Functional and molecular characterization of the frataxin homolog from Arabidopsis thaliana

Exploring the evolutionary landscape and structural resonances of ferritin with insights into functional significance in plant

The mineral iron plays a crucial role in facilitating the optimal functioning of numerous biological processes within the cellular environment. These processes involve the transportation of oxygen, energy production, immune system functioning, cognitive abilities, and muscle function. However, it is crucial to note that excessive levels of iron can result in oxidative damage within cells, primarily through Fenton reactions. Iron availability and toxicity present significant challenges that have been addressed through evolution. Ferritin is an essential protein that stores iron and is divided into different subfamilies, including DNA-binding proteins under starvation (Dps), bacterioferritin, and classical ferritin. Ferritin plays a critical role in maintaining cellular balance and protecting against oxidative damage. This study delves into ferritin's evolutionary dynamics across diverse taxa, emphasizing structural features and regulatory mechanisms. Insights into ferritin's evolution and functional diversity are gained through phylogenetic and structural analysis in bacterial Dps, bacterioferritin, and classical ferritin proteins. Additionally, the involvement of ferritin in plant stress responses and development is explored. Analysis of ferritin gene expression across various developmental stages and stress conditions provides insights into its regulatory roles. This comprehensive exploration enhances our understanding of ferritin's significance in plant biology, offering insights into its evolutionary history, structural diversity, and protective mechanisms against oxidative stress.

Genome-Wide Identification, Characterization and Expression Profiling of Potato (Solanum tuberosum) Frataxin (FH) Gene

Frataxin (FH) plays a crucial role in the biogenesis of mitochondria and the regulation of iron in the cells of various organisms. However, there has been very little research on FH in plants. In this study, the potato FH gene (StFH) was identified and characterized using a genome-wide approach, and its sequence was compared to those of FH genes from Arabidopsis, rice, and maize. The FH genes were found to have a lineage-specific distribution and were more conserved in monocots than in dicots. While multiple copies of FH genes have been reported in some species, including plants, only one isoform of FH was found in potato. The expression of StFH in leaves and roots was analyzed under two different abiotic stress conditions, and the results showed that StFH was upregulated more in leaves and that its expression levels increased with the severity of the stress. This is the first study to examine the expression of an FH gene under abiotic stress conditions.

Structural and Functional Characterization of CreFH1, the Frataxin Homolog from Chlamydomonas reinhardtii

Frataxin plays a key role in cellular iron homeostasis of different organisms. It has been implicated in iron storage, detoxification, delivery for Fe-S cluster assembly and heme biosynthesis. However, its specific role in iron metabolism remains unclear, especially in photosynthetic organisms. To gain insight into the role and properties of frataxin in algae, we identified the gene CreFH1, which codes for the frataxin homolog from Chlamydomonas reinhardtii. We performed the cloning, expression and biochemical characterization of CreFH1. This protein has a predicted mitochondrial transit peptide and a significant structural similarity to other members of the frataxin family. In addition, CreFH1 was able to form a dimer in vitro, and this effect was increased by the addition of Cu²⁺ and also attenuated the Fenton reaction in the presence of a mixture of Fe²⁺ and H₂O₂. Bacterial cells with overexpression of CreFH1 showed increased growth in the presence of different metals, such as Fe, Cu, Zn and Ni and H₂O₂. Thus, results indicated that CreFH1 is a functional protein that shows some distinctive features compared to its more well-known counterparts, and would play an important role in response to oxidative stress in C. reinhardtii.

Heme oxygenase-nitric oxide crosstalk-mediated iron homeostasis in plants under oxidative stress

Plant growth under abiotic stress conditions significantly enhances intracellular generation of reactive oxygen species (ROS). Oxidative status of plant cells is directly affected by the modulation of iron homeostasis. Among mammals and plants, heme oxygenase-1 (HO-1) is a well-known antioxidant enzyme. It catalyzes oxygenation of heme, thereby producing Fe²⁺, CO and biliverdin as byproducts. The antioxidant potential of HO-1 is primarily due to its catalytic reaction byproducts. Biliverdin and bilirubin possess conjugated π-electrons which escalate the ability of these biomolecules to scavenge free radicals. CO also enhances the ROS scavenging ability of plants cells by upregulating catalase and peroxidase activity. Enhanced expression of HO-1 in plants under oxidative stress accompanies sequestration of iron in specialized iron storage proteins localized in plastids and mitochondria, namely ferritin for Fe³⁺ storage and frataxin for storage of Fe-S clusters, respectively. Nitric oxide (NO) crosstalks with HO-1 at multiple levels, more so in plants under oxidative stress, in order to maintain intracellular iron status. Formation of dinitrosyl-iron complexes (DNICs) significantly prevents Fenton reaction during oxidative stress. DNICs also release NO upon dissociation in target cells over long distance in plants. They also function as antioxidants against superoxide anions and lipidic free radicals. A number of NO-modulated transcription factors also facilitate iron homeostasis in plant cells. Plants facing oxidative stress exhibit modulation of lateral root formation by HO-1 through NO and auxin-dependent pathways. The present review provides an in-depth analysis of the structure-function relationship of HO-1 in plants and mammals, correlating them with their adaptive mechanisms of survival under stress.

Iron-sulfur proteins in plant mitochondria: roles and maturation

Iron-sulfur (Fe-S) clusters are prosthetic groups ensuring electron transfer reactions, activating substrates for catalytic reactions, providing sulfur atoms for the biosynthesis of vitamins or other cofactors, or having protein-stabilizing effects. Hence, metalloproteins containing these cofactors are essential for numerous and diverse metabolic pathways and cellular processes occurring in the cytoplasm. Mitochondria are organelles where the Fe-S cluster demand is high, notably because the activity of the respiratory chain complexes I, II, and III relies on the correct assembly and functioning of Fe-S proteins. Several other proteins or complexes present in the matrix require Fe-S clusters as well, or depend either on Fe-S proteins such as ferredoxins or on cofactors such as lipoic acid or biotin whose synthesis relies on Fe-S proteins. In this review, we have listed and discussed the Fe-S-dependent enzymes or pathways in plant mitochondria including some potentially novel Fe-S proteins identified based on in silico analysis or on recent evidence obtained in non-plant organisms. We also provide information about recent developments concerning the molecular mechanisms involved in Fe-S cluster synthesis and trafficking steps of these cofactors from maturation factors to client apoproteins.

