Overexpression of the FRO2 ferric chelate reductase confers tolerance to growth on low iron and uncovers posttranscriptional control

Insights in to iron-based nanoparticles (hematite and magnetite) improving the maize growth (Zea mays L.) and iron nutrition with low environmental impacts

The possible potential application of Fe-NPs on Fe nutrition, heavy metals uptake and soil microbial community needs to be investigated. In the current research, a pot experiment was used to examine the implications of Fe-NPs (α-Fe2O3 and Fe3O4) on maize growth, Fe uptake and transportation, soil microbial community, and environmental risk. Fe3O4, α-Fe2O3, FeSO4 at a rate of 800 mg Fe kg-1 were applied in soils with four replications under a completely randomized design for a period of 60 days. Results showed that Fe uptake by maize roots were increased by 107-132% than control, with obvious variations across different treatments (Fe3O4> α-Fe2O3> FeSO4> control). Similarly, plant height, leaf surface area, and biomass were increased by 40-64%, 52-91% and 38-109% respectively, with lower values by FeSO4 application. The elevated level of chlorophyll contents and carotenoids and significant effects with control on antioxidant enzymes activities (i.e., catalase, and superoxide dismutase) suggested that application of Fe-NPs improved overall biochemical processes. The differential expression of important Fe transporters (i.e., ZmYS1 and ZmFER1) as compared to control indicated the plant strategic response for efficient uptake and distribution of Fe. Importantly, Fe-NPs reduced the heavy metals uptake (i.e., chromium, cadmium, arsenic, nickel, copper) by complex formation, and showed no toxicity to the soil microbial community. In summary, the application of Fe-NPs can be a promising approach for improving crop productivity and Fe nutrition without negatively affecting soil microbial community, and fostering sustainable agricultural production.

Soil and Mineral Nutrients in Plant Health: A Prospective Study of Iron and Phosphorus in the Growth and Development of Plants

Plants being sessile are exposed to different environmental challenges and consequent stresses associated with them. With the prerequisite of minerals for growth and development, they coordinate their mobilization from the soil through their roots. Phosphorus (P) and iron (Fe) are macro- and micronutrient; P serves as an important component of biological macromolecules, besides driving major cellular processes, including photosynthesis and respiration, and Fe performs the function as a cofactor for enzymes of vital metabolic pathways. These minerals help in maintaining plant vigor via alterations in the pH, nutrient content, release of exudates at the root surface, changing dynamics of root microbial population, and modulation of the activity of redox enzymes. Despite this, their low solubility and relative immobilization in soil make them inaccessible for utilization by plants. Moreover, plants have evolved distinct mechanisms to cope with these stresses and coregulate the levels of minerals (Fe, P, etc.) toward the maintenance of homeostasis. The present study aims at examining the uptake mechanisms of Fe and P, and their translocation, storage, and role in executing different cellular processes in plants. It also summarizes the toxicological aspects of these minerals in terms of their effects on germination, nutrient uptake, plant-water relationship, and overall yield. Considered as an important and indispensable component of sustainable agriculture, a separate section covers the current knowledge on the cross-talk between Fe and P and integrates complete and balanced information of their effect on plant hormone levels.

A Critical Review of Methodologies for Evaluating Iron Fertilizers Based on Iron Reduction and Uptake by Strategy I Plants

