Presented by:

o BHAUGEEROTHEE

Trilov

o BHOGUN Shivan

o GHANTY Warren

o NEMORIN Julie

o PULTOO Ruma

o RUNGHEN Rogini

o YADALLEE Jehaan

1)Introduction

a. Types of photoreceptors

b. Fluence and action spectra

2)Phytochromes

3)Cryptochromes

4)Phototropins

5)Physiological responses of photoreceptors

6)Superchromes

7)Conclusion

Photoreceptors are defined as proteins

found in a variety of organisms, which

perceive light signals and cause the organisms

to respond to them in various ways.

In plants, photoreceptors transduce these

light signals to influence their growth and

development, as well as their reproductive

strategies.

Plants have 3 basic responses to light:

Photosynthesis: Radiant energy is converted into chemical energy.

Chlorophylls and accessory pigments absorb light in the visible spectrum.

Phototropism: Movement of a plant in response to light (due to the actions of

phytohormones, e.g. auxins).

Photoperiodism: Response of plants to changes in the duration of light or darkness

in a 24-hour cycle.

o The response to a constant fluence at the 5 different wavelengths can be measured. For instance, at fluence 1 μmolm-2, it is observed that at 550 and 575 nm, the fluence is above saturation; and at 525 nm, it is close to saturation.

Fluence response curves for a hypothetical photoresponseFluence response curves for a hypothetical photoresponse

o It is better to plot the fluence necessary to generate a constant response.

Fluence and Action Spectra:

o Plotting the response to this fluence as a function of wavelengths would give the above red curve, with flattened peaks.

Action spectra for a hypothetical photoresponse constructed from the data in the previous figure

o Plotting the 5 fluence values v/s their respective wavelengths would yield the above blue curve, i.e. our action spectrum. A well-defined peak which represents the wavelength at which the response is most effective.

Action spectra for a hypothetical photoresponse constructed from the data in the previous figure

1) Biological responses vary in their fluence requirements.

Very low fluence responses (VLFRs) – Initiated

and saturated at very low fluences (~ 0.1 nmolm-2)

Low fluence responses (LFRs) – Initiated at

higher fluences (1.0 μmolm-2)

High irradiance responses (HIRs) – Saturated at

much higher fluences (10 mmolm-2), and especially

require long exposure. 2) Biological responses vary in their action spectra.

The most effective wavelengths are those requiring

the fewest number of photons to elicit a certain level

of response.

2 points to note…

Discovery of the photoreceptors:

Phytochromes:

From late 1940’s to early 1960’s, S. Hendricks (a

physical chemist) and H. Borthwick (a botanist)

discovered that red light was very effective for boosting

germination and flowering, and that far-red light

reversed these responses.

In 1959, it was discovered that the substance

involved in these antagonistic responses cycled between

2 absorbing forms: one in red region, and the other in

far-red region.

In 1983, the substance was purified and found to be a

chromoprotein. It was named “phytochrome”. The red-

absorbing form was called Pr, and the far-red one Pfr.

Cryptochromes and Phototropins:

Discovered in the 1990’s.

Phytochrome 3 (phy3):

Discovered more recently (in 1998) by Nozue and co-

workers; occasionally termed “superchrome” (Briggs

and Olney, 2001).

System of Nomenclature

Types of Phytochrome

Structure and Properties of Phytochrome

Structure and Properties of Phytochrome

Structure and Properties of Phytochrome

Synthesis of Phytochromes

Synthesis of Phytochromes

Synthesis of Phytochromes• The C-3 double bond of PΦB is then

isomerised. It is thought that phy itself can catalyse this reaction.

• Phy apoprotein binds to the 3E- PΦB in the cytoplasm to yield the Pr form of the photoreceptor. This reaction requires the bilin lyase domain (BLD) of the photoreceptor.

• Absorbtion of red light triggers a “Z” to “E” isomerisation in the C-15 double bond between the C and D rings of the linear tetrapyrrole, resulting in the far-red light-absorbing form Pfr .

