Photomorphogenesis by Vidan Biology

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1. 1 PHOTOMORPHOGENESIS Light is vital for photosynthesis but it is also necessary for plants growth and development. “The light mediated growth and development in plants independent of photosynthesis are called Photomorphogenesis”.

2. 2 PHOTOMORPHOGENESIS Light is important for photosynthesis, but is also necessary for plant growth and development.   The light mediated growth and development in the plants is Photomorphogenesis.   Light act as a signal to initiate and regulate photomorphogenesis.

3. 3 TYPES OF PHOTORECEPTORS 1. Phytochrome (Red Light Photoreceptor) Plays a role in seed germination, flowering and stem elongation etc. 2. Cryptochrome (Blue Light Photoreceptor) Plays a role in flowering, circadian rhythms and inhibition of stem elongation etc. 3. Phototropin (Blue Light Photoreceptor) Plays a role in chloroplast movement and phototropism etc 4. Zeitlupe family (Blue Light Photoreceptor) plays a role in circadian rhythms and control of flowering. There are 4 major types of photoreceptors involved in photomorphogenesis:

4. 4 Cryptochrome Phototropin Zeitlupe Phytochrome

5. Phytochrome was discovered by Sterling Hendricks & Harry Borthwick.

6. Phytochrome is red light photoreceptor of plants synthesize in dark entirely as Pr form in the cytoplasm. Phytochrome is a photomorphogenetic chromoprotein that absorbs red and far red light strongly & causes photomorphogenesis. It is a blue protein pigment with a molecular mass of about 125 kDa . It is present in almost all plants, Fungi, some Algae and Bacteria. Phytochrome

7. 7 Red light absorbing (666nm) Pr form Far red light absorbing (730nm) Pfr form. The Phytochrome is present in 2 Photoreversible forms: Absorption of red light by Pr convert it to Pfr form and absorption of Far red light by Pfr convert it to Pr form .

8. 8 Pfr Form (Far-Red Light Absorbing Form) When Pfr absorbs far red light it is converted into Pr form It is the active form which initiates biological Pfr form can also spontaneously revert to the Pr form in the dark (night time) overtime, It is called Dark Reversion.

9. 9 Pr Pfr Red Light Far Red Light Initiated Biological responses Dark Reversion Phytochrome Also absorb Blue Light, hence phytochrome effects can also be mediated by blue light Eg - PHY A.

10. 10 Pr Pfr Red Light Far Red Light Dark Reversion Day Event Active Inactive

11. 11 PHYTOCHROME • It is a Photomorphogenic pigment .   Phytochrome consist of chromophore • Absorb red and far-red light and cause photomorphogenesis. Chromophore= P hytochromobilin • Found in most plant , regulate growth and development process like induction of flowering, seed germination, stem elongation etc.

12. 12 Phytochromobilin GAF PHY PAS PRD PRD HKRD PAS GAF PHY PRD PRD HKRD Phytochromobilin Photosensory Region Regulatory Region Hinge Region Hinge Region Thiother linkage Covalent bond Thiother linkage Covalent bond N N Dimer Form C C PHY- Phytochrome -specific GAF related domain. PRD-PAS Related domain . HKRD- Histidine kinase related domain

13. 13 The N-terminal half contain PAS domain, GAF domain which binds the chromophore through a covalent bond and PHY domain which stabilizes the Phytochrome in Pfr form. The PAS-GAF-PHY domains comprises Photosensory Region of phytochrome. The C-terminal half called Regulatory Region, it contains 2 PAS-related domains (PRD) that mediates phytochrome dimerization and nuclear targeting & non-functional Histidine kinase related domain (HKRD ).

14. 14 When chromophore absorbs light there is slight change in its structure this causes a change in conformation of the protein to the form that initiates responses. N-terminal region perceives photosensory signals and C-terminal region do the transmission of photosensory signal to signal transduction pathway. It has been suggested that phytochrome are Auto phosphorylating serine/threonine kinases. Phytochrome protein are encoded by phytochrome gene family termed PHY. Its 5 members are PHY A, PHY B, PHY C, PHY D & PHY E.

