regulation of Pigment Biosynthesis in Flowers The induction of the pigment biosynthetic genes during flowering requires both developmental and environmental signals. Although factors such as temperature and water stress influence flower coloration, light is the principle environmental signal. Light, through direct exposure of the flowers or the leaves, has been shown to be required for full floral coloration in many species. However, there is little data on the molecular mechanisms that may be involved in mediating the light signals in flowers. [...]
[...] Regulation of flavonoid production in flowers Regulation of the production of flavonoid precursors The production of anthocyanins in flowers requires not only the activity of the specific flavonoid genes but also those of the general phenylpropanoid pathway for production of HCAs. Furthermore, the route to malonyl-CoA formation may also need to be upregulated. Detailed studies on the regulation of the genes required for HCA production, however, have been conducted principally in relation to their induction in response to biotic and abiotic (e.g. [...]
[...] Pigments may appear just prior to the flower becoming fertile; or in some cases, flower color may fade or the flower may turn from white to colored or even change in color following fertilization. These transformations may be the result of changes in petal cell pH as the flower ages, degradation of the pigments or de novo pigment biosynthesis. An example of the latter can be found in Lantana. The flowers are yellow initially when fertile but change to purple over an ageing [...]
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