RAVEN 9/e

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 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

40.1 Physical Defenses

  • Identify the compounds produced by the epidermis to protect against invasion.
  • Outline the steps taken by a fungus to invade a plant leaf.
  • Describe two beneficial associations between plants and microorganisms.

40.2 Toxin Defenses

  • Describe the role of secondary metabolites in plant defense.
  • Define alleleopathy.
  • List three examples of the medicinal value of secondary metabolites.

40.3 Animals that Protect Plants

  • Describe the benefit Acacia trees receive from ants that live in them.
  • Explain how some plants use parasitoid wasps to destroy caterpillars.

40.4 Systemic Responses to Invaders

  • Outline the sequence of events that leads to the production of a wound response.
  • Describe the gene-for-gene hypothesis.
  • Define systemic acquired resistance.

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.

  • 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.

Application /
  • Have students describe why plant defenses against insects also ward off viral diseases.
  • Ask students to explain why many plant defense chemicals are found in seeds.
  • Ask students to explain the possible organism that poison ivy defends itself against using the irritating oils.

Analysis /
  • Have students explain relationship between plant structure and the defense used to protect the structure.
  • Have students explain how a plant would have the need to defend itself from other plants.
  • Have students compare the relative effectiveness of dermal defenses versus chemical defenses to protect leaves.

Synthesis /
  • Ask students design an experiment to determine if prior exposure to a disease induces stronger defenses in a plant.
  • Have students design an experiment to test is damaged plants are able to stimulate stronger defenses in nearby plants.
  • Ask the students develop a commercial application for the knowledge that plant toxins called lectins are capable of killing fungi that attack plants.

Evaluation /
  • Ask students evaluate the benefits and consequences of breeding crop plants so that they are lacking their natural defenses.
  • Ask students to evaluate the feasibility of using natural plant defenses to replace traditional pesticides used in agriculture.
  • 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

Introduction

This video demonstration shows students how Azteca ants are exploited by plants as a defense mechanism.

Materials

  • Computer with Media Player and Internet access
  • LCD hooked up to computer
  • Web browser linked to You Tube Broadcast at

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.

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

  1. 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.
  2. Then explain that they will be using brine shrimp amoebocytes as a model system for looking at the effectiveness of cell-killing power.
  3. Tell students that they will be investigating the conditions needed for fungal spore germination in two types of fungi.
  4. Provide students with the following materials
  5. Large brine shrimp in chilled water
  6. Microscope
  7. Microscope slides
  8. Plastic pipette
  9. 0.5% Trypan Blue solution in dropper bottle
  10. Sharp scalpel
  11. Standard Cytotoxic Solution of household bleach in a dropper
  12. 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
  13. Marigold stem
  14. Green tea
  15. Ginseng (from health food tablets)
  16. Periwinkle or Vinca
  17. Castor bean
  18. Instruct students how to collect amoebocytes form the tail of a brine shrimp:
  19. Place the cooled shrimp on a slide with minimum amount of water
  20. Carefully slice off the tail at the base of the shrimps body
  21. Quickly place the shrimp on the microscope and focus on the cut area under medium to high power.
  22. Add 2 drops of trypan blue.
  23. Observe the amoebocytes which are small ovoid cells that leak out with the blood.
  24. Healthy amoebocytes are clear and show some cytoplasmic granules.
  25. Dying and dead amoebocytes turn blue as they take up the trypan blue.
  26. 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.
  27. 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.