Light and Environmental Signals (Lecture 6)

Question 1

Why do plants need to interpret signals from their surroundings?

Answer 1

Plants need to interpret light, nutrient, threat stimuli, in order to convert them into an adaptive response. Recall that plants cannot move, and must adapt to their local environment!

Question 2

What time of information does sunlight contain?

Answer 2

It contains information about…

• Seasons: Day length
• Shade vs Direct Light: Light quality
• Directionality

Question 3

What sophisticated methods do plants use to convert these light signals?

Answer 3

They have photoreceptors, a type of protein that senses light at different wavelengths.

Question 4

What photoreceptor detects red light?

Answer 4

Phytochrome detects the presence/absence of red light.

Question 5

How does phytochrome ‘catalyze’ the germination of seeds?

Answer 5

Since seeds have very limited nutrient reserves, they germinate only when triggered by optimal conditions.

This response is regulated by the red-far-red light photoreceptor, phytochrome, which brings about a structural shift.

Question 6

At which wavelength is germination maximized?

Answer 6

660 nm, being the wavelength of red.

Question 7

What is the on/off switch-like response of germinating seeds?

Answer 7

The last flash of light, whether it be red (max germination), or the reversed far-red light (730nm), dictates whether the seed will germinate.

If the last flash is red, then the seed will germinate.
On the contrary, if the last flash is far-red, the seed will not germinate.

Question 8

What is the key feature of the phytochrome photoreceptor?

Answer 8

The red-far-red (R/FR) reversibility!

Question 9

What is light-triggered development called?

Answer 9

Photomorphogenesis

Question 10

What other response is mediated by phytochrome?

Answer 10

The de-etiolation of seedlings - their greening, straightening, expanding.

Question 11

How much light is needed to trigger a response?

Answer 11

A minuscule amount! Even a few seconds does the trick.

Question 12

What is phytochrome composed of?

Answer 12

-Chromophore: Light-absorbing molecule, with a double bond that isomerizes from a cis to trans conformation when red light hits it.

-Protein component: The structural shift of the chromophore is sensed by the protein kinase domain.
It then phosphorylates or alters other proteins, in the signal transduction cascade.

Question 13

What are the two forms of phytochrome?

Answer 13

Pr and Pfr.

• Pr absorbs red light and is immediately converted to Pfr.
• Pfr absorbs far-red light and is quickly converted to Pr.

Question 14

Which is the physiologically active form of phytochrome?

Answer 14

Pfr.
Exposure to red light converts inactive Pr form to the active Pfr form, yielding physiological activity in the plant.
Exposure to far-red light converts the active Pfr form to the inactive Pr form, inhibiting phytochrome activity.

Question 15

When in Pfr form, what types of responses occur?

Answer 15

• Seed germination
• Inhibition of vertical growth
• Stimulation of branching
• Setting internal clocks
• Control of flowering

Question 16

What form does phytochrome revert to in the dark?

Answer 16

The Pr form (inactive).

Question 17

How can plants sense that they are being shaded by other plants?

Answer 17

Light passes differentially through a leaf canopy, with more far-red light transmitted than red light. In other words, the canopy screens out more of the red light.

The plants detect this red:far-red ratio in light, thanks to their phytochrome system.

Question 18

How do plants respond to shade?

Answer 18

They induce a shade-avoidance response, by greater stem elongation!

Question 19

Why do shade-intolerant plants respond much more to light?

Answer 19

Shade tolerant species make them more competitive than they would be under more light because they can outgrow less shade-tolerant species. In other words, they compete better in shade.

Shade-intolerant species cannot capture enough light under the shade, and thus, cannot compete.

Question 20

Why is the detection of seasons and day length important?

Answer 20

Many processes require the plant to know the time in the yearly cycle, such as flowering, setting buds, making seeds/tubers.

Question 21

What is the key indicator of the season in temperate climates?

Answer 21

Day length!

Question 22

What is photoperiodism?

Answer 22

The ability of plants to measure day length, being the relative length of dark and light periods.

Question 23

How do plants control flowering?

Answer 23

Short-day plants only flower when the days are short, thus in winter.
Light period < Critical length

Long-day plants only flower when the days are long, thus in summer.
Light period > Critical length

Day-neutral plants use other signals than day length to stimulate flowering.

Question 24

Do plants measure the length of the day or that of the night?

Answer 24

The length of the night - DARK CYCLE.

SD plants flower when NIGHTS ARE LONGER THAN the critical length (usually 12-16 hours) - Minimum night length

LD plants flower when NIGHTS ARE SHORTER THAN the critical length - Maximum night length

Question 25

What happens to the photoreceptors when a flash of light interrupts the night?

Answer 25

It resets the night, making it appear short. Thus, short-day plants will not flower!
On the contrary, if the night is interrupted, long-day plants will flower!

Question 26

What are the three important consequences of photoperiodism?

Answer 26

1. Many garden flowers come into bloom at specific times in the summer. This has nothing to do with when they were planted, but rather, it has to do with the photoperiod!
2. Spinach and lettuce bolt and produce flowers in the spring. They are long-day plants.
3. Potatoes originate from the equatorial regions. Only after variants selected, that could flower under long-day conditions, could this crop be widely grown in Europe.

Question 27

What is phototropism?

Answer 27

A feature in which plants grow towards the light.

Question 28

Where is the action spectrum for phototropic growth?

Answer 28

The action spectrum is in the blue region.

Question 29

What are the two blue light receptors, and what are their structures?

Answer 29

Phototropin and cryptochrome. Both are proteins with a chromophore that absorbs blue light.

Question 30

What growth hormone is involved in phototropism?

Answer 30

Auxin.

Question 31

What other blue-light triggered responses exist?

Answer 31

Stomatal opening, at the beginning of the day.

Question 32

What photoreceptor is involved in phototropism?

Answer 32

Phototropin