1. Blue-light Responses: Morphogenesis and Stomatal Movements
BIOL3745 Plant Physiology
Unit 3
Chapter 18
Blue-light Responses:
Morphogenesis and Stomatal Movements

2. Phototropism: directional growth toward (or away from) the light
Blue light responses
Blue-light: 400 – 500 nm
Phototropism: directional growth toward (or away from) the light
Stimulate stomatal opening movement
Inhibition of hypocotyl elongation involving membrane depolarization
Gene activation
Pigment biosynthesis
Tracking the sun by leaves
Chloroplast movements within cells

3. A characteristic “three-finger” action spectrum in 400-500 nm
Figure Action spectrum for blue light–stimulated phototropism in oat coleoptiles
PP5e-Fig jpg
A characteristic “three-finger” action spectrum in nm

4. Figure 18.2 Relationship between direction of growth and unequal incident light
PP5e-Fig jpg

5. Bending is due to asymmetric growth
Figure Time-lapse photograph of a corn coleoptile growing toward unilateral blue light
Bending is due to asymmetric growth
PP5e-Fig jpg

6. Figure 18.4 Phototropism in wild-type (A) and mutant (B) Arabidopsis seedlings
PP5e-Fig jpg

7. Figure Blue light–induced changes in elongation rates of etiolated cucumber seedlings and transient membrane depolarization of hypocotyl cells
PP5e-Fig jpg

8. Figure 18.6 Light-stimulated stomatal opening in detached epidermis of Vicia faba (broad bean)
PP5e-Fig jpg

9. Figure 18.7 Stomatal opening tracks photosynthetic active radiation at the leaf surface
PP5e-Fig jpg

10. Figure 18.8 Response of stomata to blue light under a red-light background
PP5e-Fig jpg

11. Figure 18.9 Action spectrum for blue light–stimulated stomatal opening
PP5e-Fig jpg

12. Figure 18.10 Blue light–stimulated swelling of guard cell protoplasts
Blue-light stimulated stomatal movements are driven by changes in the osmoregulation of guard cells
PP5e-Fig jpg

13. Figure 18.11 Acidification of a suspension medium of guard cell protoplasts of V. faba
PP5e-Fig jpg
Blue light stimulates an H+-ATPase at the guard cell plasma membrane, generating an electrochemical potential gradient that drives ion uptake

14. Figure 18.12 Activation of the H+-ATPase at the plasma membrane of guard cell protoplasts
PP5e-Fig jpg

15. Figure 18.13 Osmoregulatory pathways
PP5e-Fig jpg
Blue light stimulates K+ and its counter ions (chloride), malate, sucrose accumulation

16. Figure 18.14 Daily course of changes in stomatal aperture
PP5e-Fig jpg

17. Figure 18.15 Role of the proton-pumping ATPase in the regulation of stomatal movement
PP5e-Fig jpg
Activation of the enzyme involves the phosphrylation of serine and threonine residues in its C-terminal domain regulatory protein binds to the phosphrylated C terminus.

18. Blue-light photoreceptors
There are three types of photoreceptors associated with blue-light responses
Cryptochromes: function primarily in the inhibition of stem elongation and flowering;
Phototropins: function primarily in phototropism;
Zeaxanthin: functions in the blue-light response of stomatal movement.

19. Figure 18.16 Blue light effects on transgenic and mutant seedlings of Arabidopsis
PP5e-Fig jpg

20. Blue-light photoreceptors
In Arabidopsis, the genes cry1 and cry2 are involved in blue light dependent inhibition of stem elongation, cotyledon expansion, anthocyanin synthesis, control of flowering, and setting of circadian rhythms;
The CRY1 protein, and to a less extent CRY2, accumulate in the nucleus and interact with ubiquitin ligase COP1, both in vivo and in vitro.
The activity of cryptochrome is influenced by its phosphorylation state. The CRY1 protein also regulates anion channel activity at the plasma membrane.
The protein phototropin aids in regulation of phototropism. Phototropin binds the flavin FMN and autophosphorylates in response to blue light.

21. Figure 18.17 Adduct formation of FMN and a cysteine residue of phototropin protein
PP5e-Fig jpg

22. Figure 18.18 Sensory transduction process of blue light–stimulated inhibition of stem elongation
PP5e-Fig jpg

23. Blue light photoreceptors
Phototropin-less mutants are defective in phototropism and in chloroplast movement.
Blue light suppression of stem elongation by blue light is initiated by phot1, with cry1, and to a limited extent cry2, modulating the response.

24. npq1: zeaxanthin-less mutant;
Figure Blue-light sensitivity and responses of Arabidopsis mutants and wild type
npq1: zeaxanthin-less mutant;
phot1/phot2: phototropin-less double mutant
No stomatal opening in the above mutants when illuminated with 10 μmol m-2 s-1 blue light.
PP5e-Fig jpg

25. Figure 18.20 Zeaxanthin content tracks photosynthetically active radiation and stomatal apertures
PP5e-Fig jpg

26. Zeaxanthin
The chloroplast carotenoid zeaxanthin is implicated in blue-light photoreception in guard cell.
Daily stomatal opening, incident radiation, zeaxanthin content of guard cells, and stomatal apertures are closely related.
The absorption spectrum of zeaxanthin matches the action spectrum for blue-light stimulated stomatal opening.
Blue light stimulated stomatal opening is blocked if zeaxanthin accumulation in guard cells is blocked. Manipulation of zeaxanthin content is guard cells permits regulation of their response to blue light.

27. Figure 18.21 Absorption spectrum of zeaxanthin in ethanol
PP5e-Fig jpg

28. Figure 18.22 Role of zeaxanthin in blue light sensing in guard cells
PP5e-Fig jpg

29. Figure 18.23 Blue–green reversibility of stomatal movements
PP5e-Fig jpg

30. Acidification of the lumen stimulates zeaxanthin formation.
Blue light signal transduction for guard cells crosses the chloroplast envelop, activates the H+-ATPase, and produces turgor buildup to accomplish stomatal opening.
Acidification of the lumen stimulates zeaxanthin formation.
The blue-light response of guard cells is reversed by green light.

31. Figure Absorption spectrum of blue–green reversal of the orange carotenoid protein (cyanobacteria)
PP5e-Fig jpg

32. Chemical structures of zeaxanthin and 3-hydroxyechinenone
PP5e-ITA-18-p538.jpg

33. Blue-light response
A carotenoid-protein complex in cyanobacteria, the orange carotenoid protein, shows blue-green reversibility and function as a light sensor. It provides a molecular model for blue light sensing by zeaxanthin in guard cells.
Guard cells respond to blue light using three different sensory transduction pathways, mediated by
A specific blue light photoreceptor;
Photosynthesis in the guard cell chloroplast;
phytochrome