Under iron (Fe)-limited conditions, plants have developed strategies for acquiring this essential micronutrient. Several Fe sources have been studied as potential fertilizers, with Fe synthetic chelates being the most used to prevent and correct Fe chlorosis in crops. The determination of the activity of the Fe chelate reductase (FCR) enzyme has long been described in the literature to understand the efficiency of Strategy I plants in acquiring Fe from fertilizers under deficient conditions. Other experiments have focused on the translocation of Fe to the plant to define the effectiveness of Fe fertilizers. Yet, both assays are relevant in knowing the capacity of a novel Fe source and other compounds alleviating Fe chlorosis in Strategy I plants. This work reviews the methodologies that are used in FCR assays to evaluate novel Fe fertilizers, including the factors modulating the results obtained for FCR assay activity, such as the Fe substrate, the Fe level during the growing period and during the FCR assay, the pH, the choice of an in vivo or in vitro method, and the plant species. A discussion of the benefits of the concurrence of FCR and Fe uptake assays is then presented alongside a proposed methodology for assessing the effectiveness of Fe fertilizers, emphasizing the importance of understanding chemical and physiological plant interactions. This methodology unifies key factors that modify FCR activity and combines these with the use of the 57Fe tracer to enhance our comprehension of the efficacy of Fe-based fertilizers' effectiveness in alleviating Fe chlorosis. This comprehensive approach not only contributes to the fundamental understanding of Fe-deficient Strategy I plants but also establishes a robust method for determining the efficiency of novel sources for correcting Fe deficiency in plants.

Role of transcription factor complex OsbHLH156-OsIRO2 in regulating manganese, copper, and zinc transporters in rice

Iron (Fe), manganese (Mn), copper (Cu), and zinc (Zn) are essential micronutrients that are necessary for plant growth and development, but can be toxic at supra-optimal levels. Plants have evolved a complex homeostasis network that includes uptake, transport, and storage of these metals. It was shown that the transcription factor (TF) complex OsbHLH156-OsIRO2 is activated under Fe deficient conditions and acts as a central regulator on Strategy II Fe acquisition. In this study, the role of the TF complex on Mn, Cu, and Zn uptake was evaluated. While Fe deficiency led to significant increases in shoot Mn, Cu, and Zn concentrations, the increases of these divalent metal concentrations were significantly suppressed in osbhlh156 and osiro2 mutants, suggesting that the TF complex plays roles on Mn, Cu, and Zn uptake and transport. An RNA-sequencing assay showed that the genes associated with Mn, Cu, and Zn uptake and transport were significantly suppressed in the osbhlh156 and osiro2 mutants. Transcriptional activation assays demonstrated that the TF complex could directly bind to the promoters of OsIRT1, OsYSL15, OsNRAMP6, OsHMA2, OsCOPT1/7, and OsZIP5/9/10, and activate their expression. In addition, the TF complex is required to activate the expression of nicotianamine (NA) and 2'-deoxymugineic acid (DMA) synthesis genes, which in turn facilitate the uptake and transport of Mn, Cu, and Zn. Furthermore, OsbHLH156 and OsIRO2 promote Cu accumulation to partially restore the Fe-deficiency symptoms. Taken together, OsbHLH156 and OsIRO2 TF function as core regulators not only in Fe homeostasis, but also in Mn, Cu, and Zn accumulation.

Ferric reduction oxidase in Lilium pumilum affects plant saline-alkaline tolerance by regulating ROS homeostasis

Ferric reduction oxidase (FRO) plays important roles in biotic and abiotic stress. However, the function of ferric reduction oxidase from Lilium pumilum in response to NaHCO3 is unknown. Here we report the functional characterization of ferric reduction oxidase 7 in Lilium pumilum (LpFRO7) in stresses. Under NaHCO3 stress, the LpFRO7 overexpression lines exhibited lower accumulation of reactive oxygen species (ROS), higher activities in antioxidant enzyme (CAT, SOD and POD) and ferrite reductase, resulting in improved tolerance compared to the wild type (WT). In order to determine the functional network of LpFRO7, it was confirmed by EMSA assays, Yeast one-hybrid assays and Dual luciferase reporter assays that LpbHLH115 transcription factor can bind to the promoter of LpFRO7. Yeast two-hybrid assays, BiFC, and LCI assays were performed to prove that LpFRO7 can interact with LpTrx. Combining these findings, we concluded that LpFRO7 affects plant saline-alkaline tolerance by regulating ROS homeostasis.

