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 2 Physical Defenses Biotic factors can be more detrimental to plants than abiotic factors -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 Physical Defenses The attack threat is enhanced with nonnative invasive species, who have no natural predators in their new environment 3 Alfalfa plant bug 4 Dermal Tissue System The first-line defense of all plants Epidermal cells throughout the plant secrete a variety of lipid material that protects plant surfaces from water loss and attack -Wax, cutin, and suberin Silica inclusions, trichomes, bark and even thorns can also offer protection Dermal Tissue System These exterior defenses can be penetrated -Mechanical wounds allow microbial entry -Bacteria can cause damage because they provide sites for ice nucleation -Parasitic nematodes use their sharp mouth parts to get through the plant cell walls -Some form tumors on roots 5 6 1
Dermal Tissue System Dermal Tissue System Fungi seek out the weak spot in the dermal system, or stomata, to enter the plant The phases of fungal invasion: 1. Windblown spore lands on leaves 2. Spore germinates & forms adhesion pad 3. Hyphae grow through cell walls and press against cell membrane 4. Hyphae differentiate into haustoria 7 8 Beneficial Microorganisms Fungi and bacteria can also be beneficial to plants -Mycorrhizal fungi -Nitrogen-fixing bacteria -Plant growth-promoting rhizobia (PGPR) -These provide various nutrients for plants 9 10 Toxin Defenses Many plants produce toxins that kill herbivores or make them ill, or repel them with strong flavors or odors Metabolic pathways needed to sustain life in plants have also lead to the production of secondary metabolites -Many of these affect herbivores as well as humans 11 12 2
Toxin Defenses Protective secondary metabolites include alkaloids (caffeine, nicotine), tannins & oils -Wild species of tobacco have elevated levels of nicotine that are lethal to tobacco hornworms 13 14 Toxin Defenses 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 15 Toxin Defenses 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 16 Toxin Effects on Humans 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 Toxin Effects on Humans 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 -It functions as a ribosome-binding protein that inhibits translation 17 18 3
Plants with Medicinal Value 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 19 20 Plants with Medicinal Value 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 21 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 22 Animals that Protect Plants Animals that Protect Plants 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 23 24 4
Animals that Protect Plants 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 25 26 A wound response occurs when a leaf is chewed or injured -Leads to rapid production of proteinase inhibitors throughout the plant -Bind to digestive enzymes in the gut of the herbivore 27 The signaling pathway involves four steps: 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 28 Salicylic acid is another molecule involved in the wound response 29 30 5
Specific Defense Responses Specific Defense Responses 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, a protective hypersensitive response develops -If no binding occurs, the plant succumbs to disease 31 32 Specific Defense Responses The hypersensitive response leads to a very rapid cell death around the site of attack -This seals off the wounded tissue to prevent the pathogen or pest from moving into rest of the plant Antimicrobial agents produced include -Hydrogen peroxide and nitric oxide -Phytoalexins 33 Specific Defense Responses Plants can also undergo a systemic response called systemic acquired resistance (SAR) -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 -However, it is neither as specific nor as long-lasting as mammalian responses 34 35 6