1. 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
General introduction to plant defense and attraction

2. - cost of prevention and eradication Cotton boll weevil
Even bigger problem for agriculture
Agricultural pests
– economic damage
- cost of prevention and eradication
Cotton boll weevil

3. Plant is more than primary metabolism: more than sugars and starch, and proteins etc, extracting compounds from the primary C pathways for biosynthetic purposes
Fig

4. Important part of plant metabolism is geared towards plant defense
Fig. 13.4

6. 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 N-containing monoterpenes Alkaloids
diterpene acids Mustard oils

7. Physical Defenses

8. Stem spines Colletia paradoxa Leaf spines- Opuntia invicta
Shoot spines- Dovyalis caffra
Otherwise known as kei apple
Drought tolerant

9. A closer look…

10. Why did spines often evolve in areas that are dry or in other ways “stressful”?
Other roles - competition, camouflage?

11. Waxes, Cutins, and Suberins

12. 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

14. Cutin composed of long fatty acid chains
a major component of plant cuticle

15. PP13020.jpg

16. PP1302B.jpg

17. Cutin Plants’ cuticles often vary with the climate in which they live.
Cactus cuticle
Cactus cuticle

18. 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.

19. PP1303A.jpg

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

22. 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

23. Suberin can form transport barriers between the soil and the roots

24. “Secondary” Metabolites- a term that has stuck

25. Secondary Compounds are so-called because: They do not play a role in photosynthesis, growth, or respiration. HOWEVER…

26. 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)

28. Constitutive vs. Induced Defenses
Constitutive defense - always present
Induced defense - synthesized in response to challenge

30. 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.

31. 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.

34. H3C CH CH CH2 H2C 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

35. 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

36. 2. As defensive compounds
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

37. PP13060.jpg
Non-volatile
Volatile

38. Non-volatile terpenes - limonene apparently distasteful to herbivores
PP1306a.jpg
Non-volatile terpenes - limonene
apparently distasteful to herbivores

39. PP1306b.jpg
Volatile terpenes such as menthol broadcast a smell that warns herbivores that the plant is toxic to them before herbivore feeding commences.

40. 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.

41. 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.

43. 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

45. 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.

46. 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.

47. Terpenes as human medicinal drugs
limonene - monoterpenoid (C10) dietary anticarcinogen
Artemisnin - sesquiterpenoid (C15) antimalarial
Taxol - diterpenoid anticancer drug from Pacific yew (Taxus brevifolia)

48. 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.