Kelompok 6Kelompok 6

1. Destri Nurul Husna (4401413064)2. Syaiful Ishlahul U (4401413065)3. Annissaa Nurjannah (4401413072)4. Ririn Muhajaroh (4401413094)5. Anifatussholihah (4401413099)

Action spectrum for blue light–stimulated

phototropism in oat coleoptiles. An action

spectrum showsthe relationship between a

biological response and thewavelengths of light

absorbed. The “three-finger” pattern

in the 400 to 500 nm region is characteristic of specific blue-light responses. (After Thimann and Curry 1960.

ACTION SPECTRUM

Phototropism (pronounced foe-TA-tro-piz-em) is the growth of a plant in the direction of its light source. Plants are very sensitive to their environment and have evolved many forms of “tropisms” in order to ensure their survival. A tropism is the growth of a plant as a response to a stimulus, and phototropism occurs when a plant responds to light by bending in the direction of the light.

Relationship between direction of growth and unequal

incident light. Cotyledons from a young seedling are shown as

viewed from the top. The arrows indicate the direction of

phototropic curvature. The diagrams illustrate how the direction of

growth varies with the location and the intensity of the light source,

but growth is always toward light. (After Firn 1994.)

Time-lapse photograph of a corn

coleoptile growing toward

unilateral blue light given from the

right. The consecutive exposures

were made 30 minutes apart.

Note the increasing angle of

curvature as the coleoptile bends.

(Courtesy of M. A. Quiñones.)

Phototropism in sporangiophores of the mold Phy-comyces

has been studied to identify genes involved in pho-totropic

responses. The sporangiophore consists of a spo-rangium (spore-

bearing spherical structure) that develops on a stalk consisting of a

long, single cell. Growth in the sporangiophore is restricted to a

growing zone just below the sporangium.

When irradiated with unilateral blue light, the sporan-

giophore bends toward the light with an action spectrum similar to

that of coleoptile phototropism (Cerda-Olmedo and Lipson 1987).

These studies of Phycomyces have led to the isolation of many

mutants with altered phototropic responses and the identification of

several genes that are required for normal phototropism.

Phototropism in wild-type (A) and mutant (B)Arabidopsis seedlings.

Unilateral light was applied from the

right. (Courtesy of Dr. Eva Huala.)

• The stems of seedlings growing in the dark elongate The stems of seedlings growing in the dark elongate very rapidly, and the inhibition of stem elongation by very rapidly, and the inhibition of stem elongation by light is a key morphogenetic response of the seedling light is a key morphogenetic response of the seedling emerging from the soil surface emerging from the soil surface

• The conversion of Pr to Pfr (the red- and far red–The conversion of Pr to Pfr (the red- and far red–absorbing forms of phy- tochrome, respectively) in absorbing forms of phy- tochrome, respectively) in etiolated seedlings causes a phytochrome-dependent, etiolated seedlings causes a phytochrome-dependent, sharp decrease in elongation rates sharp decrease in elongation rates

Distribution of transmitted, 450 nm blue light in an

etiolated corn coleoptile.

The diagram in the upper right of each

frame shows the area of the coleoptile being

measured by a fiberoptic probe. A cross section of

the tissue appears at the bottom of each frame.

The trace above it shows the amount of light

sensed by the probe at each point. A sensing

mechanism that depended on light gradients

would sense the difference in the amount of light

between the lighted and shaded sides of the

coleoptile, and this information would be

transduced into an unequal auxin concen- tration

and bending.

o However, action spectra for the decrease in elongation rate show strong activity in the blue region, which cannot be explained by the absorption properties of phytochrome. In fact, the 400 to 500 nm blue region of the action spectrum for the inhibition of stem elongation closely resembles that of phototropism.

o Blue light and phytochrome-mediated hypocotyl responses can also be distinguished by the swiftness of the response. Whereas phytochrome-mediated changes in elongation rates can be detected within 8 to 90 minutes, depending on the species, blue-light responses are rapid, and can be measured within 15 to 30 s.

o Another fast response elicited by blue light is a depo- larization of the membrane of hypocotyl cells that precedes the inhibition of growth rate

o The membrane depolarization is caused by the activation of anion channels which facilitates the efflux of anions such as chloride.

o Use of an anion channel blocker prevents the blue light–dependent membrane depolarization and decreases the inhibitory effect of blue light on hypocotyl elongation.

