Chapter 32 Summary

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Core Concepts Summary

32.1 Plants have evolved mechanisms to protect themselves from infection by pathogens, which include viruses, bacteria, fungi, worms, and even parasitic plants.

Pathogens enter plants through damaged tissue or stomata. page 668

Plant pathogens spread by growing or moving through the plant vascular system. Viruses move in the phloem, whereas bacteria and fungi move in the xylem. page 669

Biotrophic pathogens gain resources from living cells; necrotrophic pathogens kill cells before exploiting them. page 669

Plants have an immune system that allows them to detect and respond to pathogens. page 670

One part of the plant immune system acts generally and recognizes highly conserved molecules produced by broad classes of pathogens. page 670

A second part of the plant immune system acts specifically and consists of “resistance” or R genes that encode R proteins, which recognize pathogen-derived AVR proteins page 670

Plants respond to pathogens by actively killing the cells surrounding the infection (the hypersensitive response) and by sending a signal to uninfected tissues so that they can mount a defense (systemic acquired resistance). page 671

The bacterium Rhizobium radiobacter infects plants by inserting some of its genes into the plant’s genome, resulting in the formation of a tumor and providing a way to genetically engineer plants. page 673

32.2 Plants use chemical, mechanical, and ecological defenses to protect themselves from being eaten by herbivores.

Plant defenses against herbivory include dense hairs, latex, and chemicals. page 675

Alkaloids are nitrogen-bearing compounds that interact with the nervous system of animals. page 675

Terpenes, volatile compounds that give rise to many of the essential oils we associate with plants, deter herbivores. page 676

Tannins bind with proteins, reducing their digestibility. page 676

Ant-plants, such as the bullhorn acacia, provide food and shelter for ants, which defend their host plant. page 677

32.3 The production of defenses is costly, resulting in trade-offs between protection and growth.

Plants produce both constitutive defenses, which are produced whether or not a threat is present, as well as inducible defenses, which are triggered when a plant detects that it is being attacked. page 679

Plants produce volatile signals that attract insects that prey on herbivores. page 681

A trade-off is sometimes observed between plant growth and defense, in which allocation to defense results in slower growth rates. page 681

In nutrient-rich environments, species that allocate more resources to growth and less to defense can outcompete plants that invest more resources to defense and less to growth; the opposite is true on nutrient-poor soils. page 681

32.4 Interactions among plants, pathogens, and herbivores contribute to the origin and maintenance of plant diversity.

The “escape and radiate” pattern of plant evolution suggests that plants undergo a burst of diversification following the evolution of a new form of defense against a pathogen or an herbivore. page 682

The Janzen–Connell hypothesis proposes that interactions with pathogens and herbivores increase plant diversity. page 683

Herbivores and pathogens are a major concern for agriculture. page 683

Crop protection includes the use of chemical pesticides, integrated pest management, application of spores or toxins from Bacillus thuringiensis to plants, and inserting genes that encode for toxins from B. thuringiensis to make Bt-modified plants. page 684

Self-Assessment

  1. Describe how pathogens enter and move within the plant body.

    Self-Assessment 1 Answer

    Pathogens can enter plants through a variety of mechanisms. Some enter through wounds in the plant stem or leaves. Others enter through the stomata or on the piercing mouthparts of insects or nematodes. Some pathogens even secrete enzymes that weaken epidermal cell walls; they can then force their way through the weakened epidermis and enter the plant. Once inside the plant, the pathogen can grow (as do fungi) or move (as do bacteria) through cell walls. The pathogen can also move through the xylem or phloem. Viruses can move through plant cells through plasmodesmata.

  2. Distinguish between biotrophic and necrotrophic plant pathogens.

    Self-Assessment 2 Answer

    Biotrophic pathogens, such as viruses, get their resources from plant cells that are still alive. Necrotrophic pathogens, such as some bacteria, first kill the cell before obtaining resources from it.

