The potato tuber mitochondrial proteome

Evaluating the phytotoxicological effects of bulk and nano forms of zinc oxide on cellular respiration-related indices and differential gene expression in Hordeum vulgare L

The increasing use of nanoparticles is driving the growth of research on their effects on living organisms. However, studies on the effects of nanoparticles on cellular respiration are still limited. The remodeling of cellular-respiration-related indices in plants induced by zinc oxide nanoparticles (nnZnO) and its bulk form (blZnO) was investigated for the first time. For this purpose, barley (Hordeum vulgare L.) seedlings were grown hydroponically for one week with the addition of test compounds at concentrations of 0, 0.3, 2, and 10 mg mL-1. The results showed that a low concentration (0.3 mg mL-1) of blZnO did not cause significant changes in the respiration efficiency, ATP content, and total reactive oxygen species (ROS) content in leaf tissues. Moreover, a dose of 0.3 mg mL-1 nnZnO increased respiration efficiency in both leaves (17 %) and roots (38 %). Under the influence of blZnO and nnZnO at medium (2 mg mL-1) and high (10 mg mL-1) concentrations, a dose-dependent decrease in respiration efficiency from 28 % to 87 % was observed. Moreover, the negative effect was greater under the influence of nnZnO. The gene transcription of the subunits of the mitochondria electron transport chain (ETC) changed mainly only under the influence of nnZnO in high concentration. Expression of the ATPase subunit gene, atp1, increased slightly (by 36 %) in leaf tissue under the influence of medium and high concentrations of test compounds, whereas in the root tissues, the atp1 mRNA level decreased significantly (1.6-2.9 times) in all treatments. A dramatic decrease (1.5-2.4 times) in ATP content was also detected in the roots. Against the background of overexpression of the AOX1d1 gene, an isoform of alternative oxidase (AOX), the total ROS content in leaves decreased (with the exception of 10 mg mL-1 nnZnO). However, in the roots, where the pressure of the stress factor is higher, there was a significant increase in ROS levels, with a maximum six-fold increase under 10 mg mL-1 nnZnO. A significant decrease in transcript levels of the pentose phosphate pathway and glycolytic enzymes was also shown in the root tissues compared to leaves. Thus, the disruption of oxidative phosphorylation leads to a decrease in ATP synthesis and an increase in ROS production; concomitantly reducing the efficiency of cellular respiration.

Deep proteomics reveals incorporation of unedited proteins into mitochondrial protein complexes in Arabidopsis

The mitochondrial proteome consists of numerous types of proteins which either are encoded and synthesized in the mitochondria, or encoded in the cell nucleus, synthesized in the cytoplasm and imported into the mitochondria. Their synthesis in the mitochondria, but not in the nucleus, relies on the editing of the primary transcripts of their genes at defined sites. Here, we present an in-depth investigation of the mitochondrial proteome of Arabidopsis (Arabidopsis thaliana) and a public online platform for the exploration of the data. For the analysis of our shotgun proteomic data, an Arabidopsis sequence database was created comprising all available protein sequences from the TAIR10 and Araport11 databases, supplemented with sequences of proteins translated from edited and nonedited transcripts of mitochondria. Amino acid sequences derived from partially edited transcripts were also added to analyze proteins encoded by the mitochondrial genome. Proteins were digested in parallel with six different endoproteases to obtain maximum proteome coverage. The resulting peptide fractions were finally analyzed using liquid chromatography coupled to ion mobility spectrometry and tandem mass spectrometry. We generated a "deep mitochondrial proteome" of 4,692 proteins. 1,339 proteins assigned to mitochondria by the SUBA5 database (https://suba.live) accounted for >80% of the total protein mass of our fractions. The coverage of proteins by identified peptides was particularly high compared to single-protease digests, allowing the exploration of differential splicing and RNA editing events at the protein level. We show that proteins translated from nonedited transcripts can be incorporated into native mitoribosomes and the ATP synthase complex. We present a portal for the use of our data, based on "proteomaps" with directly linked protein data. The portal is available at www.proteomeexplorer.de.

Proteomes reveal the lipid metabolic network in the complex plastid of Phaeodactylum tricornutum

Phaeodactylum tricornutum plastid is surrounded by four membranes, and its protein composition and function remain mysterious. In this study, the P. tricornutum plastid-enriched fraction was obtained and 2850 proteins were identified, including 92 plastid-encoded proteins, through label-free quantitative proteomic technology. Among them, 839 nuclear-encoded proteins were further determined to be plastidial proteins based on the BLAST alignments within Plant Proteome DataBase and subcellular localization prediction, in spite of the strong contamination by mitochondria-encoded proteins and putative plasma membrane proteins. According to our proteomic data, we reconstructed the metabolic pathways and highlighted the hybrid nature of this diatom plastid. Triacylglycerol (TAG) hydrolysis and glycolysis, as well as photosynthesis, glycan metabolism, and tocopherol and triterpene biosynthesis, occur in the plastid. In addition, the synthesis of long-chain acyl-CoAs, elongation, and desaturation of fatty acids (FAs), and synthesis of lipids including TAG are confined in the four-layered-membrane plastid based on the proteomic and GFP-fusion localization data. The whole process of generation of docosahexaenoic acid (22:6) from palmitic acid (16:0), via elongation and desaturation of FAs, occurs in the chloroplast endoplasmic reticulum membrane, the outermost membrane of the plastid. Desaturation that generates 16:4 from 16:0 occurs in the plastid stroma and outer envelope membrane. Quantitative analysis of glycerolipids between whole cells and isolated plastids shows similar composition, and the FA profile of TAG was not different. This study shows that the diatom plastid combines functions usually separated in photosynthetic eukaryotes, and differs from green alga and plant chloroplasts by undertaking the whole process of lipid biosynthesis.

Mapping the castor bean endosperm proteome revealed a metabolic interaction between plastid, mitochondria, and peroxisomes to optimize seedling growth

In this work, we studied castor-oil plant Ricinus communis as a classical system for endosperm reserve breakdown. The seeds of castor beans consist of a centrally located embryo with the two thin cotyledons surrounded by the endosperm. The endosperm functions as major storage tissue and is packed with nutritional reserves, such as oil, proteins, and starch. Upon germination, mobilization of the storage reserves requires inter-organellar interplay of plastids, mitochondria, and peroxisomes to optimize growth for the developing seedling. To understand their metabolic interactions, we performed a large-scale organellar proteomic study on castor bean endosperm. Organelles from endosperm of etiolated seedlings were isolated and subjected to liquid chromatography-tandem mass spectrometry (LC-MS/MS). Computer-assisted deconvolution algorithms were applied to reliably assign the identified proteins to their correct subcellular localization and to determine the abundance of the different organelles in the heterogeneous protein samples. The data obtained were used to build a comprehensive metabolic model for plastids, mitochondria, and peroxisomes during storage reserve mobilization in castor bean endosperm.

Exploring the Potential Role of Ribosomal Proteins to Enhance Potato Resilience in the Face of Changing Climatic Conditions

Potatoes have emerged as a key non-grain crop for food security worldwide. However, the looming threat of climate change poses significant risks to this vital food source, particularly through the projected reduction in crop yields under warmer temperatures. To mitigate potential crises, the development of potato varieties through genome editing holds great promise. In this study, we performed a comprehensive transcriptomic analysis to investigate microtuber development and identified several differentially expressed genes, with a particular focus on ribosomal proteins-RPL11, RPL29, RPL40 and RPL17. Our results reveal, by protein-protein interaction (PPI) network analyses, performed with the highest confidence in the STRING database platform (v11.5), the critical involvement of these ribosomal proteins in microtuber development, and highlighted their interaction with PEBP family members as potential microtuber activators. The elucidation of the molecular biological mechanisms governing ribosomal proteins will help improve the resilience of potato crops in the face of today's changing climatic conditions.

Effect of the metal ion-induced carbonylation modification of mitochondrial membrane channel protein VDAC on cell vitality, seedling growth and seed aging

Introduction

Seeds are the most important carrier of germplasm preservation. However, an irreversible decrease in vigor can occur after the maturation of seeds, denoted as seed aging. Mitochondrion is a crucial organelle in initiation programmed cell death during seed aging. However, the underlying mechanism remains unclear.

Methods

Our previous proteome study found that 13 mitochondria proteins underwent carbonylation modification during the aging of Ulmus pumila L. (Up) seeds. This study detected metal binding proteins through immobilized metal affinity chromatography (IMAC), indicating that metal binding proteins in mitochondria are the main targets of carbonization during seed aging. Biochemistry, molecular and cellular biology methods were adopted to detect metal-protein binding, protein modification and subcellular localization. Yeast and Arabidopsis were used to investigate the biological functions in vivo.

