ED 25. Light

Photoreceptors

Photoreceptors are the ‘Detectors’:

• PSII

• Cryptochrome

• Phototropin

• Zeaxanthin

• Phytochrome

Arabidopsis thaliana

• model organism - ‘botanists fruit fly’

• analysis of mutants lacking some responses to light

• - nph1 (non-phototropic hypocotyl) - phototropin 1 gene

• - phototropins are blue light sensing chromo-proteins

Loss of photoreceptor function dramatically alters phenotype

Colour pic Figure 3. De-etiolation in Arabidopsis seedlings. Genotypes shown are: wild type (A), and mutants in phyA (B), phyB (C), phyBDE (D), phyABE (E) and phyABDE (F). 

Phytochromes are red-light photoreceptors 

• phytochromes are light absorbing molecules found in all green plants 

• they respond mainly to red and far-red light

• dimetric proteins 

• tetrapyrrole chromophore - phytochromobilin

Action spectra help identify chromophores

• measure germination of seedlings exposed to different coloured lights

• compare this to phytochrome absorbance spectrum

Photomorphogenesis

• photomorphogenesis - this is the control of growth and morphology by light

• De-(etiolation

• Circadian rhythms

• Flowering

High irradiance response

• measures quantity of light 

• irradiance is quantum flux - units micromol photons per m² per second 

• tells the plants to start chlorophyll synthesis and stop etiolation response 

• Phytocrhome-A and cryptochrome 

Low fluence response

• measures quality of light (ratio of red to far-red), mainly Phytochrome-B

• Fluence - > micromol photons per m² (time not so important)

• lettuce seeds need light to germinate

• red light stimulated germination but far red reversed the effect

Red to far-red ratio

Mechanism of LFR 

• phytochromes exist in two forms that are interconverted by red and far-red light 

• phytochromes are synthesised as inactive P_R which absorbed red light 

• P_FR is the active form, and absorbs far-red 

• High [P_FR] → lack of shading → stimulates germination → adaptive 

• P_FR slowly reverts to inactive P_R form

• Photoreversible process 

Phytochrome signalling

• P_FR has exposed NLS

• P_FR kinase activity starts a signalling cascade

• P_FR acts on transcription factors

Biological clock

• circadian rhythms are controlled buy a biological clock

• can anticipate the needs of the day → stomata open before dawn, flower opening (photonasty) to match pollinator activity

Period 

• the oscillator usually runs with an approximately 24hr period

Free-running

• if plants are grown under constant conditions, they will free run

• many plants show free running period faster or slower than 24hr

Attentuation

• if the plants are kept in a constant environment for a long time, circadian rhythms fade away

Real data: stomatal rhythm under constant light 

The clock is entrained by Zeitgebers

• normal zeitgeberis daybreak which resets the clock daily

• can also shift its phase

Genetic observations

• We cannot ‘see’ the biological clock directly

• look at functions it controls

• free-running period is genetically controlled

Clock genes and mutants suggest a mechanism - three genes at once 

Many genes show rhythmic transcription

Connecting photoreceptors and the clock

• circadian transcription of flowering gene CO

• PHYB causes CO protein degradation

• PHYA and CRY stabilise CO protein in long days

• CO protein induces FT expression

• FT protein is the flowering hormone