Differential transcription of plastid genes during barley chloroplast development

Abstract

Plastid transcription activity and DNA copy number were quantified during chloroplast development in the first foliage leaf of dark-grown and illuminated barley seedlings. Plastids isolated from cells located in the basal meristem of four-d-old seedlings were small (~2 μm in diameter), exhibited low transcription activity and contained approximately 130 copies of plastid DNA per organelle. In a 1 to 3 cm region located adjacent to the leaf basal meristem, transcription activity per plastid increased 10-fold and DNA copy number increased from 130 to 210. Plastid transcription activity declined rapidly in illuminated plants with increasing leaf cell age and plastid DNA copy number also declined but with a slower time course. In dark-grown seedlings, plastid transcription activity declined more slowly than in illuminated plants while DNA copy number remained constant with increasing cell age. These data show that plastid transcriptional activity and DNA copy number increase early in chloroplast development and that transcriptional activity per DNA template varies up to 5-fold during barley leaf biogenesis. The dependence of nuclear cab and rbcS mRNA accumulation on plastid transcription activity during leaf cell development was assessed. Upon inhibition of the initial increase in barley plastid transcription activity via the plastid RNA polymerase inhibitor tagetitoxin, the cab and rbcS transcripts did not accumulate to detectable levels. These results were also observed in a barley pigment-deficient mutant, alb-f^17, and in plants containing photo-oxidized plastids which show similar reductions in plastid transcription activity. This implies that the activation of plastid transcription during the early stages of chloroplast biogenesis is necessary for the proper expression of rbcS and cab. Plastid DNA levels were unaffected by tagetitoxin treatment, suggesting that plastid DNA replication is regulated independently of plastid transcription. Chloroplast genomes encode rRNAs, tRNAs and proteins involved in transcription, translation and photosynthesis...