Plant Ferritin Research Articles

Structure, Function, and Nutrition of Phytoferritin: A Newly Functional Factor for Iron Supplement

Ferritins are members of the superfamily of iron storage and detoxification proteins present in all living organisms and play important roles in controlling cellular iron homeostasis. In contrast to animal ferritin, relatively little information is available on the structure and function of phytoferritin. Phytoferritin is observed in plastids whereas animal ferritins are largely found in the cytoplasm of cell. Compared to animal ferritin, phytoferritin exhibits two major distinctive features in structure. First, phytoferritin contains a specific extension peptide (EP) at the N-terminal while animal ferritin lacks it. The EP is located on the exterior surface of protein, which recently has been found to act as a second ferroxidase center for iron-binding and oxidation, and regulate iron release during the germination and early growth of seedlings. Second, only H-type subunit has been identified in phytoferritin, which is usually a heteropolymer consisting of two different subunits, H-1 and H-2, sharing ∼80% amino acid sequence identity. These two subunits in phytoferritin play a positively cooperative role in iron oxidative deposition in protein. Iron deficiency anemia (IDA) is the most common and widespread nutritional disorder in the world, so it is crucial to explore a safe and efficient functional factor for iron supplement. Fortunately, phytoferritin seems to be a suitable candidate. In legume seeds, more than 90% of iron is stored in the form of ferritin in amyloplasts. Recently, some studies at different levels have demonstrated that plant ferritin could be used as novel, utilizable, plant-based forms of iron for populations with a low iron status. This review focuses on recent progress in structure, function, and nutrition of phytoferritin.

NADH induces iron release from pea seed ferritin: A model for interaction between coenzyme and protein components in foodstuffs

Plant ferritin from legume seeds co-exists with coenzyme NADH (a reduced form of nicotinamide-adenine dinucleotide) in many foodstuffs. In the present study, the interaction of NADH with apo pea seed ferritin (PSF) was investigated by fluorescence resonance energy transfer (FRET), fluorescence titration, transmission electron microscope (TEM), and isothermal titration calorimetry (ITC). We found that NADH molecules bound on the outer surface of PSF close to the 4-fold channels, which was 1.58nm from tryptophan residue (Trp). Consequently, such binding facilitates iron release from holo PSF, which might have a negative effect on PSF as an iron supplement, while NADH was oxidised into NAD+. However, the binding of NADH to the protein does not affect the entry of toxic ferrous ions into the apo protein shell, where these ferrous ions were oxidised into less toxic ferric ions. Moreover, NADH binding markedly affects the tertiary structure around Trp residues of PSF. These findings advanced our understanding of the interactions between different naturally occurring components in a complex food system.

Rethinking Red Meat as a Prevention Strategy for Iron Deficiency

The possibility that iron-deficiency anemia may lead to neurodevelopmental deficits has led some to recommend red meat as an early complementary food for infants; however, 2 new lines of evidence put this advice in a new light. First, recent studies show an association between consumption of red meat and cancer, diabetes, cardiovascular disease, and overall mortality. Second, new research has elucidated the role of plant ferritin as a readily available source of iron. There is an abundance of options for plant-based iron-rich foods that are consistent with long-term healthful eating that can meet infants’ nutritional needs while sidestepping red meat’s potential adverse health effects.

Stability and iron oxidation properties of a novel homopolymeric plant ferritin from adzuki bean seeds: A comparative analysis with recombinant soybean seed H-1 chain ferritin

