Light Regulation of Plant Development: HY5 Genomic Binding Sites

Photomorphogenesis is a critical developmental process in plants involving numerous signaling pathways that coordinately regulate the inhibition of stem elongation, differentiation of chloroplasts, accumulation of chlorophyll, and leaf expansion that accompany the transition from dark to light as a seedling emerges from the soil. Arabidopsis HY5 encodes a bZIP transcription factor that is a positive regulator of photomorphogenesis, and previous studies suggest that it is a key regulator of numerous genes during photomorphogenesis (Oyama et al., 1997; Ang et al., 1998). In the dark, HY5 activity is repressed by the key negative regulator of photomorphogenesis, COP1, an E3 ubiquitin ligase that interacts directly with HY5 in the nucleus and targets it for degradation via the ubiquitinproteasome pathway (Osterlund et al., 2000; Saijo et al., 2003). In hypocotyls of light-grown seedlings, COP1 is initially inactivated and subsequently excluded from the nucleus, allowing derepression of HY5 (reviewed in Quail, 2002). This suggests that HY5 acts at or near the top of the transcriptional cascade that follows the dark-to-light transition during photomorphogenesis. In this issue of The Plant Cell, Lee et al. (pages 731–749) use in vivo chromatin immunoprecipation combined with microarray analysis (ChIP-chip) to map genomewide in vivo HY5 binding sites in Arabidopsis. HY5 binding sites were mapped to 3894 genes, including numerous genes encoding early light-responsive genes and other transcription factors, and binding activity was observed preferentially in promoter regions of transcribed genes. The work thus supports a model wherein HY5 functions as a high hierarchical regulator of numerous transcriptional cascades operating during photomorphogenesis.