Inhibition of the iron-induced ZmFer1 maize ferritin gene expression by antioxidants and serine/threonine phosphatase inhibitors

A Plant Model of α-Synucleinopathy: Expression of α-Synuclein A53T Variant in Hairy Root Cultures Leads to Proteostatic Stress and Dysregulation of Iron Metabolism

Synucleinopathies, typified by Parkinson's disease (PD), entail the accumulation of α-synuclein (αSyn) aggregates in nerve cells. Various αSyn mutants, including the αSyn A53T variant linked to early-onset PD, increase the propensity for αSyn aggregate formation. In addition to disrupting protein homeostasis and inducing proteostatic stress, the aggregation of αSyn in PD is associated with an imbalance in iron metabolism, which increases the generation of reactive oxygen species and causes oxidative stress. This study explored the impact of αSyn A53T expression in transgenic hairy roots of four medicinal plants (Lobelia cardinalis, Artemisia annua, Salvia miltiorrhiza, and Polygonum multiflorum). In all tested plants, αSyn A53T expression triggered proteotoxic stress and perturbed iron homeostasis, mirroring the molecular profile observed in human and animal nerve cells. In addition to the common eukaryotic defense mechanisms against proteostatic and oxidative stresses, a plant stress response generally includes the biosynthesis of a diverse set of protective secondary metabolites. Therefore, the hairy root cultures expressing αSyn A53T offer a platform for identifying secondary metabolites that can ameliorate the effects of αSyn, thereby aiding in the development of possible PD treatments and/or treatments of synucleinopathies.

Emerging roles of protein phosphorylation in plant iron homeostasis

Remarkable progress has been made in dissecting the molecular mechanisms involved in iron (Fe) homeostasis in plants, especially the identification of key transporter and transcriptional regulatory networks. But how the protein activity of these master players is regulated by Fe status remains underexplored. Recent studies show that major players toggle switch their properties by protein phosphorylation under different Fe conditions and consequently control the signaling cascade and metabolic adjustment. Moreover, Fe deficiency causes changes of multiple kinases and phosphatases. Here, we discuss how these findings highlight the emergence of the protein phosphorylation-dependent regulation for rapid and precise responses to Fe status to attain Fe homeostasis. Further studies will be needed to fully understand the regulation of these intricate networks.

Transcriptional regulation of chickpea ferritin CaFer1 influences its role in iron homeostasis and stress response

Ferritin, ubiquitous among all living organisms except yeast, exhibits iron-regulated expression. In plants, this regulation is applied through transcriptional control. Previous studies established the presence of two types of cis-acting elements in the promoter region: the iron regulatory element (FRE) in soybean and the iron-dependent regulatory sequence (IDRS) in maize and Arabidopsis. Adverse environmental conditions (e.g. water-deficit and oxidative stress) are known to modulate the expression of phytoferritin genes. In this study, we cloned and investigated the promoter sequence of a chickpea ferritin, designated CaFer1. Phylogenetic analysis of the CaFer1 promoter revealed its evolutionary relationship with other phytoferritins. The CaFer1 promoter exhibited several putative regulatory elements including two known transcription factor (TF) binding sites, Athb-1 and Myb.Ph. Electrophoretic mobility shift assay confirmed the sequence-specific binding of Athb-1 and Myb.Ph on the CaFer1 promoter. The TF-binding dynamics of CaFer1 showed high induction under conditions of iron-deficiency and water-deficit. We also demonstrated the possible interaction of CaFer1 with IRT1, a key component of the iron uptake system in plants, indicating its involvement in maintaining cellular iron levels. These results provide new insights into the underlying mechanisms of function of these interacting factors in CaFer1-mediated iron homeostasis and the stress response in plants.

Trace metal metabolism in plants

Many trace metals are essential micronutrients, but also potent toxins. Due to natural and anthropogenic causes, vastly different trace metal concentrations occur in various habitats, ranging from deficient to toxic levels. Therefore, one focus of plant research is on the response to trace metals in terms of uptake, transport, sequestration, speciation, physiological use, deficiency, toxicity, and detoxification. In this review, we cover most of these aspects for the essential micronutrients copper, iron, manganese, molybdenum, nickel, and zinc to provide a broader overview than found in other recent reviews, to cross-link aspects of knowledge in this very active research field that are often seen in a separated way. For example, individual processes of metal usage, deficiency, or toxicity often were not mechanistically interconnected. Therefore, this review also aims to stimulate the communication of researchers following different approaches, such as gene expression analysis, biochemistry, or biophysics of metalloproteins. Furthermore, we highlight recent insights, emphasizing data obtained under physiologically and environmentally relevant conditions.

