Plants and Their Environment I

Question 1

What is dormancy?

Answer 1

An adaptive train ensuring that germination occurs in a suitable environment at a suitable time.
A dormant seed is generally defined as one that doesn’t germinate under normal growth conditions. An extra signal is required

Question 2

Why is dormancy necessary?

Answer 2

Allows plant to wait until the environment is right for growth.

Water – need water to grow, seedlings much more sensitive to drought than adult plants.

Light
Day length is critical as the plant needs to generate enough photosynthate to produce seed or survive.
Don’t want to germinate in shady places, e.g; canopy cover.

Not a passive process- actively suppressing germination.

Question 3

What are some mechanisms of dormancy?

Answer 3

Physical:

• Needs breaking to germinate (freezing, high temp, digestive tract, drying) as otherwise water can’t get in.
• Can be one cycle or multiple lasting months to years.
• Barrier is usually to water.

Chemical:
-Inhibitors present in the seed coat – need to be washed out before germination can occur. Heavy rain can wash these out.

Physiological:
-Hormone antagonism-
Abscisic acid (ABA)- promotes dormancy vs Giberellins (GA)- promotes germination.

Question 4

How long can dormancy last for?

Answer 4

Seeds can persist and remain dormant for years.

Question 5

What happens to ABA and GA in seed ripening?

Answer 5

GA levels remain very low, ABA levels rise to ripening, then fall.

Question 6

What is an imbibed seed?

How is ABA and GA affected?

Answer 6

Imbibed seed is one that has taken up water, but not yet germinated.
Rise in ABA- ensures germination does not happen.
Slight rise in GA.

Question 7

How is a dormancy signal broken?

What do DELLAs do?

Answer 7

Production of hormones works in the same way as sensitivity- sensitivity to ABA high at first, so production of ABA high. As ABA sensitivity production/sensitivity decreases, production/sensitivity to GA increases. Results in germination.
DELLAs actively supress germination. Block any germination TFs.

Question 8

How is germination triggered?

Answer 8

GA is produced by the seed after imbibition but the seed isn’t sensitive to GA.

Light (and cold or other stimuli) promote increased production of GA and increased sensitivity to GA by repressing ABA synthesis and promoting ABA catabolism.

Eventually GA production causes inhibition of ABA synthesis and promotes degradation of ABA.

Question 9

What are strigalactones and what do they do to break dormancy?

Answer 9

Secreted molecules with 2 primary plant functions:

• Interaction signal for arbuscular mycorrhizal fungi.
• Supress shoot branching.

Striga is a parasitic plant on the roots of many cultivated cereals and uses strigolactones as a germination cue to indicate that a plant is nearby.

Question 10

How do heat, fire and smoke break dormancy? What are some examples?

Answer 10

Ceanothus spp.
Woody perennial:
-Forms thick scrubland. Have all scrub cleared by fire before germination.
-Seeds require high heat (to the extent of burning) followed by chilling over winter to germinate the following spring.
-Seeds >100 years old have been found to be viable.

Papaver californicum
Annual:
-Seeds require chemicals from smoke to germinate. Chemicals in ash are washed into the soil by rain after a fire, and these promote germination.
-Can remain dormant in the soil for many years between fires.

These are both evolutionary strategies.

Question 11

How has agriculture affected dormancy?

Answer 11

Many crops have been bred to have no dormancy e.g. Wheat.
Advantage:
Immediate synchronous germination straight after planting .
Disadvantage:
Rain fall on ripe crops can induce pre-harvest germination.
Germination in storage if conditions are damp.

Pre-harvest germination due to no dormancy.

Question 12

What is photomorphogenesis?

Answer 12

Light regulation of plant growth.

Question 13

How does light perception affect plant growth?

Answer 13

Prime determinant of how a plant grows.
Regulates:
-Germination
-Post-emergence dark growth (skotomorphogenesis)
-De-etiolation
-Photomorphogenesis
-Shade avoidance
-Flowering time

UV – UVR8, Cryptochromes.
Blue – Phototropins, Cryptochromes, FKF1, (Phytochrome).
Red – Phytochromes.
Far red – Phytochromes.
All of these are proteins that absorb the light wavelengths stated, cause signalling to which the plant responds.

Question 14

How is plant growth affected in dark and light conditions?

Answer 14

Dark growth: -Skotomorphogenesis.

• Seedling trait during conditions unable to support photosynthesis.
• Adaptive, light seeking response.
• No functional chloroplasts.
• All energy into elongation to get to light. Virtually no root.

Light growth:

• Photomorphogenesis.
• De-etiolation.
• Chloroplasts activated.
• Switch to photomorphogenesis requires minimal light.
• Plant has roots and leaves.

All regulated by presence or absence of light.
One way switch.

Question 15

What are the main players in photomorphogenesis?

Answer 15

Phytochromes.

Question 16

What are phytochromes?

Answer 16

5 forms in Arabidopsis: PHYA, B, C, D and E- A and b are the main players in skotomorphogenesis and photomorphogenesis.

Made in red light absorbing inactive PR form.

Converted by absorbing red light into active PFR form.

Photoreversible switches-
Red light (650-680nm) induces many responses.
Far red (710-740nm) light inhibits them.

Ratio of R:FR forms mediates size of response.

Question 17

Where do phytochromes operate?

Answer 17

Operate in:
VLFR: Very Low Fluence Response – <5ms of sunlight, very few molecules converted to PFr form, essentially irreversible, PHYA in response to Red. Switch from skotomorphogenesis to photomorphogenesis.

LFR: Low Fluence Response – Short exposure (response saturated after ~1s of sunlight), reversible, PHYB. Intermediate- in normal conditions, all will be in active form because it is saturated.

HIR: High Irradiance Response – Long exposure, reversible, PHYA in response to Far Red and PHYB in response to Red.

Different modes.
All of these combines result in plant growth patterns.

Question 18

What do PHYA and PHYB do?

Answer 18

PHYA and B control switch from skoto- to photomorphogenesis.

PHYA highly abundant in dark grown seedlings.

PFR form rapidly degraded but allows for a burst of signaling.

Acts as a Far Red VLFR sensor in this instance and detects soil surface.

PHYB acts in LFR mode and takes over once initial switch has occurred.

PHYA in PR form inhibits photomorphogenesis.
PHYA degraded. PHYA in PFR form activates photomophogenesis.
Rest of response regulated by other PHY.

PHYB is particularly involved in plant shade avoidance. Elongation of plant in the dark to find light.

Question 19

What is the main phytochrome in photomorphogenesis?

Answer 19

PHYB is the dominant phytochrome affecting photomorphogenesis.

PHYA mutants don’t show many changes.
PHYB mutants- constitutive shade avoidance phenotype- always think they’re in shade.

Question 20

How does plant shade avoidance work?

Answer 20

In non shaded conditions:
High red : far red ratio.
What allows plants to grow in normal manner.
Supresses shade avoidance response.

Small plants get shaded by bigger plants. Depleted in RED light. Low R:FR.
Plant grows longer and away from shading plant, towards light.
PHYB acting in LFR mode.

Question 21

How does light quality affect germination?

Answer 21

PHYA acting in HIR mode (sustained period of far red enriched light) suppresses germination.
PHYB overridden.
Seed can differentiate between different types of shading by measuring the R:FR ratio.
Suppresses germination- all in PR form.

If the big shading plant dies or falls over:
High R:FR ration for other plants.
PHYA PFR formed and germination occurs. PHYB also acts.

Shading by a wall won’t change ratio, but shading by a plant will.
Seed can sense this.