Plastid Gene Expression during Amyloplast Formation in Cultured Tobacco Cells

Abstract

Summary We analyzed changes in plastid gene expression during amyloplast formation in cultured tobacco (Nicotiana tabacum L.) cells. When cultured tobacco cells (line BY 2) in the stationary phase were transferred to auxin-depleted culture medium that contained cytokinin, leucoplast-like plastids in the BY-2 cells were converted to amyloplasts within two days. The cell number, number of plastids per cell, and the relative amount of plastid DNA per cell remained nearly constant throughout this process. During amyloplast formation, transcripts for most plastid genes increased only transiently, and stopped increasing during the late phase of amyloplast development, while transcript levels for psbA (a gene for D 1 protein of photosystem 11) and psaAlB (genes for P700 apoprotein Al/A2 of photosystem 1) continued to increase. Assay of transcription in vitro using isolated plastid-nuclei (nucleoids) showed that the transcriptional activities of most plastid genes decreased dramatically during this process, whereas the transcription of psbA and psaAl B continued at a considerable rate, showing a correlation between transcription and transcript accumulation. However, the fact that increases in transcript abundance were observed for all the plastid genes investigated here in spite of the marked decrease in their transcriptional activity suggests that the degradation of transcripts may also be inactive in the amyloplasts of BY-2 cells.