• Conformational changes in the protein backbone are required to maintain this high energy state of the photoreceptor.

• Pfr is converted back to Pr either through a slow dark reversion (not induced by light) or via a fast reaction upon absorption of far-red light.

Phytochrome Evolution in Plants and Evolutionary Relationships among PHYGenes

Phytochrome Evolution in Plants and Evolutionary Relationships among PHYGenes

Absorption spectra of Phytochromes

• Pr has a high absorbance at 660 nm and 667 nm which is the red region and has a little absorbance in the far red light region.

• On the other hand Pfr has optimum absorbance in the far red region (~730nm) and also rather considerable absorption at the red region.

• Since both absorb in the red and far red region as shown in fig, there is never complete phototransformation but rather a dynamic equilibrium known as the photostationary state between the Pr and Pfr.

• ф symbol denotes the photostationary state which is the ratio of Pfr to the total phytochromes (P) in a light environment.

Pfr/P• This ratio varies among species but it is usually

0.8 except in sunlight as there are a large number of intermediates produced from the multi-step conversion of Pfr to Pr and vice-versa.

Light stability and selective degradation of phytochrome A

• Phytochrome A is the most unstable among all the phytochromes. It has a half life of 100hr in the dark in the inactive form and a half life of 60 hr in red or white light when converted to the active Pfr A form.

• Prior to degradation, PHYA are gathered into large particles of about 1µm.

• Both the lysine rich residues and the highly conserved PEST sequence (rich in proline (P), glutamic acid (E), serine (S), and threonine (T)) are exposed.

• Degradation occurs through ubiquitination which is the marking of the PHYA apoprotein by ubiquitin tails and then the protein is hydrolysed by a 26s proteasome which will cut the PHYA into fragments.

Cryptochromes

Structure of Cryp1 and Cryp2Absorption spectra

Cryptochromes• originally a generic term to describe blue light

receptors; today it now refers to the photolyase-like proteins(Cashmore AR (2003);Lin C, Shalitin D (2003);Sancar A (2003)).

• can be subdivided into three groups: plant cryptochrome, animal cryptochrome and CRY – DASH found in humans (Brautigam, CA et al., 2004)

• The organism whereby most research has been carried out on cryptochrome is the plant Arabidopsis thaliana

CRYs in Arabidopsis thaliana

• at least three cryptochromes have been identified namely cry1, cry2 and cry 3. Cry1 and cry2 are coded by the genes CRY 1 and CRY 2 respectively (Ahmad et al., 2002).

• Cry1 and cry2 are found predominantly in the nucleus and the cytoplasm whereas cry 3 is found in the organelles.

Structure of CRYs

• Cry 1 and cry 2 are structurally similar proteins (Srivastava, 2001).

• They both possess an N- terminal domain, also referred as the PHR domain (homology photolyase domain) which is involved in light perception and a C- terminal domain, also referred as the CCT domain, which is involved in signaling.

Cry 1• N- terminal domain: bears some similarity to bacterial photolyases in that it

possesses two chromophores namely pterin and flavin adenine dinucleotide (FAD) (Lin et al, 1995a; Malhotra et al,1995) which are non-covalently bounded to it. However, cry 1 has no photolyase activity this is thought to be mainly due to the features of the protein and the dissimilarity of a surface cavity.

The FAD bound to the CRY 1 apoprotein is believed to alternate between the oxidized state and a semi reduced but semiquinone form of FADH˚ not the fully reduced FADH2.

• C-terminal domain:absent from the photolyase has no strong

sequence similarity with known protein domains.

The C-terminal normally varies in size and amino acid sequence depending on the type of cryptochrome.

Cry 2

• The identity of the chromophores present in Arabidopsis are still unknown, however it is thought to be a flavin supplement which shows the absence of photolyase activity.

