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: 1.required for proper leaf development 2.inhibits stem elongation in the emerging seedling 3.promotes flowering (photoperiodism) 4.promotes (or inhibits) seed germination

2. Molecular Biol. of Leaf Development 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 hypocotyl cotyledon Dark-grownLight-grown Arabidopsis Skotomorphogenesis – seedling development in darkness

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

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 Pchlrd (Protochlorophyllide reductase) – enzyme that catalyzes the reduction of protochlorophyllide; it over-accumulates in dark- grown plants, and is down-regulated by light. Step in chlorophyll synthesis that requires light ProtochlorophyllideChlorophyllide

6. D – psbA gene product, other proteins were described in a preceding slide Protein synthesis and select mRNA levels in plastids from dark-grown barley and after illumination. 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. rbcS – small subunit of RuBPCase cab/lhc – light-harvesting Chl- apoproteins of PSII pcr- protochlorophyliide reductase Transcription run-off in isolated nuclei of selected genes from dark-grown barley, and after the indicated light treatments. 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 ?: T he photoreceptor(s) Plants See: - Light Intensity - Light Direction - Colors

13. PHYTOCHROME (PHY) Some major phytochrome-controlled processes: 1.Surface seed germination 2.Inhibition of stem elongation in young seedlings 3.Promoting leaf development in young seedlings 4.promotes stomatal opening Phy exists in two interconvertible forms: P r - inactive, absorbs mainly red light (660 nm) P fr - active, absorbs far-red light (730 nm) P fr  P r slowly in dark

14. More Phytochrome properties : 1.Protein subunit of 125,000 Daltons (~1100 amino acids). 2.Chromophore is a linear tetrapyrrole, attached covalently to a cysteine. 3.Native Phy is a dimer. 4.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

15. - 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

16. 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