Tracing the role of plant proteins in the response to metal toxicity: a comprehensive review

Plants are sessile in nature, but are capable to evade from high level concentration of heavy metals like Cd, Hg, Cu, through various metabolic pathways. Some of the pathways regulate normal metabolism in plants, whereas others are required for for their survival under metal toxicity. Different plant proteins act as transporters to transfer metal from one organelle to the other and further eliminate it out from the plants. Initially, exposure of heavy metals/metalloids to plants lead to over expression of proteins which in turn stimulate other stress-related genes. Further, they activate signalling mechanism like MAPK cascade, Cd-Calmodulin signalling pathway, and oxidation signalling pathway that lead to generation of ROS (reactive oxygen species). Once these ROS (highly unstable) are formed, they generate free radicals which react with macromolecules like proteins and DNA. This has negative impact on plant growth and leads to ageing and, eventually, cell death. The uncontrolled, destructive processes damage plants physiologically and ultimately lead to oxidative stress. Activation of antioxidant enzymes like SOD (superoxide dismutase) and CAT (catalase) allows plants to cope under oxidative stress conditions. Among plant proteins, some of the antioxidant enzymes like glutathione, and APX (ascorbate peroxidase) play defensive roles against abiotic stress in plants. Chaperones help in protein folding to maintain protein stability under stress conditions. With this background, the present review gives a brief account of the functions, localization and expression pattern of plant proteins against metal/metalloid toxicity. Moreover, the aim of this review is also to summarize the cutting edge research of plant protein and metal interfaces and their future prospects.

Iron- and ferritin-dependent reactive oxygen species distribution: impact on Arabidopsis root system architecture

Iron (Fe) homeostasis is integrated with the production of reactive oxygen species (ROS), and distribution at the root tip participates in the control of root growth. Excess Fe increases ferritin abundance, enabling the storage of Fe, which contributes to protection of plants against Fe-induced oxidative stress. AtFer1 and AtFer3 are the two ferritin genes expressed in the meristematic zone, pericycle and endodermis of the Arabidopsis thaliana root, and it is in these regions that we observe Fe stained dots. This staining disappears in the triple fer1-3-4 ferritin mutant. Fe excess decreases primary root length in the same way in wild-type and in fer1-3-4 mutant. In contrast, the Fe-mediated decrease of lateral root (LR) length and density is enhanced in fer1-3-4 plants due to a defect in LR emergence. We observe that this interaction between excess Fe, ferritin, and root system architecture (RSA) is in part mediated by the H2O2/O2·- balance between the root cell proliferation and differentiation zones regulated by the UPB1 transcription factor. Meristem size is also decreased in response to Fe excess in ferritin mutant plants, implicating cell cycle arrest mediated by the ROS-activated SMR5/SMR7 cyclin-dependent kinase inhibitors pathway in the interaction between Fe and RSA.

ROS homeostasis in halophytes in the context of salinity stress tolerance

Halophytes are defined as plants that are adapted to live in soils containing high concentrations of salt and benefiting from it, and thus represent an ideal model to understand complex physiological and genetic mechanisms of salinity stress tolerance. It is also known that oxidative stress signalling and reactive oxygen species (ROS) detoxification are both essential components of salinity stress tolerance mechanisms. This paper comprehensively reviews the differences in ROS homeostasis between halophytes and glycophytes in an attempt to answer the questions of whether stress-induced ROS production is similar between halophytes and glycophytes; is the superior salinity tolerance in halophytes attributed to higher antioxidant activity; and is there something special about the specific 'pool' of enzymatic and non-enzymatic antioxidants in halophytes. We argue that truly salt-tolerant species possessing efficient mechanisms for Na(+) exclusion from the cytosol may not require a high level of antioxidant activity, as they simply do not allow excessive ROS production in the first instance. We also suggest that H2O2 'signatures' may operate in plant signalling networks, in addition to well-known cytosolic calcium 'signatures'. According to the suggested concept, the intrinsically higher superoxide dismutase (SOD) levels in halophytes are required for rapid induction of the H2O2 'signature', and to trigger a cascade of adaptive responses (both genetic and physiological), while the role of other enzymatic antioxidants may be in decreasing the basal levels of H2O2, once the signalling has been processed. Finally, we emphasize the importance of non-enzymatic antioxidants as the only effective means to prevent detrimental effects of hydroxyl radicals on cellular structures.

