Light regulation of plant development

Light and Plant Development• Plants detect parts of the light spectrum that are relevant for

photosynthesis.

• Classes of major plant photoreceptors:

– 1) Phytochromes: detect red light

– 2) Cryptochromes: detect blue light

– 3) Phototropins: detect blue light

Fig. 10-5, p. 152

Wavelength (nm)

400 500 600 7000

20

40

60

80

100

chlorophyll b

chlorophyll a

Per

cent

of

light

abs

orbe

dLight wavelengths detected by plant light receptors

PhytochromesCryptochromes and Phototropins

Red light detection:

Phytochromes

Red Light and Plant Development

• To maximize photosynthesis

Phytochromes :

1) promote seed germination2) promote de-etiolation3) control shade avoidance4) control circadian entrainment5) control flowering

Fig. 15-22, p. 253

long day,short night

short day,long night

short day,interrupted night

short day,red followedby far-red

short day,red interruption

white light

red light

red far-red

flowers

flowers

History of Phytochrome discovery

Short-day plants flower only when nights are sufficiently long. When long nights are interrupted by a short dose of white light, flowering is again delayed. The active wavelength for this light-response was found to be red light. Moreover, the effect of the red light treatment could be suppressed by treatment with far red light.

Suggests the existence of a receptor protein that is activated by red light and inhibited by far red light.

History of Phytochrome discovery

Phytochrome was also shown to control the germination of seeds. Red light (activates the receptor) promotes seed germination and far red light suppresses the red light effect.

The predicted properties of the receptor

A protein linked to a chromophore.

The chromophore (a tetrapyrrole compound) allows phytochrome to change in response to red or far-red light.

Red light

Far Red light

leading to a change in its activity.

,

Active version of Phytochrome:

Promotes seed germination, shade avoidance, and controls circadian entrainment, flowering, etc…

Inactive version of Phytochrome

Fig. 10-5, p. 152

Wavelength (nm)

400 500 600 7000

20

40

60

80

100

chlorophyll b

chlorophyll a

Per

cent

of

light

abs

orbe

d

Absorption spectra of Chlorophyll a and b

660 730The ratio of Red (660 nm) to Far Red (730 nm) light will be low underneath green leaves that absorb light between 640 and 700 nm.

Phytochrome promotes de-etiolationSeedlings grown in the dark display an etiolated growth

pattern:1) yellow unexpanded cotyledons2) apical hook 3) Long hypocotyl

Seedlings grown in red light (or white light) display a de-etiolated growth pattern (opposite to etiolated):

• Green expanded cotyledons• No apical hook• Short hypocotyl

Red light promotes chloroplast development and leaf expansion. Leaves (cotyledons) are also growing in upright position, allowing optimal light impact. Active phytochrome promotes seedling development that is optimal for photosynthesis.

Phytochrome controls shade avoidance

Seedlings that are shaded by larger (taller) plants that grow above them will show a shade avoidance response.

A shade avoidance response involves increased elongation growth (stems and petioles) and inhibition of leaf expansion.

As a result, the seedling will grow “above” of what causes the shade and will now be able to perform more efficient photosynthesis.

As soon as the seedling is not anymore shaded, shade-avoidance growth stops.

The shade avoidance response is controlled by Phytochromes and results from changes in the ratio of red to far-red light.

Chlorophyl from plants that grow above the shaded

seedling absorb blue and red light (but not far red light). The result is a lower ratio of red to far-red light received by the shaded plant. Lower levels of red light compared to far-red light means a lower level of active Phytochrome (Pfr) compared to inactive Phytochrome (Pr).

Lower level of active Phytochrome will lead to more elongation growth (see etiolation versus de-etiolation) and less leaf expansion.

Phytochrome controls shade avoidance

Shade avoidance and Red:Far Red ratio

Active phytochrome

Blue light detection:

Phototropins

Blue Light and Plant Development

• To maximize photosynthesis

Phototropins promote:

1) Phototropism2) Chloroplast movement3) Stomatal opening

See also lecture on auxin effects on plant development.

Chloroplasts move towards the source of light (too maximalize light harvest)

Chloroplasts move away from the source of light (to minimize damage by the excess light energy).

(more energy reaches the leaf)(too much light)

High-light avoidance

The Chinese character for "light" on an Arabidopsis leaf. This image was created by exploiting the plant chloroplasts' protective response to strong light. Upon selective irradiation of the area within the character, chloroplasts in this region move from the cell surface to the side walls when light is detected by the blue light receptor NPL1. The leaf surface then appears paler in color in the irradiated area. [Image: M. Wada]

Phototropins and stomatal opening

Light affects the opening of stomata. In dim or no light, the stomata are closed; as the light intensity increases, the stomata open up to some maximum value.

The blue part of the light spectrum is responsible for this response.

Blue light is perceived by phototropins that then promote the increase in solute concentration of guard cells starting with the conversion of starch into malic acid (see lectures on absorption and transportation) .

Fig. 11-9b, p. 170

Blue light detection:

Cryptochromes

Blue Light and Plant Development

• To maximize photosynthesis

Cryptochromes :

1) promote de-etiolation2) control circadian entrainment3) control flowering

Cryptochromes promote de-etiolation

Similar to Phytochromes, Cryptochromes promote the de-etiolation of seedlings and control the timing of flowering. However, in this case the response depends on blue light (not red).

The combined effects of red and blue light in promoting de-etiolation is stronger than treatments with only red or only blue light.

Cryptochromes and Phytochromes enhance each others effects in promoting seedling de-etiolation.

When plants are exposed to both red and blue light, their growth responses become optimal for light harvesting. Light harvesting is done from the red and blue parts of the light spectrum.