In plants iron–sulfur (Fe–S) proteins are found in the plastids, mitochondria, cytosol and nucleus, where they are essential for numerous physiological and developmental processes. Recent mutant ...studies, mostly in
Arabidopsis thaliana, have identified three pathways for the assembly of Fe–S clusters. The plastids harbor the SUF (sulfur mobilization) pathway and operate independently, whereas cluster assembly in the cytosol depends on the emerging CIA (cytosolic iron–sulfur cluster assembly) pathway and mitochondria. The latter organelles use the ISC (iron–sulfur cluster) assembly pathway. In all three pathways the assembly process can be divided into a first stage where S and Fe are combined on a scaffold protein, and a second stage in which the Fe–S cluster is transferred to a target protein. The second stage might involve different carrier proteins with specialized functions.
Plants are the ultimate source of iron in our diet, either directly as staple crops and vegetables or indirectly via animal fodder. Increasing the iron concentration of edible parts of plants, known ...as biofortification, is seen as a sustainable approach to alleviate iron deficiency which is a major global health issue. Advances in sequencing and gene technology are accelerating both forward and reverse genetic approaches. In this review, we summarize recent progress in iron biofortification using conventional plant breeding or transgenics. Interestingly, some of the gene targets already used for transgenic approaches are also identified as genetic factors for high iron in genome-wide association studies. Several quantitative trait loci and transgenes increase both iron and zinc, due to overlap in transporters and chelators for these two mineral micronutrients. Research efforts are predominantly aimed at increasing the total concentration of iron but enhancing its bioavailability is also addressed. In particular, increased biosynthesis of the metal chelator nicotianamine increases iron and zinc levels and improves bioavailability. The achievements to date are very promising in being able to provide sufficient iron in diets with less reliance on meat to feed a growing world population.
Iron cofactor assembly in plants Balk, Janneke; Schaedler, Theresia A
Annual review of plant biology,
01/2014, Letnik:
65
Journal Article
Recenzirano
Iron is an essential element for all photosynthetic organisms. The biological use of this transition metal is as an enzyme cofactor, predominantly in electron transfer and catalysis. The main forms ...of iron cofactor are, in order of decreasing abundance, iron-sulfur clusters, heme, and di-iron or mononuclear iron, with a wide functional range. In plants and algae, iron-sulfur cluster assembly pathways of bacterial origin are localized in the mitochondria and plastids, where there is a high demand for these cofactors. A third iron-sulfur cluster assembly pathway is present in the cytosol that depends on the mitochondria but not on plastid assembly proteins. The biosynthesis of heme takes place mainly in the plastids. The importance of iron-sulfur cofactors beyond photosynthesis and respiration has become evident with recent discoveries of novel iron-sulfur proteins involved in epigenetics and DNA metabolism. In addition, increased understanding of intracellular iron trafficking is opening up research into how iron is distributed between iron cofactor assembly pathways and how this distribution is regulated.
Iron plays a crucial role in biochemistry and is an essential micronutrient for plants and humans alike. Although plentiful in the Earth's crust it is not usually found in a form readily accessible ...for plants to use. They must therefore sense and interact with their environment, and have evolved two different molecular strategies to take up iron in the root. Once inside, iron is complexed with chelators and distributed to sink tissues where it is used predominantly in the production of enzyme cofactors or components of electron transport chains. The processes of iron uptake, distribution and metabolism are overseen by tight regulatory mechanisms, at the transcriptional and post-transcriptional level, to avoid iron concentrations building to toxic excess. Iron is also loaded into seeds, where it is stored in vacuoles or in ferritin. This is important for human nutrition as seeds form the edible parts of many crop species. As such, increasing iron in seeds and other tissues is a major goal for biofortification efforts by both traditional breeding and biotechnological approaches.
Increasing the intrinsic nutritional quality of crops, known as biofortification, is viewed as a sustainable approach to alleviate micronutrient deficiencies. In particular, iron deficiency anemia is ...a major global health issue, but the iron content of staple crops such as wheat (Triticum aestivum) is difficult to change because of genetic complexity and homeostasis mechanisms. To identify target genes for the biofortification of wheat, we functionally characterized homologs of the VACUOLAR IRON TRANSPORTER (VIT). The wheat genome contains two VIT paralogs, TaVIT1 and TaVIT2, which have different expression patterns but are both low in the endosperm. TaVIT2, but not TaVIT1, was able to rescue the growth of a yeast (Saccharomyces cerevisiae) mutant defective in vacuolar iron transport. TaVIT2 also complemented a manganese transporter mutant but not a vacuolar zinc transporter mutant. By overexpressing TaVIT2 under the control of an endosperm-specific promoter, we achieved a greater than 2-fold increase in iron in white flour fractions, exceeding minimum legal fortification levels in countries such as the United Kingdom. The antinutrient phytate was not increased and the iron in the white flour fraction was bioavailable in vitro, suggesting that food products made from the biofortified flour could contribute to improved iron nutrition. The single-gene approach impacted minimally on plant growth and also was effective in barley (Hordeum vulgare). Our results show that by enhancing vacuolar iron transport in the endosperm, this essential micronutrient accumulated in this tissue, bypassing existing homeostatic mechanisms.
