After germination, Arabidopsis seedlings follow one of two developmental pathways: skotomorphogenesis in darkness and photomorphogenesis in the light. Activation of the photoreceptor phytochrome A (phyA) by far-red light activates a developmental program that causes the inhibition of hypocotyl elongation, the opening of the apical hook, cotyledon expansion, chloroplast development and transcriptional changes in light-regulated genes. Whereas cop1 mutants undergo partial photomorphogenesis in darkness, phyA mutants undergo skotomorphogenesis in far-red light. Mutants lacking the transcription factor LAF1 are blocked in a subset of phyA responses.