Cry1 v/s cry 2

Cry 1• Stable in light

• Active at higher fluences of blue light

• Plays a role in the setting of biological clock

Cry 2• Do not accumulate, degrade

quickly in green, blue and UV-A light

• Active at lower fluences of blue light

• Involved in flowering response

Action spectrum of CRYs

• The absorption spectrum for cryptochromes are quite difficult to determine mainly because multiple photoreceptors are active in blue light (wavelength between 400nm to 500nm)

unrelated flavoproteins have been identified to absorb maximally at 450 nm.

phytochromes which normally respond to red/ far- red light have also shown some absorption of blue light

• In higher plants, an action spectrum for CRYs are even more difficult to determine as it has been observed that phytochromes are necessary for the full activity of cryptochromes in hypocotyl growth inhibition and anthocyanin accumulation.

• In a study carried out by Ahmad et al. in 2002, an action spectrum was determined in-vivo for a cry-mediated high-irradiance response i.e. the blue light dependent inhibition of hypocotyl elongation In Arabidopsis.

Action spectrum for cryptochrome-dependent hypocotyl growth inhibtion in Arabidopsis

(Ahmad et al., 2002)• The growth response of wild-type Arabidopsis

seedlings were compared to that of double cryptochrome cry1cry2 mutants to determine an activity spectrum for cryptochrome.

• Possible wavelength-specific effects was determined by evaluating the responsivity of seedlings overexpressing CRY 1 protein since it showed hypersensitive response to blue light under broad condition.

• Observations: In darkness, the length of the various hypocotyls were almost the same

(refer to figure 4.) Cry1cry2 double mutant seedlings did not show growth inhibition at any

wavelength of monochromatic light tested between 311 nm and 550nm Both wild-type and CRY1-overexpressing seedlings showed maximal

growth inhibition between 380 nm and 500 nm Little response was seen below 365 nm, using the light fluence 0.5 to 1.0

µmol/ m2/ s Little growth inhibition of wild type was observed from 500nm to 550nm At wavelengths longer than 570 nm, there were no detectable differences

between light-dependent hypocotyl growth inhibition between cryptochrome-deficient and cryptochrome overexpressing seedlings even at high light intensities.

• According to those results it can be concluded that crytochrome is active principally in the range from 365 nm to 550 nm.

Phototropin

• Phototropin belong to the AGC family of kinase (cAMP-dependent protein kinase, cGMP-dependent protein kinase G & phospholipid dependent protein kinase C) and are members of the AGC-VIII subfamily.

• Phototropin are light activated serine/threonine kinase. The original discovery of phototropin was made in the Arabidopsis thaliana mutant nph1 (non-phototropic hypocotyls mutant 1) (Liscum & Briggs, 1995 & Huala et al, 1997).

• The mutant was deficient in blue light & induces phototropic curvature. Hence, the gene responsible for the nph1 phenotype was named phototropin.

• The effect of blue light on plant development have been identified for almost two centuries (Briggs, 2006)

• Phototropins are blue-light receptors controlling a range of responses that serve to optimize the photosynthesis efficiency in plants.

• These include phototropism, light-induced stomatal opening, & chloroplast movement in response to changes in light intensity.

• Phototropin is ubiquitous in higher plants and have been identified in several plants:Araddopsis thaliana, Avena sativa (oats), Oryza sativa (rice) and Zea mays (corn).

• The phototropin are called Phot. In most phototropin containing species there are two phototropins; Phot 1 & Phot 2 present. Arabidopsis contains two phototropin: phot 1 and phot 2 (Briggs WR, Christine JM, 2002)

The diagram shows Phot1 & Phot 2 activated by blue light and overlap in function to mediate several

responses.

Biological functions of phototropin

• (a) Phototropic response of wild-type Arabidopsis seedlings (top panel) and a phototropin-deficient mutant (bottom panel) irradiated with blue light from the right.

