Chapter 38

RECAP 38.1

  1. Avr2Avr3  Healthy  Healthy  Diseased
    Avr1Avr4  Healthy  Healthy  Healthy

  2. Both general and specific immunity involve similar signaling pathways. For general immunity, the researcher could look for PAMPs on the pathogen, and a weaker response. For specific immunity, the researcher could look for gene-for-gene resistance (Avr genes) and programmed cell death and the formation of a necrotic lesion at the site of pathogen invasion.

  3. Salicylate initiates systemic resistance. Application of this molecule at the site of a wound would trigger the production of protective PR proteins against further infection. This would be a long-term effect.

RECAP 38.2

  1. It indicates that latex is important in defense against herbivores such as slugs.

  2. Because nicotine is a secondary metabolite that is always present in tobacco leaves, it is a constitutive defense that can deter many herbivores. Induced responses would be triggered by herbivores that are not affected by the nicotine.

    1. This is an induced defense, as the frass contains an elicitor.

    2. Chemicals in the frass act as elicitors that trigger signal transduction pathways. One pathway involves changes in electric potential of the cell membrane, which is transmitted via the symplast to the whole plant. Another pathway triggers production of reactive oxygen species, which act as signaling molecules in pathways that lead to change in gene expression. Yet another pathway leads to production of jasmonate and other hormones, which trigger systemic defenses, and may even travel to neighboring plants to stimulate their defense responses.

  4. Jasmonates are made in response to an elicitor from herbivory. Jasmonates can induce the synthesis of volatile molecules, which diffuse through the air to other plants or organs in the same plant.

RECAP 38.3

    1. The effects of reduced rainfall could include dehydration and osmotic stress. Genetic responses might include alterations in leaf anatomy, with a thicker cuticle to reduce evaporation; a more extensive root system to obtain water; and accumulation of solutes in the roots, which would reduce root water potential and result in more water uptake in dry soils.

    2. Flooding reduces the amount of O2 available to the plants and results in reduced respiration. Genetic responses might include increased production of pneumatophores or aerenchyma to supply air to submerged plant tissues.

  2. She could add genes encoding proteins that produce salt glands or genes that encode membrane proteins that store excess salt in the vacuole. Sequestering salt in the vacuole keeps it away from the rest of the cell, where it can harm chemicals and reactions. (See Key Concept 18.5 for salt-tolerant plants.)

    1. The lack of a plant cover would allow the accumulation of toxic ions in the soil.

    2. The genes involved might encode proteins that keep the toxic ions away from the plant cells (e.g., store the ions in glands) or away from the plant cells’ cytoplasm (e.g., store the ions in vacuoles).

WORK WITH THE DATA, P. 809

  1. Wild-type Sr33 conferred rust resistance. Either a deletion mutant of Sr33 (bar E5) or a point mutation of Sr33 (bars E6–E9) resulted in rust susceptibility. Therefore Sr33 can confer rust resistance.

    A-40

  2. Fielder with high-expressing Sr33 was rust-resistant, while Fielder without Sr33 was rust-susceptible. This reinforces the conclusion in the earlier experiments that Sr33 confers resistance to wheat rust.

  3. Sr35 confers resistance in this relative of cultivated wheat.

  4. Sr35 also confers resistance in bread wheat. Therefore there are two genes that can confer resistance.

FIGURE QUESTIONS

Figure 38.1 Many human diseases are caused by pathogens. Here we suggest just one of many correct answers for each type of pathogen.

Virus: Influenza

Bacterium: Tuberculosis

Fungus: Candidiasis

Nematode worm: Hookworm

Figure 38.2 Both plant organs are exposed to all types of pathogens, although their identities may be different. So both root and leaf cells exposed to the environment would express the defense molecules.

Figure 38.3 Recognition is by shape (like a lock and key) and by chemical interactions between the molecules, such as hydrophobic interactions and ionic interactions.

Figure 38.12 LEA proteins might include chaperonins that bind to target proteins and stabilize them so that the targeted proteins remain in their active state, and other proteins that stabilize membranes and keep them from drying out.

APPLY WHAT YOU’VE LEARNED

  1. The wound alone causes the release of a large amount of nicotine. Caterpillar herbivory (which includes the production of FACs) causes slightly less nicotine to be released. This suggests that FACs might have a slight inhibiting effect on nicotine release. Nicotine is a constitutive response; it is present naturally in the tobacco plant and does not have to be induced as a result of gene expression.

  2. Jasmonic acid production is stimulated by the production of elicitors when herbivory occurs. Jasmonic acid then causes changes in gene expression that lead to the production of secondary metabolites, such as protein inhibitors (see Figure 38.5). These protein inhibitors limit the effectiveness of the herbivore. Jasmonic acid differs from nicotine in that it does not directly affect the herbivore but stimulates production of another substance that has the effect (i.e., jasmonic acid is an intermediate substance).

  3. The burst of jasmonic acid production is much greater with the addition of FACs from the caterpillars. This stronger response suggests that FACs are elicitors that Nicotiana must recognize to trigger genetically controlled defense responses in the plant.

  4. Jasmonic acid triggers induced responses that limit hornworm herbivory in the tobacco plant. Ethylene suppresses nicotine production, making it easier for hornworm herbivory to occur. These are essentially opposite reactions. This could represent an example of the “evolutionary arms race” between plant and predator.