  3. Name and describe the two components of the plant immune system.

    Self-Assessment 3 Answer

    The first component of the plant immune system consists of receptors on the plant cell membrane that recognize broad features of pathogens such as bacterial flagellin. Recognition triggers a defense response, which, when successful, results in basal resistance. The second component of the plant immune system consists of intracellular receptors called R proteins that recognize specific avirulence (AVR) proteins secreted by the pathogens. This interaction between the R and AVR proteins elicits a defense response against the pathogen. The ability to defend against specific pathogens is referred to as specific resistance.

  4. Describe and contrast the hypersensitive response and systemic acquired resistance.

    Self-Assessment 4 Answer

    The hypersensitive response against plant pathogens is one in which the plant cells surrounding the infected tissue make an excess amount of reactive oxygen species, reinforce their cell wells, and finally die. The dead cells form a barrier that prevents the pathogen from leaving the area and infecting the rest of the plant. The spread of biotrophic pathogens is stopped by a hypersensitive response, and the growth of necrotrophic pathogens is substantially slowed.

    Systemic acquired resistance (SAR) is a plant defense mechanism used in response to necrosis resulting from either a hypersensitive response or a necrotrophic pathogen. A chemical signal is sent from the infected region to noninfected regions through the phloem. This signal triggers the expression of genes that “prep” the noninfected cells for defense in case the pathogen spreads to their area. Thus, systemic acquired resistance increases the ability of uninfected tissues to resist infection.

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  5. Describe a feature of Rhizobium radiobacter that makes it a useful tool in biotechnology.

    Self-Assessment 5 Answer

    Rhizobium radiobacter is a bacterium that is especially good at inserting its DNA into the genome of its host plant. This characteristic has allowed scientists to use the bacterium to insert genes of interest into test plants.

  6. Describe three ways that plants protect themselves from being eaten by herbivores.

    Self-Assessment 6 Answer

    Plants protect themselves from being eaten by herbivores through many mechanisms. These mechanisms may be mechanical, such as the presence of hairs on leaves or spines on tree trunks or hard silica plates in grass epidermal cells. Plants can also produce a variety of chemicals that can damage anything from the insect’s nervous system to its development. Some plants have even evolved ecological defenses against herbivory. In exchange for food or shelter, insects, such as ants, will defend the plant from other herbivores. Grasses are not destroyed by grazing because their low-to-the-ground apical meristems are not damaged.

  7. Explain why there are often trade-offs between plant growth and plant defense.

    Self-Assessment 7 Answer

    There are often trade-offs between plant growth and plant defense because a plant only has a certain amount of energy and resources to expend at one time. If the plant uses most of its energy and resources in defense against pathogens and herbivores, it may not grow very tall or fast or produce many seeds. Depending on the environment the plant is in, it will allocate its resources to best fit its needs at the time.

  8. Explain how pathogens and herbivores can increase plant diversity.

    Self-Assessment 8 Answer

    The diversity of plants is affected by the number and kinds of pathogens and herbivores that feed upon them. Plants evolve new forms of defenses, and pathogens and herbivores evolve new mechanisms to overcome these defenses, leading to another round of new plant defenses. A novel defense may allow a plant population to expand into new areas, increasing its potential to evolve into new species.

    Diversity is also increased through the mechanism of the Janzen‒Connell hypothesis, in which seedlings that are far from an individual of the same species are more likely to escape being infected by pathogens or consumed by herbivores. This gives rare species an advantage over more common ones, thus promoting the coexistence for many otherwise similar species in a small area.

  9. Draw a phylogenetic tree that illustrates an “escape and radiate” pattern of diversification for plants that evolve novel defenses.

    Self-Assessment 9 Answer

  10. Describe one benefit and one disadvantage of herbicide and pesticide use in agriculture.

    Self-Assessment 10 Answer

    One disadvantage of herbicide and pesticide use in agriculture is that it is a strong selective force for the evolution of resistance in pathogens. One benefit of these compounds is that they allow farmers to protect their crops and produce vast quantities of products.