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CHAPTER 40
LECTURE
SLIDES
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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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• Even greater daily threats exist in the form
of viruses, bacteria, fungi, animals, and
other plants
– 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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• One of the greatest problems with
nonnative invasive species, such as the
alfalfa plant bug, is the lack of natural
predators in the new environment
Alfalfa plant bug
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• Dermal tissue system
– First-line defense of all plants
– Epidermal cells throughout the plant secrete
wax to protect plant surfaces from water loss
and attack
– Above-ground parts also covered with cutin
– Suberin is found in cell walls of subterranean
plant organs
– 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
– Parasitic nematodes use their sharp mouth
parts to get through the plant cell walls
• Some form tumors on roots
– In some cases simply having bacteria on the
leaf surface can increase the risk of frost
damage
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• Fungi seek out the weak spot in the
dermal system, or stomata, to enter the
plant
• Phases of fungal invasion
1. Windblown spore lands on leaves
2. Spore germinates and forms adhesion pad
3. Hyphae grow through cell walls and press
against cell membrane
4. Hyphae differentiate into haustoria
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• Fungi and bacteria can also be beneficial
to plants
– Mycorrhizal fungi
– Nitrogen-fixing bacteria like Rhizobium
– Plant growth-promoting rhizobia (PGPR)
• Bacteria provide substances that support plant
growth
• Can also limit the growth of pathogenic soil
bacteria
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Toxin Defenses
• Many plants produce toxins that kill
herbivores, make them ill, or repel them
with strong flavors or odors
• Some are unique to plants
• Defensins are found in plants and animals
– Small, cysteine-rich peptides with
antimicrobial properties
• Reveals ancient origin of innate immunity
– In some cases defensins limit protein
synthesis
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• Secondary metabolites
– Metabolic pathways needed to sustain life are
modified
– Alkaloids
• Wild tobacco has elevated nicotine levels lethal to
tobacco hornworms
– Tannins
– Animals, including humans, can avoid many
of the cumulative toxic effects of secondary
metabolites by eating a varied diet
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• 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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• 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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• 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
• A pinhead-sized metal sphere was injected from
an umbrella tip into his thigh
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• 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
– A single seed can kill a small child
– It functions as a ribosome-binding protein that
inhibits translation
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• 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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• 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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Ants attacking a katydid to protect “their” Acacia
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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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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
– One outcome leads to rapid production of
proteinase inhibitors throughout the plant
– Bind to digestive enzymes in the gut of the
herbivore
– Signaling pathway involves
• Jasmonic acid
• Salicylic acid
• Cell fragments
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• Wound response signaling pathway
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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• 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, plant can mount defenses
that keep pathogen avirulent
– If no binding occurs, the plant succumbs to
disease
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• Recognition of the pathogen by the R
gene product leads to hypersensitive
response
– Leads to a very rapid cell death around the
site of attack
– Also to longer term, whole plant resistance
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• Rapid cell death due to hypersensitive
response
– Seals off the wounded tissue to prevent the
pathogen or pest from moving into rest of the
plant
– Hydrogen peroxide and nitric oxide produced
• May signal cascade of chemical events resulting in
localized host cell death
– Phytoalexins – antimicrobial chemical defense
agents
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• Systemic acquired resistance (SAR)
– systemic response by plants
– Several pathways lead to broad-ranging
resistance that lasts for a period of days
– 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
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