Gene Expression in Higher Plant Mitochondria. In: Day DA, Millar AH, Whelan J, editors. Plant Mitochondria: From Genome to Function. Dordrecht: Springer Netherlands; 2004. pp. 55-81. [DOI: 10.1007/978-1-4020-2400-9_4]

Evolution of mitochondrial RNA editing in extant gymnosperms

To unveil the evolution of mitochondrial RNA editing in gymnosperms, we characterized mitochondrial genomes (mitogenomes), plastid genomes, RNA editing sites, and pentatricopeptide repeat (PPR) proteins from 10 key taxa representing four of the five extant gymnosperm clades. The assembled mitogenomes vary in gene content due to massive gene losses in Gnetum and Conifer II clades. Mitochondrial gene expression levels also vary according to protein function, with the most highly expressed genes involved in the respiratory complex. We identified 9132 mitochondrial C-to-U editing sites, as well as 2846 P-class and 8530 PLS-class PPR proteins. Regains of editing sites were demonstrated in Conifer II rps3 transcripts whose corresponding mitogenomic sequences lack introns due to retroprocessing. Our analyses reveal that non-synonymous editing is efficient and results in more codons encoding hydrophobic amino acids. In contrast, synonymous editing, although performed with variable efficiency, can increase the number of U-ending codons that are preferentially utilized in gymnosperm mitochondria. The inferred loss-to-gain ratio of mitochondrial editing sites in gymnosperms is 2.1:1, of which losses of non-synonymous editing are mainly due to genomic C-to-T substitutions. However, such substitutions only explain a small fraction of synonymous editing site losses, indicating distinct evolutionary mechanisms. We show that gymnosperms have experienced multiple lineage-specific duplications in PLS-class PPR proteins. These duplications likely contribute to accumulated RNA editing sites, as a mechanistic correlation between RNA editing and PLS-class PPR proteins is statistically supported.

Mitochondrial atp9 genes from petaloid male-sterile and male-fertile carrots differ in their status of heteroplasmy, recombination involvement, post-transcriptional processing as well as accumulation of RNA and protein product

Petaloid cytoplasmic male-sterile carrots exhibit overexpression of the mitochondrial atp9 genes which is associated with specific features in organization and expression of these sequences. In carrots, the Sp-cytoplasm causes transformation of stamens into petal-like organs, while plants carrying normal N-cytoplasm exhibit normal flower morphology. Our work was aimed at characterization of distinct features both cytoplasms display with respect to organization and expression of the mitochondrial atp9 genes. We show that two carrot atp9 genes, previously reported as cytoplasm-specific, in fact occur in heteroplasmic condition. In the Sp-cytoplasm the atp9-1 version dominates over atp9-3, while in N-cytoplasmic plants this proportion is reversed. Herein, we also indicate the presence and recombination activity of a 130-/172-bp sequence repeat which likely shaped the present organization of carrot atp9 loci. Furthermore, cDNA sequence examination revealed that the atp9 open reading frames (ORFs) were C to U edited in 4 nucleotide positions. One of the editing events turns a glutamine triplet into the stop codon, thereby equalizing ORFs of atp9-1 and atp9-3. A certain fraction of partially edited molecules was identified-they all represented the atp9-3 sequence. In either Sp- or N-cytoplasmic plants multiple 5' transcript termini were observed. Of these, the ones mapping more distantly from the atp9 ORF were more pronounced in case of petaloid accessions. It was also shown that despite comparable copy number of the genomic atp9 sequences, the level of the respective mRNAs was approximately 3 times higher in case of petaloid carrots. The latter fact corresponded to the elevated content of the ATP9 protein in plants carrying Sp-cytoplasm. The semi-fertile phenotype of such plants is associated with a drop in ATP9 accumulation.

Plant mitochondrial genes can be expressed from mRNAs lacking stop codons

The mRNAs of the nad6 and ccmC genes of Arabidopsis and cauliflower were found to be processed upstream of the inframe stop codons. This result was confirmed by northern hybridization and by RT-PCR. There is no evidence that an alternative stop codon is created post-transcriptionally, either by RNA editing or by polyadenylation. The non-stop mRNAs are found in the high molecular weight polysomal fractions, suggesting that they are translated. Using antibodies directed against CcmC, the corresponding protein was detected in Arabidopsis mitochondrial extracts. These observations raise the question of how the plant mitochondrial translation system deals with non-stop mRNAs.

A role for mitochondria in the establishment and maintenance of the maize root quiescent center

Mitochondria in the oxidizing environment of the maize (Zea mays) root quiescent center (QC) are altered in function, but otherwise structurally normal. Compared to mitochondria in the adjacent, rapidly dividing cells of the proximal root tissues, mitochondria in the QC show marked reductions in the activities of tricarboxylic acid cycle enzymes. Pyruvate dehydrogenase activity was not detected in the QC. Use of several mitochondrial membrane potential (DeltaPsi(m)) sensing probes indicated a depolarization of the mitochondrial membrane in the QC, which suggests a reduction in the capacity of QC mitochondria to generate ATP and NADH. We postulate that modifications of mitochondrial function are central to the establishment and maintenance of the QC.