• (b) Chloroplast positioning in leaf cells of Arabidopsis. Microscope images represent cells viewed from above. Accumulation movement of chloroplasts to the cell surface (top panel) occurs when leaves are irradiated from above with low-intensity blue light. Avoidance movement of chloroplasts (bottom panel) to the cell sidewalls occurs when leaves are irradiated with high-intensity blue light.

• (c) Leaf positioning in wild-type (left) and phototropin-deficient Arabidopsis plants (right) as indicated by the white arrows in response to irradiation from above.

• (d) Rosette leaves from mature wild-type (left) and phototropin-deficient Arabidopsis (right). The leaf from the mutant was unexpanded and curled and therefore lies on its side.

In each phototropin (Phot), there are two LOV domains (light, oxygen, voltage sensitive domain). They are subset of proteins within the PAS domain super family.

Phototropin structure

• Lov domain binds the co-factor flavin mononucleotide (FMN) & function as blue light sensors for the protein. Each Lov domain binds oxidized FMN as chromophore. The FMN is non-covalently bound to the Lov domain.

Photo-excitation of the Lov domains by blue light leads to phototropin receptor autophosphorylation.

Lov domain structure & function (Christie,2007, Phototropin Blue Light Receptors).

• Genetic analysis using Arabidopsis has been instrumental in identifying the molecular nature of the phototropin & establishing their roles as blue light receptors.

• Purification of milligram quantities of bacterially expressed Lov domains has greatly facilitated the spectral & structural analysis of blue light sensing motifs.

• Upon illumination Lov domains undergo a photocycle that can be monitored by absorbance of fluorescence spectroscopy.

Lov domain structure & photochemistry

• In darkness, Lov domains bind FMN non-covalently forming a spectral species, designated Lov447 which absorbs maximally near 447 nm.

• Irradiation of the domain induces a unique model of photochemistry that involves the formation of a covalent adduct between the FMN chromophore and conserved cystein residue within the lov domain.

• Irradited with light drives the production of a highly reactive triplet- state flavin (Lov660) that leads to the formation of a covalent bond between the carbon of the FMN chromophore & conserved cystein residue within the (Lov390 )

• Light driven FMN-cysteinyl adduct formation occurs producing a spectral species (Lov390 ) that absorb maximally at 390nm. For phototropin Lov domains, formation of Lov390 is fully reversible in darkness, returning Lov to its initial ground state (Lov447 ).

• Therefore, Lov domains cycle between active Lov390 and inactive Lov447 states depend on light conditions. Illumination with UV light revert Lov390 to its initial dark state but the biology significance of the UV mediated reversible with respect to receptor is not yet clear

Functional role of Lov 1 & Lov 2• Lov1 & Lov 2 exhibit different quantum efficiencies and

photochemical reaction kinetics, and this imply that the two domains have different light sensing roles in regulating phototropin activity.

• Lov 2 has a higher quantum efficiency for light induced cysteinyl adduct formation than that of Lov 1 for both Phot1 & Phot 2.

• Lov 2 plays an important role in regulating phototropin activity whereby the exact role of Lov 1 is unknown.

Phototropin structure and mode of activation.

In the dark, phototropin is unphosphorylated and inactive. Absorption of light involves a photochemical reaction within the LOV domains.

• Photoexcitation of the main light sensor LOV2 causes disordering of the Jα-helix and activation of the C-terminal kinase domain, which consequently leads to autophosphorylation of the photoreceptor. The relative positions of known phosphorylation sites are indicated.

Despite these findings, the biochemical consequences of receptor are poorly understood. Autophosphorylation requires more light that is needed for a number of phototropin mediated responses.

PhyA, phyB to phyECry1, cry2

Limiting Factors

1. Light quality (wavelength)2. Light Quantity (fluence)3. Direction (unidirectional or diffused)4. Duration of exposure (length of day)

Wavelength and response

Photoreceptors

• Light capturing molecules: photoreceptors• These are phytochrome (phyA to phyE),

cryptochrome (cry1 and cry2 mainly), phototropins (phot1 and phot2)

Investigation of these photoreceptors.

