Chloroplast differentiation, as a vital part of the photomorphogenetic program in plants, is to establish photosynthetic plastids that enable plants for photoautotrophic growth in the light. This process is tightly controlled by environmental light cues, as dark-grown seedlings develop nongreen etioplasts, light triggers a rapid greening process turning on chloroplast differentiation. Although it is well known that the red and far-red absorbing phytochromes are the major photoreceptors mediating photomorphogenesis through transcription regulation in the nucleus, the molecular mechanisms by which phytochromes initiate chloroplast differentiation is largely unknown. Recently, the P.I. identified a novel and essential component for phytochrome signaling, HEMERA (HMR), in the model plant species Arabidopsis. Surprisingly, HMR is also
required for chloroplast differentiation. The discovery of the hmr mutant, which is the first photomorphogenetic mutant exhibiting an albino phenotype under all light conditions, provides an unprecedented opportunity to unravel the mechanistic links between phytochrome signaling and chloroplast differentiation. HMR encodes a highly conserved protein in land plants, and it does not share any homology to known protein classes. HMR was previously suggested to be associated with plastidic transcriptional machinery. However, the P.I.?s preliminary evidence showed that HMR is required for phytochrome signaling events in the nucleus, and thus is more likely a nuclear protein. To solve this dilemma, the proposed research will focus on characterizing the subcellular localization of HMR and further determine HMR?s role in chloroplast differentiation. Objective 1. Determination of the subcellular localization pattern of HMR using biochemical and cell biological approaches. Objective 2. Investigation of the
relationship between HMR subcellular partitioning and chloroplast differentiation. Objective 3. Identification and characterization of hmr suppressors to further decipher the roles of HMR in chloroplast differentiation. Intellectual merit: Although it is well known that chloroplast differentiation is one of the main phytochrome mediated responses, the molecular mechanism linking phytochrome signaling and the initiation of chloroplast differentiation is still missing. Further characterization of HMR, a novel factor required for both phytochrome signaling and chloroplast differentiation, will start to uncover the long-sought mechanistic links between light signaling and chloroplast differentiation. Broader impacts: The proposed research will provide excellent training in molecular genetic, cell biological, and biochemical approaches for postdoctoral fellows, graduate students, and undergraduate students. The elucidation of
molecular mechanisms in chloroplast differentiation will contribute significantly to our basic knowledge of how nuclear and organellar genomes coordinate their responses to environmental cues in eukaryotic organisms. In addition, because most living organisms on this planet, including human being, depend on energy generated by photosynthesis in the chloroplast, a better understanding of the regulation of chloroplast differentiation will have deep implications in agriculture and practical applications using plant biomass as a renewable energy source.