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

- 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

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. 11.13

Important part of plant metabolism is geared towards plant defense 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 N-containing monoterpenes Alkaloids diterpene acids 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

PP13020.jpg

PP1302B.jpg

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.

PP1303A.jpg

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.

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

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

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

PP13060.jpg Non-volatile Volatile

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

PP1306b.jpg 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 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

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.