Ferredoxins (Fd) are small iron-sulphur proteins, with sub-types that have evolved for specific redox functions. Ferredoxin C2 (FdC2) proteins are essential Fd homologues conserved in all ...photosynthetic organisms and a number of different FdC2 functions have been proposed in angiosperms. Here we use RNAi silencing in Arabidopsis thaliana to generate a viable fdC2 mutant line with near-depleted FdC2 protein levels. Mutant leaves have ~50% less chlorophyll a and b, and chloroplasts have poorly developed thylakoid membrane structure. Transcriptomics indicates upregulation of genes involved in stress responses. Although fdC2 antisense plants show increased damage at photosystem II (PSII) when exposed to high light, PSII recovers at the same rate as wild type in the dark. This contradicts literature proposing that FdC2 regulates translation of the D1 subunit of PSII, by binding to psbA transcript. Measurement of chlorophyll biosynthesis intermediates revealed a build-up of Mg-protoporphyrin IX, the substrate of the aerobic cyclase. We localise FdC2 to the inner chloroplast envelope and show that the FdC2 RNAi line has a disproportionately lower protein abundance of antennae proteins, which are nuclear-encoded and must be refolded at the envelope after import.
Organisms need to balance sufficient uptake of iron (Fe) with possible toxicity. In plant roots, a regulon of uptake genes is transcriptionally activated under Fe deficiency, but it is unknown how ...this response is inactivated when Fe becomes available. Here we describe the function of 2 partially redundant E3 ubiquitin ligases, BRUTUS-LIKE1 (BTSL1) and BTSL2, in Arabidopsis thaliana and provide evidence that they target the transcription factor FIT, a key regulator of Fe uptake, for degradation. The btsl double mutant failed to effectively down-regulate the transcription of genes controlled by FIT, and accumulated toxic levels of Fe in roots and leaves. The C-terminal domains of BTSL1 and BTSL2 exhibited E3 ligase activity, and interacted with FIT but not its dimeric partner bHLH39. The BTSL proteins were able to poly-ubiquitinate FIT in vitro and promote FIT degradation in vivo. Thus, posttranslational control of FIT is critical to prevent excess Fe uptake.
An ATP-binding cassette transporter located in the inner mitochondrial membrane is involved in iron-sulfur cluster and molybdenum cofactor assembly in the cytosol, but the transported substrate is ...unknown. ATM3 (ABCB25) from Arabidopsis thaliana and its functional orthologue Atm1 from Saccharomyces cerevisiae were expressed in Lactococcus lactis and studied in inside-out membrane vesicles and in purified form. Both proteins selectively transported glutathione disulfide (GSSG) but not reduced glutathione in agreement with a 3-fold stimulation of ATPase activity by GSSG. By contrast, Fe2+ alone or in combination with glutathione did not stimulate ATPase activity. Arabidopsis atm3 mutants were hypersensitive to an inhibitor of glutathione biosynthesis and accumulated GSSG in the mitochondria. The growth phenotype of atm3-1 was strongly enhanced by depletion of the mitochondrion-localized, GSH-dependent persulfide oxygenase ETHE1, suggesting that the physiological substrate of ATM3 contains persulfide in addition to glutathione. Consistent with this idea, a transportomics approach using mass spectrometry showed that glutathione trisulfide (GS-S-SG) was transported by Atm1. We propose that mitochondria export glutathione polysulfide, containing glutathione and persulfide, for iron-sulfur cluster assembly in the cytosol.
During seed germination, iron (Fe) stored in vacuoles is exported by the redundant NRAMP3 and NRAMP4 transporter proteins. A double nramp3 nramp4 mutant is unable to mobilize Fe stores and does not ...develop in the absence of external Fe. We used RNA sequencing to compare gene expression in nramp3 nramp4 and wild type during germination and early seedling development. Even though sufficient Fe was supplied, the Fe-responsive transcription factors bHLH38, 39, 100, and 101 and their downstream targets FRO2 and IRT1 mediating Fe uptake were strongly upregulated in the nramp3 nramp4 mutant. Activation of the Fe deficiency response was confirmed by increased ferric chelate reductase activity in the mutant. At early stages, genes important for chloroplast redox control (FSD1 and SAPX), Fe homeostasis (FER1 and SUFB), and chlorophyll metabolism (HEMA1 and NYC1) were downregulated, indicating limited Fe availability in plastids. In contrast, expression of FRO3, encoding a ferric reductase involved in Fe import into the mitochondria, was maintained, and Fe-dependent enzymes in the mitochondria were unaffected in nramp3 nramp4. Together, these data show that a failure to mobilize Fe stores during germination triggered Fe deficiency responses and strongly affected plastids, but not mitochondria.
Eukaryotic organisms have evolved a set of strategies to safeguard genome integrity, but the underlying mechanisms remain poorly understood. Here, we report that ASYMMETRIC LEAVES1/2 ENHANCER7 (AE7), ...an Arabidopsis thaliana gene encoding a protein in the evolutionarily conserved Domain of Unknown Function 59 family, participates in the cytosolic iron-sulfur (Fe-S) cluster assembly (CIA) pathway to maintain genome integrity. The severe ae7-2 allele is embryo lethal, whereas plants with the weak ae7 (ae7-1) allele are viable but exhibit highly accumulated DNA damage that activates the DNA damage response to arrest the cell cycle. AE7 is part of a protein complex with CIA1, NAR1, and MET18, which are highly conserved in eukaryotes and are involved in the biogenesis of cytosolic and nuclear Fe-S proteins. ae7-1 plants have lower activities of the cytosolic 4Fe-4S enzyme aconitase and the nuclear 4Fe-4S enzyme DNA glycosylase ROS1. Additionally, mutations in the gene encoding the mitochondrial ATP binding cassette transporter ATM3/ABCB25, which is required for the activity of cytosolic Fe-S enzymes in Arabidopsis, also result in defective genome integrity similar to that of ae7-1. These results indicate that AE7 is a central member of the CIA pathway, linking plant mitochondria to nuclear genome integrity through assembly of Fe-S proteins.
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