Variation in mitochondrial transcript profiles of protein-coding genes during early germination and seedling...

We examined RNA profiles of wheat mitochondrial genes during the developmental period when seeds leave dormancy, germinate and develop into seedlings. Mitochondrial RNAs isolated from 0 h to 6 days post-imbibition were subjected to Northern analysis, using coding-specific and intron-specific probes. Stable, edited mRNAs were observed in dormant seeds and precursor RNAs were subsequently detected early in embryo germination. The respiratory chain genes (nad7, cox1, cox2, atp6) showed mRNA profiles which paralleled those of the ribosomal RNAs, whereas ribosomal protein genes (rps2, rps3, rps7) had proportionately lower steady-state mRNA levels in later stages of seedling development. The relative levels of precursors compared with the respective mRNAs shifted down during development, consistent with transcription outpacing RNA processing in the early stages but co-ordination being more effective several days after imbibition. In the case of multiply split genes containing group II introns, complex patterns of splicing intermediates were observed, suggesting a lack of strict polarity of intron removal, although splicing efficiency appears to differ among introns. Excised intron RNAs typically are relatively more abundant in embryos than seedlings. These observations are consistent with a transient imbalance of RNA-processing machinery at the onset of seed germination, which is a period of rapid mitochondrial biogenesis.