Natural phlobaphenes are the common
bark,
pericarp,
cob glume and
seed coat (
testa)
pigments. They have not been found in flowers, unless the brown and black pigments in the involucrum of certain compositae are found to be of the phlobaphene type. In bark, phlobaphenes accumulate in the
phellem layer of cork cambium, part of the
suberin mixture.
Occurrences Many
cinchona barks contain a particular tannin,
cinchotannic acid, which by oxidation rapidly yields a dark-coloured phlobaphene called
red cinchonic,
cinchono-fulvic acid or
cinchona red. They are common in
redwoods barks like
Sequoia sempervirens or in oak barks where the chief constituent,
quercitannic acid, a molecule also present in
quercitron, is an unstable substance, having a tendency to give off water to form anhydrides (phlobaphenes), one of which is called
oak-red (C28H22O11).
Cuscuta europaea L., the European dodder, is reported to contain 30,000 ppm in the root. Phlobaphenes can be extracted from the root of the
common tormentil (
Potentilla erecta) as
tormentil red. Phlobaphens can be found in the
kola nut (where they are called
kola red),
chocolate liquor (called
cocoa red), or in the red skins or testa of the peanut. They are also reported in the fruits of the genus
Crataegus (
Fructus Crataegi) or can be extracted from
hop flowers. The chief constituent of
kino is
kinotannic acid, of which it contains 70 to 80 per cent. It also contains kino red, a phlobaphene produced from kinotannic acid by oxidation. Phlobaphenes are not present in the model plant
Arabidopsis thaliana but can be studied as the pigment responsible for the red color in some monocot cereals, including
wheat,
maize or
sorghum.
Biosynthesis In maize, phlobaphenes are synthesized in the flavonoids synthetic pathway from polymerisation of
flavan-4-ols by the expression of maize pericarp color1 (p1) gene which encodes an R2R3
myb-like
transcriptional activator of the A1 gene encoding for the
dihydroflavonol 4-reductase (reducing
dihydroflavonols into flavan-4-ols) while another gene (Suppressor of Pericarp Pigmentation 1 or SPP1) acts as a
suppressor. The p1 gene encodes an Myb-homologous transcriptional activator of genes required for biosynthesis of red phlobaphene pigments, while the P1-wr allele specifies colorless kernel
pericarp and red
cobs, and unstable factor for orange1 (Ufo1) modifies P1-wr expression to confer pigmentation in kernel pericarp, as well as vegetative tissues, which normally do not accumulate significant amounts of phlobaphene pigments. In the sorghum, the corresponding yellow seed 1 gene (y1) also encodes a R2R3 type of Myb domain protein that regulates the expression of
chalcone synthase,
chalcone isomerase and
dihydroflavonol reductase genes required for the biosynthesis of
3-deoxyflavonoids. == Chemically formed phlobaphenes ==