Increased accumulation of intron-containing transcripts in rice mitochondria caused by low temperature: is cold-sensitive RNA editing implicated?

Abstract

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.

Post-transcriptional Regulation in Mitochondria of Rice and Wheat at Low Temperatures

“…The amount of intron-containing transcript before splicing increased over time at 12˚C, while the amount of transcripts harboring no intronic sequence after splicing remained unchanged. Similar results were also obtained for the transcript profiles of rps3 (ribosomal protein S3) and ccmF C , (cytochrome c maturation related protein) 41 ( Fig. 2).…”

“…It is indicated that the accumulation of intron-containing transcripts increase at low temperatures in rice mitochondria as shown in the profile of the cox2 (cytochrome c oxidase subunit 2) transcript by Northern blots 41 ( Fig. 2).…”

“…The abundance of fourteen types of a total of twenty-three type intron-adjoining cDNAs increases and none decrease at 12˚C, while post-splicing transcripts remain mostly unchanged (Table 1). There seems to be no correlation between the cold-induced accumulation of intron-containing transcripts and organization of introns: the splicing manner (cis or trans), intron length, splicesite sequence, and length of domains V and VI 41 . Therefore, the association of nuclear genes with mitochondrial transcription and their cold responses should be investigated.…”

“…It has been widely reported that cold-inducible genes also respond to a water deficit 27,81 . Thus, the effects of ABA, NaCl, and water deficit on the cox2 transcriptional pattern were preliminarily examined; however, no accumulation change in the cox2 intron-containing transcript was observed, suggesting no direct relationship between the increase in the cox2 precursor transcript and osmotic stress 41 . The profiles of all other genes with introns were analyzed by semi-quantitative RT-PCR 41 and summarized in Table 1.…”

“…Thus, the effects of ABA, NaCl, and water deficit on the cox2 transcriptional pattern were preliminarily examined; however, no accumulation change in the cox2 intron-containing transcript was observed, suggesting no direct relationship between the increase in the cox2 precursor transcript and osmotic stress 41 . The profiles of all other genes with introns were analyzed by semi-quantitative RT-PCR 41 and summarized in Table 1. The abundance of fourteen types of a total of twenty-three type intron-adjoining cDNAs increases and none decrease at 12˚C, while post-splicing transcripts remain mostly unchanged (Table 1).…”