Abstract
During the plant life cycle, arguably the most critical environmental signal is light, which is perceived by a suite of wavelength-specific photoreceptors including the red/far-red sensing phytochrome (phy) family. At the cellular level, one of the earliest light responses is the translocation of active phy from the cytoplasm to the nucleus. In the nucleus, phys interact with and are colocalized with a group of phy-interactcing bHLH transcription factors (PIFs) in subnuclear foci termed phy nuclear bodies (phyNBs). The function of phyNBs in relation to other phy signaling events is still enigmatic. However, the identification of a novel phy signaling component, HEMERA (HMR), links phyNBs to protein degradation. Light-grown hmr mutants are impaired in their ability to form large phyNBs, and they are defective in all phy-mediated responses including hypocotyl growth inhibition and chloroplast differentiation, suggesting that phyNBs are essential for normal phy signaling. Additionally, light-grown hmr seedlings accumulate light-labile proteins, including phyA, PIF1, and PIF3. HMR is structurally similar to the yeast protein RAD23, which shuttles ubiquitylated proteins to the proteasome. In accordance with this, HMR can partially complement the rad23D mutant in yeast. Together, these data suggest that phyNBs are required for normal phy signaling, and that they are sites for protein degradation. To identify other components of the HMR-mediated phy signaling pathway, we performed a hmr suppressor screen on a weak hmr allele. This screen led to the identification of three dominant missense alleles of the same gene, SUPPRESSOR OF HEMERA (SOH). SOH encodes an unknown protein. Remarkably, loss-of-function soh mutations lead to a hmr-like phenotype, including aberrant phyNB formation and impaired phy signaling responses. These results suggest that both HMR and SOH work in concert as part of the early phytochrome signaling mechanisms involving phyB localization to phyNBs, as well as subsequent light-dependent proteolytic events.
