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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

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slide1

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

slide2

Agricultural pests

– economic damage

- cost of prevention and eradication

Cotton boll weevil

slide6

Classes of plant defenses

PHYSICAL DEFENSES

Spines, thorns

cutins

waxes

suberins

SECONDARY METABOLITES

PhenolicsDefense-related proteins

phenolic glycosides peroxidases

bound phenolics polyphenol oxidase

lignin? PAL

condensed tannins hydrolysable tannins

TerpenesN-containing monoterpenes Alkaloids

diterpene acids Mustard oils

slide8

Stem spines Colletia paradoxa

Leaf spines- Opuntia invicta

Shoot spines- Dovyalis caffra

Otherwise known as kei apple

Drought tolerant

slide10
Why did spines often evolve in areas that are dry or in other ways “stressful”?
  • Other roles - competition, camouflage?
cutin waxes suberins are made of hydrophobic compounds
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
Cutin
  • composed of long fatty acid chains
  • a major component of plant cuticle
cutin1
Cutin
  • Plants’ cuticles often vary with the climate in which they live.

Cactus cuticle

Cactus cuticle

waxes
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
Suberin
  • Also formed from fatty acids but has a different structure from cutin.
  • A cell wall constituent.
suberin1
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

slide25
Secondary Compounds are so-called because:They do not play a role in photosynthesis, growth, or respiration. HOWEVER…
sec plant secondary metabolites ondary compounds
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)
slide28

Constitutive vs. Induced Defenses

Constitutive defense - always present

Induced defense - synthesized in response to challenge

terpenes
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.
terpenes1
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.
slide34

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

slide35

Terpene functions

  • 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
slide36

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
slide37

Non-volatile

Volatile

slide38

Non-volatile terpenes - limonene

apparently distasteful to herbivores

slide39

Volatile terpenes such as menthol broadcast a smell that warns herbivores that the plant is toxic to them before herbivore feeding commences.

slide40
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.
slide41
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.
slide43

Terpenes that act against vertebrate herbivores

Triterpenes

1. cardenolides (glycosides) - acutely toxic

influence Na+/K+ ATPase of heart muscle

medicinal application - digitalis (from foxglove), used to

treat heart disease. Can slow and strengthen heart beat

2. Saponins (soaplike) - steroid, triterpenes glycosides

have lipid and water soluble parts of molecule

toxicity related to sterol binding, membrane

disruption

slide45

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.

slide46

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.

slide47

Terpenes as human medicinal drugs

limonene - monoterpenoid (C10) dietary anticarcinogen

Artemisnin - sesquiterpenoid (C15) antimalarial

Taxol - diterpenoid anticancer drug from Pacific yew (Taxus brevifolia)

slide48

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.