Copper and iron homeostasis in plants: the challenges of oxidative stress

SIGNIFICANCE

Photosynthesis, the process that drives life on earth, relies on transition metal (e.g., Fe and Cu) containing proteins that participate in electron transfer in the chloroplast. However, the light reactions also generate high levels of reactive oxygen species (ROS), which makes metal use in plants a challenge.

RECENT ADVANCES

Sophisticated regulatory networks govern Fe and Cu homeostasis in response to metal ion availability according to cellular needs and priorities. Molecular remodeling in response to Fe or Cu limitation leads to its economy to benefit photosynthesis. Fe toxicity is prevented by ferritin, a chloroplastic Fe-storage protein in plants. Recent studies on ferritin function and regulation revealed the interplay between iron homeostasis and the redox balance in the chloroplast.

CRITICAL ISSUES

Although the connections between metal excess and ROS in the chloroplast are established at the molecular level, the mechanistic details and physiological significance remain to be defined. The causality/effect relationship between transition metals, redox signals, and responses is difficult to establish.

FUTURE DIRECTIONS

Integrated approaches have led to a comprehensive understanding of Cu homeostasis in plants. However, the biological functions of several major families of Cu proteins remain unclear. The cellular priorities for Fe use under deficiency remain largely to be determined. A number of transcription factors that function to regulate Cu and Fe homeostasis under deficiency have been characterized, but we have not identified regulators that mediate responses to excess. Importantly, details of metal sensing mechanisms and cross talk to ROS-sensing mechanisms are so far poorly documented in plants.

AtFer4 ferritin is a determinant of iron homeostasis in Arabidopsis thaliana heterotrophic cells

In plants, the iron storage protein ferritin can be targeted to both chloroplasts and mitochondria. To investigate the role of Arabidopsis ATFER4 ferritin in mitochondrial iron trafficking, atfer4-1 and atfer4-2 mutant knock-outs for the AtFer4 gene were grown in heterotrophic suspension cultures. Both mutants showed altered cell size and morphology, reduced viability, higher H₂O₂ content and reduced O₂ consumption rates when compared to wt. Although no reduction in total ferritin or in mitochondrial ferritin was observed in atfer4 mutants, total iron content increased in atfer4 cells and in atfer4 mitochondria. Transcript correlation analysis highlighted a partial inverse relationship between the transcript levels of the mitochondrial ferric reductase oxidase FRO3, putatively involved in mitochondrial iron import/export, and AtFer4. Consistent with this, FRO3 transcript levels were higher in atfer4 cells. We propose that the complex molecular network maintaining Fe cellular homeostasis requires, in Arabidopsis heterotrophic cells, a proper balance of the different ferritin isoforms, and that alteration of this equilibrium, such as that occurring in atfer4 mutants, is responsible for an altered Fe homeostasis resulting in a change of intraorganellar Fe trafficking.

New insights into ferritin synthesis and function highlight a link between iron homeostasis and oxidative stress in plants

BACKGROUND

Iron is an essential element for both plant productivity and nutritional quality. Improving plant iron content was attempted through genetic engineering of plants overexpressing ferritins. However, both the roles of these proteins in plant physiology, and the mechanisms involved in the regulation of their expression are largely unknown. Although the structure of ferritins is highly conserved between plants and animals, their cellular localization differs. Furthermore, regulation of ferritin gene expression in response to iron excess occurs at the transcriptional level in plants, in contrast to animals which regulate ferritin expression at the translational level.

SCOPE

In this review, an overview of our knowledge of bacterial and mammalian ferritin synthesis and functions is presented. Then the following will be reviewed: (a) the specific features of plant ferritins; (b) the regulation of their synthesis during development and in response to various environmental cues; and (c) their function in plant physiology, with special emphasis on the role that both bacterial and plant ferritins play during plant-bacteria interactions. Arabidopsis ferritins are encoded by a small nuclear gene family of four members which are differentially expressed. Recent results obtained by using this model plant enabled progress to be made in our understanding of the regulation of the synthesis and the in planta function of these various ferritins.

CONCLUSIONS

Studies on plant ferritin functions and regulation of their synthesis revealed strong links between these proteins and protection against oxidative stress. In contrast, their putative iron-storage function to furnish iron during various development processes is unlikely to be essential. Ferritins, by buffering iron, exert a fine tuning of the quantity of metal required for metabolic purposes, and help plants to cope with adverse situations, the deleterious effects of which would be amplified if no system had evolved to take care of free reactive iron.

