Role of CIA2 and CIL in the regulation of chloroplast photomorphogenesis in Arabidopsis

Chloroplast development plays a crucial role in plant de-etiolation, a process in which plants switch from growth in darkness to light-driven development, known as photomorphogenesis. This study provides evidence that CIA2 (Chloroplast Import Apparatus 2) and CIL (CIA2-Like) contribute to chloroplast biogenesis, likely by affecting and regulating PSII assembly and related gene expression. Although their precise molecular roles remain unclear, our findings support their involvement in chloroplast development. This is indicated by deregulation of foliar chlorophyll content, chlorophyll a fluorescence parameters, chloroplast size, and gene expression of PSII molecular markers in cia2cil double mutant during de-etiolation. Chlorophyll a fluorescence and quantitative gene expression analysis during de-etiolation revealed a significant reduction in PSII maximal efficiency and non-photochemical quenching, as well as deregulated transcription of genes such as LHCB2.1 and psbA. According to the immunoblotting and microscopy imaging results, there is an impaired assembly of PSII and a compromised ultrastructure of the chloroplast membranes in cia2cil plants. However, in CIA2p::CIA2cia2cil and 35Sp::CIA2cia2cil complementation lines, reversion of this phenotype was observed. These results suggest a supporting role for CIA2 and CIL in the plant de-etiolation process, expanding our understanding of chloroplast biogenesis regulation.