Plant Defense and Secondary metabolism

1. Plant Defense and Secondary metabolism

2. Plant Defense: Multitrophic Interactions Secondary carnivores Carnivores Herbivores Pollinators Shoots and flowers Competitor plants Pathogens Aboveground Belowground Roots Pathogens Parasitic plants Herbivores Symbionts Carnivores Modified from Bruinsima & Dicke 2008

3. Plants respond to attacks by herbivores and pathogens Plants use defense systems to deter herbivory, prevent infection, and combat pathogens • Herbivory, animals eating plants, is a stress that plants • Plants counter excessive herbivory with physical defenses such as thorns and chemical defenses such as distasteful or toxic compounds • Some plants even “recruit” predatory animals that help defend against specific herbivores

4. Plant Response to Herbivores Plant Defense Traits

5. Plant Response to Herbivores • Plants damaged by insects can release volatile chemicals to warn other plants of the same species • These volatile molecules can also function as an “early warning system” for nearby plants of the same species. • Methyljasmonic acid can activate the expression of genes involved in plant defenses

6. Jasmonic Acid • Levels of jasmonic acid rise in response to damage • This hormone can trigger many types of plant defenses including terpenes and alkaloids • The action of jasmonic acid induces the transcription of many genes involved in plant defense • Jasmonic acid turns on genes for proteinase inhibitor.

7. Systemic Response

8. Plant defense traits Plants use a variety of mechanical (toughness, spines), chemical (alkaloids, phenolics, terpenoids, latex – the realm of chemical ecology), developmental, and phenological defenses Defenses may also be classified with reference to their production: 1. Constitutive – produced by & present in the plant irrespective of attack 2. Induced – produced by & present in the plant in response to attack

9. Plant defense traits • Resistance traits • Those traits that “reduce herbivory” • Avoidance (antixenosis) traits • Those traits that “affect herbivore behavior;” i.e., deter or repel herbivores • b. Antibiosis traits • Those that “reduce herbivore performance” • 2. Tolerance traits • Those traits that “reduce the impact of herbivory on fitness”

10. Resistant vs Tolerant Resistant Tolerant Susceptible

11. Resistant vs Tolerant Benefits of defense are obvious in the presence of herbivores Resistant Tolerant Susceptible

12. Resistant vs Tolerant Costs of defense are obvious in the absence of herbivores Resistant Tolerant Susceptible Slide courtesy of Alyssa Stocks Hakes; modified from the original

13. Resistance Traits DIRECT INDIRECT Slide courtesy of Amanda Accamando; modified from the original

15. Direct Defense Morphological Characters Secondary metabolites Metabolites E.g., Tannins

16. Cutin, Waxes, Suberins • They are made of hydrophobic compounds which have water-repelling properties • These compounds are non-polar • Fatty acids are one type of hydrophobic compound All plant parts exposed to the atmosphere are coated with layers of lipid material that reduce water loss and help block the entry of pathogen fungi and bacteria The principal types of coating are cutin, suberin and waxes

17. Cutin • It is found most above ground • It is a macromolecule, a polymer composed of long fatty acid chains that are attached to each other by ester linkage, creating a rigid three dimensional network • It was a major component of plant cuticle, a multilayered secreted structure that coats the outer cell wall of epidermis on the areal parts • Plants’ cuticles is composed of a top coating of wax, often vary with the climate in which they live.

19. Waxes • Complex mixtures of long-chain lipids that are extremely hydrophobic. • The most common components of waxes are straight chain alkanes and alcohol of 25 to 35 carbon atoms. • They are synthesized by epidermal cells. • They exuded through pores in the epidermal cell wall by an unknown mechanism.