Fe-S Protein Synthesis in Green Algae Mitochondria

Iron and sulfur are two essential elements for all organisms. These elements form the Fe-S clusters that are present as cofactors in numerous proteins and protein complexes related to key processes in cells, such as respiration and photosynthesis, and participate in numerous enzymatic reactions. In photosynthetic organisms, the ISC and SUF Fe-S cluster synthesis pathways are located in organelles, mitochondria, and chloroplasts, respectively. There is also a third biosynthetic machinery in the cytosol (CIA) that is dependent on the mitochondria for its function. The genes and proteins that participate in these assembly pathways have been described mainly in bacteria, yeasts, humans, and recently in higher plants. However, little is known about the proteins that participate in these processes in algae. This review work is mainly focused on releasing the information on the existence of genes and proteins of green algae (chlorophytes) that could participate in the assembly process of Fe-S groups, especially in the mitochondrial ISC and CIA pathways.

Iron-Sulfur Cluster Complex Assembly in the Mitochondria of Arabidopsis thaliana

In plants, the cysteine desulfurase (AtNFS1) and frataxin (AtFH) are involved in the formation of Fe-S groups in mitochondria, specifically, in Fe and sulfur loading onto scaffold proteins, and the subsequent formation of the mature Fe-S cluster. We found that the small mitochondrial chaperone, AtISD11, and AtFH are positive regulators for AtNFS1 activity in Arabidopsis. Moreover, when the three proteins were incubated together, a stronger attenuation of the Fenton reaction was observed compared to that observed with AtFH alone. Using pull-down assays, we found that these three proteins physically interact, and sequence alignment and docking studies showed that several amino acid residues reported as critical for the interaction of their human homologous are conserved. Our results suggest that AtFH, AtNFS1 and AtISD11 form a multiprotein complex that could be involved in different stages of the iron-sulfur cluster (ISC) pathway in plant mitochondria.

Essential and Detrimental - an Update on Intracellular Iron Trafficking and Homeostasis

Chloroplasts, mitochondria and vacuoles represent characteristic organelles of the plant cell, with a predominant function in cellular metabolism. Chloroplasts are the site of photosynthesis and therefore basic and essential for photoautotrophic growth of plants. Mitochondria produce energy during respiration and vacuoles act as internal waste and storage compartments. Moreover, chloroplasts and mitochondria are sites for the biosynthesis of various compounds of primary and secondary metabolism. For photosynthesis and energy generation, the internal membranes of chloroplasts and mitochondria are equipped with electron transport chains. To perform proper electron transfer and several biosynthetic functions, both organelles contain transition metals and here iron is by far the most abundant. Although iron is thus essential for plant growth and development, it becomes toxic when present in excess and/or in its free, ionic form. The harmful effect of the latter is caused by the generation of oxidative stress. As a consequence, iron transport and homeostasis have to be tightly controlled during plant growth and development. In addition to the corresponding transport and homeostasis proteins, the vacuole plays an important role as an intracellular iron storage and release compartment at certain developmental stages. In this review, we will summarize current knowledge on iron transport and homeostasis in chloroplasts, mitochondria and vacuoles. In addition, we aim to integrate the physiological impact of intracellular iron homeostasis on cellular and developmental processes.

Copper redox chemistry of plant frataxins

The presence of a conserved cysteine residue in the C-terminal amino acid sequences of plant frataxins differentiates these frataxins from those of other kingdoms and may be key in frataxin assembly and function. We report a full study on the ability of Arabidopsis (AtFH) and Zea mays (ZmFH-1 and ZmFH-2) frataxins to assemble into disulfide-bridged dimers by copper-driven oxidation and to revert to monomers by chemical reduction. We monitored the redox assembly-disassembly process by electrospray ionization mass spectrometry, electrophoresis, UV-Vis spectroscopy, and fluorescence measurements. We conclude that plant frataxins AtFH, ZmFH-1 and ZmFH-2 are oxidized by Cu2+ and exhibit redox cysteine monomer - cystine dimer interexchange. Interestingly, the tendency to interconvert is not the same for each protein. Through yeast phenotypic rescue experiments, we show that plant frataxins are important for plant survival under conditions of excess copper, indicating that these proteins might be involved in copper metabolism.

Plant Frataxin in Metal Metabolism

Frataxin is a highly conserved protein from prokaryotes to eukaryotes. Several functions related to iron metabolism have been postulated for this protein, including Fe-S cluster and heme synthesis, response to oxidative damage and oxidative phosphorylation. In plants, the presence of one or two isoforms of this protein with dual localization in mitochondria and chloroplasts has been reported. Frataxin deficiency affects iron metabolism in both organelles, leading to an impairment of mitochondrial respiration, and chlorophyll and photosynthetic electron transport deficiency in chloroplasts. In addition, plant frataxins can react with Cu²⁺ ions and dimerize, which causes the reduction of free Cu ions. This could provide an additional defense mechanism against the oxidation of Fe-S groups by Cu ions. While there is a consensus on the involvement of frataxin in iron homeostasis in most organisms, the interaction of plant frataxins with Cu ions, the presence of different isoforms, and/or the localization in two plant organelles suggest that this protein might have additional functions in vegetal tissues.

Identification of two frataxin isoforms in Zea mays: Structural and functional studies

Frataxin is a ubiquitous protein that plays a role in Fe-S cluster biosynthesis and iron and heme metabolism, although its molecular functions are not entirely clear. In non-photosynthetic eukaryotes, frataxin is encoded by a single gene, and the protein localizes to mitochondria. Here we report the presence of two functional frataxin isoforms in Zea mays, ZmFH-1 and ZmFH-2. We confirmed our previous findings regarding plant frataxins: both proteins have dual localization in mitochondria and chloroplasts. Physiological, biochemical and biophysical studies show some differences in the expression pattern, protection against oxidants and in the aggregation state of both isoforms, suggesting that the two frataxin homologs would play similar but not identical roles in plant cell metabolism. In addition, two specific features of plant frataxins were evidenced: their ability to form dimers and their tendency to undergo conformational change under oxygen exposure.

