Dynamic Regulation of Chloroplast Transcription

Plastids are ubiquitous DNA-containing organelles present in plant cells. These organelles carry out numerous metabolic functions including photosynthesis, starch synthesis, and steps in lipid, terpenoid, amino acid, tetrapyrrole, and plant hormone biosynthesis. Some of these functions are highly expressed in differentiated plastids. For example, chloroplasts are specialized for photosynthesis, chromoplasts for carotenoid biosynthesis and storage, and amyloplasts for starch biosynthesis. Although basal levels of plastid gene expression are observed in a11 plastid types, a large increase in plastid transcription is observed during chloroplast biogenesis. This suggests that one central regulatory point of chloroplast differentiation is the activation of chloroplast transcription. However, chloroplast development requires coordinated expression of plastid and nuclear genes, and, therefore, activation of plastid gene transcription must be coordinated with the activation of nuclear gene expression by signals that can be transmitted between plastids and the nucleus. The study of chloroplast gene expression also provides an opportunity to understand how plants sense and alter gene expression in response to light. Light is a substrate for photosynthesis, a trigger of plant developmental processes, and a potential source of injury that modulates chloroplast transcription.