Tissue-dependent plastid RNA splicing in maize: transcripts from four plastid genes are predominantly unspliced in leaf meristems and roots.

Most plastid gene products do not accumulate to high levels in meristem proplastids or in the specialized plastids of roots. To assess whether a modulation of plastid splicing activities might play a role in this tissue-dependent expression of the plastid genome, the ratio of spliced to unspliced transcripts from the atpF, petB, petD, and rpl16 genes was compared between several tissues of maize. Although these transcripts are predominantly spliced in green leaf tissue (both bundle sheath and mesophyll cells), spliced atpF, petB, and petD transcripts are underrepresented relative to their unspliced precursors in roots and leaf meristems. The ratio of spliced to unspliced rpl16 transcripts varies in a similar fashion, but the magnitude of the differences between tissues is not as great. The proportion of RNA that is spliced reflects the tissue of origin and not photosynthetic competency, chlorophyll content, or exposure to light since the leaves of photosynthetic mutants and of seedlings grown in the absence of light contain spliced and unspliced transcripts in normal ratios. These results raise the possibility that low RNA splicing activities are in part responsible for the limited expression of the plastid genome in meristematic and root tissue.