Constitutive transcription and regulation of gene expression in non-photosynthetic plastids of higher plants.

The plastid genome in higher plants contains >50 genes for rRNAs, tRNAs and proteins for transcriptional and translational functions, besides the genes encoding photosynthetic proteins. Considering the totipotency of most higher plant cells and the differentiation capacity of plastids, it can be inferred that at least the genes for genetic functions must be constitutively expressed in all plant organs, including non-photosynthetic roots, to maintain a basal level of transcriptional and translational activities. To test this hypothesis, transcription, RNA accumulation and polysome formation were analyzed in root amyloplasts, and in plastids from hypocotyls and cotyledons of dark-grown spinach seedlings. The results for 10 representative genes show that they are constitutively transcribed at relative rates which are similar in root amyloplasts and leaf chloroplasts. The differential accumulation of their mRNAs in roots and other non-photosynthetic plant organs is controlled at the post-transcriptional level by a developmental program. Although mRNAs for photosynthetic proteins are detectable in root amyloplasts, some of them are specifically depleted from polysomes relative to mRNAs for ribosomal proteins. This translational discrimination does not result from modifications in splicing or 5'- and 3' -end processing of mRNAs for photosynthetic proteins, since processing is identical in root amyloplasts and leaf chloroplasts. The results support the model of constitutive transcription of the plastid genome, and indicate that the expression of most plastid genes in spinach plants is controlled primarily by post-transcriptional and translational mechanisms.