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Franklin (2009) describes how plants use the ratio of the red wavelength (660-670nm) over the far-red wavelength (725-735nm) (R:FR) in order to avoid shading.

My question is: which receptor is stimulated by the red and which receptor is stimulated by the far-red?

In his paragraph on Phytochromes (3rd page, right column), K. Franklin seems to say that PhyB is responsible for measuring this ratio but I am not sure.

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As far as I know there are 5 receptors for far-red and red light which are the phytochroms(phyA-phyE) Its all about the ratio between red and far-red light.

Each phytochrom has an inactive(PR) and an active(PFr) conformation. phyA is the only phytochrom which is activated by far-red light, so its active state is PR. (Only if the ratio between red and far-red light is low.) The other phytochroms, are activated by red light (high ratio between red and far-red).

An active phytochrom blocks the COP1/SPA complex. This complex is a E3 ubiquitin ligase which ubiquinates transcription factors for the light answer like HY5 or HFR1.

Example: Under normal light conditions, phyB-phyE is active. They block the COP1/SPA complex so the transcription factors for the light answer are not getting ubiquinated. The plant can get a light phenotyp. In the case that a plant grows under another plant, it gets less red light because the higher growing plant uses it for its photosynthesis. phyB-phyE become inactive. COP1/SPA can ubiquinate the transcriptions factors. The plant gets an low light phenotyp by trying to grow out of the shadow.

The function of phyA is to produce a light phenotyp if there is a lot of far-red light but almost no red light. Than it is getting activated and blocks the COP1/SPA complex. Under red light phyA is not only inactivated it also gets degraded.

I just found a paper for further reading: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2828699/

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  • $\begingroup$ So the mechanism seems much simpler than what I expected. The ratio is directly "computed" by the phytochrome and the ratio is not "calculated" from the interaction between the molecular pathways activated by the phytochromes. Thanks a lot. I will clearly have a look to the paper your attached. $\endgroup$ – Remi.b Jan 29 '15 at 21:14

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