BackgroundAll reported plant ferritins are heteropolymers comprising two different H-type subunits. Whether or not homopolymeric plant ferritin occurs in nature is an open question. MethodsA homopolymeric phytoferritin from adzuki bean seeds (ASF) was obtained by various protein purification techniques for the first time, which shares the highest identity (89.6%) with soybean seed H-1 ferritin (rH-1). Therefore, we compared iron oxidation activity and protein stability of ASF with those of rH-1 by stopped-flow combined with light scattering or UV/Vis spectrophotography, SDS- and native- PAGE analyses. Additionally, a new rH-1 variant (S68E) was prepared by site-directed mutagenesis approach to elucidate their difference in protein stability. ResultsAt high iron loading of protein, the extension peptide (EP) of plant ferritin was involved in iron oxidation, and the EP of ASF exhibited a much stronger iron oxidative activity than that of rH-1. Besides, ASF is more stable than rH-1 during storage, which is ascribed to one amino acid residue, Ser68. ConclusionsASF exhibits a different mechanism in iron oxidation from rH-1 at high iron loading of protein, and a higher stability than rH-1. These differences are mainly stemmed from their different EP sequences. General significanceThis work demonstrates that plant cells have evolved the EP of phytoferritin to control iron chemistry and protein stability by exerting a fine tuning of its amino acid sequence.

Atypical iron storage in marine brown algae: a multidisciplinary study of iron transport and storage in Ectocarpus siliculosus.

Iron is an essential element for all living organisms due to its ubiquitous role in redox and other enzymes, especially in the context of respiration and photosynthesis. The iron uptake and storage systems of terrestrial/higher plants are now reasonably well understood, with two basic strategies for iron uptake being distinguished: strategy I plants use a mechanism involving induction of Fe(III)-chelate reductase (ferrireductase) and Fe(II) transporter proteins, while strategy II plants utilize high-affinity, iron-specific, binding compounds called phytosiderophores. In contrast, little is known about the corresponding systems in marine, plant-like lineages, particularly those of multicellular algae (seaweeds). Herein the first study of the iron uptake and storage mechanisms in the brown alga Ectocarpus siliculosus is reported. Genomic data suggest that Ectocarpus may use a strategy I approach. Short-term radio-iron uptake studies verified that iron is taken up by Ectocarpus in a time- and concentration-dependent manner consistent with an active transport process. Upon long-term exposure to 57Fe, two metabolites have been identified using a combination of Mössbauer and X-ray absorption spectroscopies. These include an iron–sulphur cluster accounting for ~26% of the total intracellular iron pool and a second component with spectra typical of a polymeric (Fe3+O6) system with parameters similar to the amorphous phosphorus-rich mineral core of bacterial and plant ferritins. This iron metabolite accounts for ~74% of the cellular iron pool and suggests that Ectocarpus contains a non-ferritin but mineral-based iron storage pool.

A multidisciplinary study of iron transport and storage in the marine green alga Tetraselmis suecica

The iron uptake and storage systems of terrestrial/higher plants are now reasonably well understood with two basic strategies being distinguished: strategy I involves the induction of a Fe(III)-chelate reductase (ferrireductase) along with Fe(II) or Fe(III) transporter proteins while strategy II plants have evolved sophisticated systems based on high-affinity, iron specific, binding compounds called phytosiderophores. In contrast, there is little knowledge about the corresponding systems in marine, plant-like lineages. Herein we report a study of the iron uptake and storage mechanisms in the green alga Tetraselmis suecica. Short term radio-iron uptake studies indicate that iron is taken up by Tetraselmis in a time and concentration dependent manner consistent with an active transport process. Based on inhibitor and other studies it appears that a reductive–oxidative pathway such as that found in yeast and the green alga Chlamydomonas reinhardtii is likely. Upon long term exposure to 57Fe we have been able, using a combination of Mössbauer and X-ray absorption spectroscopies, to identify three metabolites. The first exhibits Mössbauer parameters typical of a [Fe4S4]2+ cluster and which accounts for approximately 10% of the total intracellular iron pool. The second displays a spectrum typical of a [FeIIO6] system accounting for approximately 2% of the total pool. The largest component (ca. 85+%) consists of polymeric iron-oxo mineral species with parameters between that of the crystalline ferrihydrite core of animal ferritins and the amorphous hydrated ferric phosphate of bacterial and plant ferritins.

