Involved in plastid gene expression (PubMed:16326926). Required in the nucleus for the initiation of photomorphogenesis mediated by phytochromes (PHYs) (e.g. PHYA and PHYB) by mediating PHYs localization to photobodies, especially in response to red and far-red light, and implicating phytochrome nuclear bodies as sites of proteolysis for PHYs and PIFs proteins (e.g. PIF1 and PIF3). Acts downstream of PHYs and upstream of DET1 (PubMed:20603003, PubMed:22895253). Component of the transcriptionally active chromosome (TAC) complexes (PubMed:21949211, PubMed:22010110). Interacts with PTAC14 and PTAC7 (PubMed:22010110, PubMed:23082802). Binds to photoactivated phytochromes (e.g. PHYA and PHYB) via their photosensory domains; these interactions stimulate its light-mediated accumulation (PubMed:22895253). Localizes to subnuclear domains in the nucleolus and in the nucleoplasm, and in chloroplasts. Mostly expressed in cotyledons, leaves, stems and flowers, but barely in roots. In young seedlings, expressed predominantly in cotyledons, at the top of the hypocotyl, and in the root tip, regardless of light conditions. In older seedlings, confined to unexpanded cotyledons of dark-grown seedlings. Regulated by light at the post-translational level; constitutively expressed at transcript level, but accumulates in response to light (e.g. red, far red, blue and white light) at protein level. Seedling lethal without exogenous carbon sources (PubMed:16326926, PubMed:20603003). Decrease of total chlorophyll content as well as a decrease in the chlorophyll a:b ratio. Absence of grana thylakoids. Altered expression profiles of plastid genes (PubMed:16326926). Impaired in phytochrome (both PHYA and PHYB) responses, especially to red and far-red light, such as hypocotyl growth inhibition, proteolysis of phytochrome A and phytochrome-interacting transcription factors (PubMed:20603003).