A new family of ferritin genes from Lupinus luteus--comparative analysis of plant ferritins, their gene structure, and evolution

Ferritins are one of the most important elements of cellular machinery involved in iron management. Despite extensive studies conducted during the last decade, many factors regulating the expression of ferritin genes in plants remain unknown. To broaden our knowledge about the mechanisms controlling ferritin production in plant cells, we have identified and characterized a new family of ferritin genes (from yellow lupine). We have also inventoried all available plant ferritins and their genes and subjected them to a complex bioinformatic analysis. It showed that the conservative structure of ferritin genes was established much earlier than it was thought before. The first introns in ferritin genes appeared already in green algae. The number and location of introns have been finally established in mosses, over 400 million years ago, and are strictly preserved in all plants from bryophytes to dicots. Comparison of ferritin gene promoters revealed that the 14-bp-long iron-dependent regulatory sequence (IDRS), identified earlier in Arabidopsis and maize, is characteristic for all higher plants. Moreover, we found that a highly conserved IDRS can be extended (extIDRS) up to 22 bp. Phylogenetic analysis of plant ferritins showed that polypeptides of the eudicot clade can be divided into two subclasses (eudicot-1 and eudicot-2). Interestingly, we found that genes encoding proteins classified as eudicot-1 and eudicot-2 are equipped with class-specific promoters. This suggests that eudicot ferritins are structurally and perhaps functionally diverse. Based on the above observations, we were able to identify conservative elements (ELEM1--6) other than extIDRS within plant ferritin gene promoters. We also found E-boxes and iron-responsive sequence elements FeRE1 and 2, characteristically distributed within ferritin promoters. Because most of the identified conserved sequences are located within or in close proximity of extIDRS, we named this fragment of the plant ferritin gene promoter the regulatory element rich region.

Crystal structure of plant ferritin reveals a novel metal binding site that functions as a transit site for metal transfer in ferritin

Ferritins are important iron storage and detoxification proteins that are widely distributed in living kingdoms. Because plant ferritin possesses both a ferroxidase site and a ferrihydrite nucleation site, it is a suitable model for studying the mechanism of iron storage in ferritin. This article presents for the first time the crystal structure of a plant ferritin from soybean at 1.8-A resolution. The soybean ferritin 4 (SFER4) had a high structural similarity to vertebrate ferritin, except for the N-terminal extension region, the C-terminal short helix E, and the end of the BC-loop. Similar to the crystal structures of other ferritins, metal binding sites were observed in the iron entry channel, ferroxidase center, and nucleation site of SFER4. In addition to these conventional sites, a novel metal binding site was discovered intermediate between the iron entry channel and the ferroxidase site. This site was coordinated by the acidic side chain of Glu(173) and carbonyl oxygen of Thr(168), which correspond, respectively, to Glu(140) and Thr(135) of human H chain ferritin according to their sequences. A comparison of the ferroxidase activities of the native and the E173A mutant of SFER4 clearly showed a delay in the iron oxidation rate of the mutant. This indicated that the glutamate residue functions as a transit site of iron from the 3-fold entry channel to the ferroxidase site, which may be universal among ferritins.

Ferritins control interaction between iron homeostasis and oxidative stress in Arabidopsis

Ferritin protein nanocages are the main iron store in mammals. They have been predicted to fulfil the same function in plants but direct evidence was lacking. To address this, a loss-of-function approach was developed in Arabidopsis. We present evidence that ferritins do not constitute the major iron pool either in seeds for seedling development or in leaves for proper functioning of the photosynthetic apparatus. Loss of ferritins in vegetative and reproductive organs resulted in sensitivity to excess iron, as shown by reduced growth and strong defects in flower development. Furthermore, the absence of ferritin led to a strong deregulation of expression of several metal transporters genes in the stalk, over-accumulation of iron in reproductive organs, and a decrease in fertility. Finally, we show that, in the absence of ferritin, plants have higher levels of reactive oxygen species, and increased activity of enzymes involved in their detoxification. Seed germination also showed higher sensitivity to pro-oxidant treatments. Arabidopsis ferritins are therefore essential to protect cells against oxidative damage.

