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

Light wavelengths detected by plant light receptors 100 chlorophyll b 80 60 Percent of light absorbed chlorophyll a 40 20 Figure 10.5: Absorption spectra of chlorophylls a and b at different wavelengths of light. Graph shows the fraction of received light that is absorbed when the pigment is exposed to various wavelengths of light. The relation between wavelength and color is also shown. 400 500 600 700 Fig. 10-5, p. 152 Wavelength (nm) Cryptochromes and Phototropins Phytochromes

Red light detection: Phytochromes

Red Light and Plant Development To maximize photosynthesis Phytochromes : 1) promote seed germination 2) promote de-etiolation 3) control shade avoidance 4) control circadian entrainment 5) control flowering

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. long day, short night short day, long night flowers white light short day, interrupted night Figure 15.22: Effect of night length on short-day plants. From top to bottom: (a–c) Experiments demonstrate that the length of night, not day, is the critical signal that stimulates flowering. (d,e) Experiments show that phytochrome is the receptor by which the plant perceives an interruption of night. red light short day, red interruption red far-red short day, red followed by far-red flowers Fig. 15-22, p. 253

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.

, leading to a change in its activity. Far Red light Red light

Active version of Phytochrome: Promotes seed germination, shade avoidance, and controls circadian entrainment, flowering, etc… Inactive version of Phytochrome Figure 15.19: The red/far-red response. (b) Light changes the form of phytochromes.

Absorption spectra of Chlorophyll a and b Fig. 10-5, p. 152 Wavelength (nm) 400 500 600 700 20 40 60 80 100 chlorophyll b chlorophyll a Percent of light absorbed Figure 10.5: Absorption spectra of chlorophylls a and b at different wavelengths of light. Graph shows the fraction of received light that is absorbed when the pigment is exposed to various wavelengths of light. The relation between wavelength and color is also shown. 660 730 The 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-etiolation Seedlings grown in the dark display an etiolated growth pattern: yellow unexpanded cotyledons apical hook 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.

Phytochrome controls shade avoidance 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.

Shade avoidance and Red:Far Red ratio Active phytochrome

Blue light detection: Phototropins

Blue Light and Plant Development To maximize photosynthesis Phototropins promote: 1) Phototropism 2) Chloroplast movement 3) Stomatal opening

See also lecture on auxin effects on plant development.

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

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 Fig. 11-9b, p. 170 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) .

Blue light detection: Cryptochromes

Blue Light and Plant Development To maximize photosynthesis Cryptochromes : 1) promote de-etiolation 2) control circadian entrainment 3) 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.