L-galactono-1,4-lactone dehydrogenase (GLDH) forms part of three subcomplexes of mitochondrial complex I in Arabidopsis thaliana

Structure, mechanism, and evolution of the last step in vitamin C biosynthesis

Photosynthetic organisms, fungi, and animals comprise distinct pathways for vitamin C biosynthesis. Besides this diversity, the final biosynthetic step consistently involves an oxidation reaction carried out by the aldonolactone oxidoreductases. Here, we study the origin and evolution of the diversified activities and substrate preferences featured by these flavoenzymes using molecular phylogeny, kinetics, mutagenesis, and crystallographic experiments. We find clear evidence that they share a common ancestor. A flavin-interacting amino acid modulates the reactivity with the electron acceptors, including oxygen, and determines whether an enzyme functions as an oxidase or a dehydrogenase. We show that a few side chains in the catalytic cavity impart the reaction stereoselectivity. Ancestral sequence reconstruction outlines how these critical positions were affixed to specific amino acids along the evolution of the major eukaryotic clades. During Eukarya evolution, the aldonolactone oxidoreductases adapted to the varying metabolic demands while retaining their overarching vitamin C-generating function.

Structural insights into the Smirnoff-Wheeler pathway for vitamin C production in the Amazon fruit camu-camu

l-Ascorbic acid (AsA, vitamin C) is a pivotal dietary nutrient with multifaceted importance in living organisms. In plants, the Smirnoff-Wheeler pathway is the primary route for AsA biosynthesis, and understanding the mechanistic details behind its component enzymes has implications for plant biology, nutritional science, and biotechnology. As part of an initiative to determine the structures of all six core enzymes of the pathway, the present study focuses on three of them in the model species Myrciaria dubia (camu-camu): GDP-d-mannose 3',5'-epimerase (GME), l-galactose dehydrogenase (l-GalDH), and l-galactono-1,4-lactone dehydrogenase (l-GalLDH). We provide insights into substrate and cofactor binding and the conformational changes they induce. The MdGME structure reveals a distorted substrate in the active site, pertinent to the catalytic mechanism. Mdl-GalDH shows that the way in which NAD+ association affects loop structure over the active site is not conserved when compared with its homologue in spinach. Finally, the structure of Mdl-GalLDH is described for the first time. This allows for the rationalization of previously identified residues which play important roles in the active site or in the formation of the covalent bond with FAD. In conclusion, this study enhances our understanding of AsA biosynthesis in plants, and the information provided should prove useful for biotechnological applications.

The ascorbate biosynthesis pathway in plants is known, but there is a way to go with understanding control and functions

Ascorbate (vitamin C) is one of the most abundant primary metabolites in plants. Its complex chemistry enables it to function as an antioxidant, as a free radical scavenger, and as a reductant for iron and copper. Ascorbate biosynthesis occurs via the mannose/l-galactose pathway in green plants, and the evidence for this pathway being the major route is reviewed. Ascorbate accumulation is leaves is responsive to light, reflecting various roles in photoprotection. GDP-l-galactose phosphorylase (GGP) is the first dedicated step in the pathway and is important in controlling ascorbate synthesis. Its expression is determined by a combination of transcription and translation. Translation is controlled by an upstream open reading frame (uORF) which blocks translation of the main GGP-coding sequence, possibly in an ascorbate-dependent manner. GGP associates with a PAS-LOV protein, inhibiting its activity, and dissociation is induced by blue light. While low ascorbate mutants are susceptible to oxidative stress, they grow nearly normally. In contrast, mutants lacking ascorbate do not grow unless rescued by supplementation. Further research should investigate possible basal functions of ascorbate in severely deficient plants involving prevention of iron overoxidation in 2-oxoglutarate-dependent dioxygenases and iron mobilization during seed development and germination.

Using cryo-EM to understand the assembly pathway of respiratory complex I

Complex I (proton-pumping NADH:ubiquinone oxidoreductase) is the first component of the mitochondrial respiratory chain. In recent years, high-resolution cryo-EM studies of complex I from various species have greatly enhanced the understanding of the structure and function of this important membrane-protein complex. Less well studied is the structural basis of complex I biogenesis. The assembly of this complex of more than 40 subunits, encoded by nuclear or mitochondrial DNA, is an intricate process that requires at least 20 different assembly factors in humans. These are proteins that are transiently associated with building blocks of the complex and are involved in the assembly process, but are not part of mature complex I. Although the assembly pathways have been studied extensively, there is limited information on the structure and molecular function of the assembly factors. Here, the insights that have been gained into the assembly process using cryo-EM are reviewed.

Modulation in gene expression and enzyme activity suggested the roles of monodehydroascorbate reductase in development and stress response in bread wheat

Monodehydroascorbate reductase (MDHAR) is a crucial enzymatic antioxidant of the ascorbate-glutathione pathway involved in reactive oxygen species scavenging. Herein, we identified 15 TaMDHAR genes in bread wheat. Phylogenetic analysis revealed their clustering into three groups, which are also related to the subcellular localization in the peroxisome matrix, peroxisome membrane, and chloroplast. Each TaMDHAR protein consisted of two conserved domains; Pyr_redox and Pyr_redox_2 of the pyridine nucleotide disulfide oxidoreductase family. The occurrence of diverse groups of cis-regulatory elements in the promoter region and their interaction with numerous transcription factors suggest assorted functions of TaMDHARs in growth and development and in light, phytohormones, and stress responses. Expression analysis in various tissues further revealed their importance in vegetative and reproductive development. In addition, the differential gene expression and enhanced enzyme activity during drought, heat, and salt treatments exposed their role in abiotic stress response. Interaction of MDHARs with various antioxidant enzymes and biochemicals related to the ascorbate-glutathione cycle exposed their synchronized functioning. Interaction with auxin indicated the probability of cross-talk between antioxidants and auxin signaling. The miR168a, miR169, miR172 and others interaction with various TaMDHARs further directed their association with developmental processes and stress responses. The current study provides extensive information about the importance of TaMDHARs, moreover, the precise role of each gene needs to be established in future studies.

Antioxidants of Non-Enzymatic Nature: Their Function in Higher Plant Cells and the Ways of Boosting Their Biosynthesis

Plants are exposed to a variety of abiotic and biotic stresses leading to increased formation of reactive oxygen species (ROS) in plant cells. ROS are capable of oxidizing proteins, pigments, lipids, nucleic acids, and other cell molecules, disrupting their functional activity. During the process of evolution, numerous antioxidant systems were formed in plants, including antioxidant enzymes and low molecular weight non-enzymatic antioxidants. Antioxidant systems perform neutralization of ROS and therefore prevent oxidative damage of cell components. In the present review, we focus on the biosynthesis of non-enzymatic antioxidants in higher plants cells such as ascorbic acid (vitamin C), glutathione, flavonoids, isoprenoids, carotenoids, tocopherol (vitamin E), ubiquinone, and plastoquinone. Their functioning and their reactivity with respect to individual ROS will be described. This review is also devoted to the modern genetic engineering methods, which are widely used to change the quantitative and qualitative content of the non-enzymatic antioxidants in cultivated plants. These methods allow various plant lines with given properties to be obtained in a rather short time. The most successful approaches for plant transgenesis and plant genome editing for the enhancement of biosynthesis and the content of these antioxidants are discussed.

The conditional mitochondrial protein complexome in the Arabidopsis thaliana root and shoot

Protein complexes are important for almost all biological processes. Hence, to fully understand how cells work, it is also necessary to characterize protein complexes and their dynamics in response to various cellular cues. Moreover, the dynamics of protein interaction play crucial roles in regulating the (dis)association of protein complexes and, in turn, regulating biological processes such as metabolism. Here, mitochondrial protein complexes were investigated by blue native PAGE and size-exclusion chromatography under conditions of oxidative stress in order to monitor their dynamic (dis)associations. Rearrangements of enzyme interactions and changes in protein complex abundance were observed in response to oxidative stress induced by menadione treatment. These included changes in enzymatic protein complexes involving γ-amino butyric acid transaminase (GABA-T), Δ-ornithine aminotransferase (Δ-OAT), or proline dehydrogenase 1 (POX1) that are expected to affect proline metabolism. Menadione treatment also affected interactions between several enzymes of the tricarboxylic acid (TCA) cycle and the abundance of complexes of the oxidative phosphorylation pathway. In addition, we compared the mitochondrial complexes of roots and shoots. Considerable differences between the two tissues were observed in the mitochondrial import/export apparatus, the formation of super-complexes in the oxidative phosphorylation pathway, and specific interactions between enzymes of the TCA cycle that we postulate may be related to the metabolic/energetic requirements of roots and shoots.