Use of1. Overexpression of phy- and cryp- tochrome

in transgenic plants2. Mutants defect in specific known

photoreceptors (e.g. phyA mutant is a mutant whereby its phytochrome A has been inactivated)

Methods1. Overexpression of genes• Difficult to analyze and less reliable2. Mutants• Generation of mutants is possible by growing them in

etiolated or de-etiolated condition, whereby they will be deficient in a specific known photoreceptor.

• They will be identified as photomorphogenic response (light structural and developmental purposes requiring specific photoreceptors)

• Mutants not responding to the condition set, indicates a defective photoreceptor not achieving its signal transduction pathway

Isolation of the photoreceptors

Class 1 photoreceptor

• Of the four classes, only the last class, .i.e. class 4 mutants are de-etiolated phenotype

• Class 1 mutants are chromophore defective mutants– Non-etiolated+exogenus biliverdin promote

chromophore synthesis– Etiolated + exogenus supply of biliverdin

absence of class 1 photoreceptor

Class 2 (Apoprotein deficient)

• If illumination under different wavelength gives null or impaired response, the plant is said to be deficient in class 2 photoreceptors.

• Better identified when using multiple mutants combination. (e.g. phyAphyB mutants gives an attenuated response to R light whereas both phyA and phyB holotypes shows relatively good response to R light)

Class 3 Mutants defective in signal transduction

• The presence of photoreceptor is known to be functional but no response is achieved because of defective protein which is formed during signal transduction process.

• Absence of response indicate class 3 mutants, but can be also because of class 2 mutants. To resolve this issue, cloning of the protein and the sequences are compared quick difference will be noted if there had been mutation or not.

• E.g. Arabidopsis hy5 mutant

Class 4 De-etiolated seedlings

• Dark grown seedlings are exposed to light• Synthesis of protein for repressing de-

etiolation which will act as negative regulators

Physiological responses of Photoreceptors

1. Seed germination2. De-etiolation response3. Seedling establishment

1. Seed germination

• Seeds possess food reserves which can absorb light signals (FR) which has high penetrating ability than R light.

• As the light signals hit the phytochrome molecules, they stimulate seed germination processes, depending on the time and appropriate distance of the seed location in the soil.

• If Pfr/P ratio ≈ 0.1% VLFR promote some seeds to germinate

• If Pfr/P ratio >1.0% LFR promote other types of seed to germinate

Seed germination with Pfr/P ratio

• Casal et al., 1997

Seed location?

Red light from sun penetrates to seed.

No light from sun to this deep seed.

Seed germinates. No germination.

Red light to seed = near surface

Figure 5 (a) Germination of seed in different conditions:

A. Wild typeB. phyA mutantC phyB mutant

Figure 5 (b) Germination according to photon fluence range with part A showing incubation period and B shows the response with varying concentrations of red light (at 667 nm)

phyB controls seed germination in dark LFR

phyA controls seed germination in VLFR

Shinomura et al. (1994) Shinomura et al. (1996)

PhyA and phyB

phyA• Controls germination actively

in light• Requires VLFR range• Irreversible

phyB• Controls germination in dark

via PfrB form• LFR range• Reversible

Both are required for germination under different fluence range.

Real life situation, under large forest canopy phyA is stimulated by VLFR and seed germinates.The other light stable (phyB and other photoreceptors) are activated by LFR (where the R:FR ratio varies for different seeds)

2. De-etiolation response• Inhibition of stem elongation (hypo- and/or epi-

cotyl)• Cotyledon opening and their expansion• Etioplast conversion into chloroplasts.• Concomitant expression of photosynthetic genes• Defense mechanism (anthocyanin production)• HIR Gene expression biosynthesis of

flavonoid• Responses are regulated via multiple

photoreceptors.