Isolation and characterization of a new phytoferritin from broad bean (Vicia faba) seed with higher stability compared to pea seed ferritin

Plant ferritin widely occurring in legume seeds represents a novel alternative dietary iron source for treatment of iron deficiency anemia. However, recent studies have shown that known plant ferritins such as SSF and PSF are unstable because they contain a higher amount of the H-1 subunit. Therefore, it is of special interest to explore a new plant ferritin with a lower amount of the H-1. In the present study, a new ferritin (BBSF) was purified from broad bean seeds by consecutive anion exchange and size exclusion chromatography to homogeneity. The apparent molecular mass of BBSF was found to be 560kDa by native-PAGE analysis. N-terminal sequence, MALDI-TOF-MS, and SDS-PAGE analyses indicate that BBSF consists of H-1 and H-2 subunits with a H1/H-2 ratio of 1:6, and shares very high sequence identity with PSF. In contrast, the ratio of H-1 to H-2 in pea seed ferritin (PSF) is 1:2 and in soybean seed ferritin (SSF) is 1:1. Thus, this new protein has quite different subunit composition from PSF and SSF, which might affect its activity in iron uptake. As expected, PSF exhibits a higher catalyzing activity than BBSF, and the association rate of PSF is faster than BBSF upon high iron loading of the protein. More importantly, the stability of BBSF is much higher than that of PSF and SSF under the same experimental conditions, which is beneficial to its application for iron supplement.

A novel strategy of natural plant ferritin to protect DNA from oxidative damage during iron oxidation

Plant ferritin is a naturally occurring heteropolymer in plastids, where Fe2+ is oxidatively deposited into the protein. However, the effect of this process on the coexistence of DNA and plant ferritin in the plastids is unknown. To investigate this effect, we built a system in which various plant ferritins and DNA coexist, followed by treatment with ferrous ions under aerobic conditions. Interestingly, naturally occurring soybean seed ferritin (SSF), a heteropolymer with an H-1/H-2 ratio of 1 to 1 in the apo form, completely protected DNA from oxidative damage during iron oxidative deposition into protein, and a similar result was obtained with its recombinant form, but not with its homopolymeric counterparts, apo rH-1 and apo rH-2. We demonstrate that the difference in DNA protection between heteropolymeric and homopolymeric plant ferritins stems from their different strategies to control iron chemistry during the above oxidative process. For example, the detoxification reaction occurs only in the presence of apo heteropolymeric SSF (hSSF), thereby preventing the production of hydroxyl radicals. In contrast, hydroxyl radicals are apparently generated via the Fenton reaction when apo rH-1 or rH-2 is used instead of apo hSSF. Thus, a combination of H-1 and H-2 subunits in hSSF seems to impart a unique DNA-protective function to the protein, which was previously unrecognized. This new finding advances our understanding of the structure and function of ferritin and of the widespread occurrence of heteropolymeric plant ferritin in nature.

The extension peptide of plant ferritin from sea lettuce contributes to shell stability and surface hydrophobicity

Plant ferritins have some unique structural and functional features. Most of these features can be related to the plant-specific "extension peptide" (EP), which exists in the N-terminus of the mature region of a plant ferritin. Recent crystallographic analysis of a plant ferritin revealed the structure of the EP, however, two points remain unclear: (i) whether the structures of well-conserved EP of plant ferritins are common in all plants, and (ii) whether the EP truly contributes to the shell stability of the plant ferritin oligomer. To clarify these matters, we have cloned a green-plant-type ferritin cDNA from a green alga, Ulva pertusa, and investigated its crystal structure. Ulva pertusa ferritin (UpFER) has a plant-ferritin-specific extension peptide composed of 28 amino acid residues. In the crystal structure of UpFER, the EP lay on and interacted with the neighboring threefold symmetry-related subunit. The amino acid residues involved in the interaction were very highly conserved among plant ferritins. The EPs masked the hydrophobic pockets on the ferritin shell surface by lying on them, and this made the ferritin oligomer more hydrophilic. Furthermore, differential scanning calorimetric analysis of the native and its EP-deletion mutant suggested that the EP contributed to the thermal stability of the plant ferritin shell. Thus, the shell stability and surface hydrophobicity of plant ferritin were controlled by the presence or absence of the plant-ferritin-specific EP. This regulation can account for those processes such as shell stability, degradation, and association of plant ferritin, which are significantly related to iron utilization in plants.

