Chapter 39 - Plant Responses To Internal And External Signals

Question 1

What are plant hormones?

Answer 1

Chemical signals that modify or control one or more specific physiological process within a place; at very low concentrations plant hormones can have a profound effect in plant growth and development depending on the amount, concentration, and combination of different hormones.

Question 2

What is a hormone?

Answer 2

A signaling molecule that is produced in low concentrations by one part of an organism’s body and transported to other parts, where it binds to a specific receptor and triggers responses in target cells and tissues.

Question 3

What are the major responses if Auxin?

Answer 3

Stimulates cell elongation
Regulates branching and organ bending
At higher concentrations, may inhibit cell elongation by inducing production of ethylene.
Phototropism and leaf abscission

Question 4

What is phototropism?

Answer 4

The growth of a shoot toward light or away from it

Question 5

What is Auxin?

Answer 5

A chemical substance that promotes elongation of coleoptiles produced predominantly in shoot tips and moves only from top to base.

Question 6

What are the major responses to Cytokinins?

Answer 6

Stimulate plant cell division
Promote later bud growth
Slow organ dominance

Question 7

What is apical dominance?

Answer 7

The ability of the apical bud to suppress the development of a ill art buds

Question 8

What are the major responses to Gibberellins?

Answer 8

Promote stem elongation
Help seeds break dormancy and use stored reserves
Larger fruits

Question 9

What are the major responses to ethylene?

Answer 9

Mediates fruit ripening and the triple response

Abscission

Question 10

How does the action of ethylene work?

Answer 10

Enzymatic breakdown of cell walls makes fruit soft.

Ripening = positive feedback ( ethylene triggers ripening and ripening triggers more ethylene)

Question 11

What is gravitropism?

Answer 11

Response to gravity

Question 12

Where is gravitropism seen?

Answer 12

Positive gravitropism is seen in roots ( more auxin = less cell elongation)

Negative gravitropism is seen in shoots (more auxin = more cell elongation)

Question 13

What are statoliths?

Answer 13

Dense cytoplasmic components that settle under the influence of gravity to the lower portions of the cell.

Question 14

What is thigmotropism?

Answer 14

Directional growth in response to touch

Question 15

What is thigmomorphogenesis?

Answer 15

Changes in form that result from mechanical perturbation (i.e. wind)

Question 16

How do plants respond to dry environments (drought)?

Answer 16

Drought avoiders: grow in shade
Xerophytes: plant’s that need very little water; minimize water loss but take in less CO2 meaning less photosynthesis and slower growth.

Question 17

How do xerophytes conserve water?

Answer 17

Thick waxy leaves and epidermal hairs to reduce evaporation

Stomatocrypts reduce transpiration

Question 18

Leaf adaptations to dry environments?

Answer 18

Thick, water conserving leaves = succulents
Only grow when water is available = ocotillo
Spines (stem does photosynthesis; reflects solar radiation) = cactus

Question 19

Root adaptations to dry environments?

Answer 19

Taproot system = deep roots that search for water deeply

Shallow root system = roots grow horizontally

Question 20

What are alternate strategies responding to dry environments?

Answer 20

Reducing surface area of leaves (roll up leaves to reduce evaporation)
Hang downward to avoid direct sunlight (ie eucalyptus)
Accumulate proline and other solutes in vacuoles (ie xerophyte)

Question 21

Why are hot environments bad for plants?

Answer 21

Heat can denature proteins

Question 22

Adaptations to responses for hot environments?

Answer 22

Similar to dry environments

CAM plants have a specialized form of photosynthesis found in succulents that allows photosynthesis at night.

Question 23

Why are wet environments bad for plants and what are some adaptations?

Answer 23

Soil may lack airspace meaning less oxygen for the roots.

Adaptations: shallow, slow growing roots, pneumatophores (come out of water to get oxygen like snorkels)

Question 24

Why are salty environments bad for plants?

Answer 24

Lower water potential and salt is toxic to plants. Halophytes can live here.

Question 25

What are adaptations for salty environments?

Answer 25

1. Restricting the entry of toxic ions at root level (exclusion)
2. Transporting the toxic ions to stem, leaf sheath or older leaves (plant level compartmentation)
3. Excretion is salt through salt glands, salt-hairs or bladders (in most halophytes)
4. Sequestration of the toxic ions to vacuole or cell wall - cell level compartmentation

Question 26

What is a plant’s first line of defense against infection?

Answer 26

The barrier presented by the epidermis and periderm (mechanical defense)
Pathogens can enter through wounds or natural openings (ie stomata)

Question 27

What is PAMP-Triggered immunity?

Answer 27

Depends on the plant’s ability to recognize pathogen associated molecular patterns (molecular sequences that are specific to certain pathogens)
Starts a chain is signaling events (protein extension, blocking of plasmodesmata, Lignin: mechanical barrier and toxic precursor) leading to the production of antimicrobial chemicals (phytoalexins) and toughen plant cell walls with Lignin

Question 28

What is the Hypersensitive response?

Answer 28

A localized and specific, short term response that causes cell and tissue death near the infection sight, induces the production of enzyme that attacks the pathogen, stimulates changes in the cell wall that confine the pathogen.

Question 29

What is system acquired resistance?

Answer 29

Causes systemic expression of defense genes and is long lasting and nonspecific response.
Methylsalicylic acid is synthesized around the infection site and carried in the phloem to other remote sites where it is converted to salicylic acid which triggers the defense system to respond rapidly to another infection.

Question 30

What are some defenses against herbivores?

Answer 30

Physical defenses (thorns and trichomes) and chemical defenses (distasteful/toxic compounds)

Question 31

What are molecular level defenses?

Answer 31

Chemical compounds including terpenoids, phenolics, and alkaloids can be produced to deter attackers - called secondary metabolites
(BT is a natural insecticide)

Question 32

What are some chemical defenses?

Answer 32

They have no role in primary plant functions and are often secondary metabolites that attack the nervous system of insects
ie: atropine, cocaine, codeine and morphine, tetrodotoxin, nicotine, and erythromycin which we use as an antibiotic

Question 33

Cellular level defenses?

Answer 33

Cells may be specialized to form trichomes which store chemical deterrents, or produce irritants like raphides which are common in dung cane

Question 34

Tissue level defenses?

Answer 34

Leaves may be toughened with sclerenchyma tissue ie. artichoke

Question 35

Organ level defenses?

Answer 35

Leaves can be modified into spines and bristles; stems can become thorns; some species have leaves that appear partially eaten (making herbivores think the “good parts” are gone); some have structures that mimic insect eggs to turn off herbivores from eating or other insects from laying eggs.

Question 36

Organisms level defense?

Answer 36

Plants may respond to attack by altering flowering time

Question 37

Population level defenses?

Answer 37

Some plants release chemicals in response to herbivore attack that trigger defense responses in nearby members of population

Question 38

Community level defenses?

Answer 38

Some plants “recruit” predatory animals that help defend against specific herbivores
Ex. Parasitic wasps inject their eggs in caterpillars attacking the plant