20. Suberin • It was formed from fatty acids but has a different structure from cutin. • It was often within roots. • It can protect against pathogens and other damage. • It can form transport barriers between the soil and the roots • Older parts of roots more suberized • A cell wall constituent • Endodermis has suberin side walls

22. Organic compounds that appear to have no direct function in photosynthesis, growth, or respiration, but 1. They protect primary metabolism by deterring herbivores, reduce tissue loss and avoid infection by microbial pathogen 2. They also attract pollinators and seed-dispersing animals, 3. They act as an agent of plant-plant competition4. They are formed from the byproducts or intermediates of primary metabolism Secondary metabolites End points of metabolism with no strictly defined function

23. Three Principal Groups of Secondary Metabolites • Terpenes Lipid synthesized from acetyl CoA or from basic intermediates of glycolysis • Phenolic compounds Aromatic substances formed via the shikimic acid pathway or the malonic acid pathway • Nitrogen containing secondary products (alkaloids) Alkaloids which are synthesized primary from amino acids

26. Terpenes • They produced from the mevalonic acid pathway • They function as herbivore deterrents • They can be produced in response to herbivore feeding, and to attract predatory insects and parasites of the feeding herbivore. • They are constituents of essential oils • Building block- 5 C isoprene unit • They are classified by the number of isoprene units: monoterpenes-1, diterpenes-4

29. Isoprene The basic building block of the terpenes (terpenes also called “isoprenoids”) H3C CH CH CH2 Terpenes: 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 H2C

30. 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

31. 2. Defensive compounds • Toxins and feeding deterrents to insects and mammals • Examples • 1. Resins of conifers are monoterpenes • 2. Essential oils - peppermint, limon,

32. Non-volatile Volatile

33. Non-volatile terpenes – limonene apparently distasteful to herbivores

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

35. 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.

37. 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

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

39. Phenolics • Plants produce a variety of compounds that contain one or more phenol groups - called phenolics • Thousands of phenolics occur in plants

40. Phenolic Compounds • Secondary metabolites which contain a hydroxyl functional group on an aromatic ring • They are heterogenous group: • Some are water soluble only in organic solvents • Some are water soluble carboxylic acids and glycosides • Some are insoluble polymer • Many serves as defense compounds against herbivores and pathogens • Other function in attracting pollinators and fruit dispensers

41. Types of Phenolic Compounds • Benzoic acid derivatives • Caffeic acid and other simple phenylpropanoids • Flavones • Isiplavones (Isoplavonoids) • Flavonoids • Lignin

42. N-containing secondary compounds • Those are encountered less commonly in plants than the phenolics and terpenoids • Those are important in view of their bioactivity as drugs and toxins • They are synthesized from aliphatic and aromatic amino acids • Aliphatics via TCA cycle • Aromatics via shikimic acid pathway

43. Classes of N-containing 2o compounds • Alkaloids, • Cyanogenic glycosides, • Glucosinolates, • Nonprotein amino acids

44. 1. ALKALOIDS • The most important nitrogen containing secondary products • They are being found in more than 15,000 compounds found in 20% of vascular plants. • Nitrogen is usually part of a heterocyclic ring with N and C atoms

46. ALKALOIDS • Large pharmacological effects on animals • Most effective at deterring mammalian herbivores • Livestock deaths due to over-consumption of alkaloid containing plants such as lupines and groundsels • Often alkaloids are used as medicines for humans • Some examples: morphine, codeine, and scopolamine • cocaine, nicotine, and caffeine used as stimulants and sedatives.

47. Wild tobacco (Nicotiana sylvestrus) Wild tobacco can “sense” which herbivore is feeding on it. It normally produces nicotine (an alkaloid) in response to herbivore feeding. But if nicotine-tolerant caterpillars are feeding, the tobacco produces terpenes instead. These terpenes can attract the predators of the herbivore.

48. 2. CYANOGENIC GLYCOSIDES • Release the toxic gas hydrogen cyanide. • plants must have enzymes to break down the compounds and release a sugar molecule yielding a compound that can decompose to form HCN. • glycosides and enzymes which break them down are usually spatially separated (in different cellular compartments or different tissues)

49. The degradation process is stimulated by herbivore feeding Cyanogenic Glycosides

50. S. American native peoples eat cassava (Manihot esculenta), has high levels of cyanogenic glycosides. Chronic cyanide poisoning are not uncommon. Cyanogenic Glycosides