Mitochondrial ferredoxin-like is essential for forming complex I-containing supercomplexes in Arabidopsis

In eukaryotes, mitochondrial ATP is mainly produced by the oxidative phosphorylation (OXPHOS) system, which is composed of 5 multiprotein complexes (complexes I-V). Analyses of the OXPHOS system by native gel electrophoresis have revealed an organization of OXPHOS complexes into supercomplexes, but their roles and assembly pathways remain unclear. In this study, we characterized an atypical mitochondrial ferredoxin (mitochondrial ferredoxin-like, mFDX-like). This protein was previously found to be part of the bridge domain linking the matrix and membrane arms of the complex I. Phylogenetic analysis suggested that the Arabidopsis (Arabidopsis thaliana) mFDX-like evolved from classical mitochondrial ferredoxins (mFDXs) but lost one of the cysteines required for the coordination of the iron-sulfur (Fe-S) cluster, supposedly essential for the electron transfer function of FDXs. Accordingly, our biochemical study showed that AtmFDX-like does not bind an Fe-S cluster and is therefore unlikely to be involved in electron transfer reactions. To study the function of mFDX-like, we created deletion lines in Arabidopsis using a CRISPR/Cas9-based strategy. These lines did not show any abnormal phenotype under standard growth conditions. However, the characterization of the OXPHOS system demonstrated that mFDX-like is important for the assembly of complex I and essential for the formation of complex I-containing supercomplexes. We propose that mFDX-like and the bridge domain are required for the correct conformation of the membrane arm of complex I that is essential for the association of complex I with complex III2 to form supercomplexes.

Genetic and biochemical strategies for regulation of L-ascorbic acid biosynthesis in plants through the L-galactose pathway

Vitamin C (L-ascorbic acid, AsA) is an essential compound with pleiotropic functions in many organisms. Since its isolation in the last century, AsA has attracted the attention of the scientific community, allowing the discovery of the L-galactose pathway, which is the main pathway for AsA biosynthesis in plants. Thus, the aim of this review is to analyze the genetic and biochemical strategies employed by plant cells for regulating AsA biosynthesis through the L-galactose pathway. In this pathway, participates eight enzymes encoded by the genes PMI, PMM, GMP, GME, GGP, GPP, GDH, and GLDH. All these genes and their encoded enzymes have been well characterized, demonstrating their participation in AsA biosynthesis. Also, have described some genetic and biochemical strategies that allow its regulation. The genetic strategy includes regulation at transcriptional and post-transcriptional levels. In the first one, it was demonstrated that the expression levels of the genes correlate directly with AsA content in the tissues/organs of the plants. Also, it was proved that these genes are light-induced because they have light-responsive promoter motifs (e.g., ATC, I-box, GT1 motif, etc.). In addition, were identified some transcription factors that function as activators (e.g., SlICE1, AtERF98, SlHZ24, etc.) or inactivators (e.g., SlL1L4, ABI4, SlNYYA10) regulate the transcription of these genes. In the second one, it was proved that some genes have alternative splicing events and could be a mechanism to control AsA biosynthesis. Also, it was demonstrated that a conserved cis-acting upstream open reading frame (5'-uORF) located in the 5'-untranslated region of the GGP gene induces its post-transcriptional repression. Among the biochemical strategies discovered is the control of the enzyme levels (usually by decreasing their quantities), control of the enzyme catalytic activity (by increasing or decreasing its activity), feedback inhibition of some enzymes (GME and GGP), subcellular compartmentation of AsA, the metabolon assembly of the enzymes, and control of AsA biosynthesis by electron flow. Together, the construction of this basic knowledge has been establishing the foundations for generating genetically improved varieties of fruits and vegetables enriched with AsA, commonly used in animal and human feed.

Structural insights into the assembly and the function of the plant oxidative phosphorylation system

One of the key functions of mitochondria is the production of ATP to support cellular metabolism and growth. The last step of mitochondrial ATP synthesis is performed by the oxidative phosphorylation (OXPHOS) system, an ensemble of protein complexes embedded in the inner mitochondrial membrane. In the last 25 yr, many structures of OXPHOS complexes and supercomplexes have been resolved in yeast, mammals, and bacteria. However, structures of plant OXPHOS enzymes only became available very recently. In this review, we highlight the plant-specific features revealed by the recent structures and discuss how they advance our understanding of the function and assembly of plant OXPHOS complexes. We also propose new hypotheses to be tested and discuss older findings to be re-evaluated. Further biochemical and structural work on the plant OXPHOS system will lead to a deeper understanding of plant respiration and its regulation, with significant agricultural, environmental, and societal implications.

How does light facilitate vitamin C biosynthesis in leaves?

Plants store ascorbate in high concentrations, particularly in their leaves. Ascorbate is an excellent antioxidant that acts as an indispensable photoprotectant. The D-mannose/L-galactose pathway is responsible for ascorbate biosynthesis in plants. Light facilitates ascorbate biosynthesis in a light intensity-dependent manner to enhance ascorbate pool size in leaves, and photosynthesis is required for this process. Light- and photosynthesis-dependent activation of the rate-limiting enzyme GDP-L-galactose phosphorylase (GGP) plays a critical role in ascorbate pool size regulation. In addition, the tight regulation of ascorbate biosynthesis by ascorbate itself has been proposed. Ascorbate represses GGP translation in a dose-dependent manner through the upstream open reading frame in the 5'-untranslated regions of the gene, which may compete with the light-dependent activation of ascorbate biosynthesis. This review focuses on ascorbate biosynthesis based on past and latest findings and critically discusses how light activates this process.

Proteolytic regulation of mitochondrial oxidative phosphorylation components in plants

Mitochondrial function relies on the homeostasis and quality control of their proteome, including components of the oxidative phosphorylation (OXPHOS) pathway that generates energy in form of ATP. OXPHOS subunits are under constant exposure to reactive oxygen species due to their oxidation-reduction activities, which consequently make them prone to oxidative damage, misfolding, and aggregation. As a result, quality control mechanisms through turnover and degradation are required for maintaining mitochondrial activity. Degradation of OXPHOS subunits can be achieved through proteomic turnover or modular degradation. In this review, we present multiple protein degradation pathways in plant mitochondria. Specifically, we focus on the intricate turnover of OXPHOS subunits, prior to protein import via cytosolic proteasomal degradation and post import and assembly via intra-mitochondrial proteolysis involving multiple AAA+ proteases. Together, these proteolytic pathways maintain the activity and homeostasis of OXPHOS components.

Improved Enzymatic Assay and Inhibition Analysis of Redox Membranotropic Enzymes, AtGALDH and TcGAL, Using a Reversed Micellar System

Reversed micelles are helpful to solubilize otherwise insoluble membranotropic or membrane-bound enzymes in their functional form, thus enabling activity assay and inhibition analysis. However, in the case of redox enzymes, this task is further complicated by the necessity to select an appropriate electron-acceptor (EA) which, ideally, should be compatible with spectrophotometric measurements in reversed micelles. Here, we have identified such an EA and successfully used it in a reversed micellar environment to assay the activity of two homologous enzymes from mitochondria: l-galactone-1,4-lactone dehydrogenase (EC 1.3.2.3) from Arabidopsis thaliana (AtGALDH) and galactonolactone oxidase (EC 1.3.3.12) from Trypanosoma cruzi (TcGAL), differing in their membranotropic properties, with TcGAL being almost insoluble in water and particularly difficult to assay. Furthermore, we have demonstrated the possibility to use this assay for inhibition analysis, with an elucidation of the mechanism and inhibition parameters, which otherwise could not be possible. In order to perform inhibition analysis, we improved the approach for the determination of activity of such membrane enzymes based on a reversed micellar system as membrane matrix, necessary for the functioning of membrane enzymes. A number of electron acceptors (EA) were tested for AtGALDH and optimal conditions of activity determination for AtGALDH were found. The suggested method was successfully applied to the study of the inhibition of AtGALDH by lycorine, and the mixed competitive mechanism of inhibition of AtGALDH by lycorine was determined. The developed approach to inhibitor analysis was applied for TcGAL, insoluble in water membrane, and the method provides new opportunities for searching effective inhibitors that may be potential drugs. Indeed, galactonolactone oxidase from Trypanosoma cruzi (TcGAL) and AtGALDH are homologues, and the inhibition of TcGAL stops the vital biosynthesis of vitamin C in parasite Trypanosoma cruzi from causing Chagas disease. The approach proposed can be applied for the screening of inhibitors of AtGALDH and TcGAL, as well as to study properties of other membrane enzymes including determination of the mechanisms of inhibition, structure and catalytic properties, the impact of membrane components (for example lipids), and so on.

