Advances in flavonoid research since 1992.

The biochemistry and medical significance of the flavonoids.

Plant compounds that are perceived by humans to have color are generally referred to as 'pigments'. Their varied structures and colors have long fascinated chemists and biologists, who have examined their chemical and physical properties, their mode of synthesis, and their physiological and ecological roles. Plant pigments also have a long history of use by humans. The major classes of plant pigments, with the exception of the chlorophylls, are reviewed here. Anthocyanins, a class of flavonoids derived ultimately from phenylalanine, are water-soluble, synthesized in the cytosol, and localized in vacuoles. They provide a wide range of colors ranging from orange/red to violet/blue. In addition to various modifications to their structures, their specific color also depends on co-pigments, metal ions and pH. They are widely distributed in the plant kingdom. The lipid-soluble, yellow-to-red carotenoids, a subclass of terpenoids, are also distributed ubiquitously in plants. They are synthesized in chloroplasts and are essential to the integrity of the photosynthetic apparatus. Betalains, also conferring yellow-to-red colors, are nitrogen-containing water-soluble compounds derived from tyrosine that are found only in a limited number of plant lineages. In contrast to anthocyanins and carotenoids, the biosynthetic pathway of betalains is only partially understood. All three classes of pigments act as visible signals to attract insects, birds and animals for pollination and seed dispersal. They also protect plants from damage caused by UV and visible light.

https://onlinelibrary.wiley.com/doi/epdf/10.1111/j.1365-313X.2008.03447.x

Biosynthesis of plant pigments: anthocyanins, betalains and carotenoids

— Anthocyanins are water-soluble pigments found in all plant tissues throughout the plant kingdom. Our understanding of anthocyanin biosynthesis and its molecular control has greatly improved in the last decade. The adaptive advantages of anthocyanins, especially in non-reproductive tissues, is much less clear. Anthocyanins often appear transiently at specific developmental stages and may be induced by a number of environmental factors including visible and UVB radiation, cold temperatures and water stress. The subsequent production and localization of anthocyanins in root, stem and especially leaf tissues may allow the plant to develop resistance to a number of environmental stresses. This article reviews the environmental induction of anthocyanins and their proposed importance in ameliorating environmental stresses induced by visible and UVB radiation, drought and cold temperatures.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1751-1097.1999.tb01944.x

Environmental Significance of Anthocyanins in Plant Stress Responses

http://disciplinas.stoa.usp.br/pluginfile.php/87477/mod_resource/content/1/Plant%20Sci%202012%20Agati%20G.pdf

Flavonoids as antioxidants in plants: Location and functional significance

laser scanning microscopy, UV-screening pigments, Picea abies (L.) Karst., Vicia faba L., Secale cereale L., Equisetum Phenolic compounds are involved in many interactions arvense L. of plants with their biotic and abiotic environment. These substances accumulate in different plant tissues and cells during ontogenesis and under the influ- Introduction ence of various environmental stimuli, respectively. Studies on the tissue localization of phenolic com- Phenolic compounds play a major role in the interaction

/pdf/tissue-localization-of-phenolic-compounds-in-plants-by-4ka1snpfm4.pdf

Tissue localization of phenolic compounds in plants by confocal laser scanning microscopy

Epidermally located ultraviolet (UV)-absorbing phenolic compounds, flavonoids and hydroxycinnamic acid esters (HCAs), can shield the underlying tissues in plants against harmful UV-radiation. The relative importance of the two different classes of phenolic compounds for UV-screening was a matter of recent debate. Using a non-invasive method based on chlorophyll fluorescence measurements to estimate epidermal UV transmittance, the relationship between epidermal UV shielding and the content of the two different groups of secondary phenolic compounds in the epidermal layers and the underlying photosynthetic mesophyll of developing rye primary leaves grown under supplementary UV-B radiation was investigated. From the fourth to the tenth day after sowing, epidermally located flavonoids increased in an age- and irradiation-dependent manner, whereas mesophyll flavonoids and epidermal HCAs, mainly ferulic acid and p-coumaric acid esters, were constitutively present and did not vary in their contents over the observed time period. There was an excellent correlation between epidermal UV-A and UV-B absorbances as assessed by chlorophyll fluorescence measurements and contents of epidermal flavonoids. However, HCAs showed an additional contribution to UV-B shielding. In contrast, mesophyll flavonoids did not seem to play a respective role. When absorbances of the abaxial and adaxial epidermal layers were compared, it became apparent that in fully expanded primary leaves epidermal tissues from both sides were equally effective in absorption of UV-radiation. However, the earlier and more UV-exposed abaxial epidermis of young unrolling leaves showed a significantly higher absorption. It is shown that in early stages of development the epidermal HCAs are the dominant UV-B protective compounds of the primary leaf. This function is increasingly replaced by the epidermal flavonoids during leaf development and acclimation. The application of chlorophyll fluorescence measurements has been proven to be a useful tool for estimating relative contents of these compounds in epidermal tissue.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-3040.2000.00633.x

Contribution of hydroxycinnamates and flavonoids to epidermal shielding of UV‐A and UV‐B radiation in developing rye primary leaves as assessed by ultraviolet‐induced chlorophyll fluorescence measurements

Differential flavonoid response to enhanced uv-b radiation in brassica napus

Contribution of phenolic compounds to the UV-B screening capacity of developing barley primary leaves in relation to DNA damage and repair under elevated UV-B levels.

The aim of the present investigation was to define the role of soluble flavonoids as UV-B protectants in the primary leaf of barley (Hordeum vulgare L.). For this purpose we used a mutant line (Ant 287) from the Carlsberg collection of proanthocyanidin-free barley containing only 7% of total extractable flavonoids in the primary leaf as compared to the mother variety (Hiege 550/75). Seven-day-old leaves from plants grown under high visible light with or without supplementary UV-B radiation were used for the determination of UV-B sensitivity. UV-B-induced changes were assessed from parameters of chlorophyll fluorescence of photosystem II, including initial and maximum fluorescence, apparent quantum yield, and photochemical and non-photochemical quenching. A quartz fibre-optic microprobe was used to evaluate the amount of potentially harmful UV-B (310 nm radiation) penetrating into the leaf as a direct consequence of flavonoid deficiency. Our data indicate an essential role of flavonoids in UV-B protection of barley primary leaves. In leaves of the mutant line grown under supplementary UV-B, an increase in 310nm radiation in the mesophyll and a strong decrease in the quantum yield of photosynthesis were observed as compared to the corresponding mother variety. Primary leaves of liege responded to supplementary UV-B radiation with a 30% increase in the major flavonoid saponarin and a 500% increase in the minor compound lutonarin. This is assumed to be an efficient protective response since no changes in variable chlorophyll fluorescence were apparent. In addition, a further reduction in UV-B penetration into the mesophyll was recorded in these leaves.

Gottfried Weissenböck

Papers

Tissue localization of phenolic compounds in plants by confocal laser scanning microscopy

Contribution of hydroxycinnamates and flavonoids to epidermal shielding of UV‐A and UV‐B radiation in developing rye primary leaves as assessed by ultraviolet‐induced chlorophyll fluorescence measurements

Differential flavonoid response to enhanced uv-b radiation in brassica napus

Contribution of phenolic compounds to the UV-B screening capacity of developing barley primary leaves in relation to DNA damage and repair under elevated UV-B levels.

A flavonoid mutant of barley (Hordeum vulgare L.) exhibits increased sensitivity to UV‐B radiation in the primary leaf