Transcript InstrCh32PPT2016

Plant Defense
Chapter 32
Plant Defense
32.1 Plants have evolved mechanisms to protect themselves from
infection by pathogens; i.e. viruses, bacteria, fungi, worms; even
parasitic plants
• Pathogens enter plants thru damaged tissue or stomata.
Potato Blight
Irish Potato Famine (1845-1852): An oomycete protest, Phytophthora infestans,
infected the potato crop and had a devastating effect on human
Entry for Pathogens
1st line of defense for plants:
• Epidermis (thick wall and waxy
cuticle
Stomata
Pathogens enter through:
• Wounds
• Stomata
Spread through:
• Vascular system
• Bacteria and fungi move in xylem
• Viruses move in phloem
Biotrophic pathogens :gain
resources from living cells
Necrotophic pathogens: kill cells
before using them
Phytophthora
infestans
Parasitic Plants
• Parasitic plants obtain resources by infecting other plants.
• These plants produce structures that dig into the stems or roots of other plants,
eventually tapping into the host’s vascular system
• Some (Rafflesia arnoldii) lack chlorophyll entirely; while others such as mistletoe
(genus Phoradendron) are photosynthetic
• Some, like Indian pipe (Monotropa uniflora) absorb from hyphae of mycorrhizae
• Currently, over 4000 species of parasitic plants have been identified.
Plant’s Immune System
•
•
•
A plant’s immune system allows it to detect and respond to pathogens.
Not all infections have a significant effect on the host plant.
a. 
Virulent pathogens are able to overcome the host’s defenses and lead
to disease.
b. 
Avirulent pathogens damage only a small part of the plant
Plants have an innate two-part immune system, one basal and one
specific
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–
•
•
•
•
•
The basal branch consists of receptors located on the plasma membrane.
These receptors recognize molecules (i.e., flagellin, chitin) generated by a broad class of
pathogens
The specific branch…..allows plants to resist specific pathogens.
It consists of receptors, called R proteins (for “resistance”), located inside
the cell as opposed to the plasma membrane (basal immunity).
Pathogens produce proteins called AVR proteins that enter into plant cells
and facilitate infection.
Each R protein recognizes a specific AVR foreign protein.
Because this branch of the immune system depends on interactions
between specific plant and pathogen genes, it is commonly called the genefor-gene model of plant immunity.
Basal resistance
Basal Immune System
Pathogen
Receptor
protein
Plant cell
Cell wall
Plasma
membrane
Defense
response
No infection
A receptor protein in the plant cell’s plasma
membrane binds with a pathogen-derived
molecule and triggers a defense response.
Specific Immune System
Specific resistance
Pathogens
secrete AVR
proteins that
can enter
plant cells.
AVR
protein
R proteins
No defense response
Defense response
Virulent infection
Avirulent infection
In the absence of a matching R
protein, the AVR protein blocks
the plant’s basal resistance.
When an R protein binds with an
AVR protein, it prevents the AVR
protein from blocking the plant’s
basal resistance and directly
activates defensive genes.
Hypersensitive Response
• Once a pathogen has been detected, plants protect themselves by:
a. Reinforcing their natural barriers, strengthening their cell walls, closing
stomata, and plugging their xylem
b. Producing an antimicrobial compound
c. Launching a hypersensitive response: actively killing the cells
surrounding the infection
Site of initial
infection
Vascular Wilt Disease
• Virulent pathogens that move through xylem cause a variety of disorders referred to
as vascular wilt diseases. .
• Plants can defeat pathogens by isolating infected regions to a far greater degree
than is possible in most animals.
• i.e. Plants can plug xylem conduits.
• If caught in time, this can prevent pathogens from spreading.
• In some cases, the plant will block its entire vascular system, causing its own death.
Bananas infected by fungus
Grapes infected by a bacterium
Systemic Acquired Resistance
•
.
•
Plants can acquire immunity called systemic acquired resistance (SAR) to
pathogens after being exposed to the pathogen in another part of the plant.
Plant pathologist A.F. Ross infected tobacco plants with TMV (tobacco mosaic virus),
which turned leaves a mottled yellow.
•
One week later, he exposed another leaf on the same plant to the virus.
•
This second leaf showed no visible signs of infection.