The gene space of European mistletoe (Viscum album)

European mistletoe (Viscum album) is a hemiparasitic flowering plant that is known for its very special life cycle and extraordinary biochemical properties. Particularly, V. album has an unusual mode of cellular respiration that takes place in the absence of mitochondrial complex I. However, insights into the molecular biology of V. album so far are very limited. Since the genome of V. album is extremely large (estimated 600 times larger than the genome of the model plant Arabidopsis thaliana) it has not been sequenced up to now. We here report sequencing of the V. album gene space (defined as the space including and surrounding genic regions, encompassing coding as well as 5' and 3' non-coding regions). mRNA fractions were isolated from different V. album organs harvested in summer or winter and were analyzed via single-molecule real-time sequencing. We determined sequences of 39 092 distinct open reading frames encoding 32 064 V. album proteins (designated V. album protein space). Our data give new insights into the metabolism and molecular biology of V. album, including the biosynthesis of lectins and viscotoxins. The benefits of the V. album gene space information are demonstrated by re-evaluating mass spectrometry-based data of the V. album mitochondrial proteome, which previously had been evaluated using the A. thaliana genome sequence. Our re-examination allowed the additional identification of nearly 200 mitochondrial proteins, including four proteins related to complex I, which all have a secondary function not related to respiratory electron transport. The V. album gene space sequences are available at the NCBI.

Maturation and Assembly of Iron-Sulfur Cluster-Containing Subunits in the Mitochondrial Complex I From Plants

In plants, the mitochondrial complex I is the protein complex encompassing the largest number of iron-sulfur (Fe-S) clusters. The whole, membrane-embedded, holo-complex is assembled stepwise from assembly intermediates. The Q and N modules are combined to form a peripheral arm in the matrix, whereas the so-called membrane arm is formed after merging a carbonic anhydrase (CA) module with so-called Pp (proximal) and the Pd (distal) domains. A ferredoxin bridge connects both arms. The eight Fe-S clusters present in the peripheral arm for electron transfer reactions are synthesized via a dedicated protein machinery referred to as the iron-sulfur cluster (ISC) machinery. The de novo assembly occurs on ISCU scaffold proteins from iron, sulfur and electron delivery proteins. In a second step, the preformed Fe-S clusters are transferred, eventually converted and inserted in recipient apo-proteins. Diverse molecular actors, including a chaperone-cochaperone system, assembly factors among which proteins with LYR motifs, and Fe-S cluster carrier/transfer proteins, have been identified as contributors to the second step. This mini-review highlights the recent progresses in our understanding of how specificity is achieved during the delivery of preformed Fe-S clusters to complex I subunits.

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.

The scope of flavin-dependent reactions and processes in the model plant Arabidopsis thaliana

Flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) are utilized as coenzymes in many biochemical reduction-oxidation reactions owing to the ability of the tricyclic isoalloxazine ring system to employ the oxidized, radical and reduced state. We have analyzed the genome of Arabidopsis thaliana to establish an inventory of genes encoding flavin-dependent enzymes (flavoenzymes) as a basis to explore the range of flavin-dependent biochemical reactions that occur in this model plant. Expectedly, flavoenzymes catalyze many pivotal reactions in primary catabolism, which are connected to the degradation of basic metabolites, such as fatty and amino acids as well as carbohydrates and purines. On the other hand, flavoenzymes play diverse roles in anabolic reactions most notably the biosynthesis of amino acids as well as the biosynthesis of pyrimidines and sterols. Importantly, the role of flavoenzymes goes much beyond these basic reactions and extends into pathways that are equally crucial for plant life, for example the production of natural products. In this context, we outline the participation of flavoenzymes in the biosynthesis and maintenance of cofactors, coenzymes and accessory plant pigments (e. g. carotenoids) as well as phytohormones. Moreover, several multigene families have emerged as important components of plant immunity, for example the family of berberine bridge enzyme-like enzymes, flavin-dependent monooxygenases and NADPH oxidases. Furthermore, the versatility of flavoenzymes is highlighted by their role in reactions leading to tRNA-modifications, chromatin regulation and cellular redox homeostasis. The favorable photochemical properties of the flavin chromophore are exploited by photoreceptors to govern crucial processes of plant adaptation and development. Finally, a sequence- and structure-based approach was undertaken to gain insight into the catalytic role of uncharacterized flavoenzymes indicating their involvement in unknown biochemical reactions and pathways in A. thaliana.

A ferredoxin bridge connects the two arms of plant mitochondrial complex I

Mitochondrial complex I is the main site for electron transfer to the respiratory chain and generates much of the proton gradient across the inner mitochondrial membrane. Complex I is composed of two arms, which form a conserved L-shape. We report the structures of the intact, 47-subunit mitochondrial complex I from Arabidopsis thaliana and the 51-subunit complex I from the green alga Polytomella sp., both at around 2.9 Å resolution. In both complexes, a heterotrimeric γ-carbonic anhydrase domain is attached to the membrane arm on the matrix side. Two states are resolved in A. thaliana complex I, with different angles between the two arms and different conformations of the ND1 (NADH dehydrogenase subunit 1) loop near the quinol binding site. The angle appears to depend on a bridge domain, which links the peripheral arm to the membrane arm and includes an unusual ferredoxin. We propose that the bridge domain participates in regulating the activity of plant complex I.

Elevating Ascorbate in Arabidopsis Stimulates the Production of Abscisic Acid, Phaseic Acid, and to a Lesser Extent Auxin (IAA) and Jasmonates, Resulting in Increased Expression of DHAR1 and Multiple Transcription Factors Associated with Abiotic Stress Tolerance

Gene expression and phytohormone contents were measured in response to elevating ascorbate in the absence of other confounding stimuli such as high light and abiotic stresses. Young Arabidopsis plants were treated with 25 mM solutions of l-galactose pathway intermediates l-galactose (l-gal) or l-galactono-1,4-lactone (l-galL), as well as L-ascorbic acid (AsA), with 25 mM glucose used as control. Feeding increased rosette AsA 2- to 4-fold but there was little change in AsA biosynthetic gene transcripts. Of the ascorbate recycling genes, only Dehydroascorbate reductase 1 expression was increased. Some known regulatory genes displayed increased expression and included ANAC019, ANAC072, ATHB12, ZAT10 and ZAT12. Investigation of the ANAC019/ANAC072/ATHB12 gene regulatory network revealed a high proportion of ABA regulated genes. Measurement of a subset of jasmonate, ABA, auxin (IAA) and salicylic acid compounds revealed consistent increases in ABA (up to 4.2-fold) and phaseic acid (PA; up to 5-fold), and less consistently certain jasmonates, IAA, but no change in salicylic acid levels. Increased ABA is likely due to increased transcripts for the ABA biosynthetic gene NCED3. There were also smaller increases in transcripts for transcription factors ATHB7, ERD1, and ABF3. These results provide insights into how increasing AsA content can mediate increased abiotic stress tolerance.

Elevating Ascorbate in Arabidopsis Stimulates the Production of Abscisic Acid, Phaseic Acid, and to a Lesser Extent Auxin (IAA) and Jasmonates, Resulting in Increased Expression of DHAR1 and Multiple Transcription Factors Associated with Abiotic Stress Tolerance

Gene expression and phytohormone contents were measured in response to elevating ascorbate in the absence of other confounding stimuli such as high light and abiotic stresses. Young Arabidopsis plants were treated with 25 mM solutions of l-galactose pathway intermediates l-galactose (l-gal) or l-galactono-1,4-lactone (l-galL), as well as L-ascorbic acid (AsA), with 25 mM glucose used as control. Feeding increased rosette AsA 2- to 4-fold but there was little change in AsA biosynthetic gene transcripts. Of the ascorbate recycling genes, only Dehydroascorbate reductase 1 expression was increased. Some known regulatory genes displayed increased expression and included ANAC019, ANAC072, ATHB12, ZAT10 and ZAT12. Investigation of the ANAC019/ANAC072/ATHB12 gene regulatory network revealed a high proportion of ABA regulated genes. Measurement of a subset of jasmonate, ABA, auxin (IAA) and salicylic acid compounds revealed consistent increases in ABA (up to 4.2-fold) and phaseic acid (PA; up to 5-fold), and less consistently certain jasmonates, IAA, but no change in salicylic acid levels. Increased ABA is likely due to increased transcripts for the ABA biosynthetic gene NCED3. There were also smaller increases in transcripts for transcription factors ATHB7, ERD1, and ABF3. These results provide insights into how increasing AsA content can mediate increased abiotic stress tolerance.

