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Ch 13: Secondary metabolism
and plant defense
1- First line of defense: Plant
perimeter protection
2- Second line of defense:
Chemical warfare
3- Three major groups of
secondary compounds
- Terpenes
- Phenolics
- N-containing compounds
Agricultural pests
– economic damage
- cost Cotton
of prevention
and eradication
boll weevil
Fig. 11.13
Fig. 13.4
Classes of plant defenses
PHYSICAL DEFENSES
Spines, thorns
cutins
waxes
suberins
SECONDARY METABOLITES
Phenolics
Defense-related proteins
phenolic glycosides
peroxidases
bound phenolics
polyphenol oxidase
lignin?
PAL
condensed tannins
hydrolysable tannins
Terpenes
monoterpenes
diterpene acids
N-containing
Alkaloids
Mustard oils
Physical Defenses
Stem spines Colletia paradoxa
Leaf spines- Opuntia invicta
Shoot spines- Dovyalis caffra
Otherwise known as kei apple
Drought tolerant
A closer look…
• Why did spines often evolve in
areas that are dry or in other
ways “stressful”?
• Other roles - competition,
camouflage?
Waxes, Cutins, and
Suberins
Cutin, Waxes, Suberins are
made of hydrophobic
compounds
• Hydrophobic: having water-repelling
properties
• These compounds are non-polar
• Fatty acids are one type of
hydrophobic compound
Cutin
• composed of long fatty acid chains
• a major component of plant cuticle
Cutin
• Plants’ cuticles
often vary with
the climate in
which they live.
Cactus cuticle
Cactus
cuticle
Waxes
• complex mixtures of long-chain lipids
that are extremely hydrophobic.
• are synthesized by epidermal cells.
• exuded through pores in the epidermal
cell wall by an unknown mechanism.
Suberin
• Also formed from fatty acids but has a
different structure from cutin.
• A cell wall constituent.
Suberin
• often within roots.
• can protect against pathogens and other
damage.
• older parts of roots more suberized
• endodermis has suberin side walls, water
must pass through plasma membrane to get to
stele
Suberin can form transport barriers
between the soil and the roots
“Secondary” Metabolites- a
term that has stuck
Secondary Compounds are
so-called because:
They do not play a role in
photosynthesis, growth, or
respiration. HOWEVER…
Sec Plant secondary metabolites
ondary Compounds
•protect primary metabolism by deterring
herbivores, reduce tissue loss.
•also attract pollinators and seed-dispersing
animals.
•formed from the byproducts or intermediates
of primary metabolism (see figure 13.4)
Constitutive vs. Induced
Defenses
Constitutive defense always present
Induced defense - synthesized
in response to challenge
Terpenes
•Constituents of essential oils
•Building block- 5 C isoprene unit
•Terpenes are classified by the number of isoprene
units they have.
•i.e. monoterpenes-1, diterpenes-4 etc.
Terpenes
•produced from the mevalonic acid pathway
•some functions in “primary” metabolism
•function as herbivore deterrents
•can be produced in response to herbivore
feeding, and to attract predatory insects
and parasites of the feeding herbivore.
Isoprene is the basic building block of the terpenes
(terpenes also called “isoprenoids”)
H3C
CH
CH
CH2
H2C
Monoterpenes have two C5 units (10C)
Sesquiterpenes have three C5 units (15C)
Diterpenes have four C5 units (20C)
Triterpenes 30 C
Tetraterpenes 40C
Polyterpenes ([C5]n), n>8
Terpene functions
1. Growth and development
• carotenoid pigments are tetraterpenes
• chlorophyll side chain is diterpene
• giberellins (hormones) are diterpenes
• abscissic acid (hormone) is a sesquiterpene C15
• sterols are triterpenes
Terpene functions
2. As defensive compounds
•toxins and feeding deterrenets to insects and mammals
Examples
resins of conifers are monoterpenes
essential oils - peppermint, limon, basil, sage
may be in glandular hairs on epidermis
Non-volatile
Volatile
Non-volatile terpenes - limonene
apparently distasteful to herbivores
Volatile terpenes such as menthol
broadcast a smell that warns herbivores
that the plant is toxic to them before
herbivore feeding commences.
• Phytoecdysones are plant
steroids (within the
terpene class) that have
the same basic structure
as insect molting hormones
and thus interfere with
molting. These compounds
sometimes cause death of
the insect herbivore.
• Terpenes such as
pyrethrum (from
chrysanthemums) and
azadirachtin (from
the Asian and African
Neem tree) can be
used as “natural”
insecticides in
agricultural practices
or in horticulture.
Terpenes that act against vertebrate herbivores
Triterpenes
1. cardenolides (glycosides) - acutely toxic
influence Na+/K+ ATPase of heart muscle
beat
medicinal application - digitalis (from foxglove), used to
treat heart disease. Can slow and strengthen heart
2. Saponins (soaplike) - steroid, triterpenes glycosides
have lipid and water soluble parts of molecule
toxicity related to sterol binding, membrane
disruption
Genomic Organization of Plant
Terpene Synthases and
Molecular Evolutionary Implications
Trapp & Croteau, 2001
Some 30,000 known terpenes.
Phylogenetically widespread.
Common evolutionary origins
may predate angiosperms.
Extensive gene duplication events
may drive the diversity.
Prior to divergence of gymnosperms and
angiosperms, during the carboniferous, the
duplication of an ancestral terpene synthase
gene…occurred. Once copy of the duplicated
ancestral gene remained highly conserved in
structure and function, and this gene may have
contemporary descendants in the terpene
synthases involved in giberellins biosynthesis. The
second ancestral gene copy diverged in structure
and function, by adaptive evolutionary processes,
to yield a large superfamily of terpene synthases
involved in secondary metabolic pathways.
Terpenes as human medicinal drugs
limonene - monoterpenoid (C10) dietary anticarcinogen
Artemisnin - sesquiterpenoid (C15) antimalarial
Taxol - diterpenoid anticancer drug from Pacific yew (Taxus brevifolia)
Taxol from Pacific Yew, Taxus brevifolia
Taxol was isolated from bark of Pacific Yew in 1970s. Taxol interferes with
cell division by binding to the protein tubulin, a key factor in mitosis. Taxol
and related compounds now widely used in treating breast and ovarian
cancer.
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