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

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

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Physical Defenses

The attack threat is enhanced with nonnative invasive species, who have no natural predators in their new environment

Alfalfa plant bug

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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

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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

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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

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Nutrienttransfer

Plant cellmembrane

Plant cell

Fungalhypha

Fungus entering stoma

Plant epidermal cell

Germinating fungal spore

Adhesionpad

Haustorium

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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1. A volatile signal is released asthe caterpillareats a leaf.

2. Female wasp is attracted by the volatile signal,finds caterpillar, and lays eggs.

3. Wasp larvaefeed on thecaterpillar and then emerge.

4. Larvae continue to feed onthe caterpillar after it dies,but not the plant. The larvae then spin cocoons to pupate.

Volatile signal

Larvae

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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

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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

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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

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Woundedleaf

Systeminrelease

Systemin

Lipase

Membranelipids

Free linolenic acid

Jasmonic acid

Signaling pathway Activation ofproteinaseinhibitor genes

Proteinaseinhibitors

Cytoplasm

Nucleus

Membrane-boundreceptor



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Salicylic acid is another molecule involved in the wound response

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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

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1. Pathogen enters cell.

2. Proteins arereleased into cellby pathogen.

3. R gene products from the plant cell bind to

avr gene products.

4. If binding occurs, the R gene product isactivated, triggering a protective hyper-sensitive response. If no binding occurs,the plant succumbs to disease.

Virus

Bacterium

Fungus

avr

avr

R

R

R

Hypersensitiveresponse

No diseaseoccurs

No diseaseoccurs

Hypersensitiveresponse

Plantdevelopsdisease

avr


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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

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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

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Plant cells Plant cells

HR R protein

Microbialprotein

Hypersensitive Response (HR)

Local cell death seals off pathogen

Systemic Acquired Resistance (SAR)

Signalmolecule

Signalmolecule

SAR


Temporary broad-rangingresistance to pathogen

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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