Protein interaction patterns in Arabidopsis thaliana leaf mitochondria change in dependence to light

Mitochondrial biology is underpinned by the presence and activity of large protein assemblies participating in the organelle-located steps of respiration, TCA-cycle, glycine oxidation, and oxidative phosphorylation. While the enzymatic roles of these complexes are undisputed, little is known about the interactions of the subunits beyond their presence in these protein complexes and their functions in regulating mitochondrial metabolism. By applying one of the most important regulatory cues for plant metabolism, the presence or absence of light, we here assess changes in the composition and molecular mass of protein assemblies involved in NADH-production in the mitochondrial matrix and in oxidative phosphorylation by employing a differential complexome profiling strategy. Covering a mass up to 25 MDa, we demonstrate dynamic associations of matrix enzymes and of components involved in oxidative phosphorylation. The data presented here form the basis for future studies aiming to advance our understanding of the role of protein:protein interactions in regulating plant mitochondrial functions.

Atomic structure of a mitochondrial complex I intermediate from vascular plants

Respiration, an essential metabolic process, provides cells with chemical energy. In eukaryotes, respiration occurs via the mitochondrial electron transport chain (mETC) composed of several large membrane-protein complexes. Complex I (CI) is the main entry point for electrons into the mETC. For plants, limited availability of mitochondrial material has curbed detailed biochemical and structural studies of their mETC. Here, we present the cryoEM structure of the known CI assembly intermediate CI* from Vigna radiata at 3.9 Å resolution. CI* contains CI's NADH-binding and CoQ-binding modules, the proximal-pumping module and the plant-specific γ-carbonic-anhydrase domain (γCA). Our structure reveals significant differences in core and accessory subunits of the plant complex compared to yeast, mammals and bacteria, as well as the details of the γCA domain subunit composition and membrane anchoring. The structure sheds light on differences in CI assembly across lineages and suggests potential physiological roles for CI* beyond assembly.

Stress signalling dynamics of the mitochondrial electron transport chain and oxidative phosphorylation system in higher plants

BACKGROUND

Mitochondria play a diversity of physiological and metabolic roles under conditions of abiotic or biotic stress. They may be directly subjected to physico-chemical constraints, and they are also involved in integrative responses to environmental stresses through their central position in cell nutrition, respiration, energy balance and biosyntheses. In plant cells, mitochondria present various biochemical peculiarities, such as cyanide-insensitive alternative respiration, and, besides integration with ubiquitous eukaryotic compartments, their functioning must be coupled with plastid functioning. Moreover, given the sessile lifestyle of plants, their relative lack of protective barriers and present threats of climate change, the plant cell is an attractive model to understand the mechanisms of stress/organelle/cell integration in the context of environmental stress responses.

SCOPE

The involvement of mitochondria in this integration entails a complex network of signalling, which has not been fully elucidated, because of the great diversity of mitochondrial constituents (metabolites, reactive molecular species and structural and regulatory biomolecules) that are linked to stress signalling pathways. The present review analyses the complexity of stress signalling connexions that are related to the mitochondrial electron transport chain and oxidative phosphorylation system, and how they can be involved in stress perception and transduction, signal amplification or cell stress response modulation.

CONCLUSIONS

Plant mitochondria are endowed with a diversity of multi-directional hubs of stress signalling that lead to regulatory loops and regulatory rheostats, whose functioning can amplify and diversify some signals or, conversely, dampen and reduce other signals. Involvement in a wide range of abiotic and biotic responses also implies that mitochondrial stress signalling could result in synergistic or conflicting outcomes during acclimation to multiple and complex stresses, such as those arising from climate change.

The Oxidative Phosphorylation system of the mitochondria in plants

Mitochondrial Oxidative Phosphorylation (OXPHOS) provides ATP for driving cellular functions. In plants, OXPHOS takes place in the context of photosynthesis. Indeed, metabolism of mitochondria and chloroplasts is tightly linked. OXPHOS has several extra functions in plants. This review takes a view on the OXPHOS system of plants, the electron transfer chain (ETC), the ATP synthase complex and the numerous supplementary enzymes involved. Electron transport pathways are especially branched in plants. Furthermore, the "classical" OXPHOS complexes include extra subunits, some of which introduce side activities into these complexes. Consequently, and to a remarkable degree, OXPHOS is a multi-functional system in plants that needs to be efficiently regulated with respect to all its physiological tasks in the mitochondria, the chloroplasts, and beyond. Regulatory mechanisms based on posttranslational protein modifications and formation of supramolecular protein assemblies are summarized and discussed.

Vitamin C in Plants: Novel Concepts, New Perspectives, and Outstanding Issues

Significance: The concept that vitamin C (l-ascorbic acid) is at the heart of the peroxide processing and redox signaling hub in plants is well established, but our knowledge of the precise mechanisms involved remains patchy at best. Recent Advances: Ascorbate participates in the multifaceted signaling pathways initiated by both reactive oxygen species (ROS) and reactive nitrogen species. Crucially, the apoplastic ascorbate/dehydroascorbate (DHA) ratio that is regulated by ascorbate oxidase (AO) sculpts the apoplastic ROS (apoROS) signal that controls polarized cell growth, biotic and abiotic defences, and cell to cell signaling, as well as exerting control over the light-dependent regulation of photosynthesis. Critical Issues: Here we re-evaluate the roles of ascorbate in photosynthesis and other processes, addressing the question of how much we really know about the regulation of ascorbate homeostasis and its functions in plants, or how AO is regulated to modulate apoROS signals. Future Directions: The role of microRNAs in the regulation of AO activity in relation to stress perception and signaling must be resolved. Similarly, the molecular characterization of ascorbate transporters and mechanistic links between photosynthetic and respiratory electron transport and ascorbate synthesis/homeostasis are a prerequisite to understanding ascorbate homeostasis and function. Similarly, there is little in vivo evidence for ascorbate functions as an enzyme cofactor.

Assembly of Mitochondrial Complex I Requires the Low-Complexity Protein AMC1 in Chlamydomonas reinhardtii

Complex I is the first enzyme involved in the mitochondrial electron transport chain. With >40 subunits of dual genetic origin, the biogenesis of complex I is highly intricate and poorly understood. We used Chlamydomonas reinhardtii as a model system to reveal factors involved in complex I biogenesis. Two insertional mutants, displaying a complex I assembly defect characterized by the accumulation of a 700 kDa subcomplex, were analyzed. Genetic analyses showed these mutations were allelic and mapped to the gene AMC1 (Cre16.g688900) encoding a low-complexity protein of unknown function. The complex I assembly and activity in the mutant was restored by complementation with the wild-type gene, confirming AMC1 is required for complex I biogenesis. The N terminus of AMC1 targets a reporter protein to yeast mitochondria, implying that AMC1 resides and functions in the Chlamydomonas mitochondria. Accordingly, in both mutants, loss of AMC1 function results in decreased abundance of the mitochondrial nd4 transcript, which encodes the ND4 membrane subunit of complex I. Loss of ND4 in a mitochondrial nd4 mutant is characterized by a membrane arm assembly defect, similar to that exhibited by loss of AMC1. These results suggest AMC1 is required for the production of mitochondrially-encoded complex I subunits, specifically ND4. We discuss the possible modes of action of AMC1 in mitochondrial gene expression and complex I biogenesis.

A Mitochondrial LYR Protein Is Required for Complex I Assembly

Complex I biogenesis requires the expression of both nuclear and mitochondrial genes, the import of proteins, cofactor biosynthesis, and the assembly of at least 49 individual subunits. Assembly factors interact with subunits of Complex I but are not part of the final holocomplex. We show that in Arabidopsis (Arabidopsis thaliana), a mitochondrial matrix protein (EMB1793, At1g76060), which we term COMPLEX I ASSEMBLY FACTOR 1 (CIAF1), contains a LYR domain and is required for Complex I assembly. T-DNA insertion mutants of CIAF1 lack Complex I and the Supercomplex I+III. Biochemical characterization shows that the assembly of Complex I is stalled at 650 and 800 kD intermediates in mitochondria isolated from ciaf1 mutant lines.I. Yeast-two-hybrid interaction and complementation assays indicate that CIAF1 specifically interacts with the 23-kD TYKY-1 matrix domain subunit of Complex I and likely plays a role in Fe-S insertion into this subunit. These data show that CIAF1 plays an essential role in assembling the peripheral matrix arm Complex I subunits into the Complex I holoenzyme.

