Transcript Chp 40 Lecture Outline - McGraw

Plant Defense Responses
Chapter 40
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
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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
3
Physical Defenses
The attack threat is enhanced with nonnative
invasive species, who have no natural
predators in their new environment
Alfalfa plant bug
4
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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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
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Dermal Tissue System
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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
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Plant cell
membrane
Plant cell
Nutrient
transfer
Haustorium
Adhesion
pad
Germinating fungal spore
Fungal
hypha
Plant epidermal cell
Fungus entering stoma
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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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12
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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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Animals that Protect Plants
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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
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Animals that Protect Plants
1. A volatile
signal is
released as
the caterpillar
eats a leaf.
Volatile signal
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.
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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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
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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
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Systemic Response to Invaders
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Systemin
Cytoplasm
Lipase
Wounded
leaf
Systemin
release
Membranebound
receptor
Proteinase
inhibitors
Membrane
lipids
Free linolenic acid
Jasmonic acid
Signaling pathway
Nucleus
Activation of
proteinase
inhibitor genes
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Systemic Response to Invaders
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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Specific Defense Responses
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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.
avr
R
4. If binding occurs, the R gene product is
activated, triggering a protective hypersensitive response. If no binding occurs,
the plant succumbs to disease.
Hypersensitive
response
No disease
occurs
Virus
avr
Plant
develops
disease
R
Bacterium
avr
R
Fungus
Hypersensitive
response
No disease
occurs
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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
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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 34
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Hypersensitive Response (HR)
Systemic Acquired Resistance (SAR)
Local cell death seals off pathogen
Temporary broad-ranging
resistance to pathogen
Plant cells
Microbial
protein
HR
R protein
Plant cells
SAR
Signal
molecule
Signal
molecule
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