CHAPTER 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.
- 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.
- Biotrophic pathogens gain resources from living cells; necrotrophic pathogens kill cells before colonizing them.
- Plants have an innate immune system that allows them to detect and respond to pathogens.
- One part of the innate immune system acts generally and recognizes highly conserved molecules produced by broad classes of pathogens.
- A second part of the innate immune system acts specifically and consists of resistance or R genes that encode R proteins, which recognize pathogen-derived AVR proteins.
- 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).
- 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.
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.
- Alkaloids are nitrogen-bearing compounds that damage the nervous system of animals.
- Terpenes are volatile compounds that give rise to many of the essential oils we associate with plants; they deter herbivores.
- Tannins bind with proteins, reducing their digestibility.
- Ant-plants, such as the bullhorn acacia, provide food and shelter for ants, which defend their host plant.
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.
- Plants produce volatile signals that attract insects that prey on herbivores.
- Experiments show that plants growing on nutrient-rich environments allocate more resources to growth than to defense.
- A trade-off is sometimes observed between plant growth and defense, in which plants favor one or the other, but cannot have both.
32.4 INTERACTIONS AMONG PLANTS, PATHOGENS, AND HERBIVORES CONTRIBUTE TO THE ORIGIN AND MAINTENANCE OF PLANT DIVERSITY.
- The Janzen–Connell hypothesis proposes that interactions with pathogens and herbivores increase 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.
- Herbivores and pathogens are a major concern for agriculture.
- 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.
Self-Assessment Question 1
Describe how pathogens enter and move within the plant body.
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Model 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.
Self-Assessment Question 2
Distinguish between biotrophic and necrotrophic plant pathogens.
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Model 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 the resources from it.
Self-Assessment Question 3
Name and describe two components of the plant innate immune system.
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Model Answer:
Two components of the innate immune system are the basal immune system and a specific immune system. The basal 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. The specific immune system consists of 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.
Self-Assessment Question 4
Describe and contrast the hypersensitive response and systemic acquired resistance.
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Model 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 growth of necrotrophic pathogens is substantially slowed by a hypersensitive response.
Systemic acquired resistance (SAR) is a plant defense mechanism used in response to necrosis resulting either from a hypersensitive response or a necrotrophic pathogen. A chemical signal is sent from the infected region to non-infected regions through the phloem. This signal triggers the expression of genes that “prep” the non-infected cells for defense in case the pathogen spreads to their area. Thus, systemic acquired resistance increases the ability of uninfected tissues to resist infection.
Self-Assessment Question 5
Describe a feature of Rhizobium radiobacter that makes it a useful tool in biotechnology.
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Model 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.
Self-Assessment Question 6
Describe three ways that plants protect themselves from being eaten by herbivores.
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Model 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 the epidermal cells. Plants can also produce a variety of chemicals that can affect 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.
Self-Assessment Question 7
Explain why there are often trade-offs between plant growth and plant defense.
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Model 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 a lot of seeds. Depending on the environment the plant is in, it will allocate its resources to best fit its needs at the time.
Self-Assessment Question 8
Explain how pathogens and herbivores can increase plant biodiversity.
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Model Answer:
The biodiversity 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. An innovation in defense may allow a plant population to expand into new areas, increasing its potential to evolve into new species.
Biodiversity is also increased through the mechanism of the Janzen-Connell hypothesis, in which seedlings will survive better if they disperse greater distances from the parent tree because the seedling will not have to compete for resources with the parent. This model selects for high biodiversity in a small area.
Self-Assessment Question 9
Draw a phylogenetic tree that illustrates an “escape and radiate” pattern of diversification for plants that evolve novel defenses.
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Model Answer:
Self-Assessment Question 10
Describe one benefit and one disadvantage of herbicide and pesticide use in agriculture.
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Model 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.