Vitamin C in Plants: From Functions to Biofortification

Vitamin C (l-ascorbic acid) is an excellent free radical scavenger, not only for its capability to donate reducing equivalents but also for the relative stability of the derived monodehydroascorbate radical. However, vitamin C is not only an antioxidant, since it is also a cofactor for numerous enzymes involved in plant and human metabolism. In humans, vitamin C takes part in various physiological processes, such as iron absorption, collagen synthesis, immune stimulation, and epigenetic regulation. Due to the functional loss of the gene coding for l-gulonolactone oxidase, humans cannot synthesize vitamin C; thus, they principally utilize plant-based foods for their needs. For this reason, increasing the vitamin C content of crops could have helpful effects on human health. To achieve this objective, exhaustive knowledge of the metabolism and functions of vitamin C in plants is needed. In this review, the multiple roles of vitamin C in plant physiology as well as the regulation of its content, through biosynthetic or recycling pathways, are analyzed. Finally, attention is paid to the strategies that have been used to increase the content of vitamin C in crops, emphasizing not only the improvement of nutritional value of the crops but also the acquisition of plant stress resistance.

Nitric oxide-dependent regulation of sweet pepper fruit ripening

Ripening is a complex physiological process that involves changes in reactive nitrogen and oxygen species that govern the shelf-life and quality of fruits. Nitric oxide (NO)-dependent changes in the sweet pepper fruit transcriptome were determined by treating fruits at the initial breaking point stage with NO gas. Fruits were also harvested at the immature (green) and ripe (red) stages. Fruit ripening in the absence of NO resulted in changes in the abundance of 8805 transcripts whose function could be identified. Among these, functional clusters associated with reactive oxygen/nitrogen species and lipid metabolism were significantly modified. NO treatment resulted in the differential expression of 498 genes framed within these functional categories. Biochemical analysis revealed that NO treatment resulted in changes in fatty acid profiling, glutathione and proline contents, and the extent of lipid peroxidation, as well as increases in the activity of ascorbate peroxidase and lipoxygenase. These data provide supporting evidence for the crucial role of NO in the ripening of pepper fruit.

Composition of Mitochondrial Complex I during the Critical Node of Seed Aging in Oryza sativa

Previous studies have documented mitochondrial dysfunction during the critical node (CN) of rice (Oryza sativa) seed aging, including a decrease in the capacity of NADH dependent O₂ consumption. This raises the hypothesis that changes in the activity of NADH:ubiquinone oxidoreductase (complex I) may play a role in seed aging. The composition and activity of complex I was investigated at the CN of aged rice seeds. Using BN-PAGE and SWATH-MS 52 complex I subunits were identified, nineteen for the first time to be experimentally detected in rice. The subunits of the matrix arm (N and Q modules) were reduced in abundance at the CN, in accordance with a reduction in the capacity to oxidise NADH, reducing substrate oxidation and increase ROS accumulation. In contrast, subunits in the P module increased in abundance that contains many mitochondrial encoded subunits. It is proposed that the changes in complex I abundance subunits may indicate a premature re-activation of mitochondrial biogenesis, as evidenced by the increase in mitochondrial encoded subunits. This premature activation of mitochondrial biogenesis may under-pin the decreased viability of aged seeds, as mitochondrial biogenesis is a crucial event in germination to drive growth before autotrophic growth of the seedling is established.

The assembly pathway of complex I in Arabidopsis thaliana

All present-day mitochondria originate from a single endosymbiotic event that gave rise to the last eukaryotic common ancestor more than a billion years ago. However, to date, many aspects of mitochondrial evolution have remained unresolved. Comparative genomics and proteomics have revealed a complex evolutionary origin for many mitochondrial components. To understand the evolution of the respiratory chain, we have examined both the components and the mechanisms of the assembly pathway of complex I. Complex I represents the first enzyme in the respiratory chain, and complex I deficiencies have dramatic consequences in both animals and plants. The complex is located in the mitochondrial inner membrane and possesses two arms: one embedded in the inner membrane and one protruding in the matrix. Here, we describe the assembly pathway of complex I in the model plant Arabidopsis thaliana. Using a proteomics approach called complexome profiling, we have resolved the different steps in the assembly process in plants. We propose a model for the stepwise assembly of complex I, including every subunit. We then compare this pathway with the corresponding pathway in humans and find that complex I assembly in plants follows a different, and likely ancestral, pathway compared with the one in humans. We show that the main evolutionary changes in complex I structure and assembly in humans occurred at the level of the membrane arm, whereas the matrix arm remained rather conserved.

Vitamin C Content in Fruits: Biosynthesis and Regulation

Throughout evolution, a number of animals including humans have lost the ability to synthesize ascorbic acid (ascorbate, vitamin C), an essential molecule in the physiology of animals and plants. In addition to its main role as an antioxidant and cofactor in redox reactions, recent reports have shown an important role of ascorbate in the activation of epigenetic mechanisms controlling cell differentiation, dysregulation of which can lead to the development of certain types of cancer. Although fruits and vegetables constitute the main source of ascorbate in the human diet, rising its content has not been a major breeding goal, despite the large inter- and intraspecific variation in ascorbate content in fruit crops. Nowadays, there is an increasing interest to boost ascorbate content, not only to improve fruit quality but also to generate crops with elevated stress tolerance. Several attempts to increase ascorbate in fruits have achieved fairly good results but, in some cases, detrimental effects in fruit development also occur, likely due to the interaction between the biosynthesis of ascorbate and components of the cell wall. Plants synthesize ascorbate de novo mainly through the Smirnoff-Wheeler pathway, the dominant pathway in photosynthetic tissues. Two intermediates of the Smirnoff-Wheeler pathway, GDP-D-mannose and GDP-L-galactose, are also precursors of the non-cellulosic components of the plant cell wall. Therefore, a better understanding of ascorbate biosynthesis and regulation is essential for generation of improved fruits without developmental side effects. This is likely to involve a yet unknown tight regulation enabling plant growth and development, without impairing the cell redox state modulated by ascorbate pool. In certain fruits and developmental conditions, an alternative pathway from D-galacturonate might be also relevant. We here review the regulation of ascorbate synthesis, its close connection with the cell wall, as well as different strategies to increase its content in plants, with a special focus on fruits.

Ascorbic acid metabolism and functions: A comparison of plants and mammals

Ascorbic acid is synthesised by eukaryotes, the known exceptions being primates and some other animal groups which have lost functional gulonolactone oxidase. Prokaryotes do not synthesise ascorbate and do not need an ascorbate supply, so the functions that are essential for mammals and plants are not required or are substituted by other compounds. The ability of ascorbate to donate electrons enables it to act as a free radical scavenger and to reduce higher oxidation states of iron to Fe²⁺. These reactions are the basis of its biological activity along with the relative stability of the resulting resonance stabilised monodehydroascorbate radical. The importance of these properties is emphasised by the evolution of at least three biosynthetic pathways and production of an ascorbate analogue, erythroascorbate, by fungi. The iron reducing activity of ascorbate maintains the reactive centre Fe²⁺ of 2-oxoglutarate-dependent dioxygenases (2-ODDs) thus preventing inactivation. These enzymes have diverse functions and, recently, the possibility that ascorbate status in mammals could influence 2-ODDs involved in histone and DNA demethylation thereby influencing stem cell differentiation and cancer has been uncovered. Ascorbate is involved in iron uptake and transport in plants and animals. While the above biochemical functions are shared between mammals and plants, ascorbate peroxidase (APX) is an enzyme family limited to plants and photosynthetic protists. It provides these organisms with increased capacity to remove H₂O₂ produced by photosynthetic electron transport and photorespiration. The Fe reducing activity of ascorbate enables hydroxyl radical production (pro-oxidant effect) and the reactivity of dehydroascorbate (DHA) and reaction of its degradation products with proteins (dehydroascorbylation and glycation) is potentially damaging. Ascorbate status influences gene expression in plants and mammals but at present there is little evidence that it acts as a specific signalling molecule. It most likely acts indirectly by influencing the redox state of thiols and 2-ODD activity. However, the possibility that dehydroascorbylation is a regulatory post-translational protein modification could be explored.

Structure and function of complex I in animals and plants - a comparative view

The mitochondrial NADH dehydrogenase complex (complex I) has a molecular mass of about 1000 kDa and includes 40-50 subunits in animals, fungi and plants. It is composed of a membrane arm and a peripheral arm and has a conserved L-like shape in all species investigated. However, in plants and possibly some protists it has a second peripheral domain which is attached to the membrane arm on its matrix exposed side at a central position. The extra domain includes proteins resembling prokaryotic gamma-type carbonic anhydrases. We here present a detailed comparison of complex I from mammals and flowering plants. Forty homologous subunits are present in complex I of both groups of species. In addition, five subunits are present in mammalian complex I, which are absent in plants, and eight to nine subunits are present in plant complex I which do not occur in mammals. Based on the atomic structure of mammalian complex I and biochemical insights into complex I architecture from plants we mapped the species-specific subunits. Interestingly, four of the five animal-specific and five of the eight to nine plant-specific subunits are localized at the inner surface of the membrane arm of complex I in close proximity. We propose that the inner surface of the membrane arm represents a workbench for attaching proteins to complex I, which are not directly related to respiratory electron transport, like nucleoside kinases, acyl-carrier proteins or carbonic anhydrases. We speculate that further enzyme activities might be bound to this micro-location in other groups of organisms.