Iron transport and storage in the coccolithophore: Emiliania huxleyi

Iron is an essential element for all living organisms due to its ubiquitous role in redox and other enzymes, especially in the context of respiration and photosynthesis. The iron uptake and storage systems of terrestrial/higher plants are now reasonably well understood with two basic strategies for iron uptake being distinguished: strategy I plants use a mechanism involving soil acidification and induction of Fe(III)-chelate reductase (ferrireductase) and Fe(II) transporter proteins while strategy II plants have evolved sophisticated systems based on high-affinity, iron specific, binding compounds called phytosiderophores. In contrast, there is little knowledge about the corresponding systems in marine plant-like lineages. Herein we report a study of the iron uptake and storage mechanisms in the coccolithophore Emiliania huxleyi. Short term radio-iron uptake studies indicate that iron is taken up by Emiliania in a time and concentration dependent manner consistent with an active transport process. Based on inhibitor studies it appears that iron is taken up directly as Fe(iii). However if a reductive step is involved the Fe(II) must not be accessible to the external environment. Upon long term exposure to (57)Fe we have been able, using a combination of Mössbauer and XAS spectroscopies, to identify a single metabolite which displays spectral features similar to the phosphorus-rich mineral core of bacterial and plant ferritins.

Functions of Double Subunits of a Type, Structure of Iron Core, and Kinetics of Iron Release from Membrane Ferritin of Human Placenta

Abstract Membrane ferritin of human placenta (HPMF) with electrophoresis purity was prepared in batch from human placental tissues. Several analysis methods were employed to study on the structure and functions of HPMF. The results of SDS-PAGE indicated that the protein shell of HPMF comprised different subunits, which belong to a similar type. The subunits were classified into two groups with the molecular weights of 15 and 20 kDa, which were named MF 15 and MF 20 subunits, respectively. In addition, relative amount of MF 15 subunit was about two folds more than that of M 20 subunit in SDS-PAGE gel. Both MF 15 and MF 20 subunits were identified by peptide mass fingerprinting (PMF) and showed high homology with L subunit of human ferritin. Accordingly, it was believed that HPMF was a novel ferritin of two subunits with different molecular weights belonging to a similar type. Using a matrix-assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF MS), the subunit stability of HPMF was investigated, and five mass peak's corresponding ratio of mass to charge ( m / z ) such as 5071.25, 9962.51, 15131.55 (MF 15 ), 19936.40 (MF 20 ), and 20147.61 were detected, which indicated HPMF had five subunit formula of [MF 15 ] 3+ , [MF 20 ] 2+ , [MF 15 ] + , [MF 20 ] + and [MF 20 + Fe] + , respectively. Using inductively coupled plasma-mass spectrometry (ICP-MS) to analyze the elemental composition of HPMF, only 85 Fe 3+ and 15 inorganic phosphate (P i ) ions in the iron core of per molecule of HPMF were determined, which were significantly lower than that of most mammal ferritins, but higher than that of bacterial ferritin (BF). The kinetic studies showed that iron release from the complete HPMF followed the regulation of first-order reaction, and the time taken for the whole process of iron release from HPMF was about 750 min, which was slower than that from both horse spleen ferritin (HSF) and pig pancreatic ferritin (PPF), which were about 60 min. Accordingly, based on the difference of structure and function among HPMF, mammalian ferritins, plant ferritins, and BF, we proposed that M 15 and M 20 subunits play different roles in iron transporting from maternal blood to fetal blood.

Iron homeostasis and iron acquisition in plants: maintenance, functions and consequences

Iron homeostasis and iron acquisition in plants: maintenance, functions and consequences

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.

