EP Lecture 11 - Photoreceptors

Photomorphogenesis

Light‑regulated plant developmental processes distinct from photosynthesis

Plant light responses

Plants detect light quality, quantity, direction, and duration via multiple photoreceptors

UVR8

Plant UV‑B photoreceptor that uses tryptophan residues instead of an external chromophore

UVR8 mutant phenotype

Susceptible to UV‑B damage due to failure to induce protective gene expression

HY5

Transcription factor activated by UVR8 and other photoreceptors to regulate ~3000 genes

HY5 mutant phenotype

Long hypocotyl in all light qualities due to impaired light‑regulated transcription

Phytochrome

Red/far‑red photoreceptor with Pr (inactive) and Pfr (active) interconvertible forms

Phytochromobilin

Chromophore bound by phytochrome to absorb red and far‑red light

Phytochrome photoconversion

Red light converts Pr→Pfr; far‑red converts Pfr→Pr

Phytochrome mutant phenotype

Long hypocotyl in red/far‑red light due to loss of phytochrome function

Phytochrome nuclear import

Pfr moves from cytoplasm to nucleus to regulate transcription

PIF proteins

Phytochrome‑interacting transcription factors degraded by Pfr to allow light responses

Shade avoidance

Low red:far‑red stabilises PIFs, promoting auxin biosynthesis and stem extension

Vegetation shade

Enriched in far‑red light due to chlorophyll absorption of red/blue wavelengths

Vertebrate rods

Highly light‑sensitive photoreceptors with no colour discrimination

Vertebrate cones

Less sensitive photoreceptors responsible for colour vision

Rhodopsin

Visual photoreceptor composed of opsin protein and retinal chromophore

Retinal isomerisation

Light converts 11‑cis retinal to all‑trans, triggering opsin conformational change

Transducin

G‑protein activated by rhodopsin to stimulate cGMP phosphodiesterase

cGMP phosphodiesterase

Enzyme that lowers cGMP, closing Na⁺/Ca²⁺ channels in photoreceptors

Photoreceptor hyperpolarization

Light decreases cGMP, closing channels and shifting membrane potential to −75 mV

Signal amplification

One photon activates >500 transducins and closes >1000 ion channels

Rhodopsin desensitization

Phosphorylation by rhodopsin kinase followed by arrestin binding

Cone opsins

Three opsins with different spectral sensitivities enabling colour vision

Colour blindness

Caused by mutations in cone opsin genes altering spectral absorption

Melanopsin

Non‑visual photoreceptor in ipRGCs sensitive to blue light

ipRGC function

Regulates circadian rhythms and behavioural responses to light

Melanopsin signalling

Uses a G‑protein pathway distinct from transducin, similar to invertebrate opsins

SAD and melanopsin

Low melanopsin levels associated with Seasonal Affective Disorder