Abstract
Plant photoreceptors transduce environmental light cues to downstream signaling pathways, regulating a wide array of processes during growth and development. Two major plant photoreceptors with critical roles in photomorphogenesis are phytochrome B (phyB), a red/far-red absorbing photoreceptor, and cryptochrome 1 (CRY1), a UV-A/blue photoreceptor. Despite substantial genetic evidence for cross-talk between phyB and CRY1 pathways, a direct interaction between these proteins has not been observed. Here, we report that Arabidopsis phyB interacts directly with CRY1 in a light-dependent interaction. Surprisingly, the interaction is light-dissociated; CRY1 interacts specifically with the dark/far-red (Pr) state of phyB, but not with the red light-activated (Pfr) or the chromophore unconjugated form of the enzyme. The interaction is also regulated by light activation of CRY1; phyB Pr interacts only with the unstimulated form of CRY1 but not with the photostimulated protein. Further studies reveal that a small domain extending from the photolyase homology region (PHR) of CRY1 regulates the specificity of the interaction with different conformational states of phyB. We hypothesize that in plants, the phyB/CRY1 interaction may mediate cross-talk between the red/far-red- and blue/UV-sensing pathways, enabling fine-tuning of light responses to different spectral inputs.
Highlights
Phytochrome B and cryptochrome 1 (CRY1) are plant photoreceptor proteins that mediate red and UV/blue light signaling responses, respectively
From the ϩPCB plates that were incubated in the dark, we identified full-length CRY1 as a novel, PCB-dependent interacting partner of Phytochrome B (phyB) (Fig. 1A)
One early study found that irradiating plant seedlings with blue, but not red, light caused subsequent illumination with red light to have a larger effect on hypocotyl growth [22]
Summary
Phytochrome B (phyB) and cryptochrome 1 (CRY1) are plant photoreceptor proteins that mediate red and UV/blue light signaling responses, respectively. Despite many prior studies in plants that have provided genetic evidence for cross-talk between the cryptochrome and phytochrome pathways [13,14,15,16,17,18,19,20,21,22], it has not been clear how this is mediated because evidence for a direct, light-dependent interaction between these photoreceptors has been elusive. We report a novel interaction between the phyB and CRY1 photoreceptor proteins that is specific to the Pr state of phyB and is disrupted by light stimulation of either phyB or CRY1 This light-dissociated interaction may represent a novel mechanism for cross-talk between blue and red/far-red signaling pathways
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