Genome-Wide Identification of the Ferric Chelate Reductase (FRO) Gene Family in Peanut and Its Diploid Progenitors: Structure, Evolution, and Expression Profiles

The ferric chelate reductase (FRO) family plays a vital role in metal ion homeostasis in a variety of locations in the plants. However, little is known about this family in peanut (Arachis hypogaea). This study aimed to identify FRO genes from the genomes of peanut and the two diploid progenitors (A. duranensis and A. ipaensis) and to analyze their gene/protein structures and evolution. In addition, transcriptional responses of AhFRO genes to Fe deficiency and/or Cu exposure were investigated in two peanut cultivars with different Fe deficiency tolerance (Silihong and Fenghua 1). A total of nine, four, and three FRO genes were identified in peanut, A. duranensis, and A. ipaensis, respectively, which were divided into three groups. Most AhFRO genes underwent WGD/segmental duplication, leading to the expansion of the AhFRO gene family. In general, clustered members share similar gene/protein structures. However, significant divergences occurred in AhFRO2 genes. Three out of five AhFRO2 genes were lowly expressed in all tissues under normal conditions, which may be beneficial for avoiding gene loss. Transcription analysis revealed that AhFRO2 and AhFRO7 genes might be involved in the reduction of Fe/Cu in plasma membranes and plastids, respectively. AhFRO8 genes appear to confer Fe reduction in the mitochondria. Moreover, Fe deficiency induced an increase of Cu accumulation in peanut plants in which AhFRO2.2/2.4/2.5 and FRO7.1/7.2 might be involved. Our findings provided new clues for further understanding the roles of AhFRO genes in the Fe/Cu interaction in peanut.

Ferric reduction by a CYBDOM protein counteracts increased iron availability in root meristems induced by phosphorus deficiency

To mobilize sparingly available phosphorus (P) in the rhizosphere, many plant species secrete malate to release P sorbed onto (hydr)oxides of aluminum and iron (Fe). In the presence of Fe, malate can provoke Fe over-accumulation in the root apoplast, triggering a series of events that inhibit root growth. Here, we identified HYPERSENSITIVE TO LOW P1 (HYP1), a CYBDOM protein constituted of a DOMON and a cytochrome b561 domain, as critical to maintain cell elongation and meristem integrity under low P. We demonstrate that HYP1 mediates ascorbate-dependent trans-plasma membrane electron transport and can reduce ferric and cupric substrates in Xenopus laevis oocytes and in planta. HYP1 expression is up-regulated in response to P deficiency in the proximal zone of the root apical meristem. Disruption of HYP1 leads to increased Fe and callose accumulation in the root meristem and causes significant transcriptional changes in roots. We further demonstrate that HYP1 activity overcomes malate-induced Fe accumulation, thereby preventing Fe-dependent root growth arrest in response to low P. Collectively, our results uncover an ascorbate-dependent metalloreductase that is critical to protect root meristems of P-deficient plants from increased Fe availability and provide insights into the physiological function of the yet poorly characterized but ubiquitous CYBDOM proteins.

A CYBDOM protein impacts iron homeostasis and primary root growth under phosphate deficiency in Arabidopsis

Arabidopsis primary root growth response to phosphate (Pi) deficiency is mainly controlled by changes in apoplastic iron (Fe). Upon Pi deficiency, apoplastic Fe deposition in the root apical meristem activates pathways leading to the arrest of meristem maintenance and inhibition of cell elongation. Here, we report that a member of the uncharacterized cytochrome b561 and DOMON domain (CYBDOM) protein family, named CRR, promotes iron reduction in an ascorbate-dependent manner and controls apoplastic iron deposition. Under low Pi, the crr mutant shows an enhanced reduction of primary root growth associated with increased apoplastic Fe in the root meristem and a reduction in meristematic cell division. Conversely, CRR overexpression abolishes apoplastic Fe deposition rendering primary root growth insensitive to low Pi. The crr single mutant and crr hyp1 double mutant, harboring a null allele in another member of the CYDOM family, shows increased tolerance to high-Fe stress upon germination and seedling growth. Conversely, CRR overexpression is associated with increased uptake and translocation of Fe to the shoot and results in plants highly sensitive to Fe excess. Our results identify a ferric reductase implicated in Fe homeostasis and developmental responses to abiotic stress, and reveal a biological role for CYBDOM proteins in plants.

