Keywords: chemistry: pigmentation.
The red coloration found in carnivorous plants is caused
by plant pigments known as anthocyanins. Because of the interest in pigment-free
forms of certain carnivorous plants, Barry Rice asked me to write
a brief summary of the biochemistry of these pigments. While somewhat
technical, it is a fascinating topic. If you are interested in learning
more about this topic and how it is related to the biology and evolution
of these plants, read on.
Anthocyanins are members of a class of nearly universal,
water-soluble, terrestrial plant pigments that can be classified chemically
as both flavonoid (related to flavone/isoflavone, C15H10O2)
and phenolic (related to phenol, C6H5OH). They are
found in most land plants, with the exceptions of the cacti and the group
containing the beet. They contribute colors to flowers and other plant
parts ranging from shades of red through crimson and blue to purple, including
yellow and colorless. (Every color but green has been recorded). Everyone
who has drunk cranberry juice is familiar with anthocyanin: it is the
chemical that imparts the characteristic red color!
Anthocyanins apparently play a major role in two very
different plant processes. The first is in attracting insects for the
purpose of pollination. The pigments absorb strongly in the UV (ultraviolet),
and to insects which see using UV wavelengths the flowers may be particularly
conspicuous. These pigments play major roles in both pollination and predation
in carnivorous plants, attracting insects into both the flowers and the
trap apparatus. The second role anthocyanin-related pigments serve is
as a protective UV screen. The pigments are produced in response to UV
exposure, and protect the plant's DNA from damage by sunlight. (UV causes
the paired strands of genetic material in the DNA double helix to become
cross-linked, preventing cell division and other vital cellular processes
like protein production).
In a related defense mechanism, anthocyanin production
can be induced by ionizing radiation, which can damage DNA as readily
as UV can. Chemical messengers apparently signal the damage to DNA and
induce anthocyanin production in these plants.
The biosynthesis of this class of pigment is accomplished
by a series of enzymes that are bound to cell membranes. Through a series
of discrete chemical steps, they help convert two central biochemical
building blocks (acetic acid and the amino acid phenylalanine) found in
the cell's cytoplasm into the final pigment. The pigment is then excreted
on the other side of the membrane into vacuoles in the epidermal cell
layer. Significant genetic change in the DNA coding for the production
of these enzymes results in a decrease in pigment production.
Anthocyanin pigments can be produced by growing plant
cells in tissue culture. Plants showing no pigmentation in cultivation
may produce anthocyanin in tissue culture (Bell & Charwood, 1980).
Environmental factors affecting anthocyanin production
include light (intensity and wavelength, with blue and UV being most effective),
temperature, water and carbohydrate levels, and the concentrations of
the elements nitrogen, phosphorous and boron in the growth medium. Anthocyanin
production can be induced by light, blue being the most effective color.
Low light levels also induce the formation of different flavonoid pigments,
which is another interesting adaptive response on the part of plants.
Anthocyanin-type pigments are not found in animals, marine
plants or in microorganisms. It is often theorized that anthocyanin production
is an evolutionary response to plants first venturing onto the stark primordial
landscape under intense UV radiation. (Significant screening of the Earth's
surface from the effects of UV radiation did not occur until after the
advent of terrestrial plants. Oxygen in large amounts first had to be
generated by the photosynthesis of land plants before the protective ozone
layer was formed).
The evolution of insect visions response to the
unique wavelengths of light presented by these plants is an interesting
scenario, as is the evolution of carnivorous plants to take advantage
of the insect's attraction to the sight of anthocyanin. Obviously, the
plants came first and developed anthocyanin as a defense mechanism long
before the first insect evolved. Carnivorous plants subsequently modified
the pollination attraction mechanism to serve as an effective visual lure
for their prey.
Anthocyanin pigments are assembled from two different
streams of chemical raw materials in the cell: both starting from the
C2 unit acetate (or acetic acid) derived from photosynthesis, one stream
involves the shikimic acid pathway to produce the amino acid phenylalanine
(Figure 1). The other stream (the acetic acid pathway) produces three
molecules of malonyl-Coenzyme A, a C3 unit. These streams meet and are
coupled together by the enzyme chalcone synthase (CHS), which forms an
intermediate chalcone by a polyketide folding mechanism that is commonly
found in plants. The chalcone is subsequently isomerized to the prototype
pigment naringenin, which is subsequently oxidized by enzymes like flavonoid
hydroxylase and coupled to sugar molecules to yield anthocyanins. More
than five enzymes are thus required to synthesize these pigments, each
working in concert. Any even minor disruption in any of the mechanism
of these enzymes by either genetic or environmental factors would halt
Anthocyanin production was used as a visual marker in
early studies of chemotaxonomy, which studies the relationships of organisms
based on their biochemical constituents. It gave support to the one gene-one
enzyme theory that is a central tenet in the field of molecular biology.
Figure 1: Anthocyanin biosynthesis
Mann, J. 1987, Secondary Metabolism, second edition,
Oxford Univ. Press., pp. 275-285.
Bell, E.A., and Charwood, B.V. (editors) 1980, Secondary
Plant Products, Encyclopedia of Plant Physiology, New Series, vol. 8,
Reinert, J., and Yeoman, M.M. 1982, Plant Cell and
Tissue Culture, Springer Verlag,. pp. 48-49.
Torssell, K.B.G. 1983, Natural Product Chemistry, John
Wiley & Sons, pp. 138-145.