Download CHAPTER SYNOPSIS

CHAPTER 40: PLANT DEFENSE RESPONSES
WHERE DOES IT ALL FIT IN?
Chapter 40 is a follow-up of Chapter 36 and builds up the general information on green plants
provided in Chapter 30. A quick summary of Chapter 30 is essential for success at covering Chapter
40. In addition, students should be encouraged to recall the principles of eukaryotic cell structure and
evolution associated with the particular features of plants.
SYNOPSIS
Plants, like all species, are under constant attack by other living species, especially herbivores
and microorganisms. They naturally possess a wide variety of physical self defense mechanisms,
most notably, thick cell walls and tough, waxy cuticles. Many species also have trichomes,
thorns, and durable bark of various thicknesses. If these primary defenses are inadequate, many
species possess a variety of toxic chemicals that further protect them from harm. As maintenance
of chemical arsenals is energetically costly, some plants have evolved some rapid response
defenses that may even protect them against future attacks. Presence of axillary buds may allow
plants some level of recovery following some types of damage.
Microorganisms as bacteria, fungi, and viruses as well as insects and other animals compete for
the nutrients in plant tissues. Viruses, furthermore, direct plants’ DNA replicating programs to
synthesize viral DNA. Nematodes may induce rapid mitoses and the generation of giant cells or
tumors through which they acquire high amounts of food, especially carbohydrates. Mechanical
wounding by wind, rain, and other agents allows easier entry by some pathogens while others
gain entry through stomata. Invasion and infection processes are both complex and varied. Lack
of natural enemies for many pest species allows their populations to increase with minimal
limitations.
Many plants can harm, if not kill, their enemies with chemical toxins. More than 3000 known
species of plants contain cyanide-containing compounds. Some plants produce secondary
metabolites including such alkaloids as nicotine, morphine, caffeine, cocaine and tannins that
minimize or prevent herbivory. These toxins do not harm the plants themselves as they either are
contained in membrane-bound vacuoles or their toxicity is deferred until other organisms ingest
them. Some plant species are allelopathic. They secrete certain chemicals through their roots.
These chemicals prevent germination by other plants of the same or different species, and
thereby prevent competition.
Plant toxins have been used throughout history to harm and kill humans. Today, plant secondary
metabolites are being researched for their potential benefits for human health, but this research is
difficult as these compounds evolved as self-defenses against herbivory. Furthermore, some
plants synthesize many such metabolites. Some plants, including soy, contain phytoestrogens
that are similar to the human hormone estrogen, and can bind with estrogen receptors. However,
as hormone-signaling mechanisms are complex, much more research is needed in understanding
how these compounds may affect human physiology and development. Some are known to cross
the placenta during the second trimester of pregnancy while other soy phytoestrogens may lower
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rates of prostate cancer. Discovery of a secondary metabolite, taxol, in a species of a Pacific yew
tree generally considered as trash, has lead to greater awareness of the potential pharmaceutical
value of all plant species. Taxol, now artificially synthesized, is very effective against breast
cancer. Use of quinine that is extracted from the bark of Cinchona trees has been used for
centuries by the Incas of Peru against malaria. British soldiers in India also used it during the 19th
century. Quinine, along with several other anti-malaria drugs, has since been synthesized.
However, malaria has since resurged in many places in the world, especially in sub-Saharan
Africa. The malarial agent, four types of plasmodia, has developed resistance to the synthetic
drugs, and quinine is again the drug of choice. Genetic engineering on the plasmodium as well as
the Anopheles mosquito vector, may lead to new strategies to combat malaria. Meanwhile,
preventing contact by using insecticide-treated netting to prevent mosquito bites is effective.
Greater efficiency of self-defense chemical toxins would occur if, rather than plants’ maintaining
a ready and constant supply, the chemicals were produced on demand, in other words, if the
toxic chemical response were inducible. This occurs with wound responses that occur when
leaves are chewed or otherwise injured. Proteinase inhibitors are rapidly produced throughout the
affected plant in response to a localized injury that triggers the release of a small peptide
signaling molecule, systemin. Since proteinase inhibitors are produced as a generalized response
to any wounding, even mechanical, experiments must be conducted very carefully to preclude
confounding information.
H.H.Flor first described host-pathogen specific responses about fifty years ago. He determined
that many plants have specific resistance genes (R) whose products interact with those of specific
avirulence genes (avr) in certain pathogens. This finding is known as the gene-for-gene
hypothesis, and several pairs of R and avr genes have been cloned for potential addition to
cultivated crops as natural biocides. The R gene triggers the hypersensitive reaction (HR) that
leads to rapid death of cells around the invasion site, thereby walling off the pest. It also triggers
longer-term resistance, known as systemic acquired resistance (SAR), in some species. SAR
allows plants to respond more quickly if attacked again; however, SAR is neither pest-specific
nor permanent. If no R genes are present, the HR does not occur. Phytoalexins are another group
of anti-microbial chemicals that plants may produce in response to attack.
