An increase in the unspliced cox2 transcript and accompanying decrease in the frequency of RNA editing near the exon/intron junction (intron binding site 1, IBS1) have been reported in cold-treated wheat. Here, an attempt was made to clarify whether a similar phenomenon occurs in rice. Levels of unspliced cox2 transcript increased and its editing at the IBS was abolished after cold treatment. The accumulation of COXII protein remained unaffected. The accumulation of intron-containing transcripts of another eight mitochondrial genes, 23 introns in total, was analyzed by Northern blotting and semi-quantitative RT-PCR. An increase in 14 of the 23 intron-adjoining cDNA after cold treatment was observed. Six RNA editing sites in the IBS of four genes were tested as to their status by sequencing cDNA. One of these sites in the nad7 transcript showed a close association with splicing, with editing and splicing occurring simultaneously, irrespective of cold treatment. Two other sites in the intron-containing cox2 and rps3 transcripts were sensitive to cold, where editing frequency began to decrease 1 day after cold treatment, and finally exhibited a tight association with splicing 14 days later. The other sites were efficiently edited. The intron-spliced transcripts were fully edited at all six sites.

We investigated the effects of low temperature on the expression of mitochondria-encoded genes in wheat. During the course of our study, we found that the transcriptional pattern of cox2 was affected by low temperature. Northern blot analysis indicated that the precursor transcript before intron splicing was clearly increased by cold, although the level of mature message was not changed. We also found that cold treatment severely suppressed RNA editing at intron-binding site 1 of the precursor transcript, which plays an important role in intron splicing. These results suggest that low temperature could affect the post-transcriptional regulation of wheat mitochondrial cox2 transcripts, i.e. intron splicing and/or RNA editing.

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