Results and discussion

In IMAC assay, 12 proteins were identified as Fe²+/Cu²+/Zn²+ binding proteins, including mitochondrial voltage dependent anion channels (VDAC). UpVDAC showed binding abilities to all the three metal ions. His204Ala (H204A) and H219A mutated UpVDAC proteins lost their metal binding ability, and became insensitive to metal-catalyzed oxidation (MCO) induced carbonylation. The overexpression of wild-type UpVDAC made yeast cells more sensitive to oxidative stress, retarded the growth of Arabidopsis seedlings and accelerated the seed aging, while overexpression of mutated UpVDAC weakened these effects of VDAC. These results reveal the relationship between the metal binding ability and carbonylation modification, as well as the probable function of VDAC in regulating cell vitality, seedling growth and seed aging.

Solanum tuberosum Microtuber Development under Darkness Unveiled through RNAseq Transcriptomic Analysis

Potato microtuber (MT) development through in vitro techniques are ideal propagules for producing high quality potato plants. MT formation is influenced by several factors, i.e., photoperiod, sucrose, hormones, and osmotic stress. We have previously developed a protocol of MT induction in medium with sucrose (8% w/v), gelrite (6g/L), and 2iP as cytokinin under darkness. To understand the molecular mechanisms involved, we performed a transcriptome-wide analysis. Here we show that 1715 up- and 1624 down-regulated genes were involved in this biological process. Through the protein-protein interaction (PPI) network analyses performed in the STRING database (v11.5), we found 299 genes tightly associated in 14 clusters. Two major clusters of up-regulated proteins fundamental for life growth and development were found: 29 ribosomal proteins (RPs) interacting with 6 PEBP family members and 117 cell cycle (CC) proteins. The PPI network of up-regulated transcription factors (TFs) revealed that at least six TFs-MYB43, TSF, bZIP27, bZIP43, HAT4 and WOX9-may be involved during MTs development. The PPI network of down-regulated genes revealed a cluster of 83 proteins involved in light and photosynthesis, 110 in response to hormone, 74 in hormone mediate signaling pathway and 22 related to aging.

Cross-Talk of Protein Expression and Lysine Acetylation in Response to TMV Infection in Nicotiana benthamiana

Lysine acetylation (K^ac), a reversible PTM, plays an essential role in various biological processes, including those involving metabolic pathways, pathogen resistance, and transcription, in both prokaryotes and eukaryotes. TMV, the major factor that causes the poor quality of Solanaceae crops worldwide, directly alters many metabolic processes in tobacco. However, the extent and function of K^ac during TMV infection have not been determined. The validation test to detect K^ac level and viral expression after TMV infection and Nicotinamide (NAM) treatment clarified that acetylation was involved in TMV infection. Furthermore, we comprehensively analyzed the changes in the proteome and acetylome of TMV-infected tobacco (Nicotiana benthamiana) seedlings via LC-MS/MS in conjunction with highly sensitive immune-affinity purification. In total, 2082 lysine-acetylated sites on 1319 proteins differentially expressed in response to TMV infection were identified. Extensive bioinformatic studies disclosed changes in acetylation of proteins engaged in cellular metabolism and biological processes. The vital influence of K^ac in fatty acid degradation and alpha-linolenic acid metabolism was also revealed in TMV-infected seedlings. This study first revealed K^ac information in N. benthamiana under TMV infection and expanded upon the existing landscape of acetylation in pathogen infection.

Genome-Wide Expression Analysis of Glyoxalase I Genes Under Hyperosmotic Stress and Existence of a Stress-Responsive Mitochondrial Glyoxalase I Activity in Durum Wheat (Triticum durum Desf.)

Glyoxalase I (GLYI) catalyzes the rate-limiting step of the glyoxalase pathway that, in the presence of GSH, detoxifies the cytotoxic molecule methylglyoxal (MG) into the non-toxic D-lactate. In plants, MG levels rise under various abiotic stresses, so GLYI may play a crucial role in providing stress tolerance. In this study, a comprehensive genome database analysis was performed in durum wheat (Triticum durum Desf.), identifying 27 candidate GLYI genes (TdGLYI). However, further analyses of phylogenetic relationships and conserved GLYI binding sites indicated that only nine genes encode for putative functionally active TdGLYI enzymes, whose distribution was predicted in three different subcellular compartments, namely cytoplasm, plastids and mitochondria. Expression profile by qRT-PCR analysis revealed that most of the putative active TdGLYI genes were up-regulated by salt and osmotic stress in roots and shoots from 4-day-old seedlings, although a different behavior was observed between the two types of stress and tissue. Accordingly, in the same tissues, hyperosmotic stress induced an increase (up to about 40%) of both GLYI activity and MG content as well as a decrease of GSH (up to about -60%) and an increase of GSSG content (up to about 7-fold) with a consequent strong decrease of the GSH/GSSG ratio (up to about -95%). Interestingly, in this study, we reported the first demonstration of the existence of GLYI activity in highly purified mitochondrial fraction. In particular, GLYI activity was measured in mitochondria from durum wheat (DWM), showing hyperbolic kinetics with Km and Vmax values equal to 92 ± 0.2 μM and 0.519 ± 0.004 μmol min^-1 mg^-1 of proteins, respectively. DWM-GLYI resulted inhibited in a competitive manner by GSH (Ki = 6.5 ± 0.7 mM), activated by Zn²⁺ and increased, up to about 35 and 55%, under salt and osmotic stress, respectively. In the whole, this study provides basis about the physiological significance of GLYI in durum wheat, by highlighting the role of this enzyme in the early response of seedlings to hyperosmotic stress. Finally, our results strongly suggest the existence of a complete mitochondrial GLYI pathway in durum wheat actively involved in MG detoxification under hyperosmotic stress.

Genome-Wide Expression Analysis of Glyoxalase I Genes Under Hyperosmotic Stress and Existence of a Stress-Responsive Mitochondrial Glyoxalase I Activity in Durum Wheat (Triticum durum Desf.)

Glyoxalase I (GLYI) catalyzes the rate-limiting step of the glyoxalase pathway that, in the presence of GSH, detoxifies the cytotoxic molecule methylglyoxal (MG) into the non-toxic D-lactate. In plants, MG levels rise under various abiotic stresses, so GLYI may play a crucial role in providing stress tolerance. In this study, a comprehensive genome database analysis was performed in durum wheat (Triticum durum Desf.), identifying 27 candidate GLYI genes (TdGLYI). However, further analyses of phylogenetic relationships and conserved GLYI binding sites indicated that only nine genes encode for putative functionally active TdGLYI enzymes, whose distribution was predicted in three different subcellular compartments, namely cytoplasm, plastids and mitochondria. Expression profile by qRT-PCR analysis revealed that most of the putative active TdGLYI genes were up-regulated by salt and osmotic stress in roots and shoots from 4-day-old seedlings, although a different behavior was observed between the two types of stress and tissue. Accordingly, in the same tissues, hyperosmotic stress induced an increase (up to about 40%) of both GLYI activity and MG content as well as a decrease of GSH (up to about -60%) and an increase of GSSG content (up to about 7-fold) with a consequent strong decrease of the GSH/GSSG ratio (up to about -95%). Interestingly, in this study, we reported the first demonstration of the existence of GLYI activity in highly purified mitochondrial fraction. In particular, GLYI activity was measured in mitochondria from durum wheat (DWM), showing hyperbolic kinetics with Km and Vmax values equal to 92 ± 0.2 μM and 0.519 ± 0.004 μmol min^-1 mg^-1 of proteins, respectively. DWM-GLYI resulted inhibited in a competitive manner by GSH (Ki = 6.5 ± 0.7 mM), activated by Zn²⁺ and increased, up to about 35 and 55%, under salt and osmotic stress, respectively. In the whole, this study provides basis about the physiological significance of GLYI in durum wheat, by highlighting the role of this enzyme in the early response of seedlings to hyperosmotic stress. Finally, our results strongly suggest the existence of a complete mitochondrial GLYI pathway in durum wheat actively involved in MG detoxification under hyperosmotic stress.

Eukaryotic cellular intricacies shape mitochondrial proteomic complexity

Mitochondria have been fundamental to the eco-physiological success of eukaryotes since the last eukaryotic common ancestor (LECA). They contribute essential functions to eukaryotic cells, above and beyond classical respiration. Mitochondria interact with, and complement, metabolic pathways occurring in other organelles, notably diversifying the chloroplast metabolism of photosynthetic organisms. Here, we integrate existing literature to investigate how mitochondrial metabolism varies across the landscape of eukaryotic evolution. We illustrate the mitochondrial remodelling and proteomic changes undergone in conjunction with major evolutionary transitions. We explore how the mitochondrial complexity of the LECA has been remodelled in specific groups to support subsequent evolutionary transitions, such as the acquisition of chloroplasts in photosynthetic species and the emergence of multicellularity. We highlight the versatile and crucial roles played by mitochondria during eukaryotic evolution, extending from its huge contribution to the development of the LECA itself to the dynamic evolution of individual eukaryote groups, reflecting both their current ecologies and evolutionary histories.

