PHYTOCHROME STRUCTURE AND FUNCTION BY NOUR DEEB
- 2. CHAPTER 17
- 3. 1.provides a major source of information from the environment during plant growth and development. 2. In plants, light-dependent responses are controlled by a series of photoreceptors that can be classified into three known groups—the phytochromes, cryptochromes and phototropins.
- 5. There are 4 classes of photoreceptors : 1. Phytochromes (red and far-red) 2. Cryptochrome (blue and UV-A): seedling development and flowering 3. Phototropin (blue and UV-A): differential growth in a light gradient 4. UV-B receptors photo receptor reads the information contained in the light by selectively absorbing different wave length of light.
- 6. Phytochrome: • Phytochrome is a photoreceptor, a pigment that plants, and some bacteria and fungi, use to detect light • It is sensitive to light in the red and far-red region of the visible spectrum. • In other word a blue-green pigment found in many plants, in which it regulates various developmental processes. It alters plant behavior
- 8. Biosynthesis of phytochrome.. • Phytochrome is produced in different parts of the cell and assembled from those parts. • The phytochrome binding protein is coded in nuclear genes, transcribed in the nucleus and translated on cytosolic ribosomes. • The phytochrome chromophore is produced in the plastid. These are assembled in the cytosol
- 9. Phytochrome Is a Dimer Composed of Two Polypeptides • Native phytochrome is a soluble protein • It occurs as a dimer made up of two equivalent subunits. • Together, the apoprotein and its chromophore make up the holoprotein. • In higher plants the chromophore of phytochrome is a linear tetrapyrrole termed phytochromobilin. There is only one chromophore per monomer of apoprotein, and it is attached to the protein through a thioether linkage to a cysteine residue
- 13. 1. The Pr form: •Absorbs at a peak of 660nm •Is the form synthesized in dark-grown seedlings. •When Pr absorbs red light, it is converted to the Pfr form 2. • The Pfr form: •Absorbs at a peak of 730 nm. •The Pfr form is the active form that initiates biological responses. •When Pfr absorbs far red light, it is converted to the Pr form.
- 15. What is strange about these pigments ???? When these pigments absorbed light (photon), they change chemically into the OTHER form. The two forms of phytochrome differ in their absorption spectra.
- 16. Phytochrome Is Encoded by a Multigene Family •cloning of phytochrome genes – helps in the detailed comparison of the amino acid sequences of the related proteins •phytochromes are soluble proteins •phytochrome gene family is named PHY •PHYA, PHYB, PHYC, PHYD, and PHYE •The apoprotein by itself (without the chromophore) is designated PHY •holoprotein (with the chromophore) is designated phy •Different phytochrpmes are expressed in different tissues at different times in development and mediate different light responses •PHY A and PHY B mediates red and far red responses •Phytochromes also interact with each other and also with other receptors and devolopmentala stimuli
- 17. PHY Genes Encode Two Types of Phytochrome •On the basis of their expression patterns, the products of members of the PHY gene family can be classified as •Type I •Type II phytochromes •PHYA is the only gene that encodes aType I phytochrome. •Additional studies of plants that contain mutated •forms of the PHYA gene (termed phyA alleles) have confirmed this conclusion and have given some clues about the role of this phytochrome in whole plants
- 18. • Light can be absorbed only when the polypeptide is covalently linked with phytochromobilin to form the holoprotein. Phytochromobilin is synthesized inside plastids and is derived from 5-aminolevulinic acid via a pathway that branches from the chlorophyll biosynthetic pathway . • Because the chromophore absorbs the light, conformational changes in the protein are initiated by changes in the chromophore. Upon absorption of light, the Pr chromophore undergoes a cis–trans isomerization of the double bond between carbons 15 and 16 and rotation of the C14–C15 single bond.
- 20. • The amount of light is referred to as the fluence. - number of photons hitting on a unit surface area • In addition to the fluence, some phytochrome responses are sensitive to the irradiance or fluence rate of light . • Each phytochrome response has a characteristic range of light fluences over which the magnitude of the response is proportional to the fluence
- 21. 1. VLFRs (very low fluence responses) • < 10-3 mmol photons/m2 (converts 0.01% of phytochrome) Not FR reversible Transform 0.02% of Pr into Pfr 2. HIR (high irradiance responses) • >1000 mmol photons/m2, continuous irradiation, dependent on actual fluence 3. LFRs (low fluence responses) • 1-1000 mmol photons/m2, FR light reversible Require high exposure to high fluence
- 22. 1)regulate the germination of seeds (photoblasty) 2) the synthesis of chlorophyll 3) the elongation of seedlings 4)the size, shape and number and movement of leaves and the timing of flowering in adult plants Many flowering plants use it to regulate the time of flowering based on the length of day and night (photoperiodism) and to set circadian rhythms This pigment has a regulatory role in all phases of plant growth and development (photomorphogenesis) and is apparently ubiquitous in all taxonomic groups of eukaryotic plants with the exception of fungi.
- 23. : 1.Is the process by which plant development is controlled by light . • Plants exhibit different growth habits in dark and light • In the dark they have elongated stems, undifferentiated chloroplasts and unexpanded leaves. This is called skotomorphogenesis. • Photomorphogenesis (light grown) involves the inhibition of stem elongation, the differentiation of chloroplasts and accumulation of chlorophyll, and the expansion of leaves. • Thus the same stimulus causes opposite effects on cell elongation in leaves and stems. Photomorphogenesis can be induced by red, far red and blue light. • You know that a pigment absorbs light and is altered electronically . • results in a change in other chemicals in the immediate environment to drive photosynthesis.
- 25. Seed germination is inhibited by far red light and stimulated by red light Since chlorophyll in green leaves absorbs red light but not far red light Seeds deposited under a heavy canopy of trees don’t germinated Some seeds germinate only in spring because they become exposed to red light after the leaves have fallen and decomposed and before new leaves that shade them have emerged. The phytochrome protein is generally synthesized in the Pr form when red light enters the plant some of the Pr is converted into Pfr In some plants the PFR enters the nucleus and bind to proteins that associate with the dna either activate orinhibit transcription of specific genes.