Characterization of the galactono-1,4-lactone dehydrogenase from pepper fruits and its modulation in the ascorbate biosynthesis. Role of nitric oxide

Pepper fruit is one of the highest vitamin C sources of plant origin for our diet. In plants, ascorbic acid is mainly synthesized through the L-galactose pathway, being the L-galactono-1,4-lactone dehydrogenase (GalLDH) the last step. Using pepper fruits, the full GalLDH gene was cloned and the protein molecular characterization accomplished. GalLDH protein sequence (586 residues) showed a 37 amino acids signal peptide at the N-terminus, characteristic of mitochondria. The hydrophobic analysis of the mature protein displayed one transmembrane helix comprising 20 amino acids at the N-terminus. By using a polyclonal antibody raised against a GalLDH internal sequence and immunoblotting analysis, a 56kDa polypeptide cross-reacted with pepper fruit samples. Using leaves, flowers, stems and fruits, the expression of GalLDH by qRT-PCR and the enzyme activity were analyzed, and results indicate that GalLDH is a key player in the physiology of pepper plants, being possibly involved in the processes which undertake the transport of ascorbate among different organs. We also report that an NO (nitric oxide)-enriched atmosphere enhanced ascorbate content in pepper fruits about 40% parallel to increased GalLDH gene expression and enzyme activity. This is the first report on the stimulating effect of NO treatment on the vitamin C concentration in plants. Accordingly, the modulation by NO of GalLDH was addressed. In vitro enzymatic assays of GalLDH were performed in the presence of SIN-1 (peroxynitrite donor) and S-nitrosoglutahione (NO donor). Combined results of in vivo NO treatment and in vitro assays showed that NO provoked the regulation of GalLDH at transcriptional and post-transcriptional levels, but not post-translational modifications through nitration or S-nitrosylation events promoted by reactive nitrogen species (RNS) took place. These results suggest that this modulation point of the ascorbate biosynthesis could be potentially used for biotechnological purposes to increase the vitamin C levels in pepper fruits.

Ascorbic acid deficiency leads to increased grain chalkiness in transgenic rice for suppressed of L-GalLDH

The grain chalkiness of rice (Oryza sativa L.), which determines the rice quality and price, is a major concern in rice breeding. Reactive oxygen species (ROS) plays a critical role in regulating rice endosperm chalkiness. Ascorbic acid (Asc) is a major plant antioxidant, which strictly regulates the levels of ROS. l-galactono-1, 4-lactone dehydrogenase (L-GalLDH, EC 1.3.2.3) is an enzyme that catalyzes the last step of Asc biosynthesis in higher plants. Here we show that the L-GalLDH-suppressed transgenic rice, GI-1 and GI-2, which have constitutively low (between 30% and 50%) leaf and grain Asc content compared with the wild-type (WT), exhibit significantly increased grain chalkiness. Further examination showed that the deficiency of Asc resulted in a higher lipid peroxidation and H₂O₂ content, accompanied by a lower hydroxyl radical scavenging rate, total antioxidant capacity and photosynthetic ability. In addition, changes of the enzyme activities and gene transcript abundances related to starch synthesis were also observed in GI-1 and GI-2 grains. The results we presented here suggest a close correlation between Asc deficiency and grain chalkiness in the L-GalLDH-suppressed transgenics. Asc deficiency leads to the accumulation of H₂O₂, affecting antioxidant capacity and photosynthetic function, changing enzyme activities and gene transcript abundances related to starch synthesis, finally leading to the increased grain chalkiness.

The mitochondrial complexome of Arabidopsis thaliana

Mitochondria are central to cellular metabolism and energy conversion. In plants they also enable photosynthesis through additional components and functional flexibility. A majority of those processes relies on the assembly of individual proteins to larger protein complexes, some of which operate as large molecular machines. There has been a strong interest in the makeup and function of mitochondrial protein complexes and protein-protein interactions in plants, but the experimental approaches used typically suffer from selectivity or bias. Here, we present a complexome profiling analysis for leaf mitochondria of the model plant Arabidopsis thaliana for the systematic characterization of protein assemblies. Purified organelle extracts were separated by 1D Blue native (BN) PAGE, a resulting gel lane was dissected into 70 slices (complexome fractions) and proteins in each slice were identified by label free quantitative shot-gun proteomics. Overall, 1359 unique proteins were identified, which were, on average, present in 17 complexome fractions each. Quantitative profiles of proteins along the BN gel lane were aligned by similarity, allowing us to visualize protein assemblies. The data allow re-annotating the subunit compositions of OXPHOS complexes, identifying assembly intermediates of OXPHOS complexes and assemblies of alternative respiratory oxidoreductases. Several protein complexes were discovered that have not yet been reported in plants, such as a 530 kDa Tat complex, 460 and 1000 kDa SAM complexes, a calcium ion uniporter complex (150 kDa) and several PPR protein complexes. We have set up a tailored online resource (https://complexomemap.de/at_mito_leaves) to deposit the data and to allow straightforward access and custom data analyses.

Cardiolipin Supports Respiratory Enzymes in Plants in Different Ways

In eukaryotes the presence of the dimeric phospholipid cardiolipin (CL) is limited to the mitochondrial membranes. It resides predominantly in the inner membrane where it interacts with components of the mitochondrial electron transfer chain. CL deficiency has previously been shown to affect abundances of the plant NADH-dehydrogenase complex and its association with dimeric cyctochrome c reductase. Using an Arabidopsis thaliana knock-out mutant for the final enzyme of CL biosynthesis we here extend current knowledge on the dependence of plant respiration on CL. By correlating respiratory enzyme abundances with enzymatic capacities in mitochondria isolated from wild type, CL deficient and CL complemented heterotrophic cell culture lines a new picture of the participation of CL in plant respiration is emerging. Data indicate a loss of a general reduction of respiratory capacity in CL deficient mitochondria which cannot solely be attributed to decreased abundances or capacities of mitochondrial electron transfer protein complexes and supercomplexes. Instead, it most likely is the result of a loss of the mobile electron carrier cytochrome c. Furthermore, enzymes of the tricarboxylic acid cycle are found to have lower maximum activities in the mutant, including the succinate dehydrogenase complex. Interestingly, abundance of the latter is not altered, indicative of a direct impact of CL deficiency on the enzymatic capacity of this electron transfer chain protein complex.

A novel allele of L-galactono-1,4-lactone dehydrogenase is associated with enhanced drought tolerance through affecting stomatal aperture in common wheat

In higher plants, L-galactono-1,4-lactone dehydrogenase (GLDH) plays important roles in ascorbic acid (AsA) biosynthesis and assembly of respiration complex I. Here we report three homoeologous genes (TaGLDH-A1, -B1 and -D1) encoding common wheat GLDH isozymes and a unique allelic variant (TaGLDH-A1b) associated with enhanced drought tolerance. TaGLDH-A1, -B1 and -D1 were located on chromosomes 5A, 5B and 5D, respectively, and their transcripts were found in multiple organs. The three homoeologs each conferred increased GLDH activity when ectopically expressed in tobacco. Decreasing TaGLDH expression in wheat significantly reduced GLDH activity and AsA content. TaGLDH-A1b differed from wild type allele TaGLDH-A1a by an in-frame deletion of three nucleotides. TaGLDH-A1b was biochemically less active than TaGLDH-A1a, and the total GLDH activity levels were generally lower in the cultivars carrying TaGLDH-A1b relative to those with TaGLDH-A1a. Interestingly, TaGLDH-A1b cultivars showed stronger water deficiency tolerance than TaGLDH-A1a cultivars, and TaGLDH-A1b co-segregated with decreased leaf water loss in a F₂ population. Finally, TaGLDH-A1b cultivars generally exhibited smaller leaf stomatal aperture than TaGLDH-A1a varieties in control or water deficiency environments. Our work provides new information on GLDH genes and function in higher plants. TaGLDH-A1b is likely useful for further studying and improving wheat tolerance to drought stress.

