(400-32) Light-dependent alternative promoter usage of the NCP mediates nucleus-chloroplast communication for initiating chloroplast biogenesis in Arabidopsis
Body of Abstract: Chloroplast biogenesis is critical to establishing photosynthetic green plastids that enable plants to provide food (sugars) and oxygen essential for all life on earth. Chloroplast differentiation in angiosperms requires light that is sensed by a suite of photoreceptors, including red/far-red-sensing phytochromes. Light-activated phytochromes in the nucleus send signals to plastids to induce the assembly of plastid-encoded RNA polymerase (PEP) holoenzyme complex for the activation of photosynthesis-associated plastid-encoded genes. Recent genetic screens have identified REGULATOR OF CHLOROPLAST BIOGENESIS (RCB) and NUCLEAR CONTROL OF PEP ACTIVITY (NCP) as key controllers of phytochrome-mediated PEP assembly and activity. However, the mechanism of inter-organelle communication to control light-dependent chloroplast biogenesis remains enigmatic. Here, we demonstrate that light and dark use different transcription start sites of the NCP promoter, producing two different mRNA isoforms. The longer NCP isoform including N-terminal chloroplast transit peptide localizes to chloroplasts. The short isoform lacking the transit peptide localizes to cytoplasm and the nucleus. We propose that alternative promoter usage of the NCP is the key mechanism of light-dependent nucleus-to-plastid signaling for initiating chloroplast biogenesis in Arabidopsis.