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Light not only constitutes an essential energy source for photosynthesis but also provides spatio-temporal information used by plants to trigger adaptive responses. In particular, the first exposure to light of a germinating plantlet induces chloroplast biogenesis and phototrophic growth. Within a few hours, this photomorphogenic transition involves reprogramming of thousands genes' expression in cotyledon cells, many of them undergoing pioneering rounds of transcription. This event is paralleled by dynamic changes of histone mark profiles over regulated genes, of heterochromatin organization and of nucleus size. In previous studies, we also reported a ~3-fold increase of global RNA Polymerase II activity, suggesting that a switch from relatively quiescent to more active transcriptional status is induced by light in most cotyledon nuclei. We aim at assessing the functional consequences of such higher-order changes and the molecular mechanisms driving them. We will present how the chromatin-associated photomorphogenic regulator DET1 influences both heterochromatin dynamics and histone H2B monoubiquitination homeostasis by controlling expression of a linker histone variant and the stability of a SAGA-like deubiquitination module, respectively. Hence, we describe a signaling path that modulates the epigenome landscape, hypothetically contributing to adjust chromatin status to the cell transcriptional activity.