Subcellular Proteomics as a Unified Approach of Experimental Localizations and Computed Prediction Data for Arabidopsis and Crop Plants

In eukaryotic organisms, subcellular protein location is critical in defining protein function and understanding sub-functionalization of gene families. Some proteins have defined locations, whereas others have low specificity targeting and complex accumulation patterns. There is no single approach that can be considered entirely adequate for defining the in vivo location of all proteins. By combining evidence from different approaches, the strengths and weaknesses of different technologies can be estimated, and a location consensus can be built. The Subcellular Location of Proteins in Arabidopsis database ( http://suba.live/ ) combines experimental data sets that have been reported in the literature and is analyzing these data to provide useful tools for biologists to interpret their own data. Foremost among these tools is a consensus classifier (SUBAcon) that computes a proposed location for all proteins based on balancing the experimental evidence and predictions. Further tools analyze sets of proteins to define the abundance of cellular structures. Extending these types of resources to plant crop species has been complex due to polyploidy, gene family expansion and contraction, and the movement of pathways and processes within cells across the plant kingdom. The Crop Proteins of Annotated Location database ( http://crop-pal.org/ ) has developed a range of subcellular location resources including a species-specific voting consensus for 12 plant crop species that offers collated evidence and filters for current crop proteomes akin to SUBA. Comprehensive cross-species comparison of these data shows that the sub-cellular proteomes (subcellulomes) depend only to some degree on phylogenetic relationship and are more conserved in major biosynthesis than in metabolic pathways. Together SUBA and cropPAL created reference subcellulomes for plants as well as species-specific subcellulomes for cross-species data mining. These data collections are increasingly used by the research community to provide a subcellular protein location layer, inform models of compartmented cell function and protein-protein interaction network, guide future molecular crop breeding strategies, or simply answer a specific question-where is my protein of interest inside the cell?

Advances in proteome-wide analysis of plant lysine acetylation

Lysine acetylation (LysAc) is a conserved and important post-translational modification (PTM) that plays a key role in plant physiological and metabolic processes. Based on advances in Lys-acetylated protein immunoenrichment and mass-spectrometric technology, LysAc proteomics studies have been performed in many species. Such studies have made substantial contributions to our understanding of plant LysAc, revealing that Lys-acetylated histones and nonhistones are involved in a broad spectrum of plant cellular processes. Here, we present an extensive overview of recent research on plant Lys-acetylproteomes. We provide in-depth insights into the characteristics of plant LysAc modifications and the mechanisms by which LysAc participates in cellular processes and regulates metabolism and physiology during plant growth and development. First, we summarize the characteristics of LysAc, including the properties of Lys-acetylated sites, the motifs that flank Lys-acetylated lysines, and the dynamic alterations in LysAc among different tissues and developmental stages. We also outline a map of Lys-acetylated proteins in the Calvin-Benson cycle and central carbon metabolism-related pathways. We then introduce some examples of the regulation of plant growth, development, and biotic and abiotic stress responses by LysAc. We discuss the interaction between LysAc and N^α-terminal acetylation and the crosstalk between LysAc and other PTMs, including phosphorylation and succinylation. Finally, we propose recommendations for future studies in the field. We conclude that LysAc of proteins plays an important role in the regulation of the plant life cycle.

Mitochondrial proteomic analysis reveals the regulation of energy metabolism and reactive oxygen species production in Clematis terniflora DC. leaves under high-level UV-B radiation followed by dark treatment

Clematis terniflora DC. is an important medicinal plant from the family Ranunculaceae. A previous study has shown that active ingredients in C. terniflora, such as flavonoids and coumarins, are increased under ultraviolet B radiation (UV-B) and dark treatment and that the numbers of genes related to the tricarboxylic acid cycle and mitochondrial electron transport chain (mETC) are changed. To uncover the mechanism of the response to UV-B radiation and dark treatment in C. terniflora, mitochondrial proteomics was performed. The results showed that proteins related to photorespiration, mitochondrial membrane permeability, the tricarboxylic acid cycle, and the mETC mainly showed differential expression profiles. Moreover, the increase in alternative oxidase indicated that another oxygen-consuming respiratory pathway in plant mitochondria was induced to minimize mitochondrial reactive oxygen species production. These results suggested that respiration and mitochondrial membrane permeability were deeply influenced to avoid energy consumption and maintain energy balance under UV-B radiation and dark treatment in C. terniflora leaf mitochondria. Furthermore, oxidative phosphorylation was able to regulate intracellular oxygen balance to resist oxidative stress. This study improves understanding of the function of mitochondria in response to UV-B radiation and dark treatment in C. terniflora. SIGNIFICANCE: C. terniflora was an important traditional Chinese medicine for anti-inflammatory. Previous study showed that the contents of coumarins which were the main active ingredient in C. terniflora were induced by UV-B radiation and dark treatment. In the present study, to uncover the regulatory mechanism of metabolic changes in C. terniflora, mitochondrial proteomics analysis of leaves was performed. The results showed that photorespiration and oxidative phosphorylation pathways were influenced under UV-B radiation and dark treatment. Mitochondria in C. terniflora leaf played a crucial role in energy mechanism and regulation of cellular oxidation-reduction to maintain cell homeostasis under UV-B radiation followed with dark treatment.

Plant mitochondria - past, present and future

The study of plant mitochondria started in earnest around 1950 with the first isolations of mitochondria from animal and plant tissues. The first 35 years were spent establishing the basic properties of plant mitochondria and plant respiration using biochemical and physiological approaches. A number of unique properties (compared to mammalian mitochondria) were observed: (i) the ability to oxidize malate, glycine and cytosolic NAD(P)H at high rates; (ii) the partial insensitivity to rotenone, which turned out to be due to the presence of a second NADH dehydrogenase on the inner surface of the inner mitochondrial membrane in addition to the classical Complex I NADH dehydrogenase; and (iii) the partial insensitivity to cyanide, which turned out to be due to an alternative oxidase, which is also located on the inner surface of the inner mitochondrial membrane, in addition to the classical Complex IV, cytochrome oxidase. With the appearance of molecular biology methods around 1985, followed by genomics, further unique properties were discovered: (iv) plant mitochondrial DNA (mtDNA) is 10-600 times larger than the mammalian mtDNA, yet it only contains approximately 50% more genes; (v) plant mtDNA has kept the standard genetic code, and it has a low divergence rate with respect to point mutations, but a high recombinatorial activity; (vi) mitochondrial mRNA maturation includes a uniquely complex set of activities for processing, splicing and editing (at hundreds of sites); (vii) recombination in mtDNA creates novel reading frames that can produce male sterility; and (viii) plant mitochondria have a large proteome with 2000-3000 different proteins containing many unique proteins such as 200-300 pentatricopeptide repeat proteins. We describe the present and fairly detailed picture of the structure and function of plant mitochondria and how the unique properties make their metabolism more flexible allowing them to be involved in many diverse processes in the plant cell, such as photosynthesis, photorespiration, CAM and C4 metabolism, heat production, temperature control, stress resistance mechanisms, programmed cell death and genomic evolution. However, it is still a challenge to understand how the regulation of metabolism and mtDNA expression works at the cellular level and how retrograde signaling from the mitochondria coordinates all those processes.

Isolation of Highly Purified, Intact, and Functional Mitochondria from Potato Tubers Using a Two-in-One Percoll Density Gradient

The isolation of mitochondria from potato tubers (Solanum tuberosum L.) is described, but the methodology can easily be adapted to other storage tissues. After homogenization of the tissue, filtration and differential centrifugation, the key step is a Percoll density gradient centrifugation. The Percoll gradient contains two parts: a bottom part containing Percoll in 0.3 M sucrose, and a slightly less dense top part containing Percoll in 0.3 M mannitol. After centrifugation, a density gradient is formed that is almost linear in the central part, and this is where the band containing the purified intact mitochondria is formed. This method makes it possible to process large amounts of plant material (2-6 kg) and saves at least 1.5 h on the preparation time compared to methods where two consecutive purification methods are used. Nonetheless, it yields large amounts of mitochondria (50-125 mg protein) of very high purity, intactness and functionality.

