Pfr

Pfr is the far-red-absorbing photoform of phytochrome, a photoreceptor protein that enables plants to sense red and far-red light. Phytochromes exist as two interconvertible forms: Pr, which absorbs red light, and Pfr, which absorbs far-red light. Exposure to red light converts Pr to Pfr, while exposure to far-red light converts Pfr back to Pr. In darkness, Pfr gradually reverts to Pr through thermal dark reversion, a process influenced by temperature and other factors.

On formation, Pfr acts as the signaling state that modulates plant development. It accumulates in illuminated tissues and, after translocation to the nucleus, alters the activity or stability of transcription factors and other signaling components, triggering changes in gene expression. These responses regulate seed germination, shade avoidance, stem elongation, leaf expansion, and chlorophyll biosynthesis, helping the plant adapt to the light environment and manage competition for light.

Phytochromes and their Pfr state are widespread in land plants and are also found in some algae and bacteria. The plant photoreceptor system uses the Pr↔Pfr switch to measure light quality, particularly the red to far-red ratio, and duration, thereby integrating photic information with developmental programs. The Pr–Pfr ratio, influenced by light spectrum, intensity, duration, and temperature, provides a quantitative readout of environmental light conditions that shape growth and maturation.