Chapter 40 Plant defense responses

What are the different physical defenses used by plants? Viruses, bacteria, fungi can tap into the nutrient resources of plants and use their DNA replicating mechanisms to self replicate Defense mechanisms used by plants are – Cutin and Suberin deposition protect plants from nutrient and water loss Silica intrusions, trichomes, bark and even thorns protect nutrient-rich plant interior

How do invaders/predators penetrate dermal defenses? Parasitic nematodes use their sharp mouthparts to get through cell walls Bacteria function as sites for ice nucleation and resultant ice crystals damage leaves Fungi gain entry through stomatal openings The hyphae of fungi form haustoria which absorbs nutrients from plant cell membrane

What are the different toxin defenses used by plants? Plants produce toxins that that either kill or make their predators ill Produce secondary metabolites such as alkaloids, tannins and oils to protect themselves from predators Plants produce toxins (cyanogenic glycosides) which become toxic after ingestion in the animal – HCN cyanide Allelopathy- A chemical signal secreted by roots of one plant block the germination of nearby seeds or inhibit growth of neighboring plants. Minimizes shading and competition for nutrients. Example: Black Walnut

Some metabolites act as toxins and some have medicinal values Ricin found in beans (Ricinus communis) is six times more lethal than cyanide and two times as lethal as cobra venom. Ricin acts as a ribosome binding protein, inactivates 1500 ribosomes/minute – blocks translation Soybeans (genistein acting as estrogen), Taxol (fights breast cancer), Quinine obtained from Cinchona trees (fights malaria)

Some animals protect plants Complex coevolution between plants and animals have resulted in mutualistic associations Ants and acacia trees coevolved an association in which ants physically protect trees from other invaders and the acacia provides the ants with food and shelter In some plants volatile chemicals are released during caterpillar herbivory that attracts parasitoid wasps, which lay their eggs in the caterpillar and kill it (figure 40.9)

How a wound response can protect plant from herbivore? Wounded leaves produce an 18-amino-acid peptide (systemin) Systemin travels through apoplast of wounded tissue and moves throughout plant through phloem Systemin binds to systemin receptor that produces jasmonic acid or salicylic acid Signals gene expression ---- producing proteinase inhibitor that binds to digestive enzymes in gut of herbivore (figure 40.10)

What is gene for gene hypothesis? Proposed by H.H. Flor Pathogens have an avirulence gene (avr) that can interact with plant resistance gene (R) Interaction can make plant mount defenses to ensure that avr remains avirulent Leads to hypersensitive response (HR) that leads to cell death around the site of invasion No interaction between avr and R can lead to a disease in plant

Other sources of systemic responses Phytoalexins are antimicrobial agents produced by plants Plants are capable of systemic response to pathogen or pest attack called a systemic acquired resistance (SAR)-long distance signal provided by Jasmonic or salicylic acid

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