Characterisation of antioxidants in photosynthetic and non-photosynthetic leaf tissues of variegated Pelargonium zonale plants

Hydrogen peroxide is an important signalling molecule, involved in regulation of numerous metabolic processes in plants. The most important sources of H2 O2 in photosynthetically active cells are chloroplasts and peroxisomes. Here we employed variegated Pelargonium zonale to characterise and compare enzymatic and non-enzymatic components of the antioxidative system in autotrophic and heterotrophic leaf tissues at (sub)cellular level under optimal growth conditions. The results revealed that both leaf tissues had specific strategies to regulate H2 O2 levels. In photosynthetic cells, the redox regulatory system was based on ascorbate, and on the activities of thylakoid-bound ascorbate peroxidase (tAPX) and catalase. In this leaf tissue, ascorbate was predominantly localised in the nucleus, peroxisomes, plastids and mitochondria. On the other hand, non-photosynthetic cells contained higher glutathione content, mostly located in mitochondria. The enzymatic antioxidative system in non-photosynthetic cells relied on the ascorbate-glutathione cycle and both Mn and Cu/Zn superoxide dismutase. Interestingly, higher content of ascorbate and glutathione, and higher activities of APX in the cytosol of non-photosynthetic leaf cells compared to the photosynthetic ones, suggest the importance of this compartment in H2 O2 regulation. Together, these results imply different regulation of processes linked with H2 O2 signalling at subcellular level. Thus, we propose green-white variegated leaves as an excellent system for examination of redox signal transduction and redox communication between two cell types, autotrophic and heterotrophic, within the same organ.

Cytochrome c, a hub linking energy, redox, stress and signaling pathways in mitochondria and other cell compartments

Cytochrome c (CYTc) is a soluble redox-active heme protein that transfers electrons from complex III to complex IV in the cyanide-sensitive mitochondrial respiratory pathway. CYTc biogenesis is a complex process that requires multiple steps until the mature active protein is obtained. CYTc levels and activity are finely regulated, revealing the importance of this protein not only as electron carrier but also in many other processes. In this article, we describe the role of CYTc in mitochondrial respiration, from its canonical role as electron carrier for ATP production to its involvement in protein import and the stabilization of respiratory complexes and supercomplexes. In plants, CYTc is connected to the synthesis of the antioxidant ascorbate and the detoxification of toxic compounds. Finally, CYTc is also a multi-functional signaling molecule that influences the balance between life and death, acting in energy provision for cellular functions or triggering programmed cell death. The confluence of several metabolic routes into a single protein that links redox reactions with energy producing pathways seems logical from the point of view of cellular economy, control and organization.

Plant mitochondrial Complex I composition and assembly: A review

In the mitochondrial inner membrane, oxidative phosphorylation generates ATP via the operation of several multimeric enzymes. The proton-pumping Complex I (NADH:ubiquinone oxidoreductase) is the first and most complicated enzyme required in this process. Complex I is an L-shaped enzyme consisting of more than 40 subunits, one FMN molecule and eight Fe-S clusters. In recent years, genetic and proteomic analyses of Complex I mutants in various model systems, including plants, have provided valuable insights into the assembly of this multimeric enzyme. Assisted by a number of key players, referred to as "assembly factors", the assembly of Complex I takes place in a sequential and modular manner. Although a number of factors have been identified, their precise function in mediating Complex I assembly still remains to be elucidated. This review summarizes our current knowledge of plant Complex I composition and assembly derived from studies in plant model systems such as Arabidopsis thaliana and Chlamydomonas reinhardtii. Plant Complex I is highly conserved and comprises a significant number of subunits also present in mammalian and fungal Complexes I. Plant Complex I also contains additional subunits absent from the mammalian and fungal counterpart, whose function in enzyme activity and assembly is not clearly understood. While 14 assembly factors have been identified for human Complex I, only two proteins, namely GLDH and INDH, have been established as bona fide assembly factors for plant Complex I. This article is part of a Special Issue entitled Respiratory complex I, edited by Volker Zickermann and Ulrich Brandt.

L-Galactono-1,4-lactone dehydrogenase is an assembly factor of the membrane arm of mitochondrial complex I in Arabidopsis

L-Galactono-1,4-lactone dehydrogenase (GLDH) catalyses the last enzymatic step of the ascorbate biosynthetic pathway in plants. GLDH is localised to mitochondria and several reports have shown that GLDH is associated with complex I of the respiratory chain. In a gldh knock-out mutant, complex I is not detectable, suggesting that GLDH is essential for complex I assembly or stability. GLDH has not been identified as a genuine complex I subunit, instead, it is present in a smaller, lowly abundant version of complex I called complex I*. In addition, GLDH activity has also been detected in smaller protein complexes within mitochondria membranes. Here, we investigated the role of GLDH during complex I assembly. We identified GLDH in complexes co-localising with some complex I assembly intermediates. Using a mutant that accumulates complex I assembly intermediates, we confirmed that GLDH is associated with the complex I assembly intermediates of 400 and 450 kDa. In addition, we detected accumulation of the 200 kDa complex I assembly intermediate in the gldh mutant. Taken together, our data suggest that GLDH is an assembly factor of the membrane arm of complex I. This function appears to be independent of the role of GLDH in ascorbate synthesis, as evidenced by the ascorbate-deficient mutant vtc2-1 accumulating wild-type levels of complex I. Therefore, we propose that GLDH is a dual-function protein that has a second, non-enzymatic function in complex I assembly as a plant-specific assembly factor. We propose an updated model for complex I assembly that includes complex I* as an assembly intermediate.

Depletion of the "gamma-type carbonic anhydrase-like" subunits of complex I affects central mitochondrial metabolism in Arabidopsis thaliana

"Gamma-type carbonic anhydrase-like" (CAL) proteins form part of complex I in plants. Together with "gamma carbonic anhydrase" (CA) proteins they form an extra domain which is attached to the membrane arm of complex I on its matrix exposed side. In Arabidopsis two CAL and three CA proteins are present, termed CAL1, CAL2, CA1, CA2 and CA3. It has been proposed that the carbonic anhydrase domain of complex I is involved in a process mediating efficient recycling of mitochondrial CO2 for photosynthetic carbon fixation which is especially important during growth conditions causing increased photorespiration. Depletion of CAL proteins has been shown to significantly affect plant development and photomorphogenesis. To better understand CAL function in plants we here investigated effects of CAL depletion on the mitochondrial compartment. In mutant lines and cell cultures complex I amount was reduced by 90-95% but levels of complexes III and V were unchanged. At the same time, some of the CA transcripts were less abundant. Proteome analysis of CAL depleted cells revealed significant reduction of complex I subunits as well as proteins associated with photorespiration, but increased amounts of proteins participating in amino acid catabolism and stress response reactions. Developmental delay of the mutants was slightly alleviated if plants were cultivated at high CO2. Profiling of selected metabolites revealed defined changes in intermediates of the citric acid cycle and amino acid catabolism. It is concluded that CAL proteins are essential for complex I assembly and that CAL depletion specifically affects central mitochondrial metabolism.

Antioxidative and proteolytic systems protect mitochondria from oxidative damage in S-deficient Arabidopsis thaliana

We examined the functioning of the antioxidative defense system in Arabidopsis thaliana under sulphur (S) deficiency with an emphasis on the role of mitochondria. In tissue extracts and in isolated mitochondria from S-deficient plants, the concentration of non-protein thiols declined but protein thiols did not change. Superoxide anion and hydrogen peroxide were accumulated in leaf blades and the generation of superoxide anion by isolated mitochondria was higher. Lower abundance of reduced (GSH) plus oxidized (GSSG) glutathione in the leaf and root tissues, and leaf mitochondria from S-deficient plants was accompanied by a decrease in the level of GSH and the changes in the GSH/GSSG ratios. In the chloroplasts, the total level of glutathione decreased. Lower levels of reduced (AsA) and oxidized (DHA) ascorbate were reflected in much higher ratios of AsA/DHA. Sulphur deficiency led to an increase in the activity of cytosolic, mitochondrial and chloroplastic antioxidative enzymes, peroxidases, catalases and superoxide dismutases. The protein carbonyl level was higher in the leaves of S-deficient plants and in the chloroplasts, while in the roots, leaf and root mitochondria it remained unchanged. Protease activity in leaf extracts of S-deficient plants was higher, but in root extracts it did not differ. The proteolytic system reflected subcellular specificity. In leaf and root mitochondria the protease activity was higher, whereas in the chloroplasts it did not change. We propose that the preferential incorporation of S to protein thiols and activation of antioxidative and proteolytic systems are likely important for the survival of S-deficient plants and that the mitochondria maintain redox homeostasis.