De-etiolation response

Stem elongation inhibition• Seed germinates from VLFR to LFR range

whereby hypocotyl growth and cotyledon opening and expansion are inhibited

• De-etiolation is controlled by – phyA in VLF– phyB in LF

De-etiolation response• Continuous FR, R and B lights makes phyA, phyB

and cry1 (receptors responsible for de-etiolation) to become useless when exposed to high irradiance

• Also involve gene expressions which are more easily analyzed by photosynthetic or anthocyanin biosynthesis than the complex set of observational study.

• White, R, FR and B lights control both genes in etiolated seedlings whereas in mature plants, only B light control anthocyanin genes.

Chloroplast development

• phyA and phyB promote chloroplast development by converting etioplast into chloroplast

• Proved by a brief R light pulse using phyA and phyB single mutants as well as phyAphyB double mutants.

• Double mutants provided a much lower response, conclude that both, either phyA or phyB holotypes can mediate the above mentioned effect.

Summary of De-etiolation response after 5 days (Neff and Chory, 1998)

Defense system (anthocyanin)

Red light• Production is depleted

under R light (all single mutants) and the rate is even more in some double mutants (e.g. phyAphyB or hy1phyB)

Blue light• Accumulation of

anthocyanin• Reduced in presence

of cry1, phyB, hy1phyB and phyAphyB double mutants.

PhyA observed a general intermediate range => phyA is not a major contribution to anthocyanin in B light.However in continuous far red, FRc accumulation is mainly due to phyA

Defense system (anthocyanin)• Deficient in cry1, Arabidopsis hy4 mutants

observed no difference from WT in R and FR lights

• In B lights elongated hypocotyls possessed reduced cotyledons and anthocyanin compared to WT

• This can be explained by decrease in light induced transcription of genes encoding enzymes such as chalcone synthase

• Reduced in sensitivity is observed in cry2 deficient seedlings in blue LF range

3. Seedling establishment

• Since light will inhibit hypocotyl elongation but promote opening and enlargement of cotyledon, as the seedlign grows, the light stable phytochromes (phyB to phyE) and chloroplast development will activate stomatal opening for photosynthesis.

• Potato plants have been observed dwarfness with large lateral rbanching and enhanced photosynthetic capacity if phyB is overexpressed

Thiele et al. (1999).

Inhibition of Hypocotyl elongation-Possible mechanisms concluded

from observations

• According to studies on Arabidopsis seedlings (Jensen et al, 1998; Parks 1998; Ahmad, 2002; Shcopfer 2001)

• Inhibition of Hypocotyl elongation is induced by action of blue region of electromagnetic spectrum on PhyA/B but most importantly on CRy 1/2 . The Phy A principally acts on CRY2 at the level of the indirect action on cytochrome dependent pathway. Far red Light also mediates this response But in minute scale

• Interaction of phytochrome and Cryptochrome has been described as blue-light-dependent inhibition of Hypocotyl elongation and anthocyanin production(Ahmad & Cashmore 1997)

• Possible interaction with GA’s, etc etc…not elucidated yet• NOTE- CRYPTOCRHOME refers to flavoproteins blue-light

photoreceptors

Blue light(380-500nm) high fluence rate of 30-s

Hyperpolarisation of Hypocotyl cell(cell membrane) by increase in H+ content and opening of Ca++-channel

HY4 geneOverexpression of CRY1 Phototropin* inhibited action- auxin dependent action prevents basipetal transport of IAAblue-light-dependent inhibition of Hypocotyl elongation and anthocyanin production

Role of Ca++ as secondary messenger and inhibition of H+-ATPase pump needs to be studied

*naphthylphthalamic acid (NPA)

POST TRANSLATIONAL MODIFICATION

OF BIOSYNTHETIC ENZYMES

INCREASE in lignification associated to hemicelluloses

deposition in mesocotyl elongation zone

1. Increase in cell-wall stiffness(tensile

modulus)=No cell extension

Peroxidase catalysed-H2O2 phenolic cross-linking reaction

INHIBITED INHIBITED MESOCOTYMESOCOTYL L ELONGATIELONGATIONON

?

