1 – environmental control of seed germination

The anthocyanins (glycosilated anthocyanidins) are water-soluble, vacuolar pigments, responsible for the violet, blue, purple, dark red and scarlet coloration of fruits, flowers, stems and leaves in all orders of higher plants (Swain 1976), with one exception, the order Centrospermae, where the violet-red colors are due to a different group of water soluble vacuolar pigments, the betacyanins (Piattelli 1976). The anthocyanins form one group of a class of water-soluble pigments, the flavonoids, which include, beside the anthocyanins, flavonols, flavones, flavanones, cathechins, chalkones and others. The flavonoids share a common structural unit, the C6C3C6 skeleton of flavone (Swain 1976). The flavonoids are widespread throughout the plant kingdom and are rather abundant; it has been estimated that about 1.5% of the carbon fixed annually in photosynthesis is used for the synthesis of flavonoids (Smith 1972). The flavonoids are derivatives of cinnamic acid which is formed through the deamination of phenylalanine. The early steps in the conversion of phenylalanine to derivatives of cinnamic acids are common to the pathways for the biosynthesis of cinnamate esters, flavonoids and lignins; these early steps are known as the general phenylpropanoid metabolism. The enzymes that catalyze the individual early steps are: phenalalanine ammonialyase (PAL, EC 4.3.1.5), cinnamate-4-hydroxylase (EC 1.14.13.11) and 4-courmarate: CoA ligase (EC 6.2.1.12).

The Photoregulation of Anthocyanin Synthesis

SUMMARY

The elongation of dark-grown gherkin hypocotyls has been recorded continuously in darkness and on exposure to blue, red or far-red light. Evidence was obtained that the inhibition of elongation in blue light is not mediated by the phytochrome and “high energy reactions” but that a specific blue-light absorbing pigment is involved.

Rapid growth inhibition of gherkin hypocotyls in blue light1

The biosynthesis of anthocyanins in plant tissues either requires light or is enhanced by it. Light-dependent anthocyanin synthesis has been extensively used as a model system for studies of the mechanism of photoregulation of plant development. Two components can be distinguished in the action of light on anthocyanin production. The first component is the red-far red reversible, phytochrome-mediated response induced by short irradiations; the amount of anthocyanin formed in response to a single, short irradiation is small. The second component is the response to prolonged exposures; the formation of large amounts of anthocyanin requires prolonged exposures to high fluence rates of visible and near-visible radiation (290 to 750 nm) and shows the typical properties of the “High Irradiance Reaction” (HIR) of plant photomorphogenesis. Phytochrome is involved in the photoregulation of the HIR response and is the only photoreceptor mediating the action of prolonged red and far red irradiations. The response to prolonged ultraviolet and blue radiation is probably mediated, at least in some systems, by two photoreceptors: phytochrome and cryptochrome, the latter being a specific ultraviolet-blue-light photoreceptor. The nature of the interaction between phytochrome and cryptochrome in the regulation of plant photomorphogenic responses is still unclear.

Light-dependent anthocyanin synthesis: A model system for the study of plant photomorphogenesis

An action spectrum for anthocyanin formation in dark-grown broom sorghum (Sorghum bicolor Moench, cv Acme Broomcorn and cv Sekishokuzairai Fukuyama Broomcorn) seedlings was determined over the wavelength range from 260 to 735 nanometers. The action peaks were at 290, 650, 385, and 480 nanometers in descending order of height. The action of the 290-nanometer peak was not affected by subsequently given far red light, whereas those of the other three action peaks were nullified completely. The nullification of the 385-nanometer peak action by far red light was reversible. When an irradiation at these action peaks was followed by a phytochrome-saturating fluence of red light irradiation, the action of the 290-nanometer peak remained, whereas that of the 385-nanometer peak as well as those of the 650- and 480-nanometer peaks was masked by the action of the second irradiation. These findings suggested that the 290- and 385-nanometer action peaks involved different photoreceptors, the latter being phytochrome. The blue light-absorbing photoreceptor as reported to be a prerequisite for phytochrome action in milo sorghum was not found to exist in the broom sorghums.The action spectrum deprived of the involvement of phytochrome was determined in the ultraviolet region by irradiating with far red light following monochromatic ultraviolet light. The spectrum had a single intense peak at 290 nanometers and no action at all at wavelengths longer than 350 nanometers.

https://www.jstor.org/stable/pdfplus/4267577.pdf

Ultraviolet Action Spectrum for Anthocyanin Formation in Broom Sorghum First Internodes

— Kinetic studies with the mustard seedling (Sinapis alba L.) support the hypothesis that the so-called ‘high energy reaction’ of photomorphogenesis can be understood solely on the basis of phytochrome. Light-induced anthocyanin synthesis (a typical ‘positive’ photoresponse(1) and light dependent inhibition of hypocotyl lengthening (a typical ‘negative’ photoresponse(1)) have been investigated. In order to explain the experimental data we have to assume that there are two different types of phytochrome 730 which differ greatly as far as their resistance to irreversible destruction is concerned. The existence of these two different types of phytochrome 730 has already been postulated on the basis of spectrophotometric measurements in vivo.(2)

Kinetic studies to interpret ‘high-energy phenomena’ of photomorphogenesisl on the basis of phytochrome

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Kinetic studies to interpret ‘high-energy phenomena’ of photomorphogenesisl on the basis of phytochrome