Evolution of alternative biosynthetic pathways for vitamin C following plastid acquisition in photosynthetic eukaryotes

Ascorbic acid (vitamin C) is an enzyme co-factor in eukaryotes that also plays a critical role in protecting photosynthetic eukaryotes against damaging reactive oxygen species derived from the chloroplast. Many animal lineages, including primates, have become ascorbate auxotrophs due to the loss of the terminal enzyme in their biosynthetic pathway, l-gulonolactone oxidase (GULO). The alternative pathways found in land plants and Euglena use a different terminal enzyme, l-galactonolactone dehydrogenase (GLDH). The evolutionary processes leading to these differing pathways and their contribution to the cellular roles of ascorbate remain unclear. Here we present molecular and biochemical evidence demonstrating that GULO was functionally replaced with GLDH in photosynthetic eukaryote lineages following plastid acquisition. GULO has therefore been lost repeatedly throughout eukaryote evolution. The formation of the alternative biosynthetic pathways in photosynthetic eukaryotes uncoupled ascorbate synthesis from hydrogen peroxide production and likely contributed to the rise of ascorbate as a major photoprotective antioxidant.

DOI:http://dx.doi.org/10.7554/eLife.06369.001

Animals, plants, algae and other eukaryotic organisms all need vitamin C to enable many of their enzymes to work properly. Vitamin C also protects plant and algal cells from damage by molecules called reactive oxygen species (ROS), which can be produced when these cells harvest energy from sunlight in a process called photosynthesis. Photosynthesis occurs inside structures called chloroplasts, and has evolved on multiple occasions in eukaryotes when non-photosynthetic organisms acquired chloroplasts from other algae and then had to develop improved defences against ROS.

There are several steps involved in the production of vitamin C. In many animals, an enzyme called GULO carries out the final step by converting a molecule known as an aldonolactone into vitamin C; this reaction also produces ROS as a waste product. The GULO enzyme is missing in humans, primates and some other groups of animals, so these organisms must get all the vitamin C they need from their diet.

Plants and algae use a different enzyme—called GLDH—to make vitamin C from aldonolactone. GLDH is very similar to GULO, but it does not produce ROS as a waste product. It is not clear how the different pathways have evolved, or why some animals have lost the ability to make their own vitamin C.

Here, Wheeler et al. used genetics and biochemistry to investigate the evolutionary origins of vitamin C production in a variety of eukaryotic organisms. This investigation revealed that although GULO is missing from the insects and several other groups of animals, it is present in the sponges and many other eukaryotes. This suggests that GULO evolved in early eukaryotic organisms and has since been lost by the different groups of animals. On the other hand, GLDH is only found in plants and the other eukaryotes that can photosynthesize.

Wheeler et al.'s findings suggest that GULO has been lost and replaced by GLDH in all plants and algae following their acquisition of chloroplasts. GDLH allows plants and algae to make vitamin C without also producing ROS, which could explain why vitamin C has been able to take on an extra role in these organisms. The results allow us to better understand the functions of vitamin C in photosynthetic organisms and the processes associated with the acquisition of chloroplasts during evolution.

DOI:http://dx.doi.org/10.7554/eLife.06369.002

Protein import into plant mitochondria: signals, machinery, processing, and regulation

The majority of more than 1000 proteins present in mitochondria are imported from nuclear-encoded, cytosolically synthesized precursor proteins. This impressive feat of transport and sorting is achieved by the combined action of targeting signals on mitochondrial proteins and the mitochondrial protein import apparatus. The mitochondrial protein import apparatus is composed of a number of multi-subunit protein complexes that recognize, translocate, and assemble mitochondrial proteins into functional complexes. While the core subunits involved in mitochondrial protein import are well conserved across wide phylogenetic gaps, the accessory subunits of these complexes differ in identity and/or function when plants are compared with Saccharomyces cerevisiae (yeast), the model system for mitochondrial protein import. These differences include distinct protein import receptors in plants, different mechanistic operation of the intermembrane protein import system, the location and activity of peptidases, the function of inner-membrane translocases in linking the outer and inner membrane, and the association/regulation of mitochondrial protein import complexes with components of the respiratory chain. Additionally, plant mitochondria share proteins with plastids, i.e. dual-targeted proteins. Also, the developmental and cell-specific nature of mitochondrial biogenesis is an aspect not observed in single-celled systems that is readily apparent in studies in plants. This means that plants provide a valuable model system to study the various regulatory processes associated with protein import and mitochondrial biogenesis.

Metabolic control of redox and redox control of metabolism in plants

SIGNIFICANCE

Reduction-oxidation (Redox) status operates as a major integrator of subcellular and extracellular metabolism and is simultaneously itself regulated by metabolic processes. Redox status not only dominates cellular metabolism due to the prominence of NAD(H) and NADP(H) couples in myriad metabolic reactions but also acts as an effective signal that informs the cell of the prevailing environmental conditions. After relay of this information, the cell is able to appropriately respond via a range of mechanisms, including directly affecting cellular functioning and reprogramming nuclear gene expression.

RECENT ADVANCES

The facile accession of Arabidopsis knockout mutants alongside the adoption of broad-scale post-genomic approaches, which are able to provide transcriptomic-, proteomic-, and metabolomic-level information alongside traditional biochemical and emerging cell biological techniques, has dramatically advanced our understanding of redox status control. This review summarizes redox status control of metabolism and the metabolic control of redox status at both cellular and subcellular levels.

CRITICAL ISSUES

It is becoming apparent that plastid, mitochondria, and peroxisome functions influence a wide range of processes outside of the organelles themselves. While knowledge of the network of metabolic pathways and their intraorganellar redox status regulation has increased in the last years, little is known about the interorganellar redox signals coordinating these networks. A current challenge is, therefore, synthesizing our knowledge and planning experiments that tackle redox status regulation at both inter- and intracellular levels.

FUTURE DIRECTIONS

Emerging tools are enabling ever-increasing spatiotemporal resolution of metabolism and imaging of redox status components. Broader application of these tools will likely greatly enhance our understanding of the interplay of redox status and metabolism as well as elucidating and characterizing signaling features thereof. We propose that such information will enable us to dissect the regulatory hierarchies that mediate the strict coupling of metabolism and redox status which, ultimately, determine plant growth and development.

Small kernel 1 encodes a pentatricopeptide repeat protein required for mitochondrial nad7 transcript editing and seed development in maize (Zea mays) and rice (Oryza sativa)

RNA editing modifies cytidines (C) to uridines (U) at specific sites in the transcripts of mitochondria and plastids, altering the amino acid specified by the DNA sequence. Here we report the identification of a critical editing factor of mitochondrial nad7 transcript via molecular characterization of a small kernel 1 (smk1) mutant in Zea mays (maize). Mutations in Smk1 arrest both the embryo and endosperm development. Cloning of Smk1 indicates that it encodes an E-subclass pentatricopeptide repeat (PPR) protein that is targeted to mitochondria. Loss of SMK1 function abolishes the C → U editing at the nad7-836 site, leading to the retention of a proline codon that is edited to encode leucine in the wild type. The smk1 mutant showed dramatically reduced complex-I assembly and NADH dehydrogenase activity, and abnormal biogenesis of the mitochondria. Analysis of the ortholog in Oryza sativa (rice) reveals that rice SMK1 has a conserved function in C → U editing of the mitochondrial nad7-836 site. T-DNA knock-out mutants showed abnormal embryo and endosperm development, resulting in embryo or seedling lethality. The leucine at NAD7-279 is highly conserved from bacteria to flowering plants, and analysis of genome sequences from many plants revealed a molecular coevolution between the requirement for C → U editing at this site and the existence of an SMK1 homolog. These results demonstrate that Smk1 encodes a PPR-E protein that is required for nad7-836 editing, and this editing is critical to NAD7 function in complex-I assembly in mitochondria, and hence to embryo and endosperm development in maize and rice.

Cytochrome c1 exhibits two binding sites for cytochrome c in plants

In plants, channeling of cytochrome c molecules between complexes III and IV has been purported to shuttle electrons within the supercomplexes instead of carrying electrons by random diffusion across the intermembrane bulk phase. However, the mode plant cytochrome c behaves inside a supercomplex such as the respirasome, formed by complexes I, III and IV, remains obscure from a structural point of view. Here, we report ab-initio Brownian dynamics calculations and nuclear magnetic resonance-driven docking computations showing two binding sites for plant cytochrome c at the head soluble domain of plant cytochrome c1, namely a non-productive (or distal) site with a long heme-to-heme distance and a functional (or proximal) site with the two heme groups close enough as to allow electron transfer. As inferred from isothermal titration calorimetry experiments, the two binding sites exhibit different equilibrium dissociation constants, for both reduced and oxidized species, that are all within the micromolar range, thus revealing the transient nature of such a respiratory complex. Although the docking of cytochrome c at the distal site occurs at the interface between cytochrome c1 and the Rieske subunit, it is fully compatible with the complex III structure. In our model, the extra distal site in complex III could indeed facilitate the functional cytochrome c channeling towards complex IV by building a "floating boat bridge" of cytochrome c molecules (between complexes III and IV) in plant respirasome.