Environmental Stresses and Plant Hormones (Lecture 8)

Question 1

How do plants adapt to stressful environmental conditions?

Answer 1

Plants can’t move, so they must adapt using biochemical or physiological responses.

Question 2

When do plant responses to stress occur?

Answer 2

They occur within seconds, to days.

Question 3

How do plants respond to stress?

Answer 3

Stress leads to rapid changes, such as ion fluxes and gene expressions.
Eventually, new proteins are synthesized to deal with the stress.
These responses often involve the release of plant hormones, especially abscisic acid, ethylene, and jasmonic acid.

Question 4

How do plant hormones help in dealing with stress?

Answer 4

They help coordinate and signal the stress throughout the plant.

Question 5

Abiotic Stress: Drought (aka water stress)

Answer 5

Plants close stomata, and therefore cannot undergo photosynthesis or maintain turgor.

Question 6

Abiotic Stress: Cold and Chilling

Answer 6

Cold temperatures may impact membranes.
Freezing causes ice crystal formation and physical disruption.

Plants can adjust membrane composition, and also synthesize antifreeze chemicals and proteins to stabilize biomolecules.

Question 7

Abiotic Stress: UV Light (high-intensity light stress)

Answer 7

This excessive light damages cells and proteins directly, or produce toxic oxidative chemicals.

Plants respond by accumulating UV-absorbing compounds (‘sunscreen’) and antioxidants in leaves.

Question 8

Abiotic Stress: Mechanical stress in soil

Answer 8

Seedlings and roots are impeded from expanding = Physical resistance to their growth.

Plants respond by becoming stockier and stronger.

Question 9

Abiotic Stress: Heat shock

Answer 9

Overheating causes protein denaturation.

Plants respond by producing heat shock proteins, which help to refold denatured proteins.

Question 10

Abiotic Stress: Flooding

Answer 10

Overwatering leads to a lack of oxygen (anoxia) in roots.

Plants respond by activating non-oxygenic (anaerobic) metabolism, such as fermentation and ethanol production, as well as anatomical adaptation.

Question 11

Biotic Stress: Herbivores

Answer 11

Causes loss of photosynthetic area and nutrients, and entry points for pathogens.

Plants respond by producing toxic proteins and chemicals.

Question 12

Biotic Stress: Pathogens and plant disease

Answer 12

Causes loss of nutrients, or tissue death and necrosis.

Plants respond by secreting toxins, and proteins that attack fungal cell walls.

Question 13

What is the key to combatting pathogens and plant disease?

Answer 13

Early detection!

Question 14

What is the role of abscisic acid (ABA) plant hormones in responding to drought or salt stress?

Answer 14

When lack of water in soil occurs, this low water potential is sensed by roots and leaves. As a result, ABA is transported and released to leaves.

Question 15

How does ABA work in response to drought or salt stress?

Answer 15

ABA diffuses to guard cells and triggers rapid stomatal closure: Opening of K+ channels, rapid efflux of K+.

Loss of turgor occurs in the guard cell, closing the stomatal pore.

Question 16

Why is ABA critical in seed development?

Answer 16

It promotes the maintenance of the dormant (inactive) state of cells.

Question 17

Mature seeds have ______ water content.

Answer 17

Low

Question 18

What happens to mutants deficient in ABA?

Answer 18

They germinate precociously, and are viviparous (reproducing that occurs from the buds forming plantlets while still being attached to the parent plant, or from seeds germinating within the fruit).

Question 19

Which plant hormone is ABA antagonistic to?

Answer 19

Gibberellin, which triggers seed germination and amylase secretion.

Question 20

What is the triple response, in facing mechanical stress?

Answer 20

Seedlings respond by…

1. Stocky growth
2. Not green
3. Forming and maintaining an apical hook

Question 21

Why do seedlings undertake the triple response?

Answer 21

This allows seedlings to get around obstacles and push through soil.

Question 22

What gaseous hormone is the triple response controlled by?

Answer 22

Ethylene

Question 23

What is the role of ethylene in combatting anaerobic stress (flooding of roots) and oxygen deprivation?

Answer 23

Prolonged flooding in corn roots causes ethylene build-up and signals programmed cell death in root cortex tissues.

This leads to air tubes, for extra aeration.

Question 24

What is ethylene’s role in climacteric fruit?

Answer 24

It stimulates ripening! Due to the ethylene produced after being picked, they will continue ripening.

Ripening can also be promoted using exogenous ethylene.

Examples: Tomatoes, bananas, apples.

Question 25

What component of herbivores can plants detect?

Answer 25

Components of herbivore saliva.

Question 26

What is jasmonic acid (JA)?

Answer 26

Jasmonic acid is a defense hormone produced rapidly during an herbivore attack.

Question 27

What does jasmonic acid consist of?

Answer 27

It is made from linolenic acid, a common unsaturated plant fatty acid, found in the thylakoid.

Question 28

What functions does jasmonic acid undertake?

Answer 28

• Induces many plant defense proteins and chemicals making insects or vertebrate herbivores sick. An example is protease inhibitors, which messes up insects’ protein digestion.
• Regulates defenses against nectrotrophic pathogens, which kill cells by secreting toxins.

Question 29

How fast are JA responses?

Answer 29

VERY fast! Plant defense is very active and involves many chemical changes.

Question 30

What differentiates mutant plants that cannot produce jasmonic acid?

Answer 30

They are hyper-susceptible to insect pests.

Question 31

Which plant stress responses are within minutes?

Answer 31

Drought: Stomatal closure

Question 32

Which plant stress responses are within hours?

Answer 32

Herbivore defense

Question 33

Which plant stress responses are within days?

Answer 33

Ethylene responses

Question 34

The stress responses are ____ for each type of stress encountered by plants.

Answer 34

Distinct