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Biol 352 Lecture 11 Phytochrome and Photomorphogenesis February 12, 2007

Biol 352 Lecture 11 Phytochrome and Photomorphogenesis February 12, 2007. Review: BRs Brassinosteroids (BRs) are polyhydroxylated steroid hormones. Brassinolide (BL) is the most active BR. Bioassays can be used to distinguish BRs from other plant hormones.

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Biol 352 Lecture 11 Phytochrome and Photomorphogenesis February 12, 2007

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  1. Biol 352 Lecture 11 Phytochrome and Photomorphogenesis February 12, 2007

  2. Review:BRs • Brassinosteroids (BRs) are polyhydroxylated steroid hormones. Brassinolide (BL) is the most active BR. • Bioassays can be used to distinguish BRs from other plant hormones. • Deficiency in BR biosynthesis results in dwarfism. • BRs promote both cell elongation and cell division.

  3. Major plant hormones: • Auxins: growth hormone • Gibberellins: hormone for cell elongation • Cytokinins: regulator for cell division • Ethylene: gaseous hormone • Abscisic acid: stress hormone • Brassinosteroids: sterol hormone

  4. Lecture Outline: • Photomorphogenesis • Photoreversibility • Structure of phytochrome • Physiology of phytochrome • Phytochrome-regulated gene expression • Summary

  5. Learning Objectives: • Grasp the concept of photomorphogenesis • Grasp the concept of photoreversibility • Distinguish different types of phytochromes • Determine the critical steps in phytochrome-regulated gene expression • Reading: • 3rd Ed, Plant Physiology, Taiz & Zeiger, Chapter 17: p375-402 • 4th Ed, Plant Physiology, Taiz & Zeiger, Chapter 17: p417-442

  6. Concept 1: Photomorphogenesis • Power of light: a developmental switch from dark to light growth • Light response: decrease stem elongation, beginning of apical-hook straightening, the initiation of the synthesis of pigments • …Photomorphogenesis • Light acts as a developmental trigger rather than a direct energy source • …chlorophyll is not present this time

  7. Concept 2: Photoreversibility • Phytochrome:a protein pigment that absorbs red and far-red light most strongly. • Photoreversibility:the effects of red light (650-650nm) on morphogenesis could be reversed by a subsequent irradiation with light of longer wavelength (710-740nm).

  8. Lettuce seed germination

  9. Red light Pr Pfr Far-red light Photoreversibility Red light: 650-680nm Far-red light: 710-740nm • In dark-grown plants, phytochrome is present in a red light-absorbing form, Pr. • Pr is converted by red light to a far-red-light-absorbing form, Pfr. • Pfr can be converted back to Pr by far-red light. • Pfr is the physiological active form of phytochrome.

  10. Concept 3: The Absorption Spectra of the Pfr and Pr Forms Overlap Blue light: 400-500nm • Photostationary state: the equilibrium between Pfr and Pr. • Both forms of phytochrome absorb light in the blue region of the spectrum.

  11. Concept 4:Phytochrome Responses can be Distinguished by the Amount of Light Required

  12. Concept 5: Phytochrome is a Dimer Composed of Two Polypeptides • A dimer of two equivalent subunits. • Each subunit consists of two component: (a) Chromophore, a light-absorbing pigment molecule, and (b) Apoprotein, a polypeptide chain. • Assembly of the phytochrome apoprotein with its chromophore is autocatalytic.

  13. Structure of the Pr and Pfr Forms of Phytochrome

  14. Concept 6: Phytochrome is an Autophosphorylation Protein Kinase

  15. Concept 7: Two types of phytochrome • Type I: • more abundant than type II in dark • PHYA • mRNA is unstable • PfrA protein undergoes degradation • Type II: • less abundant than type I in dark • PHYB, PHYC, PHYD, PHYE • mRNA stable • Pfr protein stable • PHY: the apoprotein by itself (without the chromophore) • phy: the holoprotein (with the chromophore)

  16. Phytochrome Specialization • phyA: required for the response to continuous far-red light, de-etiolation, VLFR germination • phyB: mediates responses to continuous red or white light, mediates shade avoidance by regulating hypocotyl length, and regulates photoreversible seed germination • phyC: least characterized • phyD: regulates leaf petiole elongation and flowering time • phyE: acts redundantly with phyB and phyD, and with phyA and phyB in inhibition of internode elongation

  17. R FR 0, 0.02, 0.2, 2, 20, 200 mol m-2s-1

  18. Concept 8:Plants Respond to the R:FR ratio R/FR ratio: photo fluence rate in 10nm band centered on 660nm divided by photo fluence rate in 10nm band centered on 730nm • Sun plants: higher FR content promotes stem extension • Shade plants: higher FR content has litter or no effect on stem extension

  19. Concept 9: Phytochrome-Regulated Gene Expression • Phytochrome is a light-regulated protein kinase. • Phytochrome is capable of autophosphorylation. • Phytochrome translocates into the nucleus in response to light. • Phytochrome regulates the expression of both primary- and secondary-response genes. • Phytochrome-induced gene expression involves protein degradation.

  20. Primary response gene Secondary response gene Light-induced gene expression

  21. Summary: • Phytochromes perceive red and far-red light of between 600 and 750 nm. • Phytochrome can interconvert between Pr and Pfr forms. • Phytochrome is a dimer composed of two polypeptides and has a light-absorbing pigment molecule. • Two types of phytochrome: type I (PHYA) and type II (PHYB-E). • Phytochrome B mediates responses to continuous red or white light, whereas phytochrome A is required for the response to continuous far-red light. • Phytochrome moves from cytoplasm to the nucleus, where it interacts with a number of transcription factors to induce light-regulated gene expression (early- and secondary-response genes).

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