The sum of all of the chemical
reactions that take place in an organism is called metabolism. Most of that
carbon, nitrogen, and energy ends up in molecules that are common to all cells
and are required for the proper functioning of cells and organisms. These
molecules, e.g., lipids, proteins, nucleic acids, and carbohydrates, are called
primary metabolites. Unlike animals, however, most plants divert a
significant proportion of assimilated carbon and energy to the synthesis of
organic molecules that may have no obvious role in normal cell function. These
molecules are known as secondary metabolites.
The distinction between primary and
secondary metabolites is not always easily made. At the biosynthetic level,
primary and secondary metabolites share many of the same intermediates and are
derived from the same core metabolic pathways. Secondary metabolites generally,
but not always, occur in relatively low quantities and their production may be
widespread or restricted to particular families, genera, or even species. They
were known, however, to have significant economic and medicinal value and were
thus of more than a passing interest to natural products chemists. In recent
years, however, it has become increasingly evident that many natural products
do have significant ecological functions, such as protection against microbial
or insect attack.
Secondary metabolites
For many years the adaptive
significance of most secondary metabolites was unknown. These compounds were
thought to be simply functionless end products of metabolism, or metabolic
wastes. Today we know that many secondary metabolites have important ecological
functions in plants:
- They protect plants against being eaten by herbivores
and against being infected by microbial pathogens.
- They serve as attractants (odor, color, taste) for
pollinators and seed-dispersing animals.
- They function as agents of plant-plant competition and
plant-microbe symbioses.
The ability of plants to compete and
survive is therefore profoundly affected by the ecological functions of their
secondary metabolites.
Secondary metabolism is also
relevant to agriculture. The very defensive compounds that increase the
reproductive fitness of plants by warding off fungi, bacteria, and herbivores
may also make them undesirable as food for humans. Many important crop plants
have been artificially selected to produce relatively low levels of these
compounds (which, of course, can make them more susceptible to insects and
disease).
Plant secondary metabolites can be
divided into three chemically distinct groups: terpenes, phenolics, and
nitrogen-containing compounds.
Terpenes
The terpenes, or terpenoids,
constitute the largest class of secondary metabolites. Most of the diverse
substances of this class are insoluble in water. Certain terpenes have
well-characterized functions in plant growth or development and so can be
considered primary rather than secondary metabolites. For example, the
gibberellins, an important group of plant hormones, are diterpenes.
Brassinosteroids, another class of plant hormones with growth-regulating
functions, originate from triterpenes. The vast majority of terpenes, however,
are secondary metabolites presumed to be involved in plant defenses.
Terpenes are toxins and feeding
deterrents to many herbivorous insects and mammals; thus they appear to play
important defensive roles in the plant kingdom. For example, monoterpene esters
called pyrethroids, found in the leaves and flowers of Chrysanthemum
species, show striking insecticidal activity. Both natural and synthetic
pyrethroids are popular ingredients in commercial insecticides because of their
low persistence in the environment and their negligible toxicity to mammals. In
conifers such as pine and fir, monoterpenes accumulate in resin ducts found in
the needles, twigs, and trunk. These compounds are toxic to numerous insects,
including bark beetles, which are serious pests of conifer species throughout
the world. Many plants contain mixtures of volatile monoterpenes and
sesquiterpenes, called essential oils, that lend a characteristic odor to their
foliage. Peppermint, lemon, basil, and sage are examples of plants that contain
essential oils. The chief monoterpene constituent of lemon oil is limonene;
that of peppermint oil is menthol (Figure 2.21). Essential oils have
well-known insect repellent properties.
Figure 2.21 Structures of limonene (A) and menthol (B): these two
well-known monoterpenes serve as defenses against insects and other organisms (source:
Taiz L., Zeiger E., 2010)
They are frequently found in glandular
hairs that project outward from the epidermis and serve to “advertise” the
toxicity of the plant, repelling potential herbivores even before they take a
trial bite. Triterpenes that defend plants against vertebrate herbivores
include cardenolides and saponins. Cardenolides are glycosides
(compounds containing an attached sugar or sugars) that taste bitter and are
extremely toxic to higher animals. Saponins are steroid and triterpene
glycosides, so named because of their soaplike properties. The presence of both
lipid-soluble (the steroid or triterpene) and water-soluble (the sugar)
elements in one molecule gives saponins detergent properties.
Phenolic compounds
Plants produce a large variety of
secondary compounds that contain a phenol group: a hydroxyl functional group on
an aromatic ring. These substances are classified as phenolic compounds, or
phenolics. Plant phenolics are a chemically heterogeneous group of nearly
10,000 individual compounds: Some are soluble only in organic solvents, some
are water-soluble carboxylic acids and glycosides, and others are large,
insoluble polymers. In keeping with their chemical diversity, phenolics play a
variety of roles in the plant. Many serve as defenses against herbivores and
pathogens. Others function in mechanical support, in attracting pollinators and
fruit dispersers, in absorbing harmful ultraviolet radiation, or in reducing
the growth of nearby competing plants.
