Photosynthesis, the conversion of solar energy absorbed by chlorophyll and carotenoid pigments into stored chemical energy, takes place in green plant tissues within a compartment of the plant cell known as the plastid. The chloroplast, the specific plastid type that performs photosynthesis, contains internal membranes called thylakoids required for this process. Nongreen plant tissues that do not photosynthesize contain other characteristic plastid types, including the etioplast in the embryonic leaves of dark-grown angiosperm seedlings. The hallmark of maturing etioplasts is a paracrystalline inner membrane of unknown function termed the prolamellar body. When dark-grown seedlings that have germinated underground emerge from the soil, light causes etioplasts to differentiate into chloroplasts. Such seedlings begin to green and form photosynthetically active thylakoids due to the strictly light-dependent synthesis of chlorophyll catalyzed by the NADPH-protochlorophyllide oxidoreductase enzyme concentrated in the prolamellar body. Mature chloroplasts are thought to contain more than 3000 distinct proteins. More than 95 % of these proteins are encoded by genes specified by DNA in the nucleus and the remainder by the genes of the much smaller plastid genome. Because nuclear-encoded plastid proteins are synthesized in the cytosol rather than the plastid, they must be imported into that cellular compartment. Multiple mechanisms have evolved to help coordinate gene expression and protein production between the nucleus and the chloroplast. For example, several distinct chloroplast-to-nucleus signaling pathways that regulate primarily nuclear genes required for photosynthesis have been identified and partially characterized. In contrast, the potential for etioplast-to-nucleus signaling in the dark to regulate nuclear gene expression before greening begins has remained virtually unexplored. The research planned encompasses three main goals: (1) To use a whole genome approach to identify all genes of Arabidopsis, as a model for other plant species, whose expression is strongly regulated by etioplast development in dark-grown seedlings, (2) To confirm the link between etioplast differentiation and the regulation of these target nuclear genes using independent assays, (3) To determine target gene expression patterns, and the subcellular localization and function of the corresponding protein products in dark-grown and greening seedlings. This research will provide a basis for exploring communication within plant cells between the etioplast and the nucleus. A thorough understanding of this process is essential because light-dependent chloroplast maturation is required to establish photosynthetic energy metabolism in germinating seedlings. An important aspect of this project will be to provide opportunities for training scientific researchers at the graduate, undergraduate and postdoctoral levels. Different aspects of the research will provide exposure to contemporary molecular-genetic and biochemical methods applicable to higher plants. The researchers involved will be encouraged to disseminate their findings through public presentations and outreach activities, and to actively participate in scientific meetings and professional societies.
