Retrograde signaling is a communication pathway from organelles to nucleus central to regulation and coordination of numerous processes in living organisms, including developmental progressions, responses to biotic and abiotic challenges, protein trafficking and remodeling of chromatin structure. Recently, we have identified the first stress-specific retrograde plastid-to-nucleus signaling metabolite, methylerythritol cyclodiphosphate (MEcPP), an isoprenoid precursor produced by the methylerythritol phosphate (MEP) pathway. This is a conserved signaling pathway that is present in plants, pathogenic bacteria, and in the non- photosynthetic plastids apicoplast of the malarial parasite, but is absent in animals. The long-term goal of our research is to gain insight into the regulatory components involved in perception and transduction of the MEcPP signaling cascade that underlies plant responses to stress. Towards this goal, in a collaborative study, we propose to target the following specific objectives: 1: Define cellular components of the MEcPP signaling pathway using genetic and biochemical approaches. 2: Define epigenetic regulators modulated during MEcPP signaling pathway by studying MEcPP regulated class II histone deacetylase (HDAC), and by global mapping of MEcPP-mediated chromatin structural modifications. 3: Determine the role of plastid stromules (stroma filled tubules) in MEcPP-mediated plastid-to-nucleus retrograde signaling using proteomics and cell biological approaches. To undertake these specific objectives we have generated foundational data and unmatched experimental tools including suppressor lines reverted in their ability to induce stress response genes in spite of high MEcPP levels, microarray-based identification of histone deacetylase gene as potential epigenetic regulators of stress responses, and plastid expressing fluorescent protein lines that enable microscopy-based identification of chloroplast tubular extensions called stromules found in plants with enhanced MEcPP levels.