The colored pigments of plants
provide visual cues that help to attract pollinators and seed dispersers. These
pigments are of two principal types: carotenoids and flavonoids.
Carotenoids are yellow, orange, and red terpenoid compounds that also serve as
accessory pigments in photosynthesis. The flavonoids also include a wide range
of colored substances. The most widespread group of pigmented flavonoids is the
anthocyanins, which are responsible for most of the red, pink, purple,
and blue colors observed in flowers and fruits. Two other groups of flavonoids
found in flowers are flavones and flavonols. These flavonoids generally
absorb light at shorter wavelengths than do anthocyanins, so they are not
visible to the human eye. However, insects such as bees, which see farther into
the ultraviolet range of the spectrum than humans do, may respond to flavones
and flavonols as visual attractant cues. Isoflavonoids, which are found
mostly in legumes, have several different biological activities. Some, such as
rotenone, can be used effectively as insecticides, pesticides (e.g., as rat
poison), and piscicides (fish poisons). Other isoflavones have anti-estrogenic
effects; for example, sheep grazing on clover rich in isoflavonoids often
suffer from infertility. The ring system of isoflavones has a three-dimensional
structure similar to that of steroids, allowing these substances to bind to
estrogen receptors. Isoflavones may also be responsible for the anticancer
benefits of foods prepared from soybeans.
A second category of plant phenolic
polymers with defensive properties, besides lignin, is the tannins. They
are general toxins that can reduce the growth and survival of many herbivores
when added to their diets. In addition, tannins act as feeding repellents to a
great variety of animals. Mammals such as cattle, deer, and apes
characteristically avoid plants or parts of plants with high tannin contents.
Unripe fruits, for instance, frequently have very high tannin levels, which
deter feeding on the fruits until their seeds are mature enough for dispersal.
Herbivores that habitually feed on tannin-rich plant material appear to possess
some interesting adaptations to remove tannins from their digestive systems.
Plant tannins also serve as defenses against microorganisms.
From leaves, roots, and decaying
litter, plants release a variety of primary and secondary metabolites into the
environment. The release of secondary compounds by one plant that have an
effect on neighboring plants is referred to as allelopathy. If a plant can
reduce the growth of nearby plants by releasing chemicals into the soil, it may
increase its access to light, water, and nutrients and thus its evolutionary
fitness. Allelopathy is currently of great interest because of its potential
agricultural applications. Reductions in crop yields caused by weeds or
residues from the previous crop may in some cases be a result of allelopathy.
An exciting future prospect is the development of crop plants genetically
engineered to be allelopathic to weeds.
Nitrogen-containing compounds
A large variety of plant secondary
metabolites have nitrogen as part of their structure. Included in this category
are such well-known antiherbivore defenses as alkaloids and cyanogenic
glycosides, which are of considerable interest because of their toxicity to
humans as well as their medicinal properties. Most nitrogenous secondary
metabolites are synthesized from common amino acids.
The alkaloids are a large
family of more than 15,000 nitrogen-containing secondary metabolites. They are
found in approximately 20% of vascular plant species. As a group, alkaloids are
best known for their striking pharmacological effects on vertebrate animals.
Alkaloids are usually synthesized from one of a few common amino acids – in
particular, lysine, tyrosine, or tryptophan. However, the carbon skeleton of
some alkaloids contains a component derived from the terpene pathway. Several
different types, including nicotine and its relatives (Figure 2.22), are
derived from ornithine, an intermediate in arginine biosynthesis. The B vitamin
nicotinic acid (niacin) is a precursor of the pyridine (six-membered) ring of
this alkaloid. Alkaloids were once thought to be nitrogenous wastes (analogous
to urea and uric acid in animals), nitrogen storage compounds, or growth
regulators, but there is little evidence to support any of these functions.
Most alkaloids are now believed to function as defenses against herbivores,
especially mammals, because of their general toxicity and deterrence
capability.
Various nitrogenous protective
compounds other than alkaloids are found in plants. Two groups of these
substances – cyanogenic glycosides and glucosinolates – are not themselves
toxic but are readily broken down to give off poisons, some of which are
volatile, when the plant is crushed. Cyanogenic glycosides release the
well-known poisonous gas hydrogen cyanide (HCN). The presence of cyanogenic
glycosides deters feeding by insects and other herbivores such as snails and
slugs. As with other classes of secondary metabolites, however, some herbivores
have adapted to feed on cyanogenic plants and can tolerate large doses of HCN.
Figure 2.22 Examples of alkaloids, a diverse group of secondary
metabolites that contain nitrogen (source: Taiz L., Zeiger E., 2010)
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