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

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

A characteristic “three-finger” action spectrum in 400-500 nm Figure 18.1 Action spectrum for blue light–stimulated phototropism in oat coleoptiles PP5e-Fig-18-01-0.jpg A characteristic “three-finger” action spectrum in 400-500 nm

Figure 18.2 Relationship between direction of growth and unequal incident light PP5e-Fig-18-02-0.jpg

Bending is due to asymmetric growth Figure 18.3 Time-lapse photograph of a corn coleoptile growing toward unilateral blue light Bending is due to asymmetric growth PP5e-Fig-18-03-0.jpg

Figure 18.4 Phototropism in wild-type (A) and mutant (B) Arabidopsis seedlings PP5e-Fig-18-04-0.jpg

Figure 18.5 Blue light–induced changes in elongation rates of etiolated cucumber seedlings and transient membrane depolarization of hypocotyl cells PP5e-Fig-18-05-0.jpg

Figure 18.6 Light-stimulated stomatal opening in detached epidermis of Vicia faba (broad bean) PP5e-Fig-18-06-0.jpg

Figure 18.7 Stomatal opening tracks photosynthetic active radiation at the leaf surface PP5e-Fig-18-07-0.jpg

Figure 18.8 Response of stomata to blue light under a red-light background PP5e-Fig-18-08-0.jpg

Figure 18.9 Action spectrum for blue light–stimulated stomatal opening PP5e-Fig-18-09-0.jpg

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-18-10-0.jpg

Figure 18.11 Acidification of a suspension medium of guard cell protoplasts of V. faba PP5e-Fig-18-11-0.jpg Blue light stimulates an H+-ATPase at the guard cell plasma membrane, generating an electrochemical potential gradient that drives ion uptake

Figure 18.12 Activation of the H+-ATPase at the plasma membrane of guard cell protoplasts PP5e-Fig-18-12-0.jpg

Figure 18.13 Osmoregulatory pathways PP5e-Fig-18-13-1.jpg Blue light stimulates K+ and its counter ions (chloride), malate, sucrose accumulation

Figure 18.14 Daily course of changes in stomatal aperture PP5e-Fig-18-14-0.jpg

Figure 18.15 Role of the proton-pumping ATPase in the regulation of stomatal movement PP5e-Fig-18-15-0.jpg Activation of the enzyme involves the phosphrylation of serine and threonine residues in its C-terminal domain. 14-3-3 regulatory protein binds to the phosphrylated C terminus.

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.

Figure 18.16 Blue light effects on transgenic and mutant seedlings of Arabidopsis PP5e-Fig-18-16-0.jpg

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.

Figure 18.17 Adduct formation of FMN and a cysteine residue of phototropin protein PP5e-Fig-18-17-0.jpg

Figure 18.18 Sensory transduction process of blue light–stimulated inhibition of stem elongation PP5e-Fig-18-18-0.jpg

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.

npq1: zeaxanthin-less mutant; Figure 18.19 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-18-19-0.jpg

Figure 18.20 Zeaxanthin content tracks photosynthetically active radiation and stomatal apertures PP5e-Fig-18-20-0.jpg

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.

Figure 18.21 Absorption spectrum of zeaxanthin in ethanol PP5e-Fig-18-21-0.jpg

Figure 18.22 Role of zeaxanthin in blue light sensing in guard cells PP5e-Fig-18-22-0.jpg

Figure 18.23 Blue–green reversibility of stomatal movements PP5e-Fig-18-23-0.jpg

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.

Figure 18.26 Absorption spectrum of blue–green reversal of the orange carotenoid protein (cyanobacteria) PP5e-Fig-18-26-0.jpg

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

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