Stomata of most plant open in the day and close at

night, while CAM plants are just the opposite.

Stomata opening are sensitive to red light and blue

light, and blue light is more effective, it stimulates opening

by a blue-light receptor: zeaxanthin.

Mechanism of stomatal openingMechanism of stomatal opening----K----K++ absorption theory absorption theory

H＋

light

K ＋

H＋

K ＋

Mal

Mal －　＋ H+

V

PM

H＋

H＋

Cl-Cl-

H+-ATPase in PM is light activated

Its function is out-pumping H+Inward rectifier K+ channel is voltage dependent, PM hyperpolarization activates the channel and carry K+ inwardCl- is transported

through Cl- /H+ symport or Cl-/OH-antiport

HCO3-+PEP

When the stomatum is opening, the [K+] rises to 0.5M, anions rise to 0.2-0.5M, the osmotic potential drops 2MPa, thus bring water in.

DARKDARK

BLUE LIGHTBLUE LIGHT

Senn (1908) documented light-induced chloroplast movement

in a number of higher plants and algae, and the phenomenon has long

been accepted as a means of optimizing photosynthetic light absorption

under changing light conditions.

Kasahara et al. (2002) recently showed that Arabidopsis

mutants defective in chloroplast high-light avoidance movement are

more susceptible than wild-type plants to photoinhibition under high-

light conditions, confirming the physiological importance of this

phenomenon.

The photoreceptors responsible for lightinduced chloroplast

movement in higher plants are phototropins, the blue light receptors that

also mediate phototropism

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 avoidanceHigh-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]

NPQ analysis of npq4 leaves depleted of chloroplast avoidance movement. (a) PsbS-minus plants were crossed with Arabidopsis knock-out lines lacking the photoprotective mechanism of chloroplast avoidance movement (phot2). Kinetics of NPQ induction and relaxation of npq4 and npq4phot2 were measured in dark-adapted leaves, as described for figure 3. (b) NPQ kinetics were measured on npq4 and npq4phot2 leaves upon illumination with either white actinic light (400 μmol photons m−2 s−1) or red light (350 μmol photons m−2 s−1, 600 < λ < 750 nm). Symbols and error bars show mean ± s.d. (n = 3). (c) Distribution of chloroplasts in the mesophyll cells of npq4 and npq4phot2 was determined by light microscopy. Leaves were dark-adapted for 1 h and then irradiated with white light at 400 µmol m−2 s−1 for 1 h. Prior to light treatment, detached leaves were infiltrated with 150 mM sorbitol containing either 50 μM nigericin, 100 μM lincomycin or 2 μM myxothiazol. (Online version in colour.)

CHLOROPLAST MOVEMENTS -CHLOROPLAST MOVEMENTS -LEMNALEMNA

DARK WEAK BLUE LIGHT STRONG BLUE LIGHT

1.1. PHOTOTROPINS : PHOTOTROPINS : phot1phot1 and and phot2phot2

- Serve as the blue light photoreceptors for phototropism, chloroplast movememnts, and stomatal opening

- Mutants lack blue light dependent phototropism, chloroplast movements, and stomatal opening

- Phototropins are flavoproteins with ser/thr protein kinases that autophosphorylate in response to blue light

2. CRYPTOCHROMES (2. CRYPTOCHROMES (cry1cry1 and and cry2cry2))

- Cryptochromes play roles in blue-light-mediated growth inhibition. Mutant hypocotyls are not inhibited by blue light.

- However, no photochemistry has yet been demonstrated, and so they may not act as photoreceptors

- Cryptochromes also found in animal cells where they function in circadian rhythms, but do not act as photoreceptors

- Cryptochromes may be involved in signal transduction downstream of other blue light photoreceptors.

3.3. ZEAXANTHIN - CAROTENOID LOCATED IN THE ZEAXANTHIN - CAROTENOID LOCATED IN THE CHLOROPLASTCHLOROPLAST

May regulate stomatal opening. Mutants have reduced response to blue light.

Zeaxanthins and photoropins may work together in guard cells

ZEAXANTHIN CAN UNDERO A CIS-TRANS ZEAXANTHIN CAN UNDERO A CIS-TRANS ISOMERIZATIONISOMERIZATION

(THIS MODEL IS STILL HYPOTHETICAL)

Trans-Zeaxanthin Cis-ZeaxanthinBLUE

GREEN

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Kelompok 6 angkatan 2013