Mass Spectrometry-Based Quantitative Cysteine Redox Proteome Profiling of Isolated Mitochondria Using Differential iodoTMT Labeling

Mitochondria are central hubs of redox biochemistry in the cell. An important role of mitochondrial carbon metabolism is to oxidize respiratory substrates and to pass the electrons down the mitochondrial electron transport chain to reduce oxygen and to drive oxidative phosphorylation. During respiration, reactive oxygen species are produced as a side reaction, some of which in turn oxidize cysteine thiols in proteins. Hence, the redox status of cysteine-containing mitochondrial proteins has to be controlled by the mitochondrial glutathione and thioredoxin systems, which draw electrons from metabolically derived NADPH. The redox status of mitochondrial cysteines can undergo fast transitions depending on the metabolic status of the cell, as for instance at early seed germination. Here, we describe a state-of-the-art method to quantify redox state of protein cysteines in isolated Arabidopsis seedling mitochondria of controlled metabolic and respiratory state by MS²-based redox proteomics using the isobaric thiol labeling reagent Iodoacetyl Tandem Mass Tag™ (iodoTMT). The procedure is also applicable to isolated mitochondria of other plant and nonplant systems.

Dioscorea Alata Tuber Proteome Analysis Uncovers Differentially Regulated Growth-associated Pathways of Tuber Development

During its life cycle, the Dioscorea tuber undergoes multiple morphological and biochemical changes. To gain a better understanding of the metabolic changes associated with tuber growth, a stage-specific gel-free proteome analysis of four distinct morphological stages namely germinating tuber (S1), degrading tuber (S2), new tuber formation (S3) and tuber maturation (S4) was done and validated by principal component analysis. A comprehensive data set identifying 78.2% of the total 3,681 proteins was generated. PANTHER and KEGG MAPPER revealed both expected (carbohydrate metabolism and redox regulation) and novel biological processes (transcription factors and hormonal regulation) characteristic for each developmental stage. Higher abundance of the enzymes of ascorbate-glutathione cycle and carbohydrate metabolism was detected during tuber germination (S1) and tuber formation stages (S3) in comparison with the mature tuber. The presence of ethylene biosynthesis components during tuber formation hints toward its probable role in postharvest shelf life. The data set comprehensively describes the proteome of Dioscorea tuber and provides growth-specific markers for tuber germination (ascorbate peroxidase, monodehydroascorbate reductase, invertase) and tuber formation (sucrose synthase), which were validated by enzyme activity assays and Western blotting. The study provides information that may influence the direction of research for improving the productivity of this under-utilized and largely neglected crop.

Comparative Proteomic Analysis of Developmental Changes in P-Type Cytoplasmic Male Sterile and Maintainer Anthers in Wheat

Cytoplasmic male sterility (CMS) plays an important role in the application of heterosis in wheat (Triticum aestivum L.). However, the molecular mechanism underlying CMS remains unknown. This study provides a comprehensive morphological and proteomic analysis of the anthers of a P-type CMS wheat line (P) and its maintainer line, Yanshi 9 hao (Y). Cytological observations indicated that the P-type CMS line shows binucleate microspore abortion. In this line, the tapetum degraded early, leading to anther cuticle defects, which could not provide the nutrition needed for microspore development in a timely manner, thus preventing the development of the microspore to the normal binucleate stage. Proteomic analysis revealed novel proteins involved in P-type CMS. Up to 2576 differentially expressed proteins (DEPs) were quantified in all anthers, and these proteins were significantly enriched in oxidative phosphorylation, glycolysis/gluconeogenesis, citrate cycle (TCA cycle), starch and sucrose metabolism, phenylpropanoid biosynthesis, and pyruvate metabolism pathways. These proteins may comprise a network that regulates male sterility in wheat. Based on the function analysis of DEPs involved in the complex network, we concluded that the P-type CMS line may be due to cellular dysfunction caused by disturbed carbohydrate metabolism, inadequate energy supply, and disturbed protein synthesis. These results provide insights into the molecular mechanism underlying male sterility and serve as a valuable resource for researchers in plant biology, in general, and plant sexual reproduction, in particular.

Isolation and comparative proteomic analysis of mitochondria from the pulp of ripening citrus fruit

Mitochondria are crucial for the production of primary and secondary metabolites, which largely determine the quality of fruit. However, a method for isolating high-quality mitochondria is currently not available in citrus fruit, preventing high-throughput characterization of mitochondrial functions. Here, based on differential and discontinuous Percoll density gradient centrifugation, we devised a universal protocol for isolating mitochondria from the pulp of four major citrus species, including satsuma mandarin, ponkan mandarin, sweet orange, and pummelo. Western blot analysis and microscopy confirmed the high purity and intactness of the isolated mitochondria. By using this protocol coupled with a label-free proteomic approach, a total of 3353 nonredundant proteins were identified. Comparison of the four mitochondrial proteomes revealed that the proteins commonly detected in all proteomes participate in several typical metabolic pathways (such as tricarboxylic acid cycle, pyruvate metabolism, and oxidative phosphorylation) and pathways closely related to fruit quality (such as γ-aminobutyric acid (GABA) shunt, ascorbate metabolism, and biosynthesis of secondary metabolites). In addition, differentially abundant proteins (DAPs) between different types of species were also identified; these were found to be mainly involved in fatty acid and amino acid metabolism and were further confirmed to be localized to the mitochondria by subcellular localization analysis. In summary, the proposed protocol for the isolation of highly pure mitochondria from different citrus fruits may be used to obtain high-coverage mitochondrial proteomes, which can help to establish the association between mitochondrial metabolism and fruit storability or quality characteristics of different species and lay the foundation for discovering novel functions of mitochondria in plants.

Magnesium Signaling in Plants

Free magnesium (Mg2+) is a signal of the adenylate (ATP+ADP+AMP) status in the cells. It results from the equilibrium of adenylate kinase (AK), which uses Mg-chelated and Mg-free adenylates as substrates in both directions of its reaction. The AK-mediated primary control of intracellular [Mg2+] is finely interwoven with the operation of membrane-bound adenylate- and Mg2+-translocators, which in a given compartment control the supply of free adenylates and Mg2+ for the AK-mediated equilibration. As a result, [Mg2+] itself varies both between and within the compartments, depending on their energetic status and environmental clues. Other key nucleotide-utilizing/producing enzymes (e.g., nucleoside diphosphate kinase) may also be involved in fine-tuning of the intracellular [Mg2+]. Changes in [Mg2+] regulate activities of myriads of Mg-utilizing/requiring enzymes, affecting metabolism under both normal and stress conditions, and impacting photosynthetic performance, respiration, phloem loading and other processes. In compartments controlled by AK equilibrium (cytosol, chloroplasts, mitochondria, nucleus), the intracellular [Mg2+] can be calculated from total adenylate contents, based on the dependence of the apparent equilibrium constant of AK on [Mg2+]. Magnesium signaling, reflecting cellular adenylate status, is likely widespread in all eukaryotic and prokaryotic organisms, due simply to the omnipresent nature of AK and to its involvement in adenylate equilibration.

CropPAL for discovering divergence in protein subcellular location in crops to support strategies for molecular crop breeding

Agriculture faces increasing demand for yield, higher plant-derived protein content and diversity while facing pressure to achieve sustainability. Although the genomes of many of the important crops have been sequenced, the subcellular locations of most of the encoded proteins remain unknown or are only predicted. Protein subcellular location is crucial in determining protein function and accumulation patterns in plants, and is critical for targeted improvements in yield and resilience. Integrating location data from over 800 studies for 12 major crop species into the cropPAL2020 data collection showed that while >80% of proteins in most species are not localised by experimental data, combining species data or integrating predictions can help bridge gaps at similar accuracy. The collation and integration of over 61 505 experimental localisations and more than 6 million predictions showed that the relative sizes of the protein catalogues located in different subcellular compartments are comparable between crops and Arabidopsis. A comprehensive cross-species comparison showed that between 50% and 80% of the subcellulomes are conserved across species and that conservation only depends to some degree on the phylogenetic relationship of the species. Protein subcellular locations in major biosynthesis pathways are more often conserved than in metabolic pathways. Underlying this conservation is a clear potential for subcellular diversity in protein location between species by means of gene duplication and alternative splicing. Our cropPAL data set and search platform (https://crop-pal.org) provide a comprehensive subcellular proteomics resource to drive compartmentation-based approaches for improving yield, protein composition and resilience in future crop varieties.

