Dynamic Features of Plastid Genome and Its Transcriptional Control in Plastid Development

Abstract

Land plants usually have different types of plastids, e.g. etioplasts, chloroplasts, amyloplasts, and chromoplasts. Although identical copies of the plastid genome are present in all plastid types, the level and pattern of accumulation of plastid transcripts varies largely among the different plastid types and during plastid differentiation and development. Plastid genomes possess many promoters of widely differing strength, and genes often have multiple initiation sites. Two distinct plastid RNA polymerases, plastid-encoded RNA polymerase (PEP) and nuclear-encoded RNA polymerase (NEP), direct such a complex transcription of plastid genes. Photosynthetic genes, e.g. psbA, psbD and rbcL are mainly transcribed by PEP. Some non-photosynthetic genes such as rpoB and accD are exclusively transcribed by NEP, and rrn and clpP genes are mutually transcribed by both PEP and NEP. The interplay of PEP and NEP results in a highly complex transcript pattern in plastids. PEP controls chloroplast development in leaves and its functional maintenance is primarily mediated by the variation of sigma factors. Arabidopsis thaliana has six nuclear-encoded plastid sigma factors (AtSIG1 to 6). In addition to the temporal dynamics of sigma factors during chloroplast development, the expression profile of each plant sigma factor in organs and cell types is diverse and probably correlated with the major function of each sigma factor. Extensive forward and reverse genetics studies revealed the role and specificity of respective sigma factors in transcription of different plastid genes involved in the biosynthesis and maintenance of the photosynthetic apparatus, during chloroplast development or under various environmental conditions such as light, salt and cold/heat stresses. Thus, transcriptional regulation in plastids, particularly in chloroplasts, is important for fine-tuned plastid gene expression.