1. Plant Defense and Secondary metabolism
General introduction to plant defense and attraction

2. Multitrophic Interactions
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
For illustration, I have a resistant, tolerant, and susceptible (or non-defended) plant represented in this cartoon. In the presence of herbivores…
Resistant Tolerant Susceptible

11. Benefits of defense are obvious in the presence of herbivores
Resistant vs Tolerant
Benefits of defense are obvious in the presence of herbivores
The benefit of investing in defense is increased fitness in the presence of herbivores relative to susceptible genotypes.
Resistant Tolerant Susceptible

12. Costs of defense are obvious in the absence of herbivores
Resistant vs Tolerant
Costs of defense are obvious in the absence of herbivores
However, the production and maintenance of defense traits and mechanisms may incur a cost to the plant when herbivores are rare or absent. So resistant and tolerant plants would be expected to have lower fitness than non-defended plants when herbivores are absent.
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
E.g., Tannins

16. Cutin, Waxes, Suberins
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
They are made of hydrophobic compounds which have water-repelling properties
These compounds are non-polar
Fatty acids are one type of hydrophobic compound

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.

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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. Secondary metabolites
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 competition 4. They are formed from the byproducts or intermediates of primary metabolism

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

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

25. Important part of plant metabolism is geared towards plant defense

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

29. Terpene functions Growth and development
carotenoid pigments are tetraterpenes
chlorophyll side chain is diterpene
giberellins (hormones) are diterpenes
abscissic acid (hormone)
sterols are triterpenes

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

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Non-volatile
Volatile

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

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

34. Terpenes such as pyrethrum (from chrysanthemums) الاقحوانand azadirachtin (from the Asian and African Neem tree) can be used as “natural” insecticides in agricultural practices or in horticulture.

36. Terpenes as human medicinal drugs
Limonene – monoterpenoid dietary anticarcinogen
Artemisnin – sesquiterpenoid antimalarial
Taxol - diterpenoid anticancer drug from Pacific yew (Taxus brevifolia) خشب الطقسوس

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

38. Phenolic Compounds
Secondary metabolites which contain a hydroxyl functional group on an aromatic ring
Many serves as defense compounds against herbivores and pathogens
Other function in attracting pollinators .

39. Types of Phenolic Compounds
Benzoic acid derivatives
Caffeic acid
Flavones
Isoplavonoids
Flavonoids
Lignin

40. 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

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42. ALKALOIDS Large pharmacological effects on animals
Most effective at deterring mammalian herbivores
Often alkaloids are used as medicines for humans
Some examples: morphine, codeine, and scopolamine
cocaine, nicotine, and caffeine used as stimulants and sedatives.

43. 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.

44. 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)

45. The degradation process is stimulated by herbivore feeding
Cyanogenic Glycosides
The degradation process is stimulated by herbivore feeding
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46. Cyanogenic Glycosides
S. American native peoples eat cassava (Manihot esculenta), has high levels of cyanogenic glycosides. Chronic cyanide poisoning are not uncommon.
PP13200.jpg

47. 3. GLUCOSINOLATES
These compounds release volatile defensive substances, “mustard oils”, (often herbivore repellents)
Plants like cabbage, broccoli, and radishes (Brassicaceae family) have these.

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49. Functions of Secondary Metabolites in Plants
The secondary metabolites have no function in the physiology of the plants
They are formed as a result of an overspill from the primary metabolism
They make a valuable contribution to the relationship between plant and their environment
Plant utilized secondary metabolites as antibiotics or signaling agent during the interaction with pathogen (SAR and Agrobacterium)
They play an important role in two resistance strategies:
a. structural level, phenyl propanoids are the major
component of wall polymers lignin and suberin
b. Inducible defence antibiotics originated from phenolics,
and terpenoids (phytoalexins)

50. Herbivore damage can elicit a Signaling Pathway (Induced Defenses)
Resistance Traits:
Indirect Defense
Induced defenses:
a. Recognition of the pathogen by the host plant; carbohydrates, fatty acids released by fungi
b. Transmission of alarm signal to host; Ca, hydrogen peroxide, enzymes.
Herbivore damage can elicit a Signaling Pathway (Induced Defenses)

51. Induced structural defenses
cytoplasmic -cytoplasm surrounds hyphae (e.g., Armillaria),
cell wall thickening
histological (cork layers, adventitious roots),
tyloses and gums,
necrotic defense (hypersensitive response).

52. Induced biochemical defenses
Hypersensitive reactions (phytoalexins, antimicrobials (important with obligates parasites – rusts, leaf spots, active oxygen radicals disrupt cell membranes, reinforcement of host cell walls)
Antimicrobials – phytoalexins, phenolics
Immunization
Local and systemic acquired resistance

53. (Phyto = “plant” and alexin = “to ward off/”)
Phytoalexins
(Phyto = “plant” and alexin = “to ward off/”)
Low molecular mass antimicrobial metabolites synthesized de novo from primary metabolites in response to infection
Structurally diverse group of metabolites with the isoflavonoids can be an example
The isoflavonoids phytolaexins are synthesized from the flavonoids branch of the phenylpropanoid pathways

54. PRODUCTION OF PHYTOALEXINS
Production of phytoalexins may be stimulated by certain compounds called elicitors.
High molecular weight substances found in the cell wall such as glucans, glycoprotein, or other polysaccharides
Gases such as ethylene (C2H4)
In susceptible plants, a pathogen may prevent the formation of phytoalexins, by the action of suppressors produced by the pathogen
The suppressor also can be a glucan, a glycoprotein, or a toxin produced by the pathogen

55. 2. H2O2 (hydrogen peroxide)
Secondary Signals
2. H2O2 (hydrogen peroxide)
Plays multiple roles:
induces defense-related genes
induces apoptosis
causes cross-linking of cell wall proteins (more resistant to wall-degrading enzymes)
may directly kill pathogens

56. Secondary Signals 3. Salicylic acid required for SAR
levels increase locally and at distance from infection
Systemic Signal? Probably not. Still unknown

57. Signaling Cascade for Defense Responses
Model derived mostly from studies in cell culture using specific elicitors.
However, there is evidence for induction in intact plants by R genes. Some aspects are also constitutive and help block most microbes (non-host resistance).

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59. Systemic Acquired Resistance
Systemic acquired resistance causes systemic expression of defense genes and is a long-lasting response
Salicylic acid is synthesized around the infection site and is likely the signal that triggers systemic acquired resistance
When a plant survives the infection of a pathogen at one site it can develop increased resistance to subsequent attacks.
Although plants don’t have “immune systems” they have signaling mechanisms that can act in this way.

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61. Genetice model for SAR Infected Tissue Non-infected Tissue INA BTH
Isd6
Cpr1?
Isd2
Isd4
nim1
npr1
Ndr1?
Cpr1
Cin2
Cin3
NahG
Local
Acquired
Resistance
Plant Pathogen
Interaction
Salicylic
Acid
SAR gene
Expression
Necrosis
INA
BTH
Non-infected Tissue
nim1
npr1
NahG
Systemic
Acquired
Resistance
Salicylic
Acid
SAR gene
Expression