Phytochrome‐regulated Gene Expression

Identification of all genes involved in the phytochrome (phy)‐mediated responses of plants to their light environment is an important goal in providing an overall understanding of light‐regulated growth and development. This article highlights and integrates the central findings of two recent comprehensive studies in Arabidopsis that have identified the genome‐wide set of phy‐regulated genes that respond rapidly to red‐light signals upon first exposure of dark‐grown seedlings, and have tested the functional relevance to normal seedling photomorphogenesis of an initial subset of these genes. The data: (a) reveal considerable complexity in the channeling of the light signals through the different phy‐family members (phyA to phyE) to responsive genes; (b) identify a diversity of transcription‐factor‐encoding genes as major early, if not primary, targets of phy signaling, and, therefore, as potentially important regulators in the transcriptional‐network hierarchy; and (c) identify auxin‐related genes as the dominant class among rapidly‐regulated, hormone‐related genes. However, reverse‐genetic functional profiling of a selected subset of these genes reveals that only a limited fraction are necessary for optimal phy‐induced seedling deetiolation.