Selective transcription and post-transcriptional processing of the heteroplasmic mitochondrial orf156 copies in the nucleus-cytoplasm hybrids of wheat.

Transcript profile and post-transcriptional processing of the mitochondrial orf156 were compared among the nucleus-cytoplasm (NC) hybrids of tetraploid and hexaploid wheat and their parental lines. These NC hybrids, in which the plasmon from the maternal Aegilops squarrosa parent is combined with the nuclear genomes of the paternal parents, and their parental lines were previously shown to have heteroplasmic copies of the polycistronic transcriptional unit nad3-orf156 in different sub-stoichiometries. Despite the heteroplasmic conditions, only single transcript types were detected among reverse transcription-PCR products of the orf156 mRNA in the respective lines. In the NC hybrids only a major maternal copy was transcribed and edited, likewise in the parental lines only single major copies were transcribed and edited. RNA gel blot analysis using the orf156 probe showed that the NC hybrids had the transcripts similar to the parental lines but unlike the maternal line. Primer extension analysis further revealed the orf156 transcripts with different 5' termini, which most probably resulted from the alternative post-transcriptional cleavages of the precursor mRNA. These cleaved transcripts were polymorphic not only among the lines but also among four different tissues. The cleavages were highly selective and the paternal-type cleavages predominated in the NC hybrids, irrespective of the tissues studied. This seemingly neutral polymorphisms in wheat could make this system convenient for further analysis of mitochondrial gene expression in plants.