FER1 and FER2 encoding two ferritin complexes in Chlamydomonas reinhardtii chloroplasts are regulated by iron

Two unlinked genes FER1 and FER2 encoding ferritin subunits were identified in the Chlamydomonas genome. An improved FER2 gene model, built on the basis of manual sequencing and incorporation of unplaced reads, indicated 49% identity between the ferritin subunits. Both FER1 and FER2 transcripts are increased in abundance as iron nutrition is decreased but the pattern for each gene is distinct. Using subunit-specific antibodies, we monitored expression at the protein level. In response to low iron, ferritin1 subunits and the ferritin1 complex are increased in parallel to the increase in FER1 mRNA. Nevertheless, the iron content of the ferritin1 complex is decreased. This suggests that increased expression results in increased capacity for iron binding in the chloroplast of iron-limited cells, which supports a role for ferritin1 as an iron buffer. On the other hand, ferritin2 abundance is decreased in iron-deprived cells, indicative of the operation of iron-nutrition-responsive regulation at the translational or post-translational level for FER2. Both ferritin subunits are plastid localized but ferritin1 is quantitatively recovered in soluble extracts of cells while ferritin2 is found in the particulate fraction. Partial purification of the ferritin1 complex indicates that the two ferritins are associated in distinct complexes and do not coassemble. The ratio of ferritin1 to ferritin2 is 70:1 in iron-replete cells, suggestive of a more dominant role of ferritin1 in iron homeostasis. The Volvox genome contains orthologs of each FER gene, indicating that the duplication of FER genes and potential diversification of function occurred prior to the divergence of species in the Volvocales.

Differential expression of endogenous ferritin genes and iron homeostasis alteration in transgenic tobacco overexpressing soybean ferritin gene

For studying the effects of endogenous ferritin gene expressions (NtFer1, GenBank accession number AY083924; and NtFer2, GenBank accession number AY141105) on the iron homeostasis in transgenic tobacco (Nicotiana tabacum L.) plants expressing soybean (Glycine max Merr) ferritin gene (SoyFer1, GenBank accession number M64337), the transgenic tobacco has been produced by placing soybean ferritin cDNA cassette under the control of the CaMV 35S promoter. The exogenous gene expression was examined by both Northern- and Western-blot analyses. Comparison of endogenous ferritin gene expressions between nontransformant and transgenic tobacco plants showed that the expression of NtFer1 was increased in the leaves of transgenic tobacco plants, whereas the NtFer2 expression was unchanged. The iron concentration in the leaves of transgenic tobacco plants was about 1.5-folds higher than that in nontransformant. Enhanced growth of transgenic tobacco was observed at the early development stages, resulting in plant height and fresh weights significantly greater than those in the nontransformant. These results demonstrated that exogenous ferritin expression induced increased expression of at least one of the endogenous ferritin genes in transgenic tobacco plants by enhancing the ferric chelate reductase activity and iron transport ability of the root, and improved the rate of photosynthesis.

Methylation of aquaporins in plant plasma membrane

A thorough analysis, using MS, of aquaporins expressed in plant root PM (plasma membrane) was performed, with the objective of revealing novel post-translational regulations. Here we show that the N-terminal tail of PIP (PM intrinsic protein) aquaporins can exhibit multiple modifications and is differentially processed between members of the PIP1 and PIP2 subclasses. Thus the initiating methionine was acetylated or cleaved in native PIP1 and PIP2 isoforms respectively. In addition, several residues were detected to be methylated in PIP2 aquaporins. Lys3 and Glu6 of PIP2;1, one of the most abundant aquaporins in the PM, occurred as di- and mono-methylated residues respectively. Ectopic expression in Arabidopsis suspension cells of PIP2;1, either wild-type or with altered methylation sites, revealed an interplay between methylation at the two sites. Measurements of water transport in PM vesicles purified from these cells suggested that PIP2;1 methylation does not interfere with the aquaporin intrinsic water permeability. In conclusion, the present study identifies methylation as a novel post-translational modification of aquaporins, and even plant membrane proteins, and may represent a critical advance towards the identification of new regulatory mechanisms of membrane transport.

An iron-induced nitric oxide burst precedes ubiquitin-dependent protein degradation for Arabidopsis AtFer1 ferritin gene expression

Ferritins play an essential role in iron homeostasis by sequestering iron in a bioavailable and non-toxic form. In plants, ferritin mRNAs are highly and quickly accumulated in response to iron overload. Such accumulation leads to a subsequent ferritin protein synthesis and iron storage, thus avoiding oxidative stress to take place. By combining pharmacological and imaging approaches in an Arabidopsis cell culture system, we have identified several elements in the signal transduction pathway leading to the increase of AtFer1 transcript level after iron treatment. Nitric oxide quickly accumulates in the plastids after iron treatment. This compound acts downstream of iron and upstream of a PP2A-type phosphatase to promote an increase of AtFer1 mRNA level. The AtFer1 gene transcription has been previously shown to be repressed under low iron conditions with the involvement of the cis-acting element iron-dependent regulatory sequence identified within the AtFer1 promoter sequence. We show here that the repressor is unlikely a transcription factor directly bound to the iron-dependent regulatory sequence; such a repressor is ubiquitinated upon iron treatment and subsequently degraded through a 26 S proteasome-dependent pathway.

