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

Cotton boll weevil

Agricultural pests – economic damage - cost of prevention and eradication

Fig. 11.13

Fig. 13.4

Classes of plant defensesClasses of plant defensesPHYSICAL DEFENSESPHYSICAL DEFENSES

Spines, thornsSpines, thorns

cutins cutins

waxeswaxes

suberinssuberins

SECONDARY METABOLITESSECONDARY METABOLITES

PhenolicsPhenolics Defense-related proteinsDefense-related proteins

phenolic glycosidesphenolic glycosides peroxidases peroxidases

bound phenolicsbound phenolics polyphenol oxidase polyphenol oxidase

lignin?lignin? PAL PAL

condensed tanninscondensed tannins hydrolysable tannins hydrolysable tannins

TerpenesTerpenes N-containingN-containing monoterpenes monoterpenes Alkaloids Alkaloids

diterpene acidsditerpene acids Mustard oils Mustard oils

Physical Defenses

Stem spines Colletia paradoxa Leaf spines- Opuntia invicta

Shoot spines- Dovyalis caffra Otherwise known as kei appleDrought 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 defense - always present

Constitutive vs. Induced Defenses

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”)

CH

H3C

H2C

CH CH2

Monoterpenes have two C5 units (10C)Sesquiterpenes have three C5 units (15C)Diterpenes have four C5 units (20C)Triterpenes 30 CTetraterpenes 40CPolyterpenes ([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

Examplesresins of conifers are monoterpenes

essential oils - peppermint, limon, basil, sagemay 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

Triterpenes1. 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, membranedisruption

Genomic Organization of Plant Terpene Synthases andMolecular Evolutionary ImplicationsTrapp & 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.

limonene - monoterpenoid (C10) dietary anticarcinogen

Artemisnin - sesquiterpenoid (C15) antimalarial

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

Terpenes as human medicinal drugs

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Taxol from Pacific Yew, Taxus brevifolia

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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.