39.1 Morphological and physiological features protect plants from invasion.
Plants Under Attack
• Because plants are constantly under attack by viruses, bacteria, fungi, animals, and other plants, a wide array of mechanisms has evolved to block such invasions. (p. 796)
• One of the greatest problems with invasive species is the lack of natural predators. (p. 796)
• The first plant defense is the dermal tissue system. (pp. 796-797)
• Fungal invasion can include several phases, such as spores landing on leaves, spore germination, hyphae growing through cell walls, and hyphae differentiating into specialized haustoria. (pp. 796-797)
39.2 Some plant defenses act by poisoning the invader.
Chemical Toxins
• Many plants produce cyanogenic glycosides that break down into cyanide when ingested, which stops electron transport and blocks cellular respiration. (p. 798)
• Alkaloids, including caffeine, nicotine, cocaine, and morphine, can affect a wide variety of cellular processes. (p. 798)
• Tannins bind to, and inactivate, protein. (p. 798)
• Two strategies explain why toxins do not kill the plants. One is for a plant to sequester a toxin in a membrane-bound structure, and the second is to produce a compound that is not toxic unless it is metabolized in the intestine of an animal. (p. 798)
• Allelopathy occurs when a chemical signal secreted by the roots of one plant blocks the germination of nearby seeds or inhibits the growth of a neighboring plant. (p. 798)
Secondary Metabolites and Humans
• Many plant secondary metabolites, including phytoestrogens and taxol, have potential benefits to human health. (p. 800)
39.3 Some plants have coevolved with bodyguards.
Animals That Protect Plants
• Several species of ants provide small armies to protect some acacia trees from herbivory, and receive sugar from nectaries in reward. (p. 801)
• A wound response in some plants releases volatile compounds to attract female parasitoid wasps, which then attack herbivorous caterpillars. (p. 801)
39.4 Systemic responses also protect plants from invaders.
Wound Responses
• Wound responses, which occur when a leaf is chewed or injured, release substances such as proteinase inhibitors. (p. 802)
Pathogen-Specific Responses
• When a plant is attacked and gene-for-gene recognition occurs, the hypersensitive response leads to rapid cell death around the site of attack, which seals off the wounded tissue to prevent the pathogen or pest from moving further into the plant. (p. 803)
• Systemic acquired resistance (SAR) can also be induced by signals such as salicylic acid and jasmonic acid that allow the plant to respond more quickly if it is attacked again. (pp. 803-804)