Nitric Oxide Signaling in Plants

Plants have four nitric oxide synthase (NOS) enzymes. NOS1 appears mitochondrial, and inducible nitric oxide synthase (iNOS) chloroplastic. Distinct peroxisomal and apoplastic NOS enzymes are predicted. Nitrite-dependent NO synthesis is catalyzed by cytoplasmic nitrate reductase or a root plasma membrane enzyme, or occurs nonenzymatically. Nitric oxide undergoes both catalyzed and uncatalyzed oxidation. However, there is no evidence of reaction with superoxide, and S-nitrosylation reactions are unlikely except during hypoxia. The only proven direct targets of NO in plants are metalloenzymes and one metal complex. Nitric oxide inhibits apoplastic catalases/ascorbate peroxidases in some species but may stimulate these enzymes in others. Plants also have the NO response pathway involving cGMP, cADPR, and release of calcium from internal stores. Other known targets include chloroplast and mitochondrial electron transport. Nitric oxide suppresses Fenton chemistry by interacting with ferryl ion, preventing generation of hydroxyl radicals. Functions of NO in plant development, response to biotic and abiotic stressors, iron homeostasis, and regulation of respiration and photosynthesis may all be ascribed to interaction with one of these targets. Nitric oxide function in drought/abscisic acid (ABA)-induction of stomatal closure requires nitrate reductase and NOS1. Nitric oxide synthasel likely functions to produce sufficient NO to inhibit photosynthetic electron transport, allowing nitrite accumulation. Nitric oxide is produced during the hypersensitive response outside cells undergoing programmed cell death immediately prior to loss of plasma membrane integrity. A plasma membrane lipid-derived signal likely activates apoplastic NOS. Nitric oxide diffuses within the apoplast and signals neighboring cells via hydrogen peroxide (H2O2)-dependent induction of salicylic acid biosynthesis. Response to wounding appears to involve the same NOS and direct targets.

Iron-regulated expression of a cytosolic ascorbate peroxidase encoded by the APX1 gene in Arabidopsis seedlings

Iron (Fe) is an essential element for living organisms. However, under aerobic conditions, its use is complicated because of its high insolubility and its potential toxicity through reactivity with reduced forms of oxygen. In plants, Fe overload can lead to intracellular concentrations beyond the storage and detoxification capacities of cells. Such a displacement toward a pro-oxidant state can activate antioxidant defenses, including Fe-mediated expression of ascorbate peroxidase genes. In this work, we demonstrate that Fe overload specifically induces the AtAPX1 gene encoding a cytosolic ascorbate peroxidase in Arabidopsis leaves. The strong constitutive expression of the AtAPX1 gene in roots is unaffected by Fe and depends on the first 5'-untranslated region intron. Presence of an AtAPX1 expressed sequence tag in the Arabidopsis database, longer in its 5' region than what could be predicted from the published AtAPX1transcription initiation site, leads to define a new transcription initiation region for this gene. A minimal promoter sequence enabling Fe-induced expression of the AtAPX1 gene is defined by following expression of various AtAPX1::beta-glucuronidase constructs in transformed Arabidopsis plantlets. This 118-bp minimal promoter sequence contains an Fe-dependent regulatory sequence-like cis-element known to be necessary for maize (Zea mays) and Arabidopsis ferritin gene derepression in response to Fe overload. Site-directed mutagenesis of this element within the AtAPX1 promoter sequence does not abolish the Fe-dependent activation of a reporter gene, indicating that it is likely not involved in the Fe-regulated expression of the AtAPX1 gene.