•
A signal has been transmitted from the originally infected leaf to the undamaged parts
of the plant
•
The transmitted signal then triggers the development of an immune response that
protects the plant from further infection.
Systemic Acquired Resistance
7 days
TMV
Tobacco
plant
Infect one leaf
with TMV.
The first leaf exposed
to TMV shows necrotic
regions resulting from a
hypersensitive
response.
Infect a second
leaf with TMV.
The second leaf
exposed to TMV
shows no visible
sign of infection
or disease.
Systemic Acquired Resistance
The first leaf
exposed to TMV
shows necrotic
regions resulting
from a
hypersensitive
response.
The second
leaf exposed to
TMV shows no
visible sign of
infection or
disease.
Levels of Plant Defenses
• Herbivory: animals eating plants
• Plant defenses are observed at multiple levels of biological
organization:
– Molecular
– Cellular
– Tissue
– Organ
– Organism
– Population
– Community
Molecular Level Defenses
•
•
Plants produce chemical compounds to deter attackers
Typically terpenoids, phenolics, and alkaloids
• Terpenoids: volatile, vaporize easily, are many of the essential oils
associated with plants; mimic insect hormones and cause insects to
molt prematurely and die
- The distinctive smells of lemon peel, mint, sage, menthol, pine
resins, and geranium leaves are all due to terpenes.
– While the odors are pleasant to us, they are feeding deterrents to
mammals such as squirrels and moose and they also obstruct
the growth and metabolism of both fungi and insects.
• Phenolics: i.e. tannins, have an unpleasant taste and stop
digestion of proteins
– Contained in unripe fruits
– Extracts from tree bark used by humans to process animal skins
by “tanning” leather.
Molecular Level Defenses C’td
• Alkaloids: damage the nervous system of animals; i.e. opium
poppy is the source of morphine, heroine, and codeine
- Accumulate in secretory cells called laticifers, which will ooze
milky-white latex (opium) when plant is damaged
- Commonly bitter tasting, i.e. nicotine, caffeine, morphine,
theobromine (found in chocolate), quinine (treatment for
malaria), strychnine, and atropine.
Opium poppy fruit
Plant Compounds Used in Medicine
Cellular Level Defenses
• Lactifiers and central vacuoles of plant cells: serve as storage places for
chemicals that deter herbivores
• Idioblasts: specialized cells in leaves and stems of taro plant (Colocasia
esculenta) that contain raphide crystals of calcium oxalate; these penetrate soft
tissue of making it easier for an irritant to cause swelling of attackers lips, mouth,
throat. Irritant is destroyed by cooking.
Raphide crystals
Tissue Level Defenses
•
Tough Sclerenchyma Fibers: some leaves are especially tough to chew
- From heavy growth of tough, hardened sclerencyma tissue
- i.e. olive leaf (Olea europaea)
Bright red cells with thick walls are tough sclerenchyma cells through vein of olive leaf
Organ Level Defenses
• Hairs: many times these hairs are also armed with chemical
irritants.
• Silica plates on grasses: wear down insect mouthparts so the
insects feed less efficiently and grow more slowly.
• Prickles, spines (modified leaves), or thorns (modified stems) on
tropical trees
• Shape of leaves: i.e. leaf of snowflake plant (Trevesia palmata)
looks like it has been partially eaten, making it less appealing
– i.e. some plants mimic insect eggs on their leaves (Passion Flowers)
Snowflake leaf
Cactus spines
Passion flower
Organismal Level Defenses
• Mechanical damage by plant can alter its entire physiology,
stopping further attack.
• i.e. a species of wild tobacco called Nicotinia attenuate changes the
timing of its flowers as a result of an attack. Normally flowers at
night, emitting acetone, which attracts hawk-moths as pollinators.
But, the moths lay eggs on leaves as they pollinate, and the larvae
are herbivores. When the plant becomes too larvae infested, they
stop producing acetone and open their flowers at dawn, when these
moths are gone. They are then pollinated by hummingbirds.
• Research has shown that the eating larvae trigger the huge change
in timing of flower opening.
Population Level Defenses
• Coordinated behavior at the population level
• Some plants can communicate by releasing molecules that warn
nearby plants of same species
• i.e. lima bean plants (Phaseolus lunatus) infested with spider mites
release a cocktail of chemicals that signal news of attack. The
noninfested lima bean plants make biochemical changes that make
them less susceptible to attack
• Masting: a population synchronously produces a massive amount
of seeds after a long interval. In this way, some seeds escape the
attackers attention.