Altered levels of AtHSCB disrupts iron translocation from roots to shoots

Plants overexpressing AtHSCB and hscb knockdown mutants showed altered iron homeostasis. The overexpression of AtHSCB led to activation of the iron uptake system and iron accumulation in roots without concomitant transport to shoots, resulting in reduced iron content in the aerial parts of plants. By contrast, hscb knockdown mutants presented the opposite phenotype, with iron accumulation in shoots despite the reduced levels of iron uptake in roots. AtHSCB play a key role in iron metabolism, probably taking part in the control of iron translocation from roots to shoots. Many aspects of plant iron metabolism remain obscure. The most known and studied homeostatic mechanism is the control of iron uptake in the roots by shoots. Nevertheless, this mechanism likely involves various unknown sensors and unidentified signals sent from one tissue to another which need to be identified. Here, we characterized Arabidopsis thaliana plants overexpressing AtHSCB, encoding a mitochondrial cochaperone involved in [Fe-S] cluster biosynthesis, and hscb knockdown mutants, which exhibit altered shoot/root Fe partitioning. Overexpression of AtHSCB induced an increase in root iron uptake and content along with iron deficiency in shoots. Conversely, hscb knockdown mutants exhibited increased iron accumulation in shoots and reduced iron uptake in roots. Different experiments, including foliar iron application, citrate supplementation and iron deficiency treatment, indicate that the shoot-directed control of iron uptake in roots functions properly in these lines, implying that [Fe-S] clusters are not involved in this regulatory mechanism. The most likely explanation is that both lines have altered Fe transport from roots to shoots. This could be consistent with a defect in a homeostatic mechanism operating at the root-to-shoot translocation level, which would be independent of the shoot control over root iron deficiency responses. In summary, the phenotypes of these plants indicate that AtHSCB plays a role in iron metabolism.

Time-resolved functional analysis of acute impairment of frataxin expression in an inducible cell model of Friedreich ataxia

Friedreich ataxia is a neurodegenerative disease caused by a GAA triplet repeat expansion in the first intron of the frataxin gene, which results in reduced expression levels of the corresponding protein. Despite numerous animal and cellular models, therapeutic options that mechanistically address impaired frataxin expression are lacking. Here, we have developed a new mammalian cell model employing the Cre/loxP recombination system to induce a homozygous or heterozygous frataxin knockout in mouse embryonic fibroblasts. Induction of Cre-mediated disruption by tamoxifen was successfully tested on RNA and protein levels. After loss of frataxin protein, cell division, aconitase activity and oxygen consumption rates were found to be decreased, while ROS production was increased in the homozygous state. By contrast, in the heterozygous state no such changes were observed. A time-resolved analysis revealed the loss of aconitase activity as an initial event after induction of complete frataxin deficiency, followed by secondarily elevated ROS production and a late increase in iron content. Initial impairments of oxygen consumption and ATP production were found to be compensated in the late state and seemed to play a minor role in Friedreich ataxia pathophysiology. In conclusion and as predicted from its proposed role in iron sulfur cluster (ISC) biosynthesis, disruption of frataxin primarily causes impaired function of ISC-containing enzymes, whereas other consequences, including elevated ROS production and iron accumulation, appear secondary. These parameters and the robustness of the newly established system may additionally be used for a time-resolved study of pharmacological candidates in a HTS manner.

Summary: The use of a new mammalian cell model with inducible homozygous and heterozygous frataxin knockout allows new insights into the chronology and causes of the disease Friedreich ataxia.

Frataxin Is Localized to Both the Chloroplast and Mitochondrion and Is Involved in Chloroplast Fe-S Protein Function in Arabidopsis

Frataxin plays a key role in eukaryotic cellular iron metabolism, particularly in mitochondrial heme and iron-sulfur (Fe-S) cluster biosynthesis. However, its precise role has yet to be elucidated. In this work, we studied the subcellular localization of Arabidopsis frataxin, AtFH, using confocal microscopy, and found a novel dual localization for this protein. We demonstrate that plant frataxin is targeted to both the mitochondria and the chloroplast, where it may play a role in Fe-S cluster metabolism as suggested by functional studies on nitrite reductase (NIR) and ferredoxin (Fd), two Fe-S containing chloroplast proteins, in AtFH deficient plants. Our results indicate that frataxin deficiency alters the normal functioning of chloroplasts by affecting the levels of Fe, chlorophyll, and the photosynthetic electron transport chain in this organelle.

Analysis of Arabidopsis thaliana atfer4-1, atfh and atfer4-1/atfh mutants uncovers frataxin and ferritin contributions to leaf ionome homeostasis

Ferritins are iron-storage proteins involved in the environmental and developmental control of the free iron pool within cells. Plant ferritins are targeted to mitochondria as well as to chloroplasts. AtFer4 is the Arabidopsis thaliana ferritin isoform that can be also targeted to mitochondria. Frataxin is a mitochondrial protein whose role is essential for plants; lack of AtFH frataxin causes early embryo-lethality in Arabidopsis. Because of that, the Arabidopsis atfh KO mutant is propagated in heterozygosis. For exploring the functional interaction between frataxin and ferritin, Arabidopsis double mutant atfer4-1/atfh was isolated and its physiological parameters were measured, as well as its ionome profile, together with those of both atfer4 and atfh single mutants, in different conditions of Fe supply. Impairment of both ferritin and frataxin did not lead to any effect on mitochondrial respiration. However, ionomics revealed that the content of macro- and microelements, occurring when the nutritional Fe supply changes, were altered in the mutants analysed. These results suggest that both ferritin and frataxin can contribute to the composition of the leaf ionome and also confirm ionomics as an excellent tool for detecting alterations in the plant's physiology.

