Plant Defense (Lecture 9)

Question 1

What is an especially important herbivore threat for plants?

Answer 1

Caterpillars!

Question 2

Which pests are plants threatened by?

Answer 2

Pathogenic fungi, bacteria, nematodes, viruses.

Question 3

Which is the most destructive threat?

Answer 3

Fungi

Question 4

What can highly specialized pathogens do?

Answer 4

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

Question 5

How do plants defend themselves?

Answer 5

Using biochemical adaptations, or physiological responses.

Question 6

How fast do plant responses to pests occur?

Answer 6

Within seconds to days! Include rapid ion fluxes, gene expression.

Question 7

What do plants produce to deal with pest stress?

Answer 7

They synthesize new proteins.

Question 8

Which is the defense hormone that coordinates the plant’s response to pests?

Answer 8

Jasmonic acid.

Question 9

Plant pathogen defense: Cell wall

Answer 9

Acts as the first barrier to pathogen entry. Pathogenic bacteria and viruses need a wound cell to enter.

Question 10

How can pathogenic fungi enter the cell wall?

Answer 10

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.

Question 11

How does the plant cell respond to entry by pathogenic fungi?

Answer 11

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.

Question 12

Plant pathogen defense: Toxins

Answer 12

Once attacks, the plant cell makes antibiotic molecules and toxins, killing the pathogen or slowing its growth.

Question 13

Plant pathogen defense: Fungal wall digesting enzymes

Answer 13

The plant can secrete enzymes that digest the fungal cell wall.

Question 14

Plant pathogen defense: Fungal enzyme inhibitors

Answer 14

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.

Question 15

Plant pathogen defense: Hypersensitive cell death

Answer 15

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!

Question 16

Plant pathogen defense mechanisms are __________ responses, triggered by ___________.

Answer 16

Active; Attack and cell damage.

Question 17

How is an attack detected by the plant?

Answer 17

Thanks to the molecules released from the fungus or the damaged cell wall (elicitors).

Question 18

What are elicitors composed of?

Answer 18

Elicitors are small pieces of the cell wall, oligosaccharides of 5-20 sugars.

Question 19

What is the key in sensing attack?

Answer 19

TIming! Early detection is key.

Question 20

How can the most devastating pathogens, being fungi, become so aggressive?

Answer 20

They are very sneaky! They avoid detection by the plant surveillance system, inactivating it.

Question 21

Why is the world green?

Answer 21

Due to the sophistication of plant mechanisms, plant defense!

Question 22

Plant Herbivory Defense: Physical mechanisms

Answer 22

Spines, thorns, hairs filled with toxins, bark (resin, as a barrier)

Question 23

What are constitutive chemical defenses?

Answer 23

They are always ON.

Question 24

What are induced chemical defenses?

Answer 24

They are turned on as triggered.

Question 25

Chemical Herbivory Defense: Protease inhibitors

Answer 25

Anti-nutritive proteins, inhibiting digestive enzymes. They make the insects sick or grow slower.

Question 26

What are specialist insects?

Answer 26

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!

Question 27

Chemical Herbivory Defense: Steroid hormone mimics

Answer 27

Phytoestrogens, which affect vertebrates.

Phytoecdysone, which mimics steroids and interferes with development.

Question 28

Chemical Herbivory Defense: Resins

Answer 28

Physical barriers that trap insects thanks to their sticky surface, and contain toxic components (terpenoids).

Question 29

Chemical Herbivory Defense: Feeding deterrents

Answer 29

These compounds are very bitter-tasting, which therefore discourage eating. Examples include cucurbitacin, tannin.

Question 30

How can feeding deterrents be tested?

Answer 30

Using choice tests, which alternate control and experimental leaves. The insects will only feed on those that taste good!

Question 31

Define: Systemic defenses

Answer 31

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!

Question 32

What allows for systemic defenses?

Answer 32

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.

Question 33

What is the systemic induction experiment utilized for?

Answer 33

It tests a compound that is triggered and subsequently induces a response in the entire plant.

Question 34

How do plants detect insects feeding on them?

Answer 34

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.

Question 35

How can plants use indirect defenses against pests and pathogens?

Answer 35

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.

Question 36

What is chemical ecology?

Answer 36

The study of the interactions between insects, herbivores, predators, and more.

Question 37

Plant defense against pests and pathogen is _______ and _______.

Answer 37

Multifaceted; complex. It involves the upregulation of many genes and can be triggered by multiple signals.

Question 38

Plant defense involves _________ and _________ of the attacker.

Answer 38

Recognition and signaling.

Question 39

Is plant defense rapid/slow? Active/inactive?

Answer 39

It is active and rapid.

Question 40

What hormone is used to coordinate defense during herbivory?

Answer 40

Jasmonic acid.

Question 41

What molecular structures do plants use in defending themselves?

Answer 41

Small molecules (toxins) and proteins, through a variety of mechanisms.

Question 42

Do plants have allies when they are being attacked?

Answer 42

Yes! They can recruit ecological helpers = Indirect defense.