Purification and characterisation of ferritin from the Baltic blue mussel Mytilus trossulus

Baltic blue mussels Mytilus trossulus were collected from the Gulf of Gdansk (southern Baltic Sea) in order to isolate ferritin from its soft tissues, as well as to purify and characterise this protein. Proteins were isolated from the inner organs of M. trossulus (hepatopancreas, gills and soft tissue residue) by thermal denaturation (70◦C) and acidification (pH 4.5) of the homogenates, followed by ammonium sulphate ((NH4)2SO4) fractionation. The ferritin was then separated by ultracentrifugation (100 000× g, 120 min.). The protein content in the purified homogenates was determined by the Lowry method using bovine serum albumin (BSA) and horse spleen ferritin (HSF) as standards. PAGE-SDS and Western blotting analysis permitted identification of ferritin in the purified preparations. Additionally, the purified homogenates and mussel soft tissue were analysed for their heavy metal contents (especially cadmium and iron) in a Video 11 E atomic absorption spectrophotometer, following wet digestion of the samples (HNO3/HClO4). * This work was supported financially by the Committee for Scientific Research, grant No. 3/P04E/035/22, and the Institute of Oceanology statutory research grant. The complete text of the paper is available at http://www.iopan.gda.pl/oceanologia/ 526 J. Potrykus, A. Kosakowska The electrophoregrams showed that the inner organs of M. trossulus contained ferritin, which, like plant ferritin, is characterised by the presence of subunits in the electrophoregram in the 26.6–28.0 kDa range. The highest ferritin content was recorded in the hepatopancreas, followed by the gills and the soft tissue residue. With regard to the sampling stations, the highest content of ferritin was noted in the animals sampled off Sopot (station D3), and in those collected by a diver off Jastarnia (W1) and Gdynia (W4). Ferritin isolated from the inner organs of mussels collected from these stations also contained the largest quantities of heavy metals (Cd and Fe). Ferritin isolated from the inner organs of mussels collected by a diver from wrecks – sites where the concentrations of iron and other trace metals in the sea water are high – contained higher quantities of heavy metals (Cd and Fe) than the ferritin isolated from the inner organs of mussels collected with the drag. This confirms that ferritin is a protein able to store and transport not only iron, but also, though to a lesser extent, some other heavy metals, including cadmium.

Knocking out of the mitochondrial AtFer4 ferritin does not alter response of Arabidopsis plants to abiotic stresses

Ferritins are iron-storage proteins which, in Arabidopsis, have a clear role in protection against oxidative stress. Plant ferritins are localized mainly in chloroplasts, but they can also be targeted to mitochondria; the ATFER4 ferritin isoform, according to bioinformatic subcellular predictors, has the highest scores for such localization in Arabidopsis. We isolated atfer4-2 mutant KO in the AtFer4 gene and we characterized it together with a second, independent mutant atfer4-1. We show that ATFER4 is indeed localized in mitochondria of Fe-treated Arabidopsis plants; when grown under Fe excess, atfer4 plants manifest, however, the same toxicity symptoms and O 2 consumption rates as wt plants. No enhanced sensitivity to oxidative conditions was observed in atfer4 seedlings exposed to salinity, osmotic stress, cold stress or oxidative stress elicited by paraquat. The growth response of roots and aerial parts in atfer4 plants under different light conditions was the same as wt. Also, the process of natural senescence, in which AtFer1 takes active part, was not perturbed in atfer4 plants. We conclude that the ATFER4 ferritin role in counteracting the environmental or developmental oxidative conditions in Arabidopsis plants is ancillary to that of the other isoforms, regardless of its mitochondrial localization.