Interaction of frataxin, an iron binding protein, with IscU of Fe-S clusters biogenesis pathway and its upregulation in AmpB resistant Leishmania donovani

Leishmania donovani is a unicellular protozoon parasite that causes visceral leishmaniasis (VL), which is a fatal disease if left untreated. Certain Fe-S proteins of the TCA cycle and respiratory chain have been found in the Leishmania parasite but the precise mechanisms for their biogenesis and the maturation of Fe-S clusters remains unknown. Fe-S clusters are ubiquitous cofactors of proteins that perform critical cellular functions. The clusters are biosynthesized by the mitochondrial Iron-Sulphur Cluster (ISC) machinery with core protein components that include the catalytic cysteine desulphurase IscS, the scaffold proteins IscU and IscA, and frataxin as an iron carrier/donor. However, no information regarding frataxin, its regulation, or its role in drug resistance is available for the Leishmania parasite. In this study, we characterized Ld-frataxin to investigate its role in the ISC machinery of L. donovani. We expressed and purified the recombinant Ld-frataxin protein and observed its interaction with Ld-IscU by co-purification and pull-down assay. Furthermore, we observed that the cysteine desulphurase activity of the purified Ld-IscS protein was stimulated in the presence of Ld-frataxin and Ld-IscU, particularly in the presence of iron; neither Ld-frataxin nor Ld-IscU alone had significant effects on Ld-IscS activity. Interestingly, RT-PCR and western blotting showed that Ld-frataxin is upregulated in AmpB-resistant isolates compared to sensitive strains, which may support higher Fe-S protein activity in AmpB-resistant L. donovani. Additionally, Ld-frataxin was localized in the mitochondria, as revealed by digitonin fractionation and indirect immunofluorescence. Thus, our results suggest the role of Ld-frataxin as an iron binding/carrier protein for Fe-S cluster biogenesis that physically interacts with other core components of the ISC machinery within the mitochondria.

The mitochondrial proteins AtHscB and AtIsu1 involved in Fe-S cluster assembly interact with the Hsp70-type chaperon AtHscA2 and modulate its catalytic activity

Arabidopsis plants contain two genes coding for mitochondrial Hsp70-type chaperon-like proteins, AtHscA1 (At4g37910) and AtHscA2 (At5g09590). Both genes are homologs of the Ssq1 gene involved in Fe-S cluster assembly in yeast. Protein-protein interaction studies showed that AtHscA2 interacts with AtIsu1 and AtHscB, two Arabidopsis homologs of the Isu1 protein and the Jac1 yeast co-chaperone. Moreover, this interaction could modulate the activity of AtHscA2. In the presence of a 1:5:5 molar ratio of AtHscA2:AtIsu1:AtHscB we observed an increase in the V(max) and a decrease in the S(0.5) for ATP of AtHscA2. Furthermore, an increase of about 28-fold in the catalytic efficiency of AtHscA2 was also observed. Results suggest that AtHscA2 in cooperation with AtIsu1 and AtHscB play an important role in the regulation of the Fe-S assembly pathway in plant mitochondria.

Mitochondrial iron transport and homeostasis in plants

Iron (Fe) is an essential nutrient for plants and although the mechanisms controlling iron uptake from the soil are relatively well understood, comparatively little is known about subcellular trafficking of iron in plant cells. Mitochondria represent a significant iron sink within cells, as iron is required for the proper functioning of respiratory chain protein complexes. Mitochondria are a site of Fe-S cluster synthesis, and possibly heme synthesis as well. Here we review recent insights into the molecular mechanisms controlling mitochondrial iron transport and homeostasis. We focus on the recent identification of a mitochondrial iron uptake transporter in rice and a possible role for metalloreductases in iron uptake by mitochondria. In addition, we highlight recent advances in mitochondrial iron homeostasis with an emphasis on the roles of frataxin and ferritin in iron trafficking and storage within mitochondria.

The iron-sulfur cluster assembly machineries in plants: current knowledge and open questions

Many metabolic pathways and cellular processes occurring in most sub-cellular compartments depend on the functioning of iron-sulfur (Fe-S) proteins, whose cofactors are assembled through dedicated protein machineries. Recent advances have been made in the knowledge of the functions of individual components through a combination of genetic, biochemical and structural approaches, primarily in prokaryotes and non-plant eukaryotes. Whereas most of the components of these machineries are conserved between kingdoms, their complexity is likely increased in plants owing to the presence of additional assembly proteins and to the existence of expanded families for several assembly proteins. This review focuses on the new actors discovered in the past few years, such as glutaredoxin, BOLA and NEET proteins as well as MIP18, MMS19, TAH18, DRE2 for the cytosolic machinery, which are integrated into a model for the plant Fe-S cluster biogenesis systems. It also discusses a few issues currently subjected to an intense debate such as the role of the mitochondrial frataxin and of glutaredoxins, the functional separation between scaffold, carrier and iron-delivery proteins and the crosstalk existing between different organelles.

Structural and functional studies of the mitochondrial cysteine desulfurase from Arabidopsis thaliana

AtNfs1 is the Arabidopsis thaliana mitochondrial homolog of the bacterial cysteine desulfurases NifS and IscS, having an essential role in cellular Fe-S cluster assembly. Homology modeling of AtNfs1m predicts a high global similarity with E. coli IscS showing a full conservation of residues involved in the catalytic site, whereas the chloroplastic AtNfs2 is more similar to the Synechocystis sp. SufS. Pull-down assays showed that the recombinant mature form, AtNfs1m, specifically binds to Arabidopsis frataxin (AtFH). A hysteretic behavior, with a lag phase of several minutes, was observed and hysteretic parameters were affected by pre-incubation with AtFH. Moreover, AtFH modulates AtNfs1m kinetics, increasing V(max) and decreasing the S(0.5) value for cysteine. Results suggest that AtFH plays an important role in the early steps of Fe-S cluster formation by regulating AtNfs1 activity in plant mitochondria.

