Summary
Red/far-red photoreceptors toggle between forms to control germination, shade avoidance and flowering.
In depth
Red light converts inactive Pr into active Pfr, which moves to the nucleus and affects PIF transcription factors. Far-red reverses. Shade (rich in far-red) triggers shade avoidance with elongation.
Phytochromes sense red:far-red ratio, telling plants about shading by neighbours. Pfr dominates in sunlight; far-red-rich shade converts it back, inducing elongation.
Example: seedlings under leaf canopies stretch to reach light.
Phytochromes are red/far-red photoreceptors, with a chromophore (phytochromobilin) bound to a protein, existing as dimers. Pfr is the active form, and in the dark it slowly reverts. Active phytochrome in the nucleus causes degradation of PIFs, promoting photomorphogenesis (short stem, open leaves, chlorophyll). In shade, the red to far-red ratio is lower, so the Pfr level falls and plants stretch (shade avoidance), which helps them overtop competitors.
Phytochromes control seed germination (lettuce seeds respond to red light and far-red reverses it, as in Borthwick and Hendricks' experiments), flowering time and circadian clock entrainment. Other photoreceptors include cryptochromes (blue), phototropins (blue) and UVR8 (UV-B).
Example: crops planted too densely stretch and lodge because of shade signals.
Frequently asked questions
What is Phytochrome Signalling?
Red/far-red photoreceptors toggle between forms to control germination, shade avoidance and flowering.
Which area of biology covers Phytochrome Signalling, and how advanced is it?
Phytochrome Signalling is one of the 56 topics in Plant Biology. It is rated Advanced, so it suits learners who want specialist depth. Related topics in this area include Photoperiodism, Gravitropism, Auxin and Phototropism.
Can you give an example of Phytochrome Signalling?
Lettuce seed germination is promoted by red and inhibited by far-red light.
What should I learn before Phytochrome Signalling?
Work through the Beginner topics in Plant Biology first, such as Plant Kingdom, Photosynthesis, Chlorophyll, then return to Phytochrome Signalling. Advanced topics assume the Intermediate vocabulary.
What topics are related to Phytochrome Signalling?
The closest topics to Phytochrome Signalling are Photoperiodism, Gravitropism, Auxin and Phototropism, Plant Hormones. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Phytochrome Signalling?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Phytochrome Signalling. To practise the whole of Plant Biology, use the Plant Biology quiz.
How is Phytochrome Signalling different from Photoperiodism?
Phytochrome Signalling: Red/far-red photoreceptors toggle between forms to control germination, shade avoidance and flowering. Photoperiodism: Responses such as flowering triggered by the length of day and night, mediated by phytochrome. Read both side by side and note what each one explains.
How should I study Phytochrome Signalling step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Photoperiodism and explain the difference aloud. Finally, take the Plant Biology quiz and come back to this page for any question you miss.