Proteomic and Bioinformatic Profiling of Transporters in Higher Plant Mitochondria

To function as a metabolic hub, plant mitochondria have to exchange a wide variety of metabolic intermediates as well as inorganic ions with the cytosol. As identified by proteomic profiling or as predicted by MU-LOC, a newly developed bioinformatics tool, Arabidopsis thaliana mitochondria contain 128 or 143 different transporters, respectively. The largest group is the mitochondrial carrier family, which consists of symporters and antiporters catalyzing secondary active transport of organic acids, amino acids, and nucleotides across the inner mitochondrial membrane. An impressive 97% (58 out of 60) of all the known mitochondrial carrier family members in Arabidopsis have been experimentally identified in isolated mitochondria. In addition to many other secondary transporters, Arabidopsis mitochondria contain the ATP synthase transporters, the mitochondria protein translocase complexes (responsible for protein uptake across the outer and inner membrane), ATP-binding cassette (ABC) transporters, and a number of transporters and channels responsible for allowing water and inorganic ions to move across the inner membrane driven by their transmembrane electrochemical gradient. A few mitochondrial transporters are tissue-specific, development-specific, or stress-response specific, but this is a relatively unexplored area in proteomics that merits much more attention.

A Uniquely Complex Mitochondrial Proteome from Euglena gracilis

Euglena gracilis is a metabolically flexible, photosynthetic, and adaptable free-living protist of considerable environmental importance and biotechnological value. By label-free liquid chromatography tandem mass spectrometry, a total of 1,786 proteins were identified from the E. gracilis purified mitochondria, representing one of the largest mitochondrial proteomes so far described. Despite this apparent complexity, protein machinery responsible for the extensive RNA editing, splicing, and processing in the sister clades diplonemids and kinetoplastids is absent. This strongly suggests that the complex mechanisms of mitochondrial gene expression in diplonemids and kinetoplastids occurred late in euglenozoan evolution, arising independently. By contrast, the alternative oxidase pathway and numerous ribosomal subunits presumed to be specific for parasitic trypanosomes are present in E. gracilis. We investigated the evolution of unexplored protein families, including import complexes, cristae formation proteins, and translation termination factors, as well as canonical and unique metabolic pathways. We additionally compare this mitoproteome with the transcriptome of Eutreptiella gymnastica, illuminating conserved features of Euglenida mitochondria as well as those exclusive to E. gracilis. This is the first mitochondrial proteome of a free-living protist from the Excavata and one of few available for protists as a whole. This study alters our views of the evolution of the mitochondrion and indicates early emergence of complexity within euglenozoan mitochondria, independent of parasitism.

Mitochondrial proteomic analysis reveals that proteins relate to oxidoreductase activity play a central role in pollen fertility in cotton

Cotton (Gossypium hirsutum L.) is an important economic crop. Cytoplasm male sterility (CMS) has been used to develop hybrid system and to produce hybrid seeds in cotton, but the molecular mechanism of CMS remains unclear. Mitochondria are semi-autonomous organelles, which play an important role in the reproduction of flowering plants. Male sterility has been proved associated with mitochondrial dysfunction in plants. In present study, a new strategy of proteomic sequencing data-independent acquisition (DIA) was used to analysis protein abundance across CMS lines 2074A (cytoplasm of Gossypium harknessii, D_2-2) and 2074S (cytoplasm of G. hirsutum, AD₁), and their maintainer 2074B. Comparing with transcriptome results showed that there is little consistence between proteome and transcriptome. A total of 2095 protein species were identified in three materials, and 186 and 161 differentially proteins were detected in the comparisons of 2074A vs 2074B, and 2074S vs 2074B, respectively. Among them, 49 and 50 proteins were specific existed in anther, and mainly participated in oxidoreductase activity, carbohydrate metabolism, fatty acid metabolism, cell aging, wax or cutin deposition and signal transduction. Gh_A07G0770 and Gh_D05G1908 were specific up-regulated in sterility lines, and the other genes Gh_D08G1196, Gh_D12G1971, Gh_A11G1250, Gh_D08G0388 were down-regulated, which presented similar expression tendency verified by qRT-PCR, transcriptome and proteome results. These six genes related to lipid synthesis, response to oxidative stress and cell aging, suggested them being involved in CMS occurrence. Using virus-induced gene silencing (VIGS) system, sterility obtained demonstrated the silencing Gh_A11G1250 in maintainer 2074B led to partial anthers abortion. Gh_A11G1250 encoded a mitochondrial localization of peroxisomal-like protein, participated in response to reactive oxygen species (ROS). Twenty-two proteins interacting with Gh_A11G1250 mainly related to chlorophyll biosynthetic process, photoperiodism and flowering, which showed different expression pattern between the male sterile line 2074A and maintainer 2074B. This novel research based on mitochondrial proteomics comparison confirmed that DAPs related to oxidative stress are critical to pollen abortion. BIOLOGICAL SIGNIFICANCE: Cytoplasm male sterility (CMS) system is utilized widely for hybrid production in cotton. However, the genetic and molecular mechanisms of CMS still need to be further elucidated. Up till now, fewer comprehensive comparisons of the mitochondrial proteomes from cotton CMS line and maintainer line have been reported. In this study, we performed a novel comparison of mitochondrial protein profiles in two CMS lines and their common maintainer line. Based on our results, we found a potential protein related to oxidative stress led to the anthers abortion. These results accumulate data to interpret the molecular mechanisms of CMS in cotton.

Comparative acetylome analysis of wild-type and fuzzless-lintless mutant ovules of upland cotton (Gossypium hirsutum Cv. Xu142) unveils differential protein acetylation may regulate fiber development

Protein acetylation (KAC) is a significant post-translational modification, which plays an essential role in the regulation of growth and development. Unfortunately, related studies are inadequately available in angiosperms, and to date, there is no report providing insight on the role of protein acetylation in cotton fiber development. Therefore, we first compared the lysine-acetylation proteome (acetylome) of upland cotton ovules in the early fiber development stages by using wild-type as well as its fuzzless-lintless mutant to identify the role of KAC in the fiber development. A total of 1696 proteins with 2754 acetylation sites identified with the different levels of acetylation belonging to separate subcellular compartments suggesting a large number of proteins differentially acetylated in two cotton cultivars. About 80% of the sites were predicted to localize in the cytoplasm, chloroplast, and mitochondria. Seventeen significantly enriched acetylation motifs were identified. Serine and threonine and cysteine located downstream and upstream to KAC sites. KEGG pathway enrichment analysis indicated oxidative phosphorylation, fatty acid, ribosome and protein, and folate biosynthesis pathways enriched significantly. To our knowledge, this is the first report of comparative acetylome analysis to compare the wild-type as well as its fuzzless-lintless mutant acetylome data to identify the differentially acetylated proteins, which may play a significant role in cotton fiber development.

Tobacco SABP2-interacting protein SIP428 is a SIR2 type deacetylase

Salicylic acid is widely studied for its role in biotic stress signaling in plants. Several SA-binding proteins, including SABP2 (salicylic acid-binding protein 2) has been identified and characterized for their role in plant disease resistance. SABP2 is a 29 kDA tobacco protein that binds to salicylic acid with high affinity. It is a methylesterase enzyme that catalyzes the conversion of methyl salicylate into salicylic acid required for inducing a robust systemic acquired resistance (SAR) in plants. Methyl salicylic acid is one of the several mobile SAR signals identified in plants. SABP2-interacting protein 428 (SIP428) was identified in a yeast two-hybrid screen using tobacco SABP2 as a bait. In silico analysis shows that SIP428 possesses the SIR2 (silent information regulatory 2)-like conserved motifs. SIR2 enzymes are orthologs of sirtuin proteins that catalyze the NAD+-dependent deacetylation of Nε lysine-acetylated proteins. The recombinant SIP428 expressed in E. coli exhibits SIR2-like deacetylase activity. SIP428 shows homology to Arabidopsis AtSRT2 (67% identity), which is implicated in SA-mediated basal defenses. Immunoblot analysis using anti-acetylated lysine antibodies showed that the recombinant SIP428 is lysine acetylated. The expression of SIP428 transcripts was moderately downregulated upon infection by TMV. In the presence of SIP428, the esterase activity of SABP2 increased modestly. The interaction of SIP428 with SABP2, it's regulation upon pathogen infection, and similarity with AtSRT2 suggests that SIP428 is likely to play a role in stress signaling in plants.

Matrix Redox Physiology Governs the Regulation of Plant Mitochondrial Metabolism through Posttranslational Protein Modifications

Mitochondria function as hubs of plant metabolism. Oxidative phosphorylation produces ATP, but it is also a central high-capacity electron sink required by many metabolic pathways that must be flexibly coordinated and integrated. Here, we review the crucial roles of redox-associated posttranslational protein modifications (PTMs) in mitochondrial metabolic regulation. We discuss several major concepts. First, the major redox couples in the mitochondrial matrix (NAD, NADP, thioredoxin, glutathione, and ascorbate) are in kinetic steady state rather than thermodynamic equilibrium. Second, targeted proteomics have produced long lists of proteins potentially regulated by Cys oxidation/thioredoxin, Met-SO formation, phosphorylation, or Lys acetylation, but we currently only understand the functional importance of a few of these PTMs. Some site modifications may represent molecular noise caused by spurious reactions. Third, different PTMs on the same protein or on different proteins in the same metabolic pathway can interact to fine-tune metabolic regulation. Fourth, PTMs take part in the repair of stress-induced damage (e.g., by reducing Met and Cys oxidation products) as well as adjusting metabolic functions in response to environmental variation, such as changes in light irradiance or oxygen availability. Finally, PTMs form a multidimensional regulatory system that provides the speed and flexibility needed for mitochondrial coordination far beyond that provided by changes in nuclear gene expression alone.