Genome-wide identification, evolutionary and expression analyses of putative Fe-S biogenesis genes in rice (Oryza sativa)

Iron-sulfur (Fe-S) proteins are ubiquitous in nature and carry Fe-S clusters (ISCs) as prosthetic groups that are essential in maintaining basic biological processes such as photosynthesis, respiration, nitrogen fixation, and DNA repair. In the present investigation, a comprehensive genome-wide analysis was carried out to find all the genes involved in the formation of ISCs in rice ( Oryza sativa L.) through a systematic EST and genomic DNA sequence data mining. This analysis profiled 44 rice ISC genes (OsISCs) that were identified using in silico analysis. Multiple sequence alignment and phylogenetic analysis revealed that these genes were highly conserved among bacteria, fungi, animals, and plants. EST analysis and RT-PCR assays demonstrated that all OsISCs were active and that the transcript abundance of some OsISCs was tissue specific. The results of this study will assist further investigations to identify and elucidate the structural components involved in the assembly, biogenesis, and regulation of OsISCs. Thus, the outcome of the present study provides basic genomic information for the OsISC and will pave the way for elucidating the precise role of OsISCs in plant growth and development in the future. Also, it may enable us in the future to enhance the crop yield, uptake of Fe, and protection against abiotic and biotic stress.

Research on plants for the understanding of diseases of nuclear and mitochondrial origin

Different model organisms, such as Escherichia coli, Saccharomyces cerevisiae, Caenorhabditis elegans, Drosophila melanogaster, mouse, cultured human cell lines, among others, were used to study the mechanisms of several human diseases. Since human genes and proteins have been structurally and functionally conserved in plant organisms, the use of plants, especially Arabidopsis thaliana, as a model system to relate molecular defects to clinical disorders has recently increased. Here, we briefly review our current knowledge of human diseases of nuclear and mitochondrial origin and summarize the experimental findings of plant homologs implicated in each process.

Exploring frataxin function

Frataxin is a nuclear-encoded mitochondrial protein highly conserved in prokaryotes and eukaryotes. Its deficiency was initially described as the phenotype of Friedreich's ataxia, an autosomal recessive disease in humans. Although several functions have been described for frataxin, that is, involvement in Fe-S cluster and heme synthesis, energy conversion and oxidative phosphorylation, iron handling and response to oxidative damage, its precise function remains unclear. Although there is a general consensus on the participation of frataxin in the maintenance of cellular iron homeostasis and in iron metabolism, this protein may have other specific functions in different tissues and organisms.

Nitric oxide, nitrosyl iron complexes, ferritin and frataxin: a well equipped team to preserve plant iron homeostasis

Iron is a key element in plant nutrition. Iron deficiency as well as iron overload results in serious metabolic disorders that affect photosynthesis, respiration and general plant fitness with direct consequences on crop production. More than 25% of the cultivable land possesses low iron availability due to high pH (calcareous soils). Plant biologists are challenged by this concern and aimed to find new avenues to ameliorate plant responses and keep iron homeostasis under control even at wide range of iron availability in various soils. For this purpose, detailed knowledge of iron uptake, transport, storage and interactions with cellular compounds will help to construct a more complete picture of its role as essential nutrient. In this review, we summarize and describe the recent findings involving four central players involved in keeping cellular iron homeostasis in plants: nitric oxide, ferritin, frataxin and nitrosyl iron complexes. We attempt to highlight the interactions among these actors in different scenarios occurring under iron deficiency or iron overload, and discuss their counteracting and/or coordinating actions leading to the control of iron homeostasis.

Intron hairpin and transitive RNAi mediated silencing of orfH522 transcripts restores male fertility in transgenic male sterile tobacco plants expressing orfH522

The present work was aimed at developing vector construct(s) suitable for restoring fertility in transgenic male sterile tobacco plants expressing male-sterility-inducing ORFH522 in tapetal cell layer (Nizampatnam et al. Planta 229:987-1001, 2009). PTGS vectors that could produce either intron spliced hairpin RNA against the orfH522 or induce silencing of orfH522 by heterologous 3'UTR region were developed using the selected 316 bp (orf316) fragment of orfH522. The constructs were independently mobilized into Agrobacterium and used for transforming tobacco. The T(1) generation plants carrying the restorer gene cassettes in homozygous condition were identified and crossed with the male sterile transgenic tobacco plants to obtain the hybrid seeds. PCR analysis of hybrid plants indicated segregation for the sterility inducing cassette while all the plants carried the restorer cassette. Hybrid plants produced fertile pollen grains and formed normal capsules upon selfing. Further molecular analyses of these hybrid plants with RT-PCR, Northern blotting and siRNA detection, revealed that intron interrupted hairpin RNA (ihp-RNA) mediated gene silencing was more effective compared to silencing by heterologous 3'UTR (SHUTR) as indicated by the complete degradation of orfH522 transcripts and formation of higher levels of orf316 specific siRNA molecules in plants carrying ihp-RNA restorer construct. Segregation analyses of F(2) (selfed hybrid) plants confirmed the co-segregation of gene cassettes and the traits in Mendelian di-hybrid ratio (9:3:3:1). Taken together, the results established that intron hairpin and transitive RNAi mediated silencing of orfH522 transcripts restored fertility in transgenic male sterile tobacco plants expressing orfH522 and ihp-RNA was more efficient in silencing orfH522 transcripts.

Iron-sulfur clusters: biogenesis, molecular mechanisms, and their functional significance

Iron-sulfur clusters [Fe-S] are small, ubiquitous inorganic cofactors representing one of the earliest catalysts during biomolecule evolution and are involved in fundamental biological reactions, including regulation of enzyme activity, mitochondrial respiration, ribosome biogenesis, cofactor biogenesis, gene expression regulation, and nucleotide metabolism. Although simple in structure, [Fe-S] biogenesis requires complex protein machineries and pathways for assembly. [Fe-S] are assembled from cysteine-derived sulfur and iron onto scaffold proteins followed by transfer to recipient apoproteins. Several predominant iron-sulfur biogenesis systems have been identified, including nitrogen fixation (NIF), sulfur utilization factor (SUF), iron-sulfur cluster (ISC), and cytosolic iron-sulfur protein assembly (CIA), and many protein components have been identified and characterized. In eukaryotes ISC is mainly localized to mitochondria, cytosolic iron-sulfur protein assembly to the cytosol, whereas plant sulfur utilization factor is localized mainly to plastids. Because of this spatial separation, evidence suggests cross-talk mediated by organelle export machineries and dual targeting mechanisms. Although research efforts in understanding iron-sulfur biogenesis has been centered on bacteria, yeast, and plants, recent efforts have implicated inappropriate [Fe-S] biogenesis to underlie many human diseases. In this review we detail our current understanding of [Fe-S] biogenesis across species boundaries highlighting evolutionary conservation and divergence and assembling our knowledge into a cellular context.

