Mechanical Defense Against Herbivores Plant Defense Mechanical Defense Against Herbivores Plants have evolved strategies to defend against two major types of enemies: herbivores, or animals that eat plants, and pathogens. Plants hold within their cells the source of energy that feeds all the consumers in an area; other than occasionally offering fruits and nectar, however, many plants defend vigorously against being eaten by herbivores. Some defense mechanisms in plants function mechanically-clearly, animals would tend to avoid sharp spines and thorns when selecting a plant to eat. Principles of Biology

Figure 1 Barrel Cactus thorns. Plant Defense Figure 1 Barrel Cactus thorns. Not many animals look at this mass of spines and consider eating this plant. Notice how much more friendly the flowers appear, as the plant attracts pollinators. Principles of Biology

Figure 2 Teasel plant defense. This plant holds paired leaves at high angles to collect water at the base. Crawling insects have a difficult time getting past the water in the moat-like structure to reach the leaves and either retreat or drown. Principles of Biology

Chemical Defense Against Herbivores Plant Defense Chemical Defense Against Herbivores Many plants use nonspecific chemical weapons that not only protect plants, but also affect animals in complex and surprising ways. A major group of plant defense chemicals, called alkaloids, have potent effects on physiological processes in mammals. Some alkaloids mimic the chemical structure of neurotransmitters and can significantly alter neural activity. Several medicines have been derived from alkaloid plant compounds, including painkillers such as morphine and cocaine. Principles of Biology

Some plant chemicals coordinate symbiotically with other organisms. Plant Defense Some plant chemicals coordinate symbiotically with other organisms. A voracious caterpillar chews a maize plant to bits. The combination of leaf damage and a chemical in the saliva from the caterpillar triggers a signaling pathway in the leaf to produce and release a chemical into the air. The chemical compound released by maize leaves attracts parasitoid wasps. Parasitoid insects lay eggs in or on the bodies of other insects. When the eggs hatch, the young larvae feed on these host insects. Principles of Biology

Figure 3 Plant and parasitoid relationship. Plant Defense Figure 3 Plant and parasitoid relationship. Plants have evolved to produce chemicals that attract parasitoid insects to attack herbivores eating the plant. Principles of Biology

Some plants defend through communication with each other. Plant Defense Some plants defend through communication with each other. Several plant species release chemicals into the air when being eaten or invaded. Plants of the same species growing near a threatened plant recognize the signal and respond by producing compounds to protect themselves and also pass the message along to neighboring plants. Some plant species such as strawberries and clover are physically connected to each other. They grow close to the ground and propagate new plants from ground-hugging runners. Like wires connecting computers, the runners produced by strawberry and clover plants allow a pathway for chemical signals to travel quickly between plants. Principles of Biology

Defense Against Pathogens Plant Defense Defense Against Pathogens Plants have several protections against infection, starting with waxy, thick, outer layers surrounding leaves and stems, much like skin surrounds the bodies of animals. Damage from wind and herbivores can expose the interior of a plant to microbial intruders. Pathogens are generally described according to the effect they have on a plant. A virulent pathogen is one against which a plant has no directed defense, so the plant dies soon after infection. An avirulent pathogen can interfere with the growth and development of a plant without killing it. Plant viruses are classified based on the type of genetic information they carry-either DNA or RNA-and are commonly transported to plants by insects. Principles of Biology

Figure 4 Virus-infected wheat. Plant Defense Figure 4 Virus-infected wheat. Yellowing leaves indicate viral infection in a wheat plant. Principles of Biology

Defense Against Pathogens Plant Defense Defense Against Pathogens If a plant has the appropriate (R) gene, the effector triggers the defensive machinery and the pathogen is disabled. Plant defense systems can be localized to the infected site or they can be systemic, treating the entire organism. One pathogenic resistance mechanism in plants is called gene-for-gene recognition. Gene-for-gene recognition enables a plant to recognize pathogenic molecules, called effectors, using special resistance (R) genes. Principles of Biology

Plant Defense Internal plant defensive systems against pathogens can be localized and systemic. For humans, eliminating a body part is usually not a viable option in fighting a pathogen; for plants, losing a few leaves usually does no major long-term damage. A hypersensitive response in plants kills cells and tissues in and around the site of an infection. Principles of Biology

Figure 5 Hypersensitive response. Plant Defense Figure 5 Hypersensitive response. The leaf on the right used a hypersensitive response to a viral infection. By containing the infection in a single leaf, the virus could not infect the entire plant. To use this mechanism, this plant must have had the appropriate resistance gene to recognize and respond quickly to the virus. Principles of Biology

Plant Defense Internal plant defensive systems against pathogens can be localized and systemic. As part of the hypersensitive response, the responding cells produce signal molecules that alert the entire plant of the infection, triggering systemic acquired resistance, a level of defense involving the whole organism. Principles of Biology