Cytosolic aconitase and ferritin are regulated by iron in Caenorhabditis elegans

Iron regulatory protein-1 (IRP-1) is a cytosolic RNA-binding protein that is a regulator of iron homeostasis in mammalian cells. IRP-1 binds to RNA structures, known as iron-responsive elements, located in the untranslated regions of specific mRNAs, and it regulates the translation or stability of these mRNAs. Iron regulates IRP-1 activity by converting it from an RNA-binding apoprotein into a [4Fe-4S] cluster protein exhibiting aconitase activity. IRP-1 is widely found in prokaryotes and eukaryotes. Here, we report the biochemical characterization and regulation of an IRP-1 homolog in Caenorhabditis elegans (GEI-22/ACO-1). GEI-22/ACO-1 is expressed in the cytosol of cells of the hypodermis and the intestine. Like mammalian IRP-1/aconitases, GEI-22/ACO-1 exhibits aconitase activity and is post-translationally regulated by iron. Although GEI-22/ACO-1 shares striking resemblance to mammalian IRP-1, it fails to bind RNA. This is consistent with the lack of iron-responsive elements in the C. elegans ferritin genes, ftn-1 and ftn-2. While mammalian ferritin H and L mRNAs are translationally regulated by iron, the amounts of C. elegans ftn-1 and ftn-2 mRNAs are increased by iron and decreased by iron chelation. Excess iron did not significantly alter worm development but did shorten their life span. These studies indicated that iron homeostasis in C. elegans shares some similarities with those of vertebrates.

Genetic regulation of cell function in response to iron overload or chelation

Iron influences many aspects of cell function on different biochemical levels. This review considers effects mediated through iron-dependent changes in gene expression in mammalian cells. Several classes of related genes are responsive to cellular iron levels, but no clear patterns readily account for the toxicity of iron overload or the consequences of removal of iron with chelating agents. Here we group some of the genes influenced by iron status into those related to iron metabolism, oxygen and oxidative stress, energy metabolism, cell cycle regulation, and tissue fibrosis. Iron excess and chelation do not generally result in a continuous or graded transcriptional response, but indicate operation of distinct mechanisms. An emerging concept is that iron signals through generation of reactive oxygen species to activate transcription factors such as NF-kappaB, whereas iron removal mimics hypoxia, perhaps by disrupting iron-based O(2) sensors and influencing gene expression through, e.g., the hypoxia-inducible factor, HIF-1. Heme and other metalloporphyrins have other distinct mechanisms for regulating transcription. Regulation of gene expression through iron-responsive elements in mRNAs coded by several genes is one of the best understood mechanisms of translational control.

Nitric oxide mediates iron-induced ferritin accumulation in Arabidopsis

Nitric oxide (NO) is a signaling molecule that plays a critical role in the activation of innate immune and inflammatory responses in animals. During the last few years, NO has also been detected in several plant species and the increasing number of reports on its function in plants have implicated NO as an important effector of growth, development and defense. Analogously to animals, NO has been recently shown to inhibit tobacco aconitase. This suggests that NO may elevate free iron levels in the cells by converting tobacco cytoplasmic aconitase into a mRNA binding protein that negatively regulates accumulation of ferritin. We investigated the possible role of NO as a regulator of ferritin levels in Arabidopsis and found that the NO-donor sodium nitroprusside (SNP) induces accumulation of ferritin both at mRNA and protein level. Iron is not necessary for this NO-mediated ferritin transcript accumulation, since SNP is still able to induce the accumulation of ferritin transcript in Arabidopsis suspension cultures pre-treated with the iron chelants DFO or ferrozine. However, NO is required for iron-induced ferritin accumulation, as the NO scavenger CPTIO prevents ferritin transcript accumulation in Arabidopsis suspension cultures treated with iron. The pathway is ser/thr phosphatase-dependent and necessitates protein synthesis; furthermore, NO mediates ferritin regulation through the IDRS sequence of the Atfer1 promoter responsible for transcriptional repression under low iron supply. NO, by acting downstream of iron in the induction of ferritin transcript accumulation is therefore a key signaling molecule for regulation of iron homeostasis in plants.

Structure and differential expression of the four members of the Arabidopsis thaliana ferritin gene family

Four ferritin genes are found within the complete sequence of the Arabidopsis thaliana genome. All of them are expressed and their corresponding cDNA species have been cloned. The polypeptide sequences deduced from these four genes confirm all the properties of the ferritin subunits described so far, non-exhaustively, from various plant species. All are predicted to be targeted to the plastids, which is consistent with the existence of a putative transit peptide at their N-terminal extremity. They also all possess a conserved extension peptide in the mature subunit. Specific residues for ferroxidase activity and iron nucleation, which are found respectively in H-type or L-type ferritin subunits in animals, are both conserved within each of the four A. thaliana ferritin polypeptides. In addition, the hydrophilic nature of the plant ferritin E-helix is conserved in the four A. thaliana ferritin subunits. Besides this strong structural conservation, the four genes are differentially expressed in response to various environmental signals, and during the course of plant growth and development. AtFer1 and AtFer3 are the two major genes expressed in response to treatment with an iron overload. Under our experimental conditions, AtFer4 is expressed with different kinetics and AtFer2 is not responsive to iron. H(2)O(2) activates the expression of AtFer1 and, to a smaller extent, AtFer3. Abscisic acid promotes the expression of only AtFer2, which is consistent with the observation that this is the only gene of the four to be expressed in seeds, whereas AtFer1, AtFer4 and AtFer3 are expressed in various vegetative organs but not in seeds.