Co-evolution of plants and some other species may lead to protection of both species from harm.
Stinging ant species that live in enlarged thorns and protect the plants from other insect pests
inhabits some Acacia tree species. Flowers that need bees for pollination appear to produce and
secrete a chemical that deters the ants but not the bees. Another example is that of parasitoid
wasps that respond to volatile chemicals released from leaves being fed upon by caterpillars.
These wasps lay their eggs in the caterpillars. Upon hatching, the larvae consume their
caterpillar hosts.
LEARNING OUTCOMES
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Know the physical and physiological adaptations that naturally protect plants against
predation, invasion, and parasitism by other species.
Name and describe the metabolic adaptations that protect some plants from various stresses.
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Describe why research on potential human benefits of secondary plant metabolites is difficult
to perform.
Explain the gene for gene hypothesis, and relate it to HR.
Describe some examples of interspecies coevolution of mutually beneficial survival
strategies.
COMMON STUDENT MISCONCEPTIONS
There is ample evidence in the educational literature that student misconceptions of information
will inhibit the learning of concepts related to the misinformation. The following concepts
covered in Chapter 40 are commonly the subject of student misconceptions. This information on
“bioliteracy” was collected from faculty and the science education literature.
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Students are unaware that plants develop environmental adaptations
Student believe that plants only use passive defense mechanisms
Students believe that plants lack tissues and organs
Students are unaware of all of the functions of stems
Students are unaware of all of the functions of leaves
Student are unfamiliar with the chemistry of plant defensive chemicals
Students believe that all plants are safe for herbivores to eat
INSTRUCTIONAL STRATEGY PRESENTATION ASSISTANCE
Emphasize that, in plants as in all species, physical barriers are the first line of defense against
stress, including attacks by other species. Challenge students to recap the nature of these barriers.
Explain that a special, disciplinary branch of science, phytopathology, staffed by
phytopathologists, exists, allowing scientists to study plant disease caused by both biotic and
abiotic agents. See the website for the American Phytopathological Society (APS), and consider
showing the APS film “Healthy Plants – Our Future”. Other teaching materials are available
from APS.
Have students compare the self-defense strategies of plants with those of humans and/or various
other animals.
HIGHER LEVEL ASSESSMENT
Higher level assessment measures a student’s ability to use terms and concepts learned from the
lecture and the textbook. A complete understanding of biology content provides students with the
tools to synthesize new hypotheses and knowledge using the facts they have learned. The
following table provides examples of assessing a student’s ability to apply, analyze, synthesize,
and evaluate information from Chapter 40.
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Application
Analysis
Synthesis
Evaluation
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Have students describe why plant defenses against insects also ward off
viral diseases.
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Ask students to explain why many plant defense chemicals are found in
seeds.
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Ask students to explain the possible organism that poison ivy defends
itself against using the irritating oils.
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Have students explain relationship between plant structure and the
defense used to protect the structure.
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Have students explain how a plant would have the need to defend itself
from other plants.
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Have students compare the relative effectiveness of dermal defenses
versus chemical defenses to protect leaves.
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Ask students design an experiment to determine if prior exposure to a
disease induces stronger defenses in a plant.
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Have students design an experiment to test is damaged plants are able to
stimulate stronger defenses in nearby plants.
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Ask the students develop a commercial application for the knowledge that
plant toxins called lectins are capable of killing fungi that attack plants.
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Ask students evaluate the benefits and consequences of breeding crop
plants so that they are lacking their natural defenses.
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Ask students to evaluate the feasibility of using natural plant defenses to
replace traditional pesticides used in agriculture.
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Ask students to evaluate benefits and risks of using defensive plant
chemicals as medicine.
VISUAL RESOURCES
Show images of plant defenses.
Bring in or show images of various nutriceuticals or medicinal compounds made from plant
defense mechanisms.
Bring in common house plants and foods that exhibit plant defenses.
IN-CLASS CONCEPTUAL DEMONSTRATIONS
A. Ants and Plants
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Introduction
This video demonstration shows students how Azteca ants are exploited by plants as a
defense mechanism.
Materials
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Computer with Media Player and Internet access
LCD hooked up to computer
Web browser linked to You Tube Broadcast at
http://www.youtube.com/watch?v=v1CT7ZsG7O0
Procedure & Inquiry
1. Explain to the students that you want to view the interrelationship between a tropical ant
called the Azteca ant and plants.