Exogenous Melatonin Promotes Cold Tolerance in Grape Seedlings: Physiological, Transcriptomic, and Functional Evidence

Melatonin (MEL) is an antioxidant molecule that enhances plant tolerance to environmental stress. However, the mechanisms by which MEL regulates cold signaling pathways in grapes under cold stress remain elusive. Here, we investigated the physiological and transcriptomic changes in grape seedlings treated with exogenous MEL to determine their protective role under cold stress. Results showed that 150 μM MEL effectively attenuated cold-induced cell damage by reducing reactive oxygen species (ROS) and preserving the chloroplast structure and function. MEL also inhibited tannin degradation, which contributed to its protective effect. Exogenous MEL promoted the synthesis of endogenous MEL, abscisic acid, auxin, and cytokinin while inhibiting gibberellin. Transcriptomic profiling revealed 776 differentially expressed transcripts in MEL-treated samples compared to controls. Functional analysis of a candidate hub gene, VvHSFA6b, showed that its overexpression in grape calli enhances cold tolerance by activating jasmonic acid synthesis pathway genes, promoting JA accumulation, and inhibiting JAZ-repressed transcription factors.

Reactive oxygen species- and nitric oxide-dependent regulation of ion and metal homeostasis in plants

Deterioration and impoverishment of soil, caused by environmental pollution and climate change, result in reduced crop productivity. To adapt to hostile soils, plants have developed a complex network of factors involved in stress sensing, signal transduction, and adaptive responses. The chemical properties of reactive oxygen species (ROS) and reactive nitrogen species (RNS) allow them to participate in integrating the perception of external signals by fine-tuning protein redox regulation and signal transduction, triggering specific gene expression. Here, we update and summarize progress in understanding the mechanistic basis of ROS and RNS production at the subcellular level in plants and their role in the regulation of ion channels/transporters at both transcriptional and post-translational levels. We have also carried out an in silico analysis of different redox-dependent modifications of ion channels/transporters and identified cysteine and tyrosine targets of nitric oxide in metal transporters. Further, we summarize possible ROS- and RNS-dependent sensors involved in metal stress sensing, such as kinases and phosphatases, as well as some ROS/RNS-regulated transcription factors that could be involved in metal homeostasis. Understanding ROS- and RNS-dependent signaling events is crucial to designing new strategies to fortify crops and improve plant tolerance of nutritional imbalance and metal toxicity.

Histone variant HTB4 delays leaf senescence by epigenetic control of Ib bHLH transcription factor-mediated iron homeostasis

Leaf senescence is an orderly process regulated by multiple internal factors and diverse environmental stresses including nutrient deficiency. Histone variants are involved in regulating plant growth and development. However, their functions and underlying regulatory mechanisms in leaf senescence remain largely unclear. Here, we found that H2B histone variant HTB4 functions as a negative regulator of leaf senescence. Loss of function of HTB4 led to early leaf senescence phenotypes that were rescued by functional complementation. RNA-seq analysis revealed that several Ib subgroup basic helix-loop-helix (bHLH) transcription factors (TFs) involved in iron (Fe) homeostasis, including bHLH038, bHLH039, bHLH100, and bHLH101, were suppressed in the htb4 mutant, thereby compromising the expressions of FERRIC REDUCTION OXIDASE 2 (FRO2) and IRON-REGULATED TRANSPORTER (IRT1), two important components of the Fe uptake machinery. Chromatin immunoprecipitation-quantitative polymerase chain reaction analysis revealed that HTB4 could bind to the promoter regions of Ib bHLH TFs and enhance their expression by promoting the enrichment of the active mark H3K4me3 near their transcriptional start sites. Moreover, overexpression of Ib bHLH TFs or IRT1 suppressed the premature senescence phenotype of the htb4 mutant. Our work established a signaling pathway, HTB4-bHLH TFs-FRO2/IRT1-Fe homeostasis, which regulates the onset and progression of leaf senescence.

