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Photomorphogenesis (control of growth & development by light)

Photomorphogenesis (control of growth & development by light). Environmental signals (light, temperature and gravity) are important signals for plant development Light affects many aspects of plant development, for example: required for proper leaf development

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Photomorphogenesis (control of growth & development by light)

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  1. Photomorphogenesis(control of growth & development by light) • Environmental signals (light, temperature and gravity) are important signals for plant development • Light affects many aspects of plant development, for example: • required for proper leaf development • inhibits stem elongation in the emerging seedling • promotes flowering (photoperiodism) • promotes (or inhibits) seed germination

  2. Molecular Biol. of Leaf Development Arabidopsis Dark-grown Light-grown • Leaf development is light-dependent in angiosperms • Chloroplast development is the signature feature: proplastids > (etioplasts) >chloroplasts (plastid number per cell increases) • Light controls expression of important chloroplast proteins cotyledon hypocotyl Skotomorphogenesis – seedling development in darkness

  3. Barley (Hordeum vulgare) 7-10 days old Older cells (etioplasts) light Young cells w/proplastids

  4. CF1- α, β subunits of ATP synthetase PSI - photosystem I Chl-apoproteins PSII – photosystem II Chl-apoproteins Pchlrd – protochlorophyllide reductase LHCII- light-harvesting Chl-apoproteins of PSII LS - large subunit of RuBPCase SS – small subunit of RuBPCase From J. Mullet & colleagues

  5. several steps light + NADPH aminolevulinic acid -------------> protochlorophyllide  Chlorophyllide  Chl Pchlrd Step in chlorophyll synthesis that requires light Pchlrd (Protochlorophyllide reductase) – enzyme that catalyzes the reduction of protochlorophyllide; it over-accumulates in dark-grown plants, and is down-regulated by light. Protochlorophyllide Chlorophyllide

  6. Protein synthesis and select mRNA levels in plastids from dark-grown barley and after illumination. D – psbA gene product, other proteins were described in a preceding slide From J. Mullet & colleagues

  7. Regulation of Plastid Proteins by Light 1. Light induction of the chloroplast-encoded proteins is mainly at the translational and post-translational (i.e., protein stability) levels John Mullet

  8. Nuclear-encoded Cab/lhc mRNAs are not present in dark-grown plants. They are induced by white light or pulses of red light, & inhibited by pulses of far-red light. N-H. Chua and colleagues

  9. Transcription run-off in isolated nuclei of selected genes from dark-grown barley, and after the indicated light treatments. rbcS – small subunit of RuBPCase cab/lhc – light-harvesting Chl- apoproteins of PSII pcr- protochlorophyliide reductase Klaus Apel

  10. Regulation of Plastid Proteins by Light 1. Light induction of the chloroplast-encoded proteins is mainly at the translational and post-translational (i.e., protein stability) levels 2. Regulation of the nuclear-encoded genes (e.g., rbcS, cab/lhc, and pcr ) is mainly at transcription - light can down-regulate (pcr) as well as up-regulate - transcriptional control also mediated by Phytochrome

  11. How does light control gene transcription and plastid development ?: The photoreceptor(s) • Plants See: • Light Intensity • Light Direction • Colors

  12. PHYTOCHROME (PHY) Some major phytochrome-controlled processes: • Surface seed germination • Inhibition of stem elongation in young seedlings • Promoting leaf development in young seedlings • promotes stomatal opening Phy exists in two interconvertible forms: Pr- inactive, absorbs mainly red light (660 nm) Pfr- active, absorbs far-red light (730 nm) Pfr Pr slowly in dark

  13. More Phytochrome properties : • Protein subunit of 125,000 Daltons (~1100 amino acids). • Chromophore is a linear tetrapyrrole, attached covalently to a cysteine. • Native Phy is a dimer. • Has His-kinase activity. The “Red Far-Red” test for Phy control: Pulse of red light  response Pulse of far-red light no response Pulse of red lightpulse of far-redno response

  14. Phytochrome in Arabidopsis - 5 Phytochrome (PhyA-PhyE) genes - Have overlapping functions, based on mutant analysis - Vary with respect to the light intensity or light quality required for activation: - e.g., far-red responses are mediated by Phy A - Can form heterodimers

  15. Blue-light receptor for leaf development (Blue light promotes leaf development and phototropsim) • absorb in the 350-450 nm range • a.k.a. Cryptochrome • Cryptochrome gene (Cry) identified using genetic approach (Cashmore & colleagues): - hy4 mutant of Arabidopsis • chromophore = flavin (FAD) Tony Cashmore

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