Light-activated phytochrome A and B interact with members of the SPA family to promote photomorphogenesis in Arabidopsis by reorganizing the COP1/SPA complex.

Phytochromes function as red/far-red photoreceptors in plants and are essential for light-regulated growth and development. Photomorphogenesis, the developmental program in light, is the default program in seed plants. In dark-grown seedlings, photomorphogenic growth is suppressed by the action of the CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1)/SUPPRESSOR OF phyA-105 complex, which targets positive regulators of photomorphogenic growth for degradation by the proteasome. Phytochromes inhibit the complex, leading to the accumulation of transcription factors promoting photomorphogenesis; yet, the mechanism by which they inactivate COP1/duanyu1842 is still unknown. Here, we show that light-activated phytochrome A (phyA) and phytochrome B (phyB) interact with and other proteins. Fluorescence resonance energy transfer-fluorescence lifetime imaging microscopy analyses show that and phytochromes colocalize and interact in nuclear bodies. Furthermore, light-activated phyA and phyB disrupt the interaction between COP1 and resulting in reorganization of the COP1/duanyu1842 complex in planta. The light-induced stabilization of HFR1, a photomorphogenic factor targeted for degradation by correlates temporally with the accumulation of phyA in the nucleus and localization of phyA to nuclear bodies. Overall, these data provide a molecular mechanism for the inactivation of the COP1/duanyu1842 complex by phyA- and phyB-mediated light perception.

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