Chloroplastic and mitochondrial metal homeostasis

Transition metal deficiency has a strong impact on the growth and survival of an organism. Indeed, transition metals, such as iron, copper, manganese and zinc, constitute essential cofactors for many key cellular functions. Both photosynthesis and respiration rely on metal cofactor-mediated electron transport chains. Chloroplasts and mitochondria are, therefore, organelles with high metal ion demand and represent essential components of the metal homeostasis network in photosynthetic cells. In this review, we describe the metal requirements of chloroplasts and mitochondria, the acclimation of their functions to metal deficiency and recent advances in our understanding of their contributions to cellular metal homeostasis, the control of the cellular redox status and the synthesis of metal cofactors.

Nitric oxide and frataxin: two players contributing to maintain cellular iron homeostasis

BACKGROUND

Nitric oxide (NO) is a signalling and physiologically active molecule in animals, plants and bacteria. The specificity of the molecular mechanism(s) involved in transducing the NO signal within and between cells and tissues is still poorly understood. NO has been shown to be an emerging and potent signal molecule in plant growth, development and stress physiology. The NO donor S-nitrosoglutathion (GSNO) was shown to be a biologically active compound in plants and a candidate for NO storage and/or mobilization between plant tissues and cells. NO has been implicated as a central component in maintaining iron bioavailavility in plants.

SCOPE AND CONCLUSIONS

Iron is an essential nutrient for almost all organisms. This review presents an overview of the functions of NO in iron metabolism in animals and discusses how NO production constitutes a key response in plant iron sensing and availability. In plants, NO drives downstream responses to both iron deficiency and iron overload. NO-mediated improvement of iron nutrition in plants growing under iron-deficient conditions represents a powerful tool to cope with soils displaying low iron availability. An interconversion between different redox forms based on the iron and NO status of the plant cells might be the core of a metabolic process driving plant iron homeostasis. Frataxin, a recently identified protein in plants, plays an important role in mitochondria biogenesis and in maintaining mitochondrial iron homeostasis. Evidence regarding the interaction between frataxin, NO and iron from analysis of frataxin knock-down Arabidopsis thaliana mutants is reviewed and discussed.

Expression of sunflower cytoplasmic male sterility-associated open reading frame, orfH522 induces male sterility in transgenic tobacco plants

Sterility in the universally exploited PET1-CMS system of sunflower is associated with the expression of orfH522, a novel mitochondrial gene. Definitive evidence that ORFH522 is directly responsible for male sterility is lacking. To test the hypothesis that ORFH522 is sufficient to induce male sterility, a set of chimeric constructs were developed. The cDNA of orfH522 was cloned in-frame with yeast coxIV pre-sequence, and was expressed under tapetum-specific promoter TA29 (construct designated as TCON). For developing control vectors, orfH522 was cloned without the transit peptide under TA29 promoter (TON) or orfH522 was cloned with or without transit peptide under the constitutive CaMV35S promoter (SCOP and SOP). Among several independent transformants obtained with each of the gene cassettes, one third of the transgenics (6/17) with TCON were completely male sterile while more than 10 independent transformants obtained with each of the control vectors were fertile. The male sterile plants were morphologically similar to fertile plants, but had anthers that remained below the stigmatic surface at anthesis. RT-PCR analysis of the sterile plants confirmed the anther-specific expression of orfH522 and bright-field microscopy demonstrated ablation of the tapetal cell layer. Premature DNA fragmentation and programmed cell death was observed at meiosis stage in the anthers of sterile plants. Stable transmission of induced male sterility trait was confirmed in test cross progeny. This constitutes the first report at demonstrating the induction of male sterility by introducing orfH522 gene that could be useful for genetic engineering of male sterility.

Nitric oxide accumulation is required to protect against iron-mediated oxidative stress in frataxin-deficient Arabidopsis plants

Frataxin is a mitochondrial protein that is conserved throughout evolution. In yeast and mammals, frataxin is essential for cellular iron (Fe) homeostasis and survival during oxidative stress. In plants, frataxin deficiency causes increased reactive oxygen species (ROS) production and high sensitivity to oxidative stress. In this work we show that a knock-down T-DNA frataxin-deficient mutant of Arabidopsis thaliana (atfh-1) contains increased total and organellar Fe levels. Frataxin deficiency leads also to nitric oxide (NO) accumulation in both, atfh-1 roots and frataxin null mutant yeast. Abnormally high NO production might be part of the defence mechanism against Fe-mediated oxidative stress.

Friedreich ataxia: an update on animal models, frataxin function and therapies

Friedreich ataxia (FRDA) is an autosomal recessive progressively debilitating degenerative disease that principally affects the nervous system and the heart. Although FRDA is considered a rare disease, is the most common inherited ataxia. It is caused by loss-of-function mutations in the FXN gene, mainly an expanded GAA triplet repeat in the intron 1. The genetic defect results in the reduction of frataxin levels, a protein targeted to the mitochondria. Frataxin deficiency leads to mitochondrial dysfunction, oxidative damage and iron accumulation. Studies of the yeast and animal models of the disease have led to propose several different roles for frataxin. Animal models have also been important for dissecting the steps of pathogenesis in FRDA and they are essential for the development of effective therapies. Currently, antioxidant and iron chelation therapies are under evaluation in clinical trials. Gene reactivation, gene therapy and protein replacement strategies for FRDA are promising approaches. This review focuses on the current models developed for FRDA, the different roles proposed for frataxin and the progress of potential treatment strategies for the disease.

