Plant phytochrome A in the Pr state assembles as an asymmetric dimer

Dear Editor,

Phytochromes (phys) are photoreceptors responsible for a series of red/far-red light-dependent processes in plants, fungi, algae, and bacteria.¹ In plants, phys regulate various essential processes, including flowering, de-etiolation, shade avoidance, and photomorphogenic responses.^1,2 The phys exist in two distinct states: the inactive Pr state and the active Pfr state. Under red-light irradiation, the covalently bound chromophore PΦB in phys isomerizes, leading to Pr-to-Pfr conversion, which alters their interactions with multiple proteins to transduce red-light signals.^1,2,3 After far-red light detection or thermal relaxation, Pfr reverts to Pr.¹ The phy family in Arabidopsis (phyA to phyE) is encoded by five genes.¹ Among them, phyA and phyB play dominant roles in regulating physiological responses. In seeds, phyA detects very low-fluence red or far-red light to promote germination irreversibly; while phyB detects low-fluence red light to promote germination, this effect can be reversed by low-fluence far-red light. At the seedling stage, phyA detects high-fluence far-red light, and phyB detects high-fluence red light to promote de-etiolation of seedlings.^1,2 Moreover, phyB is important for temperature sensing, but phyA is not.^1,4,5 Although the functional distinctions between phyA and phyB have been well established, the molecular basis of their different functions remains a matter of debate.

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