Response and Tolerance of Macleaya cordata to Excess Zinc Based on Transcriptome and Proteome Patterns

Macleaya cordata is a dominant plant of mine tailings and a zinc (Zn) accumulator with high Zn tolerance. In this study, M. cordata seedlings cultured in Hoagland solution were treated with 200 μmol·L^-1 of Zn for 1 day or 7 days, and then, their leaves were taken for a comparative analysis of the transcriptomes and proteomes between the leaves of the control and Zn treatments. Differentially expressed genes included those that were iron (Fe)-deficiency-induced, such as vacuolar iron transporter VIT, ABC transporter ABCI17 and ferric reduction oxidase FRO. Those genes were significantly upregulated by Zn and could be responsible for Zn transport in the leaves of M. cordata. Differentially expressed proteins, such as chlorophyll a/b-binding proteins, ATP-dependent protease, and vacuolar-type ATPase located on the tonoplast, were significantly upregulated by Zn and, thus, could be important in chlorophyll biosynthesis and cytoplasm pH stabilization. Moreover, the changes in Zn accumulation, the production of hydrogen peroxide, and the numbers of mesophyll cells in the leaves of M. cordata were consistent with the expression of the genes and proteins. Thus, the proteins involved in the homeostasis of Zn and Fe are hypothesized to be the keys to the tolerance and accumulation of Zn in M. cordata. Such mechanisms in M. cordata can suggest novel approaches to genetically engineering and biofortifying crops.

ELONGATED HYPOCOTYL 5 regulates BRUTUS and affects iron acquisition and homeostasis in Arabidopsis thaliana

Iron (Fe) is an essential micronutrient for both plants and animals. Fe-limitation significantly reduces crop yield and adversely impacts on human nutrition. Owing to limited bioavailability of Fe in soil, plants have adapted different strategies that not only regulate Fe-uptake and homeostasis but also bring modifications in root system architecture to enhance survival. Understanding the molecular mechanism underlying the root growth responses will have critical implications for plant breeding. Fe-uptake is regulated by a cascade of basic helix-loop-helix (bHLH) transcription factors (TFs) in plants. In this study, we report that HY5 (Elongated Hypocotyl 5), a member of the basic leucine zipper (bZIP) family of TFs, plays an important role in the Fe-deficiency signaling pathway in Arabidopsis thaliana. The hy5 mutant failed to mount optimum Fe-deficiency responses, and displayed root growth defects under Fe-limitation. Our analysis revealed that the induction of the genes involved in Fe-uptake pathway (FIT-FER-LIKE IRON DEFICIENCY-INDUCED TRANSCRIPTION FACTOR, FRO2-FERRIC REDUCTION OXIDASE 2 and IRT1-IRON-REGULATED TRANSPORTER1) is reduced in the hy5 mutant as compared with the wild-type plants under Fe-deficiency. Moreover, we also found that the expression of coumarin biosynthesis genes is affected in the hy5 mutant under Fe-deficiency. Our results also showed that HY5 negatively regulates BRUTUS (BTS) and POPEYE (PYE). Chromatin immunoprecipitation followed by quantitative polymerase chain reaction revealed direct binding of HY5 to the promoters of BTS, FRO2 and PYE. Altogether, our results showed that HY5 plays an important role in the regulation of Fe-deficiency responses in Arabidopsis.