Phosphate facilitates Fe(II) oxidative deposition in pea seed ( Pisum sativum) ferritin

The iron core within phytoferritin interior usually contains the high ratio of iron to phosphate, agreeing with the fact that phosphorus and iron are essential nutrient elements for plant growth. It was established that iron oxidation and incorporation into phytoferritin shell occurs in the plastid(s) where the high concentration of phosphate occurs. However, so far, the role of phosphate in iron oxidative deposition in plant ferritin has not been recognized yet. In the present study, Fe(II) oxidative deposition in pea seed ferritin (PSF) was aerobically investigated in the presence of phosphate. Results indicated that phosphate did not affect the stoichiometry of the initial iron(II) oxidation reaction that takes place at ferroxidase centers upon addition of ≤48 Fe(II)/protein to apoferritin, but increased the rate of iron oxidation. At high Fe(II) fluxes into ferritin (>48 Fe(II)/protein), phosphate plays a more significant role in Fe(II) oxidative deposition. For instance, phosphate increased the rate of Fe(II) oxidation about 1–3 fold, and such an increase depends on the concentration of phosphate in the range of 0–2 mM. This effect was attributed to the ability of phosphate to improve the regeneration activity of ferroxidase centers in PSF. In addition, the presence of phosphate caused a significant decrease in the absorption properties of iron core, indicating that phosphate is involved in the formation of the iron core.

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.

Quantification of Ferritin-Bound Iron in Plant Samples by Isotope Tagging and Species-Specific Isotope Dilution Mass Spectrometry

Ferritin is nature's predominant iron storage protein. The molecule consists of a hollow protein shell composed of 24 subunits which is capable of storing up to 4500 iron atoms per molecule. Recently, this protein has been identified as a target molecule for increasing iron content in plant staple foods in order to combat dietary iron deficiency, a major public health problem in developing countries. Here, we present a novel technique for quantification of ferritin-bound iron in edible plant seeds using species-specific isotope dilution mass spectrometry (IDMS) by means of a biosynthetically produced (57)Fe-labeled ferritin spike and negative thermal ionization mass spectrometry (NTIMS). Native plant ferritin and added spike ferritin were extracted in 20 mM Tris buffer (pH 7.4) and separated by anion exchange chromatography (DEAE Sepharose), followed by isotopic analysis by thermal ionization mass spectrometry. The chosen IDMS approach was critically evaluated by assessing the (i) efficiency of analyte extraction, (ii) identical behavior of spike and analyte, and (iii) potential iron isotope exchange with natural iron. Repeatabilities that can be achieved are on the order of <5% RSD for quintuplicate analyses at an absolute detection limit of 60 ng of ferritin-bound iron for plant seeds. Studies in six different legumes revealed ferritin-iron contents ranging from 15% of total iron in red kidney beans up to 69% in lentils.

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

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. 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. 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.

Microbial Siderophores Exert a Subtle Role in Arabidopsis during Infection by Manipulating the Immune Response and the Iron Status

Siderophores (ferric ion chelators) are secreted by organisms in response to iron deficiency. The pathogenic enterobacterium Erwinia chrysanthemi produces two siderophores, achromobactin and chrysobactin (CB), which are required for systemic dissemination in host plants. Previous studies have shown that CB is produced in planta and can trigger the up-regulation of the plant ferritin gene AtFER1. To further investigate the function of CB during pathogenesis, we analyzed its effect in Arabidopsis (Arabidopsis thaliana) plants following leaf infiltration. CB activates the salicylic acid (SA)-mediated signaling pathway, while the CB ferric complex is ineffective, suggesting that the elicitor activity of this siderophore is due to its iron-binding property. We confirmed this hypothesis by testing the effect of siderophores structurally unrelated to CB, including deferrioxamine. There was no activation of SA-dependent defense in plants grown under iron deficiency before CB treatment. Transcriptional analysis of the genes encoding the root ferrous ion transporter and ferric chelate reductase, and determination of the activity of this enzyme in response to CB or deferrioxamine, showed that these compounds induce a leaf-to-root iron deficiency signal. This root response as well as ferritin gene up-regulation in the leaf were not compromised in a SA-deficient mutant line. Using the Arabidopsis-E. chrysanthemi pathosystem, we have shown that CB promotes bacterial growth in planta and can modulate plant defenses through an antagonistic mechanism between SA and jasmonic acid signaling cascades. Collectively, these data reveal a new link between two processes mediated by SA and iron in response to microbial siderophores.