2. Tell the students to pay close attention to the habitat and habits of the ant.
3. Load up the website and play the Azteca 1 video
4. Ask the students to assess the habitat of the ant and what is the relationship of plants to the
ant.
5. Load up the website and play the Azteca 2 video
6. Then ask them to explain the habits of the ant.
7. Ask the class to hypothesize the value of the ant to the plant.
8. Have the students briefly discuss the evolutionary mechanisms leading to this type of plant
defense.
USEFUL INTERNET RESOURCES
1. Images of various plant defenses are valuable for supplementing a lecture based on
Chapter 40. of environmental cleanup. The Botanical Society of America has a useful
website for obtaining images of specialized plant structures used for defense. The site is
available at http://www.botany.org/plantimages/PlantDefenseMechanisms.php.
2. People who prescribe to natural medicines rely on plant defenses as source of the medical
prevention or treatment. Student can be given assignments to assess the effectiveness and
scientific merit of natural medicines made from plants. The Natural Medicines
Comprehensive Database is a useful web reference for faculty and students to gather
information on this topic. The website is available at http://www.naturaldatabase.com/.
3. Introducing students to current research on plant defenses is an effective way to stimulate
conversation during a lecture on Chapter 40. An article in Science Daily called “Plant
Defenses Prompt Bacterial Countermeasure In The Form Of 'Island' DNA Excision” can
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stimulate pertinent discussions in class. This website is located at
http://www.sciencedaily.com/releases/2006/01/060103084321.htm.
4. Case studies are important teaching tools for reinforcing information about produces
medicines from plant defenses. The University of Buffalo has a case study called “Is That
Pill You're Taking Safe? A Case Study About the Drug Development Process” which
teaches students the process of drug development. The case study can be found at
http://ublib.buffalo.edu/libraries/projects/cases/FENPHEN.html.
LABORATORY IDEAS
A. Screening for Natural Anticancer Agents
This activity has students design an experiment to screen plants for the presence of
potential anticancer agents. I
a. Explain to students that they will be looking at a test for detecting potential of anticancer
chemical in plants. Tell them that many chemicals used to treat cancer are plant
compounds that specifically kill cancer cells.
b. Then explain that they will be using brine shrimp amoebocytes as a model system for
looking at the effectiveness of cell-killing power.
c. Tell students that they will be investigating the conditions needed for fungal spore
germination in two types of fungi.
d. Provide students with the following materials
a. Large brine shrimp in chilled water
b. Microscope
c. Microscope slides
d. Plastic pipette
e. 0.5% Trypan Blue solution in dropper bottle
f. Sharp scalpel
g. Standard Cytotoxic Solution of household bleach in a dropper
h. Plants extracts to be tested (extracts are made by grinding 1 gram of plant
material per milliliter of 1:1:1 volume water: ethanol: acetone solution
i. Marigold stem
ii. Green tea
iii. Ginseng (from health food tablets)
iv. Periwinkle or Vinca
v. Castor bean
e. Instruct students how to collect amoebocytes form the tail of a brine shrimp:
a. Place the cooled shrimp on a slide with minimum amount of water
b. Carefully slice off the tail at the base of the shrimps body
c. Quickly place the shrimp on the microscope and focus on the cut area under
medium to high power.
d. Add 2 drops of trypan blue.
e. Observe the amoebocytes which are small ovoid cells that leak out with the
blood.
f. Healthy amoebocytes are clear and show some cytoplasmic granules.
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g. Dying and dead amoebocytes turn blue as they take up the trypan blue.
f. Have the students use the bleach as a cytotoxicity control to kill the amoebocytes. This is
done by adding on drop of bleach to the cells in the trypan blue while observing the
amoebocytes under the microscope. Have them notice how the dying and dead shrimp
cells and amoebocytes turn blue.
g. Then tell the students to test the plant extracts and make conclusions about their results.
LEARNING THROUGH SERVICE
Service learning is a strategy of teaching, learning and reflective assessment that merges the
academic curriculum with meaningful community service. As a teaching methodology, it falls
under the category of experiential education. It is a way students can carry out volunteer projects
in the community for public agencies, nonprofit agencies, civic groups, charitable organizations,
and governmental organizations. It encourages critical thinking and reinforces many of the
concepts learned in a course.
1. Have students do a community presentation on organic gardening using nature plant
chemical as pesticides.
2. Have students volunteer with a garden club on a community garden project.
3. Have students give a demonstration on plant defenses to elementary students.
4. Have students volunteer at botanical garden or nature center.
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