Editorial for Special Issue “Plant Mitochondria”

The primary function of mitochondria is respiration, where catabolism of substrates is coupled to adenosine triphosphate (ATP) synthesis via oxidative phosphorylation (OxPhos). [...].

molecules that scavenge ROS and limit mitochondrial and cellular damage. However, under some conditions these defences can become overwhelmed and ROS accumulate, leading to damage of proteins, lipids, and DNA.
The number of mitochondria per cell varies with tissue type, with more active cells with high energy demands, such as those in growing meristems, generally equipped with larger numbers of mitochondria per unit cell volume and typically these show faster respiration rates. Research on plant mitochondria has rapidly developed in the last few decades with the availability of genome sequences for a wide range of model and crop plants. Recent prominent themes in the plant mitochondrial research include linking mitochondrial composition to environmental stress responses and how this oxidative stress impacts upon mitochondrial function. Similarly, interest in the signaling capacity of mitochondria (the role reactive oxygen species, retrograde, and anterograde signaling) has revealed the transcriptional changes of stress responsive genes as a framework to define specific signals emanating to and from the mitochondrion. There has also been considerable interest in RNA metabolic processes in plant mitochondria including RNA transcription, RNA editing, the splicing of group I and group II introns, and RNA degradation and translation. Despite their identification more than 100 years ago plant mitochondria remain a significant area of research in the plant sciences.
In this Special Issue, "Plant Mitochondria", a total of 19 articles were accepted with 15 original research articles and 4 review articles broadly covering the field of plant mitochondrial research (Table 1). Manuscripts focused on protein synthesis and degradation [3][4][5][6], abiotic stress [7][8][9][10], OxPhos [11][12][13][14], protein import [15][16][17], ROS and antioxidants [18], and CMS [19,20]. A number of research articles in this Special Issue focused on the responses of mitochondria to abiotic stress, with studies that examined thermal stress (both hot and cold), salinity, and drought. Arimura et al. [7] demonstrated that cold induced mitochondrial fission (which was previously thought to involve the action of both a dynamin-related protein) DRP3A and another plant specific factor ELM1, only requires DRP3A in Arabidopsis. At the same time, they showed that an ELM1 paralogue (ELM2) seemed to have only a limited role in mitochondrial fission in an elm1 mutant, suggesting that Arabidopsis has a unique, cold induced mitochondrial fission that involves only DRP3A to control the size and shape of mitochondria. The mitochondrial transcription termination factors (mTERFs) which are involved in the control of organellar gene expression (OGE) with mutations in some characterized mTERFs (resulting in plants that have altered responses to salt, high light, heat, or osmotic stress) suggesting a role for these proteins in abiotic stress tolerance. Here Robles et al. [8] showed that strong loss of function mutant mterf6-2 was hypersensitive to NaCl and mannitol during seedling establishment, while mterf6-5 showed a greater sensitivity to heat later in development.
Rurek et al. presented a pair of research papers that used physiological, proteomic, and transcript analysis approaches to examine the thermal (hot and cold) and drought responses of cauliflower mitochondria [9,10]. In the thermal studies they identified a number of proteins that were temperature responsive including components of OxPhos, photorespiration, porin isoforms, and the TCA cycle. Similarly, in the drought analysis, which examines three different cauliflower cultivars, both OxPhos components and porin isoforms were seen to change in abundance, indicating a significant differential impact on mitochondrial biogenesis between the three cultivars, giving us new insights into the abiotic stress responses of the Brassica genus.
Male sterility refers to the inability of a plant to make viable pollen. It can be mediated through nuclear genes leading to genic male sterility (GMS) or through mitochondrial proteins interacting with nuclear genes, leading to cytoplasmic male sterility (CMS). Both GMS and CMS are widely used in agricultural production for the production of hybrid crops that benefit from heterosis. In this Special Issue Štorchová [20] presents a comprehensive review of the role of non-coding RNA in the CMS of flowering plants, while Reddemann and Horn [19] presented research examining the role of atp9 in the male sterility of CMS PET2 in sunflower. Here they showed that CMS PET2, which has the potential to become an alternative CMS source for commercial breeding, has a duplicated atp9 with a 271-bp-insertion in the 5' region of one of the atp9 genes which results in two unique open reading frames (orf288 and orf231). The reduced anther-specific co-transcription of these open reading frames in fertility-restored hybrids supports their involvement in male sterility in CMS PET2.
