BIO 162: OBJ 14 (PLANT DEFENSE)

For plants and animals, distinguish between innate and acquired resistance (immunity).

innate immunity: born with it, non-specific

acquired immunity: cells ability to respond to pathogens

Give an example of innate resistance from plants

barriers: wax, cork, bark

chemicals: capsaicin and other secondary metabolites

Give an example of acquired resistance from plants

hypersensitive response, R-gene/AVR gene, SAR

Give an example of innate resistance from animals

Barriers: Hair, skin, oil, mucus, stomach (acidic environment), wax, cilia in lungs, Toll-like receptors activate macrophages and other phagocytes

Give an example of acquired resistance from animals.

B cells and T cells response to pathogens

What form of immunity (innate or acquired) do secondary metabolites fall under? What is the advantage to the plant of producing capsaicin, peppermint oil, tannins, caffeine and opium?*

innate

secondary metabolites protect plants from macro-pathogens by making it less pleasant for them to eat the plant

Describe the hypersensitive response and consider why cell death can be a good thing for a plant infected with a pathogen.

senescence in the organ infected by the pathogen kills off cells in that organ and prevents the infection from spreading throughout the plant body

For a particular plant some pathogens are virulent while others are avirulent. How do these pathogens differ in terms of their affect on the plant? (resistant, disease)

Pathogen is virulent when the plant is not resistant to it, gets disease and die. Pathogen is avirulent when the plant is resistant to it.

Consider resistance genes (R genes) and avirulence genes (avr genes). In what organism are these genes encoded? Which encode receptors, which encode ligands?

R-genes: encoded in the plant genome, encode receptors, and initiate HR when bound to ligand

AVR-genes: encoded in the pathogen genome, encode products that are ligands for the R-gene, typically a cell wall or plasma membrane protein, required for normal pathogen activity

What cellular events make up the hypersensitive response? How are these events linked to R-gene/avr gene binding? (NO (nitric oxide), ROIs, cell wall, cell death)

1) Pathogen detected (R-gene product binds AVR-gene product)

2) Plant enzymes activated. Production of reactive oxygen intermediates (NO, H2O2, O2-)

3) NO diffuses out of call and into cell wall. Causes cell wall to thickens so that pathogen can't spread to the rest of the plant.

4) Signal to other tissues in the plant

5) H2O2 and O2- causes cell death

What is systemic acquired resistance (SAR)?

hormonal transmission signal of pathogen presence from site of infection to the rest of the body; tells other cells to express R gene to be prepared for the pathogen attack

What is the evidence that supports salicylic acid as a hormone for SAR?*

levels of methyl salicylate (MeSA)—a molecule derived from salicylic acid—increase dramatically after tissues are infected with a pathogen.

Phloem sap leaving infected sites has elevated levels of MeSA.

Treatments that reduce MeSA reduce or abolish SAR.

Adding MeSA to the lower leaves of tobacco plants leads to SAR in the upper, untreated leaves.

Explain the mechanism by which plants sense and respond to herbivore attack (systemin, receptor, jasmonic acid, proteinase inhibitors...know how each of these fits into the signal transduction pathway). Does this mechanism represent innate or acquired resistance?*

1) Damaged cells release a hormone (systemin) that travels through the phloem and binds to the receptors on the membranes of undamaged cells.

2) Series of reactions produces Jasmonic acid

3)Jasmonic acid, the 2° messenger, activates transcription of proteinase inhibitors that will deter the herbivore from further attack (inhibit the breakdown of proteins in the herbivore)

Describe an experiment to test the hypothesis that plants attacked by herbivorous caterpillars actively attract parasitoid wasps. How does attracting wasps benefit the plant?*

Insect damaged leaves produce 11 molecules not produced in undamaged leaves that attracts wasps (investigators put female wasps in an arena that contained leaves damaged by insects and leaves that had suffered only mechanical damage and in more than two- thirds of the tests performed, the wasps preferred to fly toward the insect-damaged leave)

wasps are parasitoids (lay eggs in caterpillars and larvae eat them from inside out)

Some plant pathogens synthesize and release cytokinin-like molecules or induce cytokinin synthesis in infected plant cells. Review material from earlier in the class to explain how this leads to the formation of a gall. (Callus)

cytokinin promotes the differentiation of cells to form shoots while auxin promote root formation.

The bacteria infect root cells which already have high auxin concentration. Equal concentration of cytokinin and auxin promote undifferentiated tissues. These undifferentiated cell mass are galls.