**1. Introduction**

Soybean is an important cash crop for protein and edible oil. Its ability to perform symbiotic nitrogen fixation in root nodules makes it a nitrogen-rich food source. Root cells obtain photo-assimilates from source tissues and catabolize these chemical compounds through root mitochondria to generate the ATPs required for various physiological and biochemical processes. In eukaryotic cells, cellular respiration mainly takes place inside mitochondria. The tricarboxylic acid (TCA) cycle and mitochondrial electron transport chain (mETC) convert biochemical energy from nutrients into ATP, which is an important energy currency of the cell.

The energy demand in root nodules is much higher than in uninoculated roots, since nitrogen fixation consumes a tremendous amount of energy [1]. Therefore, substantial differences in carbon metabolism and mitochondrial activities between root nodules and uninoculated root cells are expected. Sucrolytic activities (invertase and sucrose synthase) are found to be 3–4-fold higher in root nodules

than in uninoculated roots and these enzymatic activities are restricted to the uninoculated cortical tissue and are absent in the infected central tissue of nodules [2]. Bacteroids of the nodules were shown to exhibit a limited capacity to utilize carbohydrates [3], with limited glycolytic activities and no sucrolytic activities. It is reported that sucrose taken up by root nodule cells is converted into malate and succinate as the main energy sources for bacteroids [1]. Hence, to facilitate the carbohydrate supply to bacteroids, the physiology of soybean mitochondria in nodules is significantly different from the uninoculated roots. Mitochondria in nodules were shown to oxidize malate at a rate 2-fold higher than cotyledon mitochondria. However, the activities of TCA cycle enzymes, except malate and succinate dehydrogenases, were lower in nodule mitochondria [4].

The vast majority of mitochondrial proteins from these crucial metabolic pathways are encoded in the nucleus and, after their translation in the cytoplasm, are imported into the mitochondria [5,6]. There are 110 predicted open reading frames in soybean mtDNA and 36 of them can be translated into proteins with known functions [7,8], including some subunits of the respiratory chain: NADH dehydrogenase (complex I), cytochrome oxidase (complex IV), ATP synthase (complex V), and cytochrome C biogenesis. Coordination of the expression and accumulation of mitochondrial proteins derived from the nuclear and mitochondrial genomes are complex, especially in the post-transcriptional regulation of mtDNA expression [9,10].

Some mitochondrial mRNA transcripts are subjected to C-to-U RNA editing, a house-keeping post-transcriptional process in plants [11]. RNA editing is carried out by the editosome complexes, which are composed of pentatricopeptide repeat (PPR) proteins, Multiple Organellar RNA Editing Factors (MORF), organelle RNA Recognition Motif-containing proteins (ORRM), and organelle zinc finger editing factor family (OZ) proteins [12]. RNA editing is complex and energy-consuming. The biological function and evolutionary significance of RNA editing in plants still remain unclear [13,14]. RNA editing may lead to amino acid substitutions and affect the function of the translated proteins [15]. Editing sites, which are edited with different efficiencies in different tissues, have also been identified in the plastids of diverse species [16,17].

RNA editing has been shown to be crucial for *nad1* splicing in *Oenothera* [18] and *nad7* splicing in maize [19]. The genomes of plant mitochondria house about 20 group-II introns and their splicing requires maturases [20–22]. In *Arabidopsis thaliana*, there is one maturase gene (*matR*) in the mitochondrial genome and four nuclear maturase genes (*nMat*1-4) in the nuclear genome [23]. The four nuclear-encoded maturases are imported into mitochondria after translation [24,25]. It has been experimentally demonstrated that nuclear-encoded maturases [20,23,26] and MatR [27] are required for the splicing of various group-II introns in Arabidopsis mitochondria and the assembly of functional complex I. Here, we compare the degrees of RNA editing and intron splicing on mitochondrial transcripts in soybean nodules and roots.
