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CHAPTER 39 PLANT RESPONSES TO INTERNAL AND EXTERNAL SIGNALS Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Section E: Plant Defense: Responses to Herbivores and Pathogens 1.Plants deter herbivores with both physical and chemical defenses 2. Plants use multiple lines of defense against pathogens
Plants do not exist in isolation, but interact with many other species in their communities. Some of these interspecific interactions - for example, associations with fungi in mycorrhizae or with insect pollinators - are mutually beneficial. Most interactions that plants have with other organisms are not beneficial to the plant. As primary producers, plants are at the base of most food webs and are subject to attack by a wide variety of plant-eating (herbivorous) animals. Plants are also subject to attacks by pathogenic viruses, bacteria, and fungi. Introduction Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Herbivory is a stress that plants face in any ecosystem. Plants counter excess herbivory with both physical defenses, such as thorns, and chemical defenses, such as the production of distasteful or toxic compounds. 1. Plants deter herbivores with both physical and chemical defenses Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
For example, some plants produce an unusual amino acid, canavanine, which resembles arginine. If an insect eats a plant containing canavanine, canavanine is incorporated into the insect’s proteins in place of arginine. Because canavanine is different enough from arginine to adversely affect the conformation and hence the function of the proteins, the insect dies. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Some plants even recruit predatory animals that help defend the plant against specific herbivores. For example, a leaf damaged by caterpillars releases volatile compounds that attract parasitoid wasps, hastening the destruction of the caterpillars. Parasitoid wasps inject their eggs into their prey, including herbivorous caterpillars. The eggs hatch within the caterpillars, and the larvae eat through their organic containers from the inside out. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 39.29
These volatile molecules can also function as an “early warning system” for nearby plants of the same species. Lima bean plants infested with spider mites release volatile chemicals that signal “news” of the attack to neighboring, noninfested lima bean plants. The leaves of the noninfested plant activate defense genes whose expression patterns are similar to that produced by exposure to jasmonic acid, an important plant defense molecule. As a result, noninfested neighbors become less susceptible to spider mites and more attractive to mites that prey on spider mites. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
A plant’s first line of defense against infection is the physical barrier of the plant’s “skin,” the epidermis of the primary plant body and the periderm of the secondary plant body. However, viruses, bacteria, and the spores and hyphae of fungi can enter the plant through injuries or through natural openings in the epidermis, such as stomata. Once a pathogen invades, the plant mounts a chemical attack as a second line of defense that kills the pathogens and prevents their spread from the site of infection. 2. Plants use multiple lines of defense against pathogens Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Plants are generally resistant to most pathogens. Plants have an innate ability to recognize invading pathogens and to mount successful defenses. In a converse manner, successful pathogens cause disease because they are able to evade recognition or suppress host defense mechanisms. Those few pathogens against which a plant has little specific defense are said to be virulent. A kind of “compromise” has coevolved between plants and most of their pathogens. Avirulent pathogens gain enough access to its host to perpetuate itself without severely damaging or killing the plant. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Specific resistance to a plant disease is based on what is called gene-for-gene recognition, because it depends on a precise match-up between a genetic allele in the plant and an allele in the pathogen. This occurs when a plant with a specific dominant resistance alleles (R) recognizes those pathogens that possess complementary avirulence (Avr) alleles. Specific recognition induces expression of certain plant genes, products of which defend against the pathogen. If the plant host does not contain the appropriate R gene, the pathogen can invade and kill the plant. There are many pathogens and plants have many R genes. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Resistance occurs if the plant has a particular dominant R allele that corresponds to a specific dominant Avr allele in the pathogen. The product of an R gene is probably a specific receptor protein inside a plant cell or at its surface. The Avr gene probably leads to production of some “signal” molecule from the pathogen, a ligand capable of binding specifically to the plant cell’s receptor. The plant is able to “key” on this molecule as an announcement of the pathogen’s presence. This triggers a signal-transduction pathway leading to a defense response in the infected plant tissue. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Disease occurs if there is no gene-for-gene recognition because (b) the pathogen has no Avr allele matching an R allele of the plant, (c) the plant R alleles do not match the Avr alleles on the pathogen, or (d) neither have recognition alleles. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 39.30
Even if a plant is infected by a virulent strain of a pathogen - one for which that particular plant has no genetic resistance - the plant is able to mount a localized chemical attack in response to molecular signals released from cells damaged by infection. Molecules called elicitors, often cellulose fragments called oligosaccharins released by cell-wall damage, induce the production of antimicrobial compounds called phytoalexins. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Infection also activates genes that produce PR proteins (for pathogenesis-related). Some of these are antimicrobial and attack bacterial cell walls. Others spread “news” of the infection to nearby cells. Infection also stimulates cross-linking of molecules in the cell wall and deposition of lignins. This sets up a local barricade that slows spread of the pathogen to other parts of the plant. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
If the pathogen is avirulent based on an R-Avr match, the localized defense response is more vigorous and is called a hypersensitive response (HR). There is an enhanced production of phytoalexins and PR proteins, and the “sealing” response that contains the infection is more effective. After cells at the site of infection mount their chemical defense and seal off the area, they destroy themselves. These areas are visible as lesions on a leaf or other infected organ, but the leaf or organ will survive, and its defense response will help protect the rest of the plant. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Part of the hypersensitive response includes production of chemical signals that spread throughout the plant, stimulating production of phytoalexins and PR proteins. This response, called systemic acquired resistance (SAR), is nonspecific, providing protection against a diversity of pathogens for days. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
The hypersensitive response, triggered by R-Avr recognition, results in localized production of antimicrobial molecules, sealing off the infected areas, and cell apoptosis. It also triggers a more general systemic acquired resistance at sites distant to the site of initial infection. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 39.31
A good candidate for one of the hormones responsible for activating SAR is salicylic acid. A modified form of this compound, acetylsalicylic acid, is the active ingredient in aspirin. Centuries before aspirin was sold as a pain reliever, some cultures had learned that chewing the bark of a willow tree (Salix) would lessen the pain of a toothache or headache. In plants, salicylic acid appears to also have medicinal value, but only through the stimulation of the systemic acquired resistance system. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
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