15. 15 It is abundant in dark grown plant presence of light its concentration decreases rapidly as a result of : PHY A is most abundant phytochrome in Dark grown (etiolated) plants. They are photoreceptor of Far Red Light. Example- PHY A. It is stable in Light and present in both light and dark grown plants (mainly functions in light) PHY B is most abundant phytochrome in light grown plants They are photoreceptors of Red Light. Example- PHY B, PHY C, PHY D, PHY E. Transcriptional inhibition mRNA degradation Proteolysis

16. 16 Functions Induction of stem elongation . Promotes induction of flowering . Leaf Senescence . Leaf abscission . Seed Germination . Nyctinąstic Movements. Chloroplast development . Promotion of leaf formation & also regulates size, number, shape of Leaves.

17. Phytochrome - Photoactivated Phytochrome Pfr after Autophosporylation migrates into the nucleus and mediate Photomorphogenic development (de-etiolation) by degrading PIFS and Inactivating COP1 activity. So the main function of the Phytochrome is the removal of negative regulators of Photomorphogenesis.   HY5 (Elongated Hypocotyl 5), LAF1 (Long After Far Red Light 1) and HFR1 (Long Hypocotyl in Far Red 1) – These are Photomorphogenesis promoting transcriptional factors. In which HY5 ( bZIP transcriptional factor) is a master regulator of Photomorphogenesis in the light the level of HY5 protein increases and in dark it decreases. Positive Regulators of Photomorphogenesis MECHANISM OF ACTION

18. Negative Regulators of Photomorphogenesis MECHANISM OF ACTION COP1 (Constitutive Photomorphogenic 1 )- COP1 is E3 Ubiquitin ligase which is involve in targeting several photomorphogenesis promoting factor like HY5, LAF1, HFR1 for degradation via. 26S proteasome pathway. In the dark COP1 is present in nucleus but in the presence of light it is exported to cytoplasm. PIFS (Phytochrome Interacting factors) are bHLH transcription factors e.g. PIF3 which promotes Skotomorphogenesis (when grown in dark plants undergo etiolated growth i.e. elongated hypocotyl, closed cotyledons with apical hook & pale yellow leaves) in dark by binding to PIFs induced genes.

19. 19 Nucleus LAF1 HY5 HFR1 COP1 SPA1 COP1 PROTEOSOME 26S PIF S PIF S induced Genes Skotomorphogenesis Photomorphogenesis Migrates Degradation SPA1- suppressor of PHY A-105 1 Pr Inactive DARK

20. 20 Nucleus LAF1 HY5 HFR1 COP1 SPA1 PIF S Degradation SPA1 COP1 Photomorphogenesis Light Responsive Gene Active PROTEOSOME 26S P P P P P Migrates Pfr Active Autophosphorhylation Light

21. PIFs in the dark accumulates in the nucleus and induces expression of PIFs-Induced Genes & directly promotes Skotomorphogenesis and also inhibits Photomorphogenesis In Dark, Phytochrome is synthesize as the inactive Pr form In the dark, COP1 migrates from the cytoplasm to nucleus & forms complex with SPA1 and becomes more active COP1-SPA1 complex constantly degrade the photomorphogenic promoting transcription factors like HY5, LAF1, HFR1 via ubiquitin mediated 26S proteasome degradation pathway to prevent photomorphogenesis ARK

22. 22 It is a Photomorphogenic pigment . Absorb red and far-red light and cause photomorphogenesis . Found in most plant,regulate growth and development process like induction of flowering, seed germination, stem elongation etc. PHYTOCHROME

23. ARK 1. COP1 remains in nucleus in dark .   2. COP1 function as an E3 ubiquitin ligase and target HY5 for degradation by 26S proteasome system. 3 . No photomorphogenesis due to lack of transcription and proteins. Light COP1 Exported in cytoplasm from nucleus. 2 . High amount of HY5,due to lack of degradation by COP1 . 3. HY5 involve in successful transcription. 4 . Photomorphogenesis occur .