Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CHAPTER 40 LECTURE SLIDES

Plant Defense Responses Chapter 40

Physical Defenses Winds can uproot a tree, or snap the main shoot of a small plant Axillary buds give plants a second chance as they grow out and replace the lost shoot 3

Even greater daily threats exist in the form of viruses, bacteria, fungi, animals, and other plants –These can tap into nutrient resources of plants or use their DNA-replicating mechanisms to self-replicate –Some kill plant cells immediately, leading to necrosis 4

One of the greatest problems with nonnative invasive species, such as the alfalfa plant bug, is the lack of natural predators in the new environment 5 Alfalfa plant bug

Dermal tissue system –First-line defense of all plants –Epidermal cells throughout the plant secrete wax to protect plant surfaces from water loss and attack –Above-ground parts also covered with cutin –Suberin is found in cell walls of subterranean plant organs –Silica inclusions, trichomes, bark, and even thorns can also offer protection 6

These exterior defenses can be penetrated –Mechanical wounds allow microbial entry –Parasitic nematodes use their sharp mouth parts to get through the plant cell walls Some form tumors on roots –In some cases simply having bacteria on the leaf surface can increase the risk of frost damage 7

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Fungi seek out the weak spot in the dermal system, or stomata, to enter the plant Phases of fungal invasion 1.Windblown spore lands on leaves 2.Spore germinates and forms adhesion pad 3.Hyphae grow through cell walls and press against cell membrane 4.Hyphae differentiate into haustoria 9

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Fungi and bacteria can also be beneficial to plants –Mycorrhizal fungi –Nitrogen-fixing bacteria like Rhizobium –Plant growth-promoting rhizobia (PGPR) Bacteria provide substances that support plant growth Can also limit the growth of pathogenic soil bacteria 11

Toxin Defenses Many plants produce toxins that kill herbivores, make them ill, or repel them with strong flavors or odors Some are unique to plants Defensins are found in plants and animals –Small, cysteine-rich peptides with antimicrobial properties Reveals ancient origin of innate immunity –In some cases defensins limit protein synthesis 12

Secondary metabolites –Metabolic pathways needed to sustain life are modified –Alkaloids Wild tobacco has elevated nicotine levels lethal to tobacco hornworms –Tannins –Animals, including humans, can avoid many of the cumulative toxic effects of secondary metabolites by eating a varied diet 13

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Plants protect themselves from toxins in two main ways 1.Sequester a toxin in a membrane-bound structure 2.Produce a compound that is not toxic until it is metabolized by attacking animal Cyanogenic glycosides break down into cyanide (HCN) when ingested 16

Allelopathic plants –Secrete chemicals to block seed germination or inhibit growth of nearby plants –This strategy minimizes competition for resources –Very little vegetation grows under a black walnut tree 17

Throughout history, humans have been intentionally poisoned with plant products –Socrates died after drinking a hemlock extract containing nerve-paralyzing alkaloid –In 1978, Georgi Markov, a Bulgarian dissident, was assassinated by KGB officers using ricin A pinhead-sized metal sphere was injected from an umbrella tip into his thigh 18

19 Ricin is an alkaloid produced by the castor bean plant (Ricinus communis) –It is six times more lethal than cyanide and twice as lethal as cobra venom –A single seed can kill a small child –It functions as a ribosome-binding protein that inhibits translation

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Many secondary metabolites have benefits to human health –Phytoestrogens of soy plants Appear to lower the rate of prostate cancer in Asian males However, questions have been raised about their effect on developing fetuses Also on babies consuming soy-based formula 21

Taxol of Pacific yew trees –Fights cancers, especially breast cancer Quinine of Cinchona trees –Effective against malaria, which is caused by four species of Plasmodium –Blocks DNA replication –Also leads to build-up of toxic hemes that poison the parasite 22

Animals that Protect Plants Complex coevolution of plants and animals has resulted in mutualistic associations –Relationships that benefit both Acacia trees and ants –Small armies of ants protect Acacia trees from harmful herbivores –Plant provides ants with food and shelter 23

24 Ants attacking a katydid to protect “their” Acacia

Parasitoid wasps, caterpillars, and leaves –As caterpillar chews away, a wound response in the plant leads to release of a volatile compound –Female parasitoid wasp is attracted –Lays fertilized eggs in caterpillar –Eggs hatch and larvae kill caterpillar 25

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Systemic Response to Invaders Static plant responses to threats have an energetic downside –Are maintained in the presence or absence of threat Energy resources would be conserved if the plant response was inducible –Defenses launched only when needed 27

A wound response occurs when a leaf is chewed or injured –One outcome leads to rapid production of proteinase inhibitors throughout the plant –Bind to digestive enzymes in the gut of the herbivore –Signaling pathway involves Jasmonic acid Salicylic acid Cell fragments 28

Wound response signaling pathway 1.Wounded leaves produce an 18-amino acid peptide called systemin 2.Systemin moves throughout the plant in the phloem 3.Cells with receptors produce jasmonic acid 4.Jasmonic acid turns on genes for proteinase inhibitor 29

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H. H. Flor’s gene-for-gene hypothesis –Plants have a plant resistance gene (R); pathogens have an avirulence gene (avr) –It is the recognition of the gene products (i.e. proteins) that is critical –If binding occurs, plant can mount defenses that keep pathogen avirulent –If no binding occurs, the plant succumbs to disease 32

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Recognition of the pathogen by the R gene product leads to hypersensitive response –Leads to a very rapid cell death around the site of attack –Also to longer term, whole plant resistance 34

Rapid cell death due to hypersensitive response –Seals off the wounded tissue to prevent the pathogen or pest from moving into rest of the plant –Hydrogen peroxide and nitric oxide produced May signal cascade of chemical events resulting in localized host cell death –Phytoalexins – antimicrobial chemical defense agents 35

Systemic acquired resistance (SAR) –systemic response by plants –Several pathways lead to broad-ranging resistance that lasts for a period of days –Long-distance inducer is likely salicylic acid –At the cellular level, jasmonic acid is involved in SAR signaling –SAR allows the plant to respond more quickly to a second attack 36

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