Tuber transcriptome profiling of eight potato cultivars with different cold-induced sweetening responses to cold storage

Tubers are vegetative reproduction organs formed from underground extensions of the plant stem. Potato tubers are harvested and stored for months. Storage under cold temperatures of 2-4 °C is advantageous for supressing sprouting and diseases. However, development of reducing sugars can occur with cold storage through a process called cold-induced sweetening (CIS). CIS is undesirable as it leads to darkened color with fry processing. The purpose of the current study was to find differences in biological responses in eight cultivars with variation in CIS resistance. Transcriptome sequencing was done on tubers before and after cold storage and three approaches were taken for gene expression analysis: 1. Gene expression correlated with end-point glucose after cold storage, 2. Gene expression correlated with increased glucose after cold storage (after-before), and 3. Differential gene expression before and after cold storage. Cultivars with high CIS resistance (low glucose after cold) were found to increase expression of an invertase inhibitor gene and genes involved in DNA replication and repair after cold storage. The cultivars with low CIS resistance (high glucose after cold) showed increased expression of genes involved in abiotic stress response, gene expression, protein turnover and the mitochondria. There was a small number of genes with similar expression patterns for all cultivars including genes involved in cell wall strengthening and phospholipases. It is proposed that the pattern of gene expression is related to chilling-induced DNA damage repair and cold acclimation and that genetic variation in these processes are related to CIS.

Single organelle function and organization as estimated from Arabidopsis mitochondrial proteomics

Mitochondria host vital cellular functions, including oxidative phosphorylation and co-factor biosynthesis, which are reflected in their proteome. At the cellular level plant mitochondria are organized into hundreds of discrete functional entities, which undergo dynamic fission and fusion. It is the individual organelle that operates in the living cell, yet biochemical and physiological assessments have exclusively focused on the characteristics of large populations of mitochondria. Here, we explore the protein composition of an individual average plant mitochondrion to deduce principles of functional and structural organisation. We perform proteomics on purified mitochondria from cultured heterotrophic Arabidopsis cells with intensity-based absolute quantification and scale the dataset to the single organelle based on criteria that are justified by experimental evidence and theoretical considerations. We estimate that a total of 1.4 million protein molecules make up a single Arabidopsis mitochondrion on average. Copy numbers of the individual proteins span five orders of magnitude, ranging from >40 000 for Voltage-Dependent Anion Channel 1 to sub-stoichiometric copy numbers, i.e. less than a single copy per single mitochondrion, for several pentatricopeptide repeat proteins that modify mitochondrial transcripts. For our analysis, we consider the physical and chemical constraints of the single organelle and discuss prominent features of mitochondrial architecture, protein biogenesis, oxidative phosphorylation, metabolism, antioxidant defence, genome maintenance, gene expression, and dynamics. While assessing the limitations of our considerations, we exemplify how our understanding of biochemical function and structural organization of plant mitochondria can be connected in order to obtain global and specific insights into how organelles work.

Proteomics: a powerful tool to study plant responses to biotic stress

In recent years, mass spectrometry-based proteomics has provided scientists with the tremendous capability to study plants more precisely than previously possible. Currently, proteomics has been transformed from an isolated field into a comprehensive tool for biological research that can be used to explain biological functions. Several studies have successfully used the power of proteomics as a discovery tool to uncover plant resistance mechanisms. There is growing evidence that indicates that the spatial proteome and post-translational modifications (PTMs) of proteins directly participate in the plant immune response. Therefore, understanding the subcellular localization and PTMs of proteins is crucial for a comprehensive understanding of plant responses to biotic stress. In this review, we discuss current approaches to plant proteomics that use mass spectrometry, with particular emphasis on the application of spatial proteomics and PTMs. The purpose of this paper is to investigate the current status of the field, discuss recent research challenges, and encourage the application of proteomics techniques to further research.

Identification of the Toxoplasma gondii mitochondrial ribosome, and characterisation of a protein essential for mitochondrial translation

Apicomplexan parasites cause diseases such as malaria and toxoplasmosis. The apicomplexan mitochondrion shows striking differences from common model organisms, including fundamental processes such as mitochondrial translation. Despite evidence that mitochondrial translation is essential for parasite survival, it is largely understudied. Progress has been restricted by the absence of functional assays to detect apicomplexan mitochondrial translation, a lack of knowledge of proteins involved in the process and the inability to identify and detect mitoribosomes. We report the localization of 12 new mitochondrial proteins, including 6 putative mitoribosomal proteins. We demonstrate the integration of three mitoribosomal proteins in macromolecular complexes, and provide evidence suggesting these are apicomplexan mitoribosomal subunits, detected here for the first time. Finally, a new analytical pipeline detected defects in mitochondrial translation upon depletion of the small subunit protein 35 (TgmS35), while other mitochondrial functions remain unaffected. Our work lays a foundation for the study of apicomplexan mitochondrial translation.

The translational landscape of Arabidopsis mitochondria

Messenger RNA translation is a complex process that is still poorly understood in eukaryotic organelles like mitochondria. Growing evidence indicates though that mitochondrial translation differs from its bacterial counterpart in many key aspects. In this analysis, we have used ribosome profiling technology to generate a genome-wide snapshot view of mitochondrial translation in Arabidopsis. We show that, unlike in humans, most Arabidopsis mitochondrial ribosome footprints measure 27 and 28 bases. We also reveal that respiratory subunits encoding mRNAs show much higher ribosome association than other mitochondrial mRNAs, implying that they are translated at higher levels. Homogenous ribosome densities were generally detected within each respiratory complex except for complex V, where higher ribosome coverage corroborated with higher requirements for specific subunits. In complex I respiratory mutants, a reorganization of mitochondrial mRNAs ribosome association was detected involving increased ribosome densities for certain ribosomal protein encoding transcripts and a reduction in translation of a few complex V mRNAs. Taken together, our observations reveal that plant mitochondrial translation is a dynamic process and that translational control is important for gene expression in plant mitochondria. This study paves the way for future advances in the understanding translation in higher plant mitochondria.

Complexome Profiling Reveals Association of PPR Proteins with Ribosomes in the Mitochondria of Plants

Mitochondrial transcripts are subject to a wealth of processing mechanisms including cis- and trans-splicing events, as well as base modifications (RNA editing). Hundreds of proteins are required for these processes in plant mitochondria, many of which belong to the pentatricopeptide repeat (PPR) protein superfamily. The structure, localization, and function of these proteins is only poorly understood. Here we present evidence that several PPR proteins are bound to mitoribosomes in plants. A novel complexome profiling strategy in combination with chemical crosslinking has been employed to systematically define the protein constituents of the large and the small ribosomal subunits in the mitochondria of plants. We identified more than 80 ribosomal proteins, which include several PPR proteins and other non-conventional ribosomal proteins. These findings reveal a potential coupling of transcriptional and translational events in the mitochondria of plants. Furthermore, the data indicate an extremely high molecular mass of the "small" subunit, even exceeding that of the "large" subunit.

DNA repair in plant mitochondria - a complete base excision repair pathway in potato tuber mitochondria

Mitochondria are one of the major sites of reactive oxygen species (ROS) production in the plant cell. ROS can damage DNA, and this damage is in many organisms mainly repaired by the base excision repair (BER) pathway. We know very little about DNA repair in plants especially in the mitochondria. Combining proteomics, bioinformatics, western blot and enzyme assays, we here demonstrate that the complete BER pathway is found in mitochondria isolated from potato (Solanum tuberosum) tubers. The enzyme activities of three DNA glycosylases and an apurinic/apyrimidinic (AP) endonuclease (APE) were characterized with respect to Mg²⁺ dependence and, in the case of the APE, temperature sensitivity. Evidence for the presence of the DNA polymerase and the DNA ligase, which complete the repair pathway by replacing the excised base and closing the gap, was also obtained. We tested the effect of oxidative stress on the mitochondrial BER pathway by incubating potato tubers under hypoxia. Protein carbonylation increased significantly in hypoxic tuber mitochondria indicative of increased oxidative stress. The activity of two BER enzymes increased significantly in response to this oxidative stress consistent with the role of the BER pathway in the repair of oxidative damage to mitochondrial DNA.