Iron-sulfur cluster biogenesis systems and their crosstalk

The biogenesis of iron-sulfur clusters ([Fe-S]) plays a very important role in many essential functions of life. Several [Fe-S] biogenesis systems have been discovered, such as the NIF (nitrogen fixation), SUF (mobilisation of sulfur) and ISC (iron-sulfur cluster) systems in bacteria, and the ISC-like and CIA (cytosolic iron-sulfur protein assembly) systems in yeast. Experimental evidence has revealed that SUF and ISC in bacteria communicate with each other partly through IscR to coordinate the utilisation of iron and cysteine. The ISC-like system in yeast is localised to the mitochondria, while the ISC-dependent CIA system is localised to the cytosol; this suggests a possible role for the ISC mitochondrial export machinery in mediating crosstalk between the two systems. Based on genetic analysis, the model plant Arabidopsis thaliana contains three [Fe-S] biogenesis systems similar to SUF, ISC and CIA named AtSUF, AtISC and AtCIA. Possible communication between these three systems has been proposed.

Ectopic expression of phosphoenolpyruvate carboxylase in Vicia narbonensis seeds: effects of improved nutrient status on seed maturation and transcriptional regulatory networks

Seed maturation responds to endogenous and exogenous signals like nutrient status, energy and hormones. We recently showed that phosphoenolpyruvate carboxylase (PEPC) overexpression in Vicia narbonensis seeds alters seed metabolism and channels carbon into organic acids, resulting in greater seed storage capacity and increased protein content. Thus, these lines represent models with altered sink strength and improved nutrient status. Here we analyse seed developmental and metabolic parameters, and C/N partitioning in these seeds. Transgenic embryos take up more carbon and nitrogen. Changes in dry to FW ratio, seed fill duration and major seed components indicate altered seed development. Array-based gene expression analysis of embryos reveals upregulation of seed metabolism, especially during the transition phase and at late maturation, in terms of protein storage and processing, amino acid metabolism, primary metabolism and transport, energy and mitochondrial activity, transcriptional and translational activity, stress tolerance, photosynthesis, cell proliferation and elongation, signalling and hormone action and regulated protein degradation. Stimulated cell elongation is in accordance with upregulated signalling pathways related to gibberellic acid/brassinosteroids. We discuss that activated organic and amino acid production leads to a wide-range activation of nitrogen metabolism, including the machinery of storage protein synthesis, amino acid synthesis, protein processing and deposition, translational activity and the methylation cycle. We suggest that alpha-ketoglutarate (alpha-KG) and/or oxalacetate provide signals for coordinate upregulation of amino acid biosynthesis. Activation of stress tolerance genes indicates partial overlap between nutrient, stress and abscisic acid (ABA) signals, indicating a common interacting or regulatory mechanism between nutrients, stress and ABA. In conclusion, analysis of PEPC overexpressing seeds identified pathways responsive to metabolic and nutrient control on the transcriptional level and its underlying signalling mechanisms.

Knockout of frataxin gene causes embryo lethality in Arabidopsis

Frataxin is present in mitochondria of all eukaryotes as well as in the cytoplasm of bacteria. In humans, reduced expression of frataxin is associated with Friedreich's ataxia, a recessive inherited neurodegenerative and cardiac disorder leading to reduced life expectancy. Experimental evidences suggest that frataxin acts as an iron-chaperone protein, donating iron to the proteins involved in [Fe-S] cluster assembly and heme synthesis. It also possibly contributes to the process of iron detoxification and storage. The frataxin homolog from Arabidopsis thaliana (AtFH) is a single nuclear-encoded gene targeted to mitochondria and sharing 65% similarity with animal frataxin. In the present work, we show that the knocking out of AtFH gene causes arrest of Arabidopsis embryo development at the globular stage. Consistently with that, we also show by in situ hybridization that AtFH is expressed, in wt Arabidopsis plants, in ovule primordia as well as in embryos at various stages of development, suggesting a key role of plant frataxin during embryogenesis.

Expression and one-step purification of recombinant Arabidopsis thaliana frataxin homolog (AtFH)

Frataxin, a nuclear-encoded mitochondrial protein, has been proposed to participate in Fe-S cluster assembly, mitochondrial energy metabolism, respiration, and iron homeostasis. However, its precise function remains elusive. Frataxin is highly conserved in living organisms with no major structural changes, in particular at the C-terminal protein domain, suggesting that it plays a key function in all organisms. Recently, a plant gene, AtFH, with significant homology to other members of the frataxin family has been described. To gain insight on the frataxin role in plants, the frataxin domain was expressed in Escherichia coli BL21-codonPlus (DE3)-RIL cells and purified using a Ni-chelating column. The purified protein, added to a mixture containing Fe(II) and H2O2, attenuates the Fenton reaction indicating that the recombinant plant frataxin is functional. The procedure described here produced high yield of 99% pure protein through only one chromatographic step, suitable for further structure-function studies.

Deficiency of Arabidopsis thaliana frataxin alters activity of mitochondrial Fe-S proteins and induces oxidative stress

Frataxin, a protein crucial for the biogenesis of mitochondria in different organisms, was recently identified in Arabidopsis thaliana. To investigate the role of frataxin in higher plants, we analyze two knock-out and one knock-down T-DNA insertion mutants. The knock-out mutants present an embryo-lethal phenotype, indicating an essential role for frataxin. The knock-down mutant has reduced frataxin mRNA and protein levels. This mutant also presents retarded growth, reduced fresh weight of fruits and reduced number of seeds per fruit. Surprisingly, transcription of aconitase and the Fe-S subunit of succinate dehydrogenase (SDH2-1) are increased in mutant plants; however, the activity of these proteins is reduced, indicating a role for frataxin in Fe-S cluster assembly or insertion of Fe-S clusters into proteins. Mutant plants also have increased CO(2) assimilation rates, exhibit increased formation of reactive oxygen species (ROS) and have increased levels of transcripts for proteins known to be involved in the ROS stress responses. These results indicate that frataxin is an essential protein in plants, required for full activity of mitochondrial Fe-S proteins and playing a protective role against oxidative damage.