The Molecular Mechanism of GhbHLH121 in Response to Iron Deficiency in Cotton Seedlings

Iron deficiency caused by high pH of saline-alkali soil is a major source of abiotic stress affecting plant growth. However, the molecular mechanism underlying the iron deficiency response in cotton (Gossypium hirsutum) is poorly understood. In this study, we investigated the impacts of iron deficiency at the cotton seedling stage and elucidated the corresponding molecular regulation network, which centered on a hub gene GhbHLH121. Iron deficiency induced the expression of genes with roles in the response to iron deficiency, especially GhbHLH121. The suppression of GhbHLH121 with virus-induced gene silence technology reduced seedlings' tolerance to iron deficiency, with low photosynthetic efficiency and severe damage to the structure of the chloroplast. Contrarily, ectopic expression of GhbHLH121 in Arabidopsis enhanced tolerance to iron deficiency. Further analysis of protein/protein interactions revealed that GhbHLH121 can interact with GhbHLH IVc and GhPYE. In addition, GhbHLH121 can directly activate the expression of GhbHLH38, GhFIT, and GhPYE independent of GhbHLH IVc. All told, GhbHLH121 is a positive regulator of the response to iron deficiency in cotton, directly regulating iron uptake as the upstream gene of GhFIT. Our results provide insight into the complex network of the iron deficiency response in cotton.

Understanding the Mechanisms of Fe Deficiency in the Rhizosphere to Promote Plant Resilience

One of the most significant constraints on agricultural productivity is the low availability of iron (Fe) in soil, which is directly related to biological, physical, and chemical activities in the rhizosphere. The rhizosphere has a high iron requirement due to plant absorption and microorganism density. Plant roots and microbes in the rhizosphere play a significant role in promoting plant iron (Fe) uptake, which impacts plant development and physiology by influencing nutritional, biochemical, and soil components. The concentration of iron accessible to these live organisms in most cultivated soil is quite low due to its solubility being limited by stable oxyhydroxide, hydroxide, and oxides. The dissolution and solubility rates of iron are also significantly affected by soil pH, microbial population, organic matter content, redox processes, and particle size of the soil. In Fe-limiting situations, plants and soil microbes have used active strategies such as acidification, chelation, and reduction, which have an important role to play in enhancing soil iron availability to plants. In response to iron deficiency, plant and soil organisms produce organic (carbohydrates, amino acids, organic acids, phytosiderophores, microbial siderophores, and phenolics) and inorganic (protons) chemicals in the rhizosphere to improve the solubility of poorly accessible Fe pools. The investigation of iron-mediated associations among plants and microorganisms influences plant development and health, providing a distinctive prospect to further our understanding of rhizosphere ecology and iron dynamics. This review clarifies current knowledge of the intricate dynamics of iron with the end goal of presenting an overview of the rhizosphere mechanisms that are involved in the uptake of iron by plants and microorganisms.

MxFRO4 confers iron and salt tolerance through up-regulating antioxidant capacity associated with the ROS scavenging

Iron is involved in various metabolic pathways of plants. Stress from iron deficiency and toxicity in the soil adversely affects plant growth. Therefore, studying the mechanism of iron absorption and transport by plants is of important for resistance to iron stress and to increase crop yield. In this study, Malus xiaojinensis (a Fe-efficient Malus plant) was used as research material. A ferric reduction oxidase (FRO) family gene member was cloned and named MxFRO4. The MxFRO4 encoded a protein of 697 amino acid residues with a predicted molecular weight of 78.54 kDa and a theoretical isoelectric point of 4.90. A subcellular localization assay showed that the MxFRO4 protein was localized on the cell membrane. The expression of MxFRO4 was enriched in immature leaves and roots of M. xiaojinensis, and was strongly affected by low-iron, high-iron, and salt treatments. After introduction of MxFRO4 into Arabidopsis thaliana, the iron and salt stress tolerance of transgenic A. thaliana was greatly improved. Under exposure to low-iron and high-iron stresses, the primary root length, seedling fresh weight, contents of proline, chlorophyll, and iron, and iron(III) chelation activity of the transgenic lines were significantly increased compared with the wild type. The contents of chlorophyll and proline, and the activities of the antioxidant enzymes superoxide dismutase, peroxidase, and catalase were significantly higher in transgenic A. thaliana overexpressing MxFRO4 under salt stress compared with the wild type, whereas the malondialdehyde content was decreased. These results suggest that MxFRO4 contributes to alleviating the effects of low-iron, high-iron, and salinity stresses in transgenic A. thaliana.

