Alloplasmic effects on mitochondrial transcriptional activity and RNA turnover result in accumulated transcripts of Arabidopsis orfs in cytoplasmic male‐sterile Brassica napus

Abstract

SummaryMitochondrial transcription was investigated in a cytoplasmic male-sterile (CMS) Brassica napus line with rearranged mitochondrial (mt) DNA mostly inherited from Arabidopsis thaliana. The transcript patterns were compared with the corresponding male-fertile progenitors, B. napus and A. thaliana, and a fertility-restored line. Transcriptional activities, gene stoichiometry and transcript steady-state levels were analysed for all protein and rRNA coding genes and for several orfs present in the A. thaliana mitochondrial genome. The transcriptional activities were highly variable when comparing the parental species, while the CMS and restored lines displayed similar activities. For several ribosomal protein genes transcriptional activity was reduced while it was increased for orf139 in comparison with the parental species. The differences in transcriptional activity observed could be related to differences in relative promoter strength, as gene stoichiometry between lines was very limited. Transcript steady-state levels were more homogenous than the transcriptional activities demonstrating RNA turnover as a compensating mechanism. In the CMS line higher transcript abundance and novel transcript patterns in comparison with the parental lines were found for several genes. Of those, the transcripts for orf139, orf240a and orf294 were less abundant in the fertilityrestored line. These putative CMS-associated transcripts were mapped by cRT-PCR. In conclusion we show that (mt) DNA from A. thaliana was non-correctly transcribed and processed/degraded in the B. napus nuclear background. Furthermore, the introgressed nuclear A. thaliana DNA in the fertility-restored line contributes to a more rapid degradation of transcripts accumulated from A. thaliana derived orfs in the CMS line.

The molecular biology of cytoplasmically inherited male sterility and prospects for its engineering

“…Moreover, somatic hybridisation between fertile and alloplasmic male sterile tobacco plants leads to the generation of cytoplasmic hybrids (cybrids) with various flower morphologies intermediate between those of the parents and of plants with recombined mitochondrial genomes [19]. This gradient of cybrid phenotypes suggests that CMS might involve several mitochondrial loci, possibly accounted for by the number of latent ORFs expressed in alloplasmic lines [20,21 ].…”

The transcription machineries of plant mitochondria and chloroplasts: Composition, function, and regulation

“…At the DNA level, unregulated differences in promoter strength may lead to differential gene expression (Muise and Hauswirth, 1992). By comparing mitochondrial transcriptional rates in Arabidopsis and a cytoplasmic male sterile and a fertile line of Brassica napus species-specific transcription rates for several genes (cox1, nad4L, nad9, ccmB, rps7, and rrn5) were demonstrated, most likely a consequence of different promoter strength (Leino et al, 2005). A study comprising all mitochondrial-encoded genes in Arabidopsis revealed distinct transcription rates of genes encoding components of the same multisubunit complex (Giegé et al, 2000).…”

Silencing of the Nuclear RPS10 Gene Encoding Mitochondrial Ribosomal Protein Alters Translation in Arabidopsis Mitochondria

“…Differences in the gene promoter strength were observed between plant mitochondrial genes (Brennicke et al, 1999;Fey and Maréchal-Drouard, 1999) even for genes coding for components of the same complex (Giegé et al, 2000). However, the transcriptional activity appears to have only a small effect on transcript abundance because it is largely counterbalanced by processes at different steps of RNA maturation and degradation (Giegé et al, 2000;Leino et al, 2005;Holec et al, 2008). Therefore, it is thought that modulation of transcription is not essential for regulation of expression of genes encoded in mitochondria and that posttranscriptional processes are more important.…”