rav65819_ch40_789-800

rav65819_ch40_789-800 - ; 40 chapter Plant Defense...

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;;;;;;;; 40 chapter Plant Defense Responses
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PLANTS ARE CONSTANTLY UNDER ATTACK by viruses, bacteria, fungi, animals, and even other plants. An amazing array of defense mechanisms has evolved to block or temper an invasion. Many plant–pest relationships undergo coevolution, with the plant winning some times and the pest winning with a new offensive adaptation at other times. The first line of plant defense is thick cell walls covered with a strong cuticle. Bark, thorns, and even trichomes can deter a hungry insect. When that first line of defense fails, a chemical arsenal of toxins is waiting. Many of these molecules have no effect on the plant. Some are modified by microbes in the intestine of an herbivore into a poisonous compound. Maintaining a toxin arsenal is energy-intensive; thus, an alternative means of defense uses induced responses to protect and prevent future attacks. introduction 40.3 Animals that Protect Plants 40.4 Systemic Responses to Invaders Wound responses protect plants from herbivores Defense responses can be pathogen-speciFc concept outline 40.1 Physical Defenses Dermal tissue provides Frst-line defense Invaders can penetrate dermal defenses Bacteria and fungi can also be beneFcial to plants 40.2 Toxin Defenses
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Plants maintain chemical arsenals Plants can poison other plants Humans are susceptible to plant toxins Secondary metabolites may have medicinal value 789 rav65819_ch40_789-800.indd 789 rav65819_ch40_789-800.indd 789 11/27/06 11:24:59 AM 11/27/06 11:24:59 AM 40.1 Physical Defenses There are no tornado shelters for trees. Storms and changing environmental conditions present life- threatening challenges to plants. Structurally, trees can often withstand high winds and the weight of ice and snow, but there are limits. Winds can uproot a tree, or snap the main shoot off a small plant. Axillary buds give many plants a second chance as they grow out and replace the lost shoot (figure 40.1). Although abiotic factors such as weather constitute genuine threats to a plant, even greater daily threats exist in the form of viruses, bacteria, fungi, animals, and other plants. These enemies can tap into the nutrient resources of plants or use their DNA-replicating mechanisms to self-replicate. Some invaders kill the plant cells immediately, leading to necrosis (brown, dead tissue). Certain insects may tap into the phloem of a plant seeking carbohydrates, but leave behind a hitchhiking virus or bacterium. The threat of these attackers is reduced when they have natural predators themselves. One of the greatest problems with fgure 40.2 ALFALFA PLANT BUG. This invasive species is an agricultural problem because it arrived without any natural predators and feeds on alfalfa. fgure 40.1
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This note was uploaded on 10/15/2010 for the course BIO BIO1 taught by Professor Lipke during the Fall '09 term at CUNY Brooklyn.

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rav65819_ch40_789-800 - ; 40 chapter Plant Defense...

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