Plant Defense (Lecture 9) Flashcards
What is an especially important herbivore threat for plants?
Caterpillars!
Which pests are plants threatened by?
Pathogenic fungi, bacteria, nematodes, viruses.
Which is the most destructive threat?
Fungi
What can highly specialized pathogens do?
The rust fungi, for example, can devastate crops such as wheat on a continental scale.
Plant pathogens have various ways of invading plants: from wound sites, via the stomata, by penetrating cuticles
How do plants defend themselves?
Using biochemical adaptations, or physiological responses.
How fast do plant responses to pests occur?
Within seconds to days! Include rapid ion fluxes, gene expression.
What do plants produce to deal with pest stress?
They synthesize new proteins.
Which is the defense hormone that coordinates the plant’s response to pests?
Jasmonic acid.
Plant pathogen defense: Cell wall
Acts as the first barrier to pathogen entry. Pathogenic bacteria and viruses need a wound cell to enter.
How can pathogenic fungi enter the cell wall?
They can force their way in using an appressorium.
A specialized cell is typical of many fungal plant pathogens that are used to infect host plants. It is a flattened, hyphal “pressing” organ, from which a minute infection peg grows and enters the host, using turgor pressure capable of punching through even Mylar.
How does the plant cell respond to entry by pathogenic fungi?
They can actively reinforce the cell wall.
Example: Powdery mildew infection of the leaf. The plant cell fights back the penetration peg by utilizing a host papilla, actively reinforcing the cell wall.
Plant pathogen defense: Toxins
Once attacks, the plant cell makes antibiotic molecules and toxins, killing the pathogen or slowing its growth.
Plant pathogen defense: Fungal wall digesting enzymes
The plant can secrete enzymes that digest the fungal cell wall.
Plant pathogen defense: Fungal enzyme inhibitors
The plant cell can secrete special proteins that block the fungal enzymes attacking the plant, by making them inactive.
This is a classic ‘arms race’, in which both fungi and the plant are secreting digestive enzymes.
Plant pathogen defense: Hypersensitive cell death
This is the last resort, for individual plant cells.
If a fungal pathogen is detected inside the plant, programmed cell death is triggered. This stops the pathogen from spreading!
Plant pathogen defense mechanisms are __________ responses, triggered by ___________.
Active; Attack and cell damage.
How is an attack detected by the plant?
Thanks to the molecules released from the fungus or the damaged cell wall (elicitors).
What are elicitors composed of?
Elicitors are small pieces of the cell wall, oligosaccharides of 5-20 sugars.
What is the key in sensing attack?
TIming! Early detection is key.
How can the most devastating pathogens, being fungi, become so aggressive?
They are very sneaky! They avoid detection by the plant surveillance system, inactivating it.
Why is the world green?
Due to the sophistication of plant mechanisms, plant defense!
Plant Herbivory Defense: Physical mechanisms
Spines, thorns, hairs filled with toxins, bark (resin, as a barrier)
What are constitutive chemical defenses?
They are always ON.
What are induced chemical defenses?
They are turned on as triggered.
Chemical Herbivory Defense: Protease inhibitors
Anti-nutritive proteins, inhibiting digestive enzymes. They make the insects sick or grow slower.
What are specialist insects?
Species that have evolved the ability to tolerate/detoxify toxins and neurotoxins chemically produced by the plant.
Some can even use the plant chemicals for their own defense!
Chemical Herbivory Defense: Steroid hormone mimics
Phytoestrogens, which affect vertebrates.
Phytoecdysone, which mimics steroids and interferes with development.
Chemical Herbivory Defense: Resins
Physical barriers that trap insects thanks to their sticky surface, and contain toxic components (terpenoids).
Chemical Herbivory Defense: Feeding deterrents
These compounds are very bitter-tasting, which therefore discourage eating. Examples include cucurbitacin, tannin.
How can feeding deterrents be tested?
Using choice tests, which alternate control and experimental leaves. The insects will only feed on those that taste good!
Define: Systemic defenses
Defenses get activated in the whole plant after distant leaves are damaged. It is a proactive measure.
This means that defenses are activated IN ADVANCE OF the pests!
What allows for systemic defenses?
Jasmonic acid, which is transported systematically.
Fast transmission of Ca+ spikes in the cells. This propagation, a super speed, and distribution of responses signal a defense response.
What is the systemic induction experiment utilized for?
It tests a compound that is triggered and subsequently induces a response in the entire plant.
How do plants detect insects feeding on them?
They detect damage to cells. Notably, they detect lipid and cell wall fragments, similar to the elicitors that are present for pathogen defense.
They also detect chemicals in insect saliva, which can be super-potent activators of defense. For example, they detect caterpillar spit.
How can plants use indirect defenses against pests and pathogens?
Plants can actively recruit beneficial insects to attack herbivorous pests.
Trees secrete sugars in extrafloral nectaries, attracting ants, which attack the pests.
Plants release volatile compounds, attracting parasitoid wasps, which attack caterpillar.
What is chemical ecology?
The study of the interactions between insects, herbivores, predators, and more.
Plant defense against pests and pathogen is _______ and _______.
Multifaceted; complex. It involves the upregulation of many genes and can be triggered by multiple signals.
Plant defense involves _________ and _________ of the attacker.
Recognition and signaling.
Is plant defense rapid/slow? Active/inactive?
It is active and rapid.
What hormone is used to coordinate defense during herbivory?
Jasmonic acid.
What molecular structures do plants use in defending themselves?
Small molecules (toxins) and proteins, through a variety of mechanisms.
Do plants have allies when they are being attacked?
Yes! They can recruit ecological helpers = Indirect defense.
