RESUMEN
Upon imbibition, dry seeds rapidly gain metabolic activity and the switching on of a germination-specific transcriptional programme in the nucleus goes ahead, with the induction of many nucleus-encoded transcripts coding for plastid-localized proteins. Dedifferentiated plastids present in dry seeds differentiate into chloroplasts in cotyledons and into amyloplasts in the root and in the hypocotyl, raising the question of whether the beginning of a new plant's life cycle is also characterized by specific changes in the plastid transcriptional programme. Here the plastid transcriptome is characterized during imbibition/stratification, germination, and early seedling outgrowth. It is shown that each of these three developmental steps is characterized by specific changes in the transcriptome profile, due to differential activities of the three plastid RNA polymerases and showing the integration of plastids into a germination-specific transcriptional programme. All three RNA polymerases are active during imbibition; that is, at 4 °C in darkness. However, activity of plastid-encoded RNA polymerase (PEP) is restricted to the rrn operon. After cold release, PEP changes specificity by also transcribing photosynthesis-related genes. The period of germination and radicle outgrowth is further characterized by remarkable antisense RNA production that diminishes during greening when photosynthesis-related mRNAs accumulate to their highest but to very different steady-state levels. During stratification and germination mRNA accumulation is not paralleled by protein accumulation, indicating that plastid transcription is more important for efficient germination than translation.