• Inhibition of hypocotyl growth favours development of photosynthetic tissues

Electrical signaling in plants

• Action potential in plants are rapidly propagated electrical changes. It can be maintained at a high velocity on a short period depending on intensity of signalIt can be due to a strong initiation phase or several firings. Unlike Animal, with inward-flowing Na+(depolarization) and outward flowing K+ (repolarization), plants depends on ca++,Cl- and K+.

• Cl- replaces Na+ and K+ plays same role• Require concentrated Ca++. Prior depolarization phase initiates entry

of this cation.• Absolute refractory periods last 0.0005 s in mammalian nerves, but 4–

40 s in the Characeae and 2–4 min in Conocephalum, while relative refractory periods are 0.001–0.01 s in mammalian nerves, 60–150 s in the Characeae and 6–8 min in Conocephalum

• Propagation depends on plant organography and type of response expected

• The voltage is highly variable in each plant species unlike general uniformity in animal kingdom.

• Transmembrane potential is the expressed change of differential initial and late membrane potential

• VPs or slow wave potentials are propagating electrical signals which also consist of a transient change in membrane potential (depolarization and subsequent repolarization).

• The main difference to APs lies in longer, delayed repolarizations and a large range of variation.

• This signal varies with the intensity of the stimulus, is non-selfperpetuating and appears to be a local change to either ahydraulic pressure wave or chemicals transmitted in theead xylem.

Membrane membrane potentials

Photoperiodism and flowering• Plants sense change in light Via photoreceptors to

elicit specific physiological response. The ratio of Red to Far red light stimulates these processes

• In addition to changes in R:FR ratio, the timing of reproductive development can be influenced by changes in daylength, or photoperiod. Sensitivity to the timing of light and darkness, termed photoperiodism

• Plant sense short day light to flower are Short day plants and the opposite is for long day plants(LDP’s)

• Arapdopsis- cry1 sense difference b/w short day light and Long day light.

• In many plants it’s a combination of phy A/b, cry 1/2 .The first pair linked to red light and second for blue light/UV-A. Phytochrome may interact with cryptochrome

• Phy A-Promotes flowering in long day plants , phy B inhibits• cry 2 regulates positively the flowering activator

CONSTANS(CO). • A co-incidence model indicates that simultaneous perception

of light signals by phy A and cry2 and high levels of CO activates floral promoter Flowering locus C. This process leads to flowering

Physiological responses of photoreceptors

Chloroplast relocation• Chloroplasts can move around within a leaf

depending on the light condition.

• In Arabidopsis two phototropins were discovered(Eckardt 2003) :

i. PHOT1 is the primary photoreceptor responsible for phototropism.

ii. PHOT2 have overlapping functions in both phototropism and chloroplast movement.

But later it was found that they both have overlapping functions.

PHOT1 response to a wide range of blue light and PHOT2 principally to high-fluence-rate blue light, it was concluded that PHOT1 is the main photoreceptor for chloroplast “accumulation movement” and PHOT2 controls light avoidance movement.

PHOT1 response to a wide range of blue light and PHOT2 principally to high-fluence-rate blue light, it was concluded that PHOT1 is the main photoreceptor for chloroplast “accumulation movement” and PHOT2 controls light avoidance movement.

• The pathway for chloroplast movement is yet to be solved but a proposed mechanism is that the gene chloroplast unusual positioning1 (chup1) encodes for actin binding proteins that are important in chloroplast relocation.

cells with mutation in the chup1 gene failed to show movement of chloroplast when exposed to high and low light fluences Wild-type chloroplasts localise along the periclinal walls under low

light, maximizing the capacity for light absorption relocate against the anticlinal walls under high light, thus minimising the potential for photoinhibition. chup1 chloroplasts remain in relatively

fixed positions under high or low light, mainly on the abaxial side of cells (Eckardt 2003).

