Regulation of chloroplast development during the greening process

Sammanfattning: Upon light exposure, the expression of one-third of the nuclear-encoded genes is changing, including genes encoding many chloroplast-targeted proteins responsible for the initiation of plastid transcription and the establishment of photosynthesis. The control of the nuclear-encoded genes involved in the development of chloroplast is referred to as anterograde signalling. In my thesis work I have focused on both cryptochrome and phytochrome signalling pathways regulating the transcription of photosynthesis-related genes during chloroplast development. The results in the thesis demonstrate that several light-responsive nuclear transcription factors play a direct role in chloroplast development, and reveal the regulatory mechanism underlying the initiation of plastid photosynthetic gene expression. In this thesis, bZIP16 was found as a CRY1 interacting factor. Together with bZIP68 and GBF1, they regulate the expression of LHCBs and SIG5 through a blue light signalling pathway, promoting cotyledon opening and chloroplast development during de-etiolation. These findings provide novel insights into the role of bZIP transcription factors during chloroplast development with the involvement of the cryptochrome signalling pathway. To further investigate how the plastid photosynthetic gene expression is initiated, I focused on the role of two plastid RNA polymerases, nuclear-encoded plastid RNA polymerase (NEP) and plastid-encoded plastid RNA polymerase (PEP). By combining the PIF3-binding motif analysis, mathematic modelling and photoreceptor mutant analysis, we demonstrated a PHYB-PIF3 mediated regulation of the initial expression of the nuclear-encoded PEP components, SIGs and PAPs with the G-box/PBE-box as potential PIF3 binding-site. I have also demonstrated the presence of a fully assembled PEP complex in both proplastids and etioplasts using 2D BN/SDS-PAGE and its importance for the basal level of psaA and psbA transcription in darkness and during the early light response.