• i.e. flowering bamboo plants
Community Level Defenses
• Some plants recruit predatory animals to help against hervbivores
• i.e. The leaf damaged by caterpillars releases compounds that attract parasitoid
wasps
• i.e. Parasitoid wasps inject their eggs into caterpillars feeding on plants, The
eggs hatch within the caterpillars, and the larvae eat through. The larvae form
cocoons on the surface of the host before emerging as adult wasps.
Symbiosis of Ants and Acacia
Ant-plants….such as the bullhorn acacia….provide food and shelter for ants, which
defend their host plant
Adaptation in Grasses
Leaf blade
Zone of cell division
and expansion
Apical meristem
Prior to flowering, the apical
meristem of grasses is close
to the ground, and there is a
persistent zone of cell division
and elongation at the base of
each leaf blade.
Therefore,
if the top
is cut off...
...the apical
meristem is not
damaged and the
leaf blades can
grow back.
Resources and Defense
32.3 The production of defenses is costly, resulting in trade-offs
between protection and growth.
Plants produce 2 types of defenses:
• Constitutive defenses…which are produced whether or not a threat is
present
• Inducible defenses….which are triggered when a plant detects that it is
being attacked
How to Attract Insect Allies
Plants produce volatile signals that attract insects that prey on herbivores
Damaged plants
(two leaves torn)
Experimental
plants
(undamaged)
Control plants
(undamaged)
Leaf tannins (% tannic
acid
equivalent/g dry weight)
Air flow
Control
plants
Damaged
plants
Experimental
plants
Growth and Defense Trade-off
Clay soil plants growing in sandy soils
Unprotected from herbivores
Protected from herbivores
Allocation to plant defense may be at the cost of slower plant growth
In the presence of
herbivores, plants
grew best in their
own soil type.
The species from clay soil
sites grew faster in both
habitats when protected
from herbivores.
Increase in leaf area
(cm2/day)
Growth
and
Defense
Trade-off
Clay origin
Sand origin
Unprotected
clay habitat
Unprotected
sandy
habitat
Protected
clay
habitat
Protected
sandy
habitat
“Escape and Radiate”
32.4 Interactions among plants, pathogens, and herbivores contribute to the
origin and maintenance of plant diversity.
• “Escape and radiate” form of plant evolution: plants undergo a burst of
diversification following evolution of a new form of defense
Seedlings of rare species are more
likely to survive because they have a
greater chance of dispersing away
from the build up of pathogens under
adult trees of the same species.
Seed of rare
species B
Seedling of
rare species B
Rare
species
B
Region where species B’s
pathogen is abundant
Seedlings of common
species are less likely to
survive because they have
a high probability of being
infected by pathogens.
Seed of
common
species A
Common
species A
Region where species A’s
pathogen is abundant
Janzen-Connell hypothesis proposes that interactions with pathogens and
herbivores increase plant diversity.
Protecting Crops from
Herbivores and Pathogens
• Herbivores and pathogens are a major concern for agriculture
• Crop protection includes the use of:
• Chemical pesticides
• Pest management
• Application of spores or toxins from Bacillus thuringiensis to plants
• Inserting genes that encode for toxins from B. thuringiensis to make Btmodified plants
Refuge,
non-Bt corn
Bt corn
Bt Crops
• Bacillus thuringiensis (Bt) is a soil-dwelling bacterium that produces proteins
toxic to insects. Used since 1920s
• In 1985, the genes that encode the toxins from Bt were inserted into tobacco.
• The introduction of Bt crops reduced the application of pesticides; however,
constant exposure to Bt toxins increases the probability that pest populations will
evolve resistance.
• To prevent the evolution of resistance, U.S. farmers are required by law to plant
a fraction of their fields with non-Bt-expressing plants.
• The proximity of non-Bt-expressing plants allows pests that are not resistant to Bt
toxins to survive and interbreed with individuals feeding on the Bt-expressing plants,
which slows the evolution of the Bt-resistance.
• Not sure if this is right procedure…the efficacy of this practice is called into question
by reports of Bt-resistant pests and the need to apply increasing amounts of industrial
pesticides to Bt crops.