Characterization of an iron-dependent regulatory sequence involved in the transcriptional control of AtFer1 and ZmFer1 plant ferritin genes by iron

In eukaryotic cells, ferritin synthesis is controlled by the intracellular iron status. In mammalian cells, iron derepresses ferritin mRNA translation, whereas it induces ferritin gene transcription in plants. Promoter deletion and site-directed mutagenesis analysis, combined with gel shift assays, has allowed identification of a new cis-regulatory element in the promoter region of the ZmFer1 maize ferritin gene. This Iron-Dependent Regulatory Sequence (IDRS) is responsible for transcriptional repression of ZmFer1 under low iron supply conditions. The IDRS is specific to the ZmFer1 iron-dependent regulation and does not mediate the antioxidant response that we have previously reported (Savino et al. (1997) J. Biol. Chem. 272, 33319-33326). In addition, we have cloned AtFer1, the Arabidopsis thaliana ZmFer1 orthologue. The IDRS element is conserved in the AtFer1 promoter region and is functional as shown by transient assay in A. thaliana cells and stable transformation in A. thaliana transgenic plants, demonstrating its ubiquity in the plant kingdom.

Identification and characterization of the iron regulatory element in the ferritin gene of a plant (soybean)

Iron increases ferritin synthesis, targeting plant DNA and animal mRNA. The ferritin promoter in plants has not been identified, in contrast to the ferritin promoter and mRNA iron-responsive element (IRE) in animals. The soybean leaf, a natural tissue for ferritin expression, and DNA, with promoter deletions and luciferase or glucuronidase reporters, delivered with particle bombardment, were used to show that an 86-base pair fragment (iron regulatory element (FRE)) controlled iron-mediated derepression of the ferritin gene. Mutagenesis with linkers of random sequence detected two subdomains separated by 21 base pairs. FRE has no detectable homology to the animal IRE or to known promoters in DNA and bound a trans-acting factor in leaf cell extracts. FRE/factor binding was abrogated by increased tissue iron, in analogy to mRNA (IRE)/iron regulatory protein in animals. Maximum ferritin derepression was obtained with 50 microm iron citrate (1:10) or 500 microm iron citrate (1:1) but Fe-EDTA was ineffective, although the leaf iron concentration was increased; manganese, zinc, and copper had no effect. The basis for different responses in ferritin expression to different iron complexes, as well as the significance of using DNA but not mRNA as an iron regulatory target in plants, remain unknown.

Oxidative stress and gene regulation

Reactive oxygen species are produced by all aerobic cells and are widely believed to play a pivotal role in aging as well as a number of degenerative diseases. The consequences of the generation of oxidants in cells does not appear to be limited to promotion of deleterious effects. Alterations in oxidative metabolism have long been known to occur during differentiation and development. Experimental perturbations in cellular redox state have been shown to exert a strong impact on these processes. The discovery of specific genes and pathways affected by oxidants led to the hypothesis that reactive oxygen species serve as subcellular messengers in gene regulatory and signal transduction pathways. Additionally, antioxidants can activate numerous genes and pathways. The burgeoning growth in the number of pathways shown to be dependent on oxidation or antioxidation has accelerated during the last decade. In the discussion presented here, we provide a tabular summary of many of the redox effects on gene expression and signaling pathways that are currently known to exist.

Plant responses to metal toxicity

Metal toxicity for living organisms involves oxidative and/or genotoxic mechanisms. Plant protection against metal toxicity occurs, at least in part, through control of root metal uptake and of long distance metal transport. Inside cells, proteins such as ferritins and metallothioneins, and glutathion-derived peptides named phytochelatins, participate in excess metal storage and detoxification. Low molecular weight organic molecules, mainly organic acids and amino acids and their derivatives, also play an important role in plant metal homeostasis. When these systems are overloaded, oxidative stress defense mechanisms are activated. Molecular and cellular knowledge of these processes will be necessary to improve plant metal resistance. Occurrence of naturally tolerant plants which hyperaccumulate metals provides helpful tools for this research.