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Presentasi berjudul: "PHYTOCHROME AND LIGHT CONTROL OF PLANT DEVELOPMENT."— Transcript presentasi:


2 FENOMENON a.Seedlings grown in the light (bean) Green Wide leaves Without hypocotyl elongation Cotyledon open b.Seedlings grown in the dark absence of greening reduced leaf size hypocotyl elongation maintenance of the apical hook  Etiolated growth

3 It takes very little light to initiate the transformation from the etiolated to green state  light act as a developmental trigger rather than a direct energy source Several developmental changes: 1.a decrease in the rate of stem elongation 2.the beginning of apical-hook straightening 3.the initiation of the synthesis of pigments that are characteristic of green plants photomorphogenesis is the initial rapid changes are induced by a distinctly different light response

4 PHYTOCHROME The pigments that can promote photomorphogenic responses in plants is phytochrome. Phytochrome is a protein pigment that absorbs red (650-680 nm) and far-red (710- 740 nm) light most strongly, but that also absorbs blue light. It plays a key role in light-regulated vegetative and reproductive development

5 PROPERTY OF PHYTOCHROME Photoreversibility is the most distinctive property of phytochrome


7 STRUCTURE OF PHYTOCHROME Phytochrome  a dimer  @ holoprotein (chromophore + apoprotein) Chromophore: a light-absorbing pigment molecule In higher plants, the chromophore is called phytochromobilin Apoprotein: a polypeptide chain Chromophore attached to apoprotein, through a thioether linkage to a cysteine residue

8 SYNTHESIS & ASSEMBLY OF PHYTOCHROME A.Synthesis (Phytochromobilin) Inside plastids 5-aminolevulinic acid  via branches of the chlorophyll biosynthetic pathway Leak out into the cytosol by a passive process. B.Assembly Assembly of the apoprotein with chromophore is autocatalytic; that is, it occurs spontaneously with no additional proteins or cofactors

9 CONFORMATIONAL CHANGE IN PHYTOCHROME Chromophore absorbs the light  conformational changes initiated in it. Absorption of light  Pr chromophore undergoes a cis–trans isomerization of the double bond between carbons 15 and 16 and rotation of the C14–C15 single bond. During the conversion of Pr to Pfr, holoprotein also undergoes conformational change.

10 TYPES OF PHYTOCHROME There are two different classes of phytochrome 1.Type I phytochromes 2.Type II phytochromes  in dark-grown seedlings  Type I > Type II  in light-grown seedlings  Type I = Type II

11 PHYTOCHROME GENE FAMILY The phytochrome gene family  PHY Five individual members: PHYA, PHYB, PHYC, PHYD, and PHYE. PHYA is the only gene that encodes a Type I phytochrome. The remaining PHY genes (PHYB through PHYE) encode the Type II phytochromes

12 PHYA in the light  inhibition of transcription, mRNA degradation, and proteolysis PHYB through PHYE not significantly changed by light

13 LOCALIZATION OF PHYTOCHROME Phytochrome can find in angiosperm, gymnosperms, ferns, mosses, and algae. Phytochromes are most abundant in young & undifferentiated tissues In etiolated seedlings, the highest levels in meristematic regions or in regions that were recently meristematic.

14 PHYTOCHROME RESPONSES Phytochrome responses are influenced by the amount of light required to induce them, called as the fluence Fluence is defined as the number of photons impinging on a unit surface area 3 types of phytochrome responses based on the fluence 1.Very-low-fluence responses (VLFRs) 0.0001 - 0.05 μmol m –2 2.Low-fluence responses (LFRs) 1.0 - 1000 μmol m –2 3.High-irradiance responses (HIRs) 100 times higher than LFRs

15 Ecological FUNCTIONS of PHYTOCHROME A.Shade Avoidance Phytochrome enables plants to sense shading by other plants. Plants increase stem extension in response to shading  shade avoidance response Shading ↑  the R:FR ratio ↓(FR↑)  (Pfr/Ptotal) ↓ (Pfr  Pr)  Pr stimulate stem extension It finds in sun plants, but does not in shade plants

16 B.Circadian Rhythms Circadian rhythms are the rhythmic changes in metabolic processes in plant e.g. nyctinasty: leaves and/or leaflets open during the day and close at night The change in leaf or leaflet is caused by rhythmic turgor changes in the cells of the pulvinus, a specialized structure at the base of the petiole  red & far-red light

17 C.Phytochrome Specialization Each of phytochromes performs distinct roles in the life of the plant 1.phyB mediates responses to continuous red or white light  shade avoidance, photoreversible seed germination 2.phyA is required for the response of continuous far-red light  plant grow tall and spindly 3.phyD and phyE help mediate the shade avoidance response 4.phyD plays a role in regulating leaf petiole elongation 5.phyE acts redundantly with phyB and phyD in leaf petiole elongation 6.phyE acts redundantly with phyA and phyB in inhibition of internode elongation

18 PHYTOCHROME INTERACTION IN SEED GERMINATION The action of constant red and far-red light absorbed separately by the phyA and phyB systems

19 The effects of phyA and phyB on seedling development in sunlight versus canopy shade

20 PHYTOCHROME FUNCTIONAL DOMAIN phytochrome as a molecule having two domains: 1.N-terminal lightsensing domain  the light specificity and stability reside 2.C-terminal domain that contains a.the signaltransmitting sequences b.the site for the formation of phytochrome dimers c.the site for the addition of ubiquitin, a tag for degradation

21 CELLULAR MECHANISM Phytochrome Regulates Membrane Potentials and Ion Fluxes e.g. nyctinasty Pulvinus has 2 cell in opposite side: ventral motor cells and dorsal motor cells In leaves closure: apoplastic pH of dorsal motor cells↓  plasma membrane H + pump active (by Pfr)  accumulate K + and Cl –  turgor pressure ↑  swell; apoplastic pH of ventral motor cells↑  plasma membrane H + pump deactive (by Pfr)  release K + and Cl –  turgor pressure ↓  shrink In leaves open: the reverse pattern of apoplastic pH change

22 In leaves closure: Pfr  K + channels of dorsal motor cells open  depolarized the membrane potential accumulate K + and Cl –  turgor pressure ↑  swell; Pfr  K+ channels of vetral motor cells close  polarized the membrane potential  release K + and Cl –  turgor pressure ↓  shrink In leaves open: the reverse pattern of K + channels

23 Changes in the bioelectric potential of cells imply changes in the flux of ions across the plasma membrane Small portion of the total phytochrome is tightly bound to various organellar membranes & associated with microtubules located directly beneath the plasma membrane

24 Molecular mechanism Phytochrome Regulates Gene Expression 1.Photoreversibility 2.Autophosphorylating 3.Phosphorylating kinase 4.Interaction with G-protein 5.Activation transcription factor 6.Phytocrome movement into nuleus 7.Transcription regulation




28 summary Photomorphogenesis is effects of light on plant development and cellular metabolism Phytochrome is the pigment involved in most photomorphogenic phenomena Phytochrome exists in two forms: Pr and Pfr Phytochrome gives rise its effect through: 1.Cellular mechanism by regulates membrane potentials and ion fluxes 2.Molecular mechanism by regulates gene expression



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