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Plastids are organelles found in photosynthetic eukaryotes and are best known as sites of photosynthesis. In addition to this role, plastids manufacture and store a myriad of biomolecules, including carbohydrates, amino acids, fatty acids, plant hormones, and nucleotides (Buchanan and Gruissem, 2000). Sulfur and nitrogen are assimilated into biologically available forms in plastids, and antioxidants such as ascorbic acid (vitamin C) and other secondary metabolites are also produced there. Plastids can differentiate into several forms depending on their function in the cell in which they reside. Among the types of plastids found in plants are chloroplasts (photosynthesis), chromoplasts (fruit pigment synthesis and storage), leucoplasts (monoterpene synthesis), amyloplasts (starch storage), and etioplasts (found in dark-grown plants).
In spite of these versatile functions, plastid genomes encode fewer than 100 proteins (Sato et al., 1999). Rather, the vast majority of plastid proteins are encoded by nuclear genes. It has been estimated that approximately 3,000 proteins are transcribed in the nucleus, translated in the cytoplasm, and then imported into plastids. Plastids are believed to have originated from the engulfment of a photosynthetic bacterium by a eukaryotic cell that already contained mitochondria, which are the by-products of a separate endosymbiotic event. After endosymbiosis, most genes were either lost or transferred to the nucleus, where they acquired the regulatory sequences for expression, as well as sequences that encode a transit peptide for targeting proteins to the plastid where they are imported and cleaved (Kleine et al., 2009). Once these proteins reach their proper compartment within the plastid, many are incorporated into multisubunit complexes that include components encoded in plastid genomes. Thus, signaling between plastids and the nucleus is required to maintain plastid biological functions.
Signaling between chloroplasts and the nucleus is bidirectional. In a process called anterograde regulation, the nucleus encodes regulators that convey information about cell type and expresses proteins that are appropriate for plastid functions within that particular cell type (Pesaresi et al., 2007; Kleine et al., 2009). In anterograde regulation, gene expression in the nucleus and chloroplasts is coordinated so that proper stoichiometry of subunits of plastid protein complexes is achieved. In retrograde signaling, nuclear gene expression is regulated as a result of signals generated from plastids. These signals reflect both the developmental and functional state of...