Chapter 32 Introduction

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CHAPTER 32

Plant Defense

Keeping the World Green

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Don Johnston/Getty Images.

Core Concepts

  1. Plants have evolved mechanisms to protect themselves from infection by pathogens, which include viruses, bacteria, fungi, worms, and even parasitic plants.
  2. Plants use chemical, mechanical, and ecological defenses to protect themselves from being eaten by herbivores.
  3. The production of defenses is costly, resulting in trade-offs between protection and growth.
  4. Interactions among plants, pathogens, and herbivores contribute to the origin and maintenance of plant diversity.

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Plants have complicated interactions with animals. As we saw in Chapter 30, many plants rely on animals for pollination or seed dispersal. In return, plant seeds provide a rich source of food for animals, and leaves furnish a vast, but lower-quality, food resource. The plant body also provides habitats for an array of viruses, bacteria, and protists, not to mention other plants, which may grow on or within their tissues. How do plants defend themselves against the animals that would eat them? How do they defend against pathogens? The answers to these questions help us understand how plant populations are distributed in nature, and they help us solve practical problems as well. In the United States alone, crop damage by herbivores and pathogens costs farmers more than $2 billion every year.

Plants can’t run away and hide as animals can. Nonetheless, they can sense danger and they are able to deploy a diverse and powerful defensive arsenal that includes physical and chemical deterrents as well as aerial surveillance, armed guards, and sticky traps.

Plants produce a wide array of chemicals that alter the metabolism and influence the behavior of animals that seek to consume them. These molecules also interact with our own biochemistry, and so compounds derived from plants have been used since antiquity as sources of medicine. Our early ancestors turned to the leaves and bark of willow trees in much the same way that we rely on aspirin—and for good reason. Willows produce a compound that deters herbivores but in humans relieves aches and pain; this compound in fact provides the chemical basis of aspirin. Quinine, derived from the tropical cinchona tree, is used in the treatment of malaria, while belladonna, a relative of the tomato, produces toxins that the ancient Romans used to eliminate rivals but in very low doses relieve gastrointestinal disorders. In fact, nearly 30% of all medicines are based on chemicals that plants produce to defend themselves.