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 development2. inhibits stem elongation in the emerging seedling3. promotes flowering (photoperiodism)4. promotes (or inhibits) seed germination

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-grown Light-grownArabidopsis

Skotomorphogenesis – seedling development in darkness

Young cells w/proplastids

Older cells (etioplasts)

Barley (Hordeum vulgare) 7-10 days old

light

CF1- α, β subunits of ATP synthetase PSI - photosystem I Chl-apoproteinsPSII – photosystem II Chl-apoproteinsPchlrd – protochlorophyllide reductaseLHCII- light-harvesting Chl-apoproteins of PSII

LS - large subunit of RuBPCaseSS – small subunit of RuBPCase

From J. Mullet & colleagues

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

Protochlorophyllide Chlorophyllide

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

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

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

rbcS – small subunit of RuBPCasecab/lhc – light-harvesting Chl-apoproteins of PSIIpcr- protochlorophyliide reductase

Transcription run-off in isolated nuclei of selected genes from dark-grown barley, and after the indicated light treatments.

Klaus Apel

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

How does light control gene transcription and plastid development ?: The

photoreceptor(s)

Plants See:- Light Intensity- Light Direction- Colors

PHYTOCHROME (PHY)

Some major phytochrome-controlled processes:1. Surface seed germination2. Inhibition of stem elongation in young seedlings3. Promoting leaf development in young seedlings4. 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

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 responsePulse of far-red light no responsePulse of red light pulse of far-red no

response

- 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

• 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