1. Plant Defense Responses
Chapter 40

2. Physical Defenses
Winds can uproot a tree, or snap the main shoot of a small plant
-Axillary buds give plants a second chance as they grow out and replace the lost shoot

3. Physical Defenses
Biotic factors can be more detrimental to plants than abiotic factors
-These can tap into nutrient resources of plants or use their DNA-replicating mechanisms to self-replicate
-Some kill plant cells immediately, leading to necrosis

4. Physical Defenses
The attack threat is enhanced with nonnative invasive species, who have no natural predators in their new environment
Alfalfa plant bug

5. Dermal Tissue System The first-line defense of all plants
Epidermal cells throughout the plant secrete a variety of lipid material that protects plant surfaces from water loss and attack
-Wax, cutin, and suberin
Silica inclusions, trichomes, bark and even thorns can also offer protection

6. Dermal Tissue System These exterior defenses can be penetrated
-Mechanical wounds allow microbial entry
-Bacteria can cause damage because they provide sites for ice nucleation
-Parasitic nematodes use their sharp mouth parts to get through the plant cell walls
-Some form tumors on roots

7. Dermal Tissue System

8. Dermal Tissue System
Fungi seek out the weak spot in the dermal system, or stomata, to enter the plant
The phases of fungal invasion:
1. Windblown spore lands on leaves
2. Spore germinates & forms adhesion pad
3. Hyphae grow through cell walls and press against cell membrane
4. Hyphae differentiate into haustoria

9. Nutrient transfer Plant cell membrane Fungal hypha Haustorium Adhesion
Fungus entering stoma
Plant epidermal cell
Germinating fungal spore
Adhesion
pad
Haustorium
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10. Beneficial Microorganisms
Fungi and bacteria can also be beneficial to plants
-Mycorrhizal fungi
-Nitrogen-fixing bacteria
-Plant growth-promoting rhizobia (PGPR)
-These provide various nutrients for plants

11. Toxin Defenses
Many plants produce toxins that kill herbivores or make them ill, or repel them with strong flavors or odors
Metabolic pathways needed to sustain life in plants have also lead to the production of secondary metabolites
-Many of these affect herbivores as well as humans

14. Toxin Defenses
Protective secondary metabolites include alkaloids (caffeine, nicotine), tannins & oils
-Wild species of tobacco have elevated levels of nicotine that are lethal to tobacco hornworms

15. Toxin Defenses Plants protect themselves from toxins in two main ways
1. Sequester a toxin in a membrane-bound structure
2. Produce a compound that is not toxic until it is metabolized by attacking animal
-Cyanogenic glycosides break down into cyanide (HCN) when ingested

16. Toxin Defenses
Allelopathic plants secrete chemicals to block seed germination or inhibit growth of nearby plants
-This strategy minimizes competition for resources
-Very little vegetation grows under a black walnut tree

17. Toxin Effects on Humans
Throughout history, humans have been intentionally poisoned with plant products
-Socrates died after drinking a hemlock extract containing nerve-paralyzing alkaloid
-In 1978, Georgi Markov, a Bulgarian dissident, was assassinated by KGB officers using ricin

18. Toxin Effects on Humans
Ricin is an alkaloid produced by the castor bean plant (Ricinus communis)
-It is six times more lethal than cyanide and twice as lethal as cobra venom
-It functions as a ribosome-binding protein that inhibits translation

20. Plants with Medicinal Value
Many secondary metabolites have benefits to human health
Phytoestrogens of soy plants
-Appear to lower the rate of prostate cancer in Asian males
-However, questions have been raised about their effect on developing fetuses
-Also on babies consuming soy-based formula

21. Plants with Medicinal Value
Taxol of Pacific yew trees
-Fights cancers, especially breast cancer
Quinine of Cinchona trees
-Effective against malaria, which is caused by four species of Plasmodium
-Blocks DNA replication
-Also leads to build-up of toxic hemes that poison the parasite

22. Animals that Protect Plants
Complex coevolution of plants and animals has resulted in mutualistic associations
-Relationships that benefit both
Acacia trees and ants
-Small armies of ants protect Acacia trees from harmful herbivores
-Plant provides ants with food and shelter

23. Animals that Protect Plants

24. Animals that Protect Plants
Parasitoid wasps, caterpillars and leaves
-As caterpillar chews away, a wound response in the plant leads to release of a volatile compound
-Female parasitoid wasp is attracted
-Lays fertilized eggs in caterpillar
-Eggs hatch and larvae kill caterpillar

25. Animals that Protect Plants
1. A volatile signal is released as
the caterpillar
eats a leaf.
2. Female wasp is attracted by the volatile signal,
finds caterpillar, and lays eggs.
3. Wasp larvae
feed on the
caterpillar and then emerge.
4. Larvae continue to feed on
the caterpillar after it dies,
but not the plant. The larvae then spin cocoons to pupate.
Volatile signal
Larvae

26. Systemic Response to Invaders
Static plant responses to threats have an energetic downside
-Are maintained in the presence or absence of threat
Energy resources would be conserved if the plant response was inducible
-Defenses launched only when needed

27. Systemic Response to Invaders
A wound response occurs when a leaf is chewed or injured
-Leads to rapid production of proteinase inhibitors throughout the plant
-Bind to digestive enzymes in the gut of the herbivore

28. Systemic Response to Invaders
The signaling pathway involves four steps:
1. Wounded leaves produce an 18-amino acid peptide called systemin
2. Systemin moves throughout the plant in the phloem
3. Cells with receptors produce jasmonic acid
4. Jasmonic acid turns on genes for proteinase inhibitor

29. Systemic Response to Invaders
Wounded
leaf
Systemin
release
Lipase
Membrane
lipids
Free linolenic acid
Jasmonic acid
Signaling pathway
Activation of
proteinase
inhibitor genes
Proteinase
inhibitors
Cytoplasm
Nucleus
Membrane-
bound
receptor
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30. Systemic Response to Invaders
Salicylic acid is another molecule involved in the wound response

31. Specific Defense Responses
H. H. Flor’s gene-for-gene hypothesis
-Plants have a plant resistance gene (R); pathogens have an avirulence gene (avr)
-It is the recognition of the gene products (i.e. proteins) that is critical
-If binding occurs, a protective hypersensitive response develops
-If no binding occurs, the plant succumbs to disease

32. Specific Defense Responses
1. Pathogen
enters cell.
2. Proteins are
released into cell
by pathogen.
3. R gene products from
the plant cell bind to
avr gene products.
4. If binding occurs, the R gene product is
activated, triggering a protective hyper-
sensitive response. If no binding occurs,
the plant succumbs to disease.
Virus
Bacterium
Fungus
avr R
Hypersensitive
response
No disease
occurs
Plant
develops
disease
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33. Specific Defense Responses
The hypersensitive response leads to a very rapid cell death around the site of attack
-This seals off the wounded tissue to prevent the pathogen or pest from moving into rest of the plant
Antimicrobial agents produced include
-Hydrogen peroxide and nitric oxide
-Phytoalexins

34. Specific Defense Responses
Plants can also undergo a systemic response called systemic acquired resistance (SAR)
-Long-distance inducer is likely salicylic acid
-At the cellular level, jasmonic acid is involved in SAR signaling
-SAR allows the plant to respond more quickly to a second attack
-However, it is neither as specific nor as long-lasting as mammalian responses

35. Temporary broad-ranging resistance to pathogen
Plant cells HR
R protein
Microbial
protein
Hypersensitive Response (HR)
Local cell death seals off pathogen
Systemic Acquired Resistance (SAR)
Signal
molecule
SAR
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Temporary broad-ranging
resistance to pathogen