A total of five papers we submitted examining OxPhos, with two of these focused on identifying non-phosphorylating bypasses of the classical ETC. Wanniarachchi et al. [14] identified and characterised the alternative oxidase (AOX) and the type II NAD(P)H dehydrogenases (NDs) of rice and barley, while Velada et al. [13] characterized the AOX1 subfamily in Olea europaea cv. Galega Vulgar (European olive). Podgórska et al. [12] examined the Complex 1 mutant fro1 (frostbite 1) that has a point mutation in the 8 kDa Fe-S subunit NDUFS4 grown on different nitrogen sources. When these plants were grown on NO 3 they showed a carbon flux towards nitrogen assimilation and energy production, whereas cellulose integration into the cell wall was restricted. In contrast they showed improved growth on NH 4 + and not the expected ammonium toxicity syndrome. Similarly, Podgórska et al. [11] showed that plants with external NADPH-dehydrogenase (NDB1) knockdown were resistant to NH 4 + treatment and had milder oxidative stress symptoms with lower ROS accumulation and induction of glutathione peroxidase-like enzymes and peroxiredoxins antoxidants. Mansilla et al. provided a comprehensive review of the composition and biogenesis of the terminal oxygen acceptor cytochome c oxidase (Complex IV) in yeast, mammals, and plants. This revealed that while plants retain many biogenesis features common to other organisms, they have also developed plant specific features.
As the majority of proteins that function in mitochondria are imported from nuclear encoded cytosolic synthesized proteins, studies understanding the process of how mitochondrial protein import is controlled and regulated is vital to alter mitochondrial functions. Here Zhao et al. [17] examined the intergenomic transfer (IGT) from a broad evolutionary perspective by accessing data from nuclear, mitochondrial, and chloroplast genomes in 24 plants, and showed that mitochondrial transfer occurs in all plants examined. Additionally, Avelange-Macherel et al. [15] used two paralogues of late embryogenesis abundant proteins (LEA) (LEA38 (mitochondrial) and LEA2 (cytosolic)) to examine the influence of amino acid sequence of mitochondrial targeting sequences (MTS) on subcellular localisation. They showed that by combining substitution, charge invasion, and segment replacement, they were able to redirect LEA2 to mitochondria, providing an explanation for the loss of mitochondrial localistion after duplication of the ancestral gene. Kolli et al. [16] provided a complete review of unique aspects of plant mitochondrial inner membrane protein insertion using Complex IV as a case study, which revealed the use of Tat machinery for membrane insertion of the Rieske Fe/S protein.
Two papers examined the mitochondrial protease FTSH4, one looking at the impact of a ftsh4 mutant on meristem proliferation [3], and another identifying physiological substrates and interaction partners using a trapping approach and mass spectrometry [4]. Dolzblasz et al. showed that plants lacking AtFTSH4 show a cessation of growth at both the shoot and root apical meristems when grown at 30 • C, and that this arrest is caused by cell cycle dysregulation and the loss of cell identity. Opalińska et al. revealed a number of novel putative targets for FTSH4 including the mitochondrial pyruvate carrier 4 (MPC4), presequence translocase-associated motor 18 (PAM18), and succinate dehydrogenase (SDH) subunits. Additionally, they showed that FTSH4 is responsible for the degradation of oxidatively damaged proteins in mitochondria. Plant mitochondria contain numerous group II introns which reside in genes. Here Zmudjak et al. [6] showed that the nMAT2 maturase and the RNA helicase PMH2 associate with their intron-RNA targets in large ribonucleoprotein particle in vivo and the splicing efficiencies of the joint intron targets of nMAT2 and PMH2 are more strongly affected in a double nmat2/pmh2 mutant-line. Together this suggests that these proteins serve as components of a proto-spliceosomal complex in plant mitochondria. Robles et al. [5] provides a thorough review of the phenotypic effects on plant development displayed by mutants of mitoribosomal proteins (mitoRPs) and how they contribute to the elucidation of plant mitoRPs function, the mechanisms that control organelle gene expression, and their contribution to plant growth and morphogenesis.
Mao et al. [18] examined the application of 0.05 mM NO in aged oat seeds and saw an improvement in seed vigor and increased H 2 O 2 scavenging ability in mitochondria. Accompanying this were higher activities of CAT, GR, MDHAR, and DHAR in the AsA-GSH scavenging system, enhanced TCA cycle-related enzymes (malate dehydrogenase, succinate-CoA ligase, fumarate hydratase), and activated alternative pathways.
Overall, the 19 contributions published in this special issue illustrate the advances in the field of plant mitochondria and I look forward to catching up with the plant mitochondrial community at the next biannual meeting in Ein Gedi, Israel (https://www.icpmb2019.com/). Acknowledgments: N.L.T. was funded as an Australian Research Council ARC Future Fellow (FT13010123).

Conflicts of Interest:
The author declares no conflict of interest.