Light and Environmental Signals (Lecture 6) Flashcards
Why do plants need to interpret signals from their surroundings?
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!
What time of information does sunlight contain?
It contains information about…
- Seasons: Day length
- Shade vs Direct Light: Light quality
- Directionality
What sophisticated methods do plants use to convert these light signals?
They have photoreceptors, a type of protein that senses light at different wavelengths.
What photoreceptor detects red light?
Phytochrome detects the presence/absence of red light.
How does phytochrome ‘catalyze’ the germination of seeds?
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.
At which wavelength is germination maximized?
660 nm, being the wavelength of red.
What is the on/off switch-like response of germinating seeds?
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.
What is the key feature of the phytochrome photoreceptor?
The red-far-red (R/FR) reversibility!
What is light-triggered development called?
Photomorphogenesis
What other response is mediated by phytochrome?
The de-etiolation of seedlings - their greening, straightening, expanding.
How much light is needed to trigger a response?
A minuscule amount! Even a few seconds does the trick.
What is phytochrome composed of?
-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.
What are the two forms of phytochrome?
Pr and Pfr.
- Pr absorbs red light and is immediately converted to Pfr.
- Pfr absorbs far-red light and is quickly converted to Pr.
Which is the physiologically active form of phytochrome?
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.
When in Pfr form, what types of responses occur?
- Seed germination
- Inhibition of vertical growth
- Stimulation of branching
- Setting internal clocks
- Control of flowering
What form does phytochrome revert to in the dark?
The Pr form (inactive).
How can plants sense that they are being shaded by other plants?
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.
How do plants respond to shade?
They induce a shade-avoidance response, by greater stem elongation!
Why do shade-intolerant plants respond much more to light?
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.
Why is the detection of seasons and day length important?
Many processes require the plant to know the time in the yearly cycle, such as flowering, setting buds, making seeds/tubers.
What is the key indicator of the season in temperate climates?
Day length!
What is photoperiodism?
The ability of plants to measure day length, being the relative length of dark and light periods.
How do plants control flowering?
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.
Do plants measure the length of the day or that of the night?
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
