Abstract The control of organellar gene expression is critical for the cellular programming of all eukaryotic organisms. While perturbing mitochondrial gene expression leads to human pathologies, including cancer, altering plastid gene expression can kill plants. However, the cell signaling mechanisms that control organellar gene expression remain poorly understood. The long-term goal of the PI’s laboratory is to utilize the light-induced plastid differentiation into photosynthetically active chloroplasts in Arabidopsis as a genetic model to interrogate cell signaling mechanisms for controlling organellar gene expression. The current data support the central hypothesis that the red and far-red photoreceptor phytochrome B promotes the degradation of a small family of phytochrome-interacting basic/helix-loop-helix transcription factors in the nucleus to generate nucleus-to-plastid (anterograde) signals that trigger the assembly and activation of the multisubunit, bacterial-type plastid RNA polymerase for transcribing plastid photosynthesis genes. Here the PI propose to utilize a combination of molecular genetics, biochemistry, and genomics approaches to (1) determine the mechanism initiating anterograde signaling in the nucleus, (2) determine the mechanism of the assembly of the multisubunit plastid RNA polymerase complex, and (3) determine the mechanism activating the plastid RNA polymerase. The proposed research is innovative because it utilizes photoreceptor signaling and chloroplast biogenesis in Arabidopsis as a genetic model to elucidate a previously uncharacterized anterograde signaling pathway. The PI has developed new forward genetic approaches and biochemical assays and identified critical components that define the framework of anterograde signaling. The proposed research is significant, because it is expected to uncover the light signaling mechanism for initiating chloroplast biogenesis - a long-standing gap in our knowledge of plant light signaling and the regulation of photosynthesis. Because the control of transcription in plastids shares intrinsic similarities with that in mitochondria, what we learn in the plastid model is expected to enhance the understanding of the general principles of cell signaling mechanisms in controlling organellar gene expression, including the regulation of mitochondrial gene expression, and therefore, will ultimately contribute to the understanding of the mechanisms underlying the misregulations of mitochondrial gene expression in human diseases.