Stomatal opening

• Opening of the stomata: a hydrogen pump draws the hydrogen ions out of the guard cell making the cell membrane potential negative.

• Potassium channels to open allowing potassium ions inside the cell and hence there is an increase in the solute potential.

• Water diffuse in leading to an increase in turgor pressure.• Cellulose microfibril rings around the cell prevent cells from

increasing in width.• The two guard cells pull away from each other to

accommodate the water inside them.

• Using wild type plant, mutant phot1, mutant phot2 and double mutant (phot1-phot2) the action of PHOT1 and PHOT2 on the hydrogen pump was observed.

• Equal amount of hydrogen ions were pumped by the ATPase in the wild type, phot1 mutant and phot2 mutant

• But a decrease in activity in the ATPase in the double mutant plant was noted (Sakai et al 2001).

Leaf expansion

• Light signals decrease the development of stems and petiole and promote growth of the leaves so as to increase their surface area to capture light for photosynthesis (Sakai et al 2001).

Decrease in the leaf surface area was approximately 50%,

therefore reducing the rate of photosynthesis. A lower

expansion in leaf in the mutant phot1 and phot2 and mutant phot1 represented by the two

bars in the middle when compared to the wild type

leaves.

Decrease in the leaf surface area was approximately 50%,

therefore reducing the rate of photosynthesis. A lower

expansion in leaf in the mutant phot1 and phot2 and mutant phot1 represented by the two

bars in the middle when compared to the wild type

leaves.

Phototropism

• Mutation in PHOT1 led to impairment of hypocotyls phototropic curvature. The mutants were unable to grow towards the low density blue light stimuli provided.

• However Sakai et al. found that plants with mutant PHOT1 showed high phototropic bending when exposed to continuous light of high fluences.

• They, therefore, discovered that the second phototropin PHOT2 was responsible for such response.

Mutant phot1 and phot2 were introduced in plant cell, it was observed that there was no response

to light. It was also concluded that PHOT1-encoded protein can respond to phototropism under low light pulses whilst PHOT2-encoded

protein react to only to high light fluences rates of continuous light.

Mutant phot1 and phot2 were introduced in plant cell, it was observed that there was no response

to light. It was also concluded that PHOT1-encoded protein can respond to phototropism under low light pulses whilst PHOT2-encoded

protein react to only to high light fluences rates of continuous light.

Curvature of the stem is mediated by the hormone auxin which causes differential growth so as the plant bends towards light. Release of auxin may be regulated by reversible protein phosphorylation. Since phototropin receptors are light-activated protein kinases, these provide useful information to link the phototropins to the auxin efflux ( Sakai et al. 2001).

Curvature of the stem is mediated by the hormone auxin which causes differential growth so as the plant bends towards light. Release of auxin may be regulated by reversible protein phosphorylation. Since phototropin receptors are light-activated protein kinases, these provide useful information to link the phototropins to the auxin efflux ( Sakai et al. 2001).

Owing to these properties, it is referred to as

Superchrome, although it was properly named

phytochrome 3 (phy3) by Nozue et al.

Superchrome plays a role in chloroplast

movement (Kawai et al. 2003).

Identified in 1998 by Nozue and his team, in the

fern Adiantum capillus-veneris.

Has phytochrome properties:

An N-terminal 566 amino acids

Red-far-red reversibility

Also has phototropin properties:

2 LOV domains

A Ser/Thr kinase domain

Photoreceptors also need other

environmental stimuli to integrate light

signals, so as to give coordinated responses to

the plant.

For instance, the integration of light and

gravity signals enables plants to orient

themselves in the soil and adjust their

architecture in the soil, as well as orient plant

organs.

Plants also integrate light and temperature

signals. This provides them with important

seasonal information, e.g. the promotion of

germination or flowering after a cold period,

which can synchronise physiological responses

with favourable environmental conditions.