Salicylic acid-induced ROS production by mitochondrial electron transport chain depends on the activity of mitochondrial hexokinases in tomato (Solanum lycopersicum L.)

The growth regulator, salicylic acid (SA) plays an important role in the induction of cell death in plants. Production of reactive oxygen species (ROS) by mitochondrial electron transport chain (mtETC), cytochrome c (cyt c) release from mitochondria and loss of mitochondrial integrity can be observed during cell death execution in plant tissues. The aim of this work was to study the putative role of hexokinases (HXKs) in the initiation of cell death using tomato (Solanum lycopersicum L.) leaves and mitochondria isolated from plants exposed to a sublethal, 0.1 mM and a cell death-inducing, 1 mM concentrations of SA. Both treatments enhanced ROS and nitric oxide (NO) production in the leaves, which contributed to a concentration-dependent loss of membrane integrity. Images prepared by transmission electron microscopy showed swelling and disorganisation of mitochondrial cristae and vacuolization of mitochondria after SA exposure. Using post-embedding immunohistochemistry, cyt c release from mitochondria was also detected after 1 mM SA treatment. Both SA treatments decreased the activity and transcript levels of HXKs in the leaves and the total mtHXK activity in the mitochondrial fraction. The role of mitochondrial hexokinases (mtHXKs) in ROS and NO production of isolated mitochondria was investigated by the addition of HXK substrate, glucose (Glc) and a specific HXK inhibitor, N-acetylglucosamine (NAG) to the mitochondrial suspension. Both SA treatments enhanced ROS production by mtETC in the presence of succinate and ADP, which was slightly inhibited by Glc and increased significantly by NAG in control and in 0.1 mM SA-treated mitochondria. These changes were not significant at 1 mM SA, which caused disorganisation of mitochondrial membranes. Thus the inhibition of mtHXK activity can contribute to the mitochondrial ROS production, but it is not involved in NO generation in SA-treated leaf mitochondria suggesting that SA can promote cell death by suppressing mtHXK transcription and activity.

Dehydration-induced proteomic landscape of mitochondria in chickpea reveals large-scale coordination of key biological processes

Mitochondria play crucial roles in regulating multiple biological processes particularly electron transfer and energy metabolism in eukaryotic cells. Exposure to water-deficit or dehydration may affect mitochondrial function, and dehydration response may dictate cell fate decisions. iTRAQ-based quantitative proteome of a winter legume, chickpea, demonstrated the central metabolic alterations in mitochondria, presumably involved in dehydration adaptation. Three-week-old chickpea seedlings were subjected to progressive dehydration and the magnitude of dehydration-induced compensatory physiological responses was monitored in terms of physicochemical characteristics and mitochondrial architecture. The proteomics analysis led to the identification of 40 dehydration-responsive proteins whose expressions were significantly modulated by dehydration. The differentially expressed proteins were implicated in different metabolic processes, with obvious functional tendencies toward purine-thiamine metabolic network, pathways of carbon fixation and oxidative phosphorylation. The linearity of dehydration-induced proteome alteration was examined with transcript abundance of randomly selected candidates under multivariate stress conditions. The differentially regulated proteins were validated through sequence analysis. An extensive sequence based localization prediction revealed >62.5% proteins to be mitochondrial resident by, at least, one prediction algorithm. The results altogether provide intriguing insights into the dehydration-responsive metabolic pathways and useful clues to identify crucial proteins linked to stress tolerance. BIOLOGICAL SIGNIFICANCE: Investigation on plant mitochondrial proteome is of significance because it would allow a better understanding of mitochondrial function in plant adaptation to stress. Mitochondria are the unique organelles, which play a crucial role in energy metabolism and cellular homeostasis, particularly when exposed to stress conditions. Chickpea is one of the cultivated winter legumes, which enriches soil nitrogen and has very low water footprint and thus contributes to fortification of sustainable agriculture. We therefore examined the dehydration-responsive mitochondrial proteome landscape of chickpea and queried whether molecular interplay of mitochondrial proteins modulate dehydration tolerance. A total of 40 dehydration-induced mitochondrial proteins were identified, predicted to be involved in key metabolic processes. Our future efforts would focus on understanding both posttranslational modification and processing for comprehensive characterization of mitochondrial protein function. This approach will facilitate mining of more biomarkers linked to the tolerance trait and contribute to crop adaptation to climate change.

HCN Regulates Cellular Processes through Posttranslational Modification of Proteins by S-cyanylation

Hydrogen cyanide (HCN) is coproduced with ethylene in plant cells and is primarily enzymatically detoxified by the mitochondrial β-CYANOALANINE SYNTHASE (CAS-C1). Permanent or transient depletion of CAS-C1 activity in Arabidopsis (Arabidopsis thaliana) results in physiological alterations in the plant that suggest that HCN acts as a gasotransmitter molecule. Label-free quantitative proteomic analysis of mitochondrially enriched samples isolated from the wild type and cas-c1 mutant revealed significant changes in protein content, identifying 451 proteins that are absent or less abundant in cas-c1 and 353 proteins that are only present or more abundant in cas-c1 Gene ontology classification of these proteins identified proteomic changes that explain the root hairless phenotype and the altered immune response observed in the cas-c1 mutant. The mechanism of action of cyanide as a signaling molecule was addressed using two proteomic approaches aimed at identifying the S-cyanylation of Cys as a posttranslational modification of proteins. Both the 2-imino-thiazolidine chemical method and the direct untargeted analysis of proteins using liquid chromatography-tandem mass spectrometry identified a set of 163 proteins susceptible to S-cyanylation that included SEDOHEPTULOSE 1,7-BISPHOSPHATASE (SBPase), the PEPTIDYL-PROLYL CIS-TRANS ISOMERASE 20-3 (CYP20-3), and ENOLASE2 (ENO2). In vitro analysis of these enzymes showed that S-cyanylation of SBPase Cys⁷⁴, CYP20-3 Cys²⁵⁹, and ENO2 Cys³⁴⁶ residues affected their enzymatic activity. Gene Ontology classification and protein-protein interaction cluster analysis showed that S-cyanylation is involved in the regulation of primary metabolic pathways, such as glycolysis, and the Calvin and S-adenosyl-Met cycles.

Characterization of mitochondrial proteomes of nonbilaterian animals

Mitochondria require ~1,500 proteins for their maintenance and proper functionality, which constitute the mitochondrial proteome (mt-proteome). Although a few of these proteins, mostly subunits of the electron transport chain complexes, are encoded in mitochondrial DNA (mtDNA), the vast majority are encoded in the nuclear genome and imported to the organelle. Previous studies have shown a continuous and complex evolution of mt-proteome among eukaryotes. However, there was less attention paid to mt-proteome evolution within Metazoa, presumably because animal mtDNA and, by extension, animal mitochondria are often considered to be uniform. In this analysis, two bioinformatic approaches (Orthologue-detection and Mitochondrial Targeting Sequence prediction) were used to identify mt-proteins in 23 species from four nonbilaterian phyla: Cnidaria, Ctenophora, Placozoa, and Porifera, as well as two choanoflagellates, the closest animal relatives. Our results revealed a large variation in mt-proteome in nonbilaterian animals in size and composition. Myxozoans, highly reduced cnidarian parasites, possessed the smallest inferred mitochondrial proteomes, while calcareous sponges possessed the largest. About 513 mitochondrial orthologous groups were present in all nonbilaterian phyla and human. Interestingly, 42 human mitochondrial proteins were not identified in any nonbilaterian species studied and represent putative innovations along the bilaterian branch. Several of these proteins were involved in apoptosis and innate immunity, two processes known to evolve within Metazoa. Conversely, several proteins identified as mitochondrial in nonbilaterian phyla and animal outgroups were absent in human, representing cases of possible loss. Finally, a few human cytosolic proteins, such as histones and cytosolic ribosomal proteins, were predicted to be targeted to mitochondria in nonbilaterian animals. Overall, our analysis provides the first step in characterization of mt-proteomes in nonbilaterian animals and understanding evolution of animal mt-proteome. © 2018 IUBMB Life, 70(12):1289-1301, 2018.

Construction of a Quantitative Acetylomic Tissue Atlas in Rice (Oryza sativa L.)

PKA (protein lysine acetylation) is a key post-translational modification involved in the regulation of various biological processes in rice. So far, rice acetylome data is very limited due to the highly-dynamic pattern of protein expression and PKA modification. In this study, we performed a comprehensive quantitative acetylome profile on four typical rice tissues, i.e., the callus, root, leaf, and panicle, by using a mass spectrometry (MS)-based, label-free approach. The identification of 1536 acetylsites on 1454 acetylpeptides from 890 acetylproteins represented one of the largest acetylome datasets on rice. A total of 1445 peptides on 887 proteins were differentially acetylated, and are extensively involved in protein translation, chloroplast development, and photosynthesis, flowering and pollen fertility, and root meristem activity, indicating the important roles of PKA in rice tissue development and functions. The current study provides an overall view of the acetylation events in rice tissues, as well as clues to reveal the function of PKA proteins in physiologically-relevant tissues.