Nuclear-encoded mitochondrial complex I gene expression is restored to normal levels by inhibition of unedited ATP9 transgene expression in Arabidopsis thaliana

Mitochondria play an important role during sporogenesis in plants. The steady state levels of the nuclear-encoded mitochondrial complex I (nCI), PSST, TYKY and NADHBP transcripts increase in flowers of male-sterile plants with impairment of mitochondrial function generated by the expression of the unedited version of ATP9 (u-ATP9). This suggests a nuclear control of nCI genes in response to the mitochondrial flaw. To evaluate this hypothesis, transgenic plants carrying the GUS reporter gene, under the control of the PSST, TYKY and NADHBP promoters, were constructed. We present evidence that suppression by antisense strategy of the expression of u-ATP9 restores the normal levels of three nCI transcripts, indicating that the increase in PSST, TYKY and NADHBP in plants with a mitochondrial flaw occurs at the transcriptional level. The data presented here support the hypothesis that a mitochondrial dysfunction triggers a retrograde signaling which induce some nuclear-encoded mitochondrial genes. Moreover, these results demonstrate that this is a valuable experimental model for studying nucleus-mitochondria cross-talk events.

Reduction of Caenorhabditis elegans frataxin increases sensitivity to oxidative stress, reduces lifespan, and causes lethality in a mitochondrial complex II mutant

Friedreich ataxia is an autosomal recessive neurological disorder caused by deficiency of the mitochondrial protein frataxin. Studies in patient cells, mouse knockout animals, and Saccharomyces cerevisiae models have suggested several hypotheses on the frataxin function, but the full physiology of frataxin in mitochondria has not been well established yet. We have characterized the genomic structure of frh-1, the Caenorhabditis elegans frataxin gene, and we have developed a transient knockdown model of C. elegans frataxin deficiency by RNA interference. frh-1(RNAi) worms show a consistent pleiotropic phenotype that includes slow growth, lethargic behavior, egg laying defects, reduced brood size, abnormal pharyngeal pumping, and altered defecation. Lifespan is significantly reduced, and worms have increased sensitivity to oxidative stress that, in turn, might explain the reduction of longevity of the worms. We also demonstrate synthetic genetic interaction between frh-1 and mev-1, the gene encoding the succinate dehydrogenase cytochrome b subunit of complex II in mitochondria, suggesting a possible role of the C. elegans frataxin in the electron transport chain; thus, the respiratory chain might be involved in the pathogenesis of the disease. We propose that this C. elegans model may be a useful biological tool for drug screening in Friedreich ataxia.

Iron-sulfur cluster biosynthesis in photosynthetic organisms

Iron-sulfur (Fe/S) cluster containing proteins are widely distributed in nature and are involved in numerous processes including electron transfer, metabolic reactions, sensing, signaling, and regulation of gene expression. The knowledge about the biogenesis of Fe/S clusters, and the assembly and maturation of Fe/S cluster containing proteins is still limited, especially in photosynthetic organisms. In most organisms analyzed so far the biogenesis of Fe/S clusters involves more than one machinery. The additional compartment in photoautotrophic organisms, the plastids, presents an additional challenge for the regulation of Fe/S cluster biogenesis. The requirement for Fe/S proteins in multiple chloroplast processes argues that Fe/S cluster assembly is an essential part of plastid functionality. This review focuses on the interesting and unique aspects of Fe/S cluster biogenesis in photosynthetic organisms and compares them to what is known in other organisms.

Frataxin interacts functionally with mitochondrial electron transport chain proteins

Frataxin deficiency is the main cause of Friedreich ataxia, an autosomal recessive neurodegenerative disorder. Frataxin function in mitochondria has not been fully explained yet. In this work, we show that Saccharomyces cerevisiae frataxin orthologue Yfh1p interacts physically with succinate dehydrogenase complex subunits Sdh1p and Sdh2p of the yeast mitochondrial electron transport chain and also with electron transfer flavoprotein complex ETFalpha and ETFbeta subunits from the electron transfer flavoprotein complex. Genetic synthetic interaction experiments confirmed a functional relationship between YFH1 and succinate dehydrogenase genes SDH1 and SDH2. We also demonstrate a physical interaction between human frataxin and human succinate dehydrogenase complex subunits, suggesting also a key role of frataxin in the mitochondrial electron transport chain in humans. Consequently, we suggest a direct participation of the respiratory chain in the pathogenesis of the Friedreich ataxia, which we propose to be considered as an OXPHOS disease.

Biogenesis of iron-sulfur proteins in plants

Iron-sulfur (Fe-S) clusters are ubiquitous prosthetic groups required to sustain fundamental life processes. The assembly of Fe-S clusters and insertion into polypeptides in vivo has recently become an area of intense research. Many of the genes involved are conserved in bacteria, fungi, animals and plants. Plant cells can carry out both photosynthesis and respiration - two processes that require significant amounts of Fe-S proteins. Recent findings now suggest that both plastids and mitochondria are capable of assembling Fe-S proteins using assembly machineries that differ in biochemical properties, genetic make-up and evolutionary origin.

Mitochondrial localization of Arabidopsis thaliana Isu Fe-S scaffold proteins

Isu are scaffold proteins involved in iron-sulfur cluster biogenesis and playing a key role in yeast mitochondria and Escherichia coli. In this work, we have characterized the Arabidopsis thaliana Isu gene family. AtIsu1,2,3 genes encode polypeptides closely related to their bacterial and eukaryotic counterparts. AtIsu expression in a Saccharomyces cerevisiae Deltaisu1Deltanfu1 thermosensitive mutant led to the growth restoration of this strain at 37 degrees C. Using Isu-GFP fusions expressed in leaf protoplasts and immunodetection in organelle extracts, we have shown that Arabidopsis Isu proteins are located only into mitochondria, supporting the existence of an Isu-independent Fe-S assembly machinery in plant plastids.