Protein Carbonylation and Glycation in Legume Nodules

Nitrogen fixation is an agronomically and environmentally important process catalyzed by bacterial nitrogenase within legume root nodules. These unique symbiotic organs have high metabolic rates and produce large amounts of reactive oxygen species that may modify proteins irreversibly. Here, we examined two types of oxidative posttranslational modifications of nodule proteins: carbonylation, which occurs by direct oxidation of certain amino acids or by interaction with reactive aldehydes arising from cell membrane lipid peroxides; and glycation, which results from the reaction of lysine and arginine residues with reducing sugars or their autooxidation products. We used a strategy based on the enrichment of carbonylated peptides by affinity chromatography followed by liquid chromatography-tandem mass spectrometry to identify 369 oxidized proteins in bean (Phaseolus vulgaris) nodules. Of these, 238 corresponded to plant proteins and 131 to bacterial proteins. Lipid peroxidation products induced most carbonylation sites. This study also revealed that carbonylation has major effects on two key nodule proteins. Metal-catalyzed oxidation caused the inactivation of malate dehydrogenase and the aggregation of leghemoglobin. In addition, numerous glycated proteins were identified in vivo, including three key nodule proteins: sucrose synthase, glutamine synthetase, and glutamate synthase. Label-free quantification identified 10 plant proteins and 18 bacterial proteins as age-specifically glycated. Overall, our results suggest that the selective carbonylation or glycation of crucial proteins involved in nitrogen metabolism, transcriptional regulation, and signaling may constitute a mechanism to control cell metabolism and nodule senescence.

Changes in the mitochondrial proteome of developing maize seed embryos

Mitochondria are required for seed development, but little information is available about their function and role during this process. We isolated the mitochondria from developing maize (Zea mays L. cv. Nongda 108) embryos and investigated the mitochondrial membrane integrity and respiration as well as the mitochondrial proteome using two proteomic methods, the two-dimensional gel electrophoresis (2-DE) and sequential windowed acquisition of all theoretical fragment ion mass spectra (SWATH). Mitochondrial membrane integrity and respiration were maintained at a high level up to 21 days after pollination (DAP) and decreased thereafter, while total mitochondrial number, cytochrome c oxidase activity and respiration per embryo exhibited a bell-shaped change with peaks at 35-45 DAP. A total of 286 mitochondrial proteins changed in abundance during embryo development. During early stages of seed development (up to 21 DAP), proteins involved in energy production, basic metabolism, protein import and folding as well as removal of reactive oxygen species dominated, while during mid or late stages (35-70 DAP), some stress- and detoxification-related proteins increased in abundance. Our study, for the first time, depicted a relatively comprehensive map of energy production by mitochondria during embryo development. The results revealed that mitochondria were very active during the early stages of maize embryo development, while at the late stages of development, the mitochondria became more quiescent, but well-protected, presumably to ensure that the embryo passes through maturation, drying and long-term storage. These results advance our understanding of seed development at the organelle level.

Protein Carbonylation and Glycation in Legume Nodules

Nitrogen fixation is an agronomically and environmentally important process catalyzed by bacterial nitrogenase within legume root nodules. These unique symbiotic organs have high metabolic rates and produce large amounts of reactive oxygen species that may modify proteins irreversibly. Here, we examined two types of oxidative posttranslational modifications of nodule proteins: carbonylation, which occurs by direct oxidation of certain amino acids or by interaction with reactive aldehydes arising from cell membrane lipid peroxides; and glycation, which results from the reaction of lysine and arginine residues with reducing sugars or their autooxidation products. We used a strategy based on the enrichment of carbonylated peptides by affinity chromatography followed by liquid chromatography-tandem mass spectrometry to identify 369 oxidized proteins in bean (Phaseolus vulgaris) nodules. Of these, 238 corresponded to plant proteins and 131 to bacterial proteins. Lipid peroxidation products induced most carbonylation sites. This study also revealed that carbonylation has major effects on two key nodule proteins. Metal-catalyzed oxidation caused the inactivation of malate dehydrogenase and the aggregation of leghemoglobin. In addition, numerous glycated proteins were identified in vivo, including three key nodule proteins: sucrose synthase, glutamine synthetase, and glutamate synthase. Label-free quantification identified 10 plant proteins and 18 bacterial proteins as age-specifically glycated. Overall, our results suggest that the selective carbonylation or glycation of crucial proteins involved in nitrogen metabolism, transcriptional regulation, and signaling may constitute a mechanism to control cell metabolism and nodule senescence.

Characterization of a Novel Mitochondrial Ascorbate Transporter From Rat Liver and Potato Mitochondria

The Mitochondrial Ascorbic Acid Transporter (MAT) from both rat liver and potato mitochondria has been reconstituted in proteoliposomes. The protein has a molecular mass in the range of 28–35 kDa and catalyzes saturable, temperature and pH dependent, unidirectional ascorbic acid transport. The transport activity is sodium independent and it is optimal at acidic pH values. It is stimulated by proton gradient, thus supporting that ascorbate is symported with H⁺. It is efficiently inhibited by the lysine reagent pyridoxal phosphate and it is not affected by inhibitors of other recognized plasma and mitochondrial membranes ascorbate transporters GLUT1(glucose transporter-1) or SVCT2 (sodium-dependent vitamin C transporter-2). Rat protein catalyzes a cooperative ascorbate transport, being involved two binding sites; the measured K_0.5 is 1.5 mM. Taking into account the experimental results we propose that the reconstituted ascorbate transporter is not a GLUT or SVCT, since it shows different biochemical features. Data of potato transporter overlap the mammalian ones, except for the kinetic parameters non-experimentally measurable, thus supporting the MAT in plants fulfills the same transport role.

Large Scale Proteomic Data and Network-Based Systems Biology Approaches to Explore the Plant World

The investigation of plant organisms by means of data-derived systems biology approaches based on network modeling is mainly characterized by genomic data, while the potential of proteomics is largely unexplored. This delay is mainly caused by the paucity of plant genomic/proteomic sequences and annotations which are fundamental to perform mass-spectrometry (MS) data interpretation. However, Next Generation Sequencing (NGS) techniques are contributing to filling this gap and an increasing number of studies are focusing on plant proteome profiling and protein-protein interactions (PPIs) identification. Interesting results were obtained by evaluating the topology of PPI networks in the context of organ-associated biological processes as well as plant-pathogen relationships. These examples foreshadow well the benefits that these approaches may provide to plant research. Thus, in addition to providing an overview of the main-omic technologies recently used on plant organisms, we will focus on studies that rely on concepts of module, hub and shortest path, and how they can contribute to the plant discovery processes. In this scenario, we will also consider gene co-expression networks, and some examples of integration with metabolomic data and genome-wide association studies (GWAS) to select candidate genes will be mentioned.

Mitochondrial Biogenesis in Diverse Cauliflower Cultivars under Mild and Severe Drought. Impaired Coordination of Selected Transcript and Proteomic Responses, and Regulation of Various Multifunctional Proteins

Mitochondrial responses under drought within Brassica genus are poorly understood. The main goal of this study was to investigate mitochondrial biogenesis of three cauliflower (Brassica oleracea var. botrytis) cultivars with varying drought tolerance. Diverse quantitative changes (decreases in abundance mostly) in the mitochondrial proteome were assessed by two-dimensional gel electrophoresis (2D PAGE) coupled with liquid chromatography-tandem mass spectrometry (LC-MS/MS). Respiratory (e.g., complex II, IV (CII, CIV) and ATP synthase subunits), transporter (including diverse porin isoforms) and matrix multifunctional proteins (e.g., components of RNA editing machinery) were diversely affected in their abundance under two drought levels. Western immunoassays showed additional cultivar-specific responses of selected mitochondrial proteins. Dehydrin-related tryptic peptides (found in several 2D spots) immunopositive with dehydrin-specific antisera highlighted the relevance of mitochondrial dehydrin-like proteins for the drought response. The abundance of selected mRNAs participating in drought response was also determined. We conclude that mitochondrial biogenesis was strongly, but diversely affected in various cauliflower cultivars, and associated with drought tolerance at the proteomic and functional levels. However, discussed alternative oxidase (AOX) regulation at the RNA and protein level were largely uncoordinated due to the altered availability of transcripts for translation, mRNA/ribosome interactions, and/or miRNA impact on transcript abundance and translation.