Impact of soybean-associated plant growth-promoting bacteria on plant growth modulation under alkaline soil conditions

Conventional strategies to manage iron (Fe) deficiency still present drawbacks, and more eco-sustainable solutions are needed. Knowledge on soybean-specific diversity and functional traits of their plant growth-promoting bacteria (PGPB) potentiates their applicability as bioinoculants to foster soybean performance under calcareous soil conditions. This work aimed to assess the efficacy of PGPB, retrieved from soybean tissues/rhizosphere, in enhancing plant growth and development as well as crop yield under alkaline soil conditions. Seventy-six bacterial strains were isolated from shoots (18%), roots (53%), and rhizosphere (29%) of soybean. Twenty-nine genera were identified, with Bacillus and Microbacterium being the most predominant. Based on distinct plant growth-promoting traits, the endophyte Bacillus licheniformis P2.3 and the rhizobacteria Bacillus aerius S2.14 were selected as bioinoculants. In vivo tests showed that soybean photosynthetic parameters, chlorophyll content, total fresh weight, and Fe concentrations were not significantly affected by bioinoculation. However, inoculation with B. licheniformis P2.3 increased pod number (33%) and the expression of Fe-related genes (FRO2, IRT1, F6'H1, bHLH38, and FER4), and decreased FC-R activity (45%). Moreover, bioinoculation significantly affected Mn, Zn, and Ca accumulation in plant tissues. Soybean harbors several bacterial strains in their tissues and in the rhizosphere with capacities related to Fe nutrition and plant growth promotion. The strain B. licheniformis P2.3 showed the best potential to be incorporated in bioinoculant formulations for enhancing soybean performance under alkaline soil conditions.

Identification and Functional Analysis of Two Mitoferrins, CsMIT1 and CsMIT2, Participating in Iron Homeostasis in Cucumber

Mitochondria are one of the major iron sinks in plant cells. Mitochondrial iron accumulation involves the action of ferric reductase oxidases (FRO) and carriers located in the inner mitochondrial membrane. It has been suggested that among these transporters, mitoferrins (mitochondrial iron transporters, MITs) belonging to the mitochondrial carrier family (MCF) function as mitochondrial iron importers. In this study, two cucumber proteins, CsMIT1 and CsMIT2, with high homology to Arabidopsis, rice and yeast MITs were identified and characterized. CsMIT1 and CsMIT2 were expressed in all organs of the two-week-old seedlings. Under Fe-limited conditions as well as Fe excess, the mRNA levels of CsMIT1 and CsMIT2 were altered, suggesting their regulation by iron availability. Analyses using Arabidopsis protoplasts confirmed the mitochondrial localization of cucumber mitoferrins. Expression of CsMIT1 and CsMIT2 restored the growth of the Δmrs3Δmrs4 mutant (defective in mitochondrial Fe transport), but not in mutants sensitive to other heavy metals. Moreover, the altered cytosolic and mitochondrial Fe concentrations, observed in the Δmrs3Δmrs4 strain, were recovered almost to the levels of WT yeast by expressing CsMIT1 or CsMIT2. These results indicate that cucumber proteins are involved in the iron transport from the cytoplasm to the mitochondria.

Protein kinase MtCIPK12 modulates iron reduction in Medicago truncatula by regulating riboflavin biosynthesis

Iron (Fe) is an essential micronutrient, and deficiency in available Fe is one of the most important limiting factors for plant growth. In some species including Medicago truncatula, Fe deficiency results in accumulation of riboflavin, a response associated with Fe acquisition. However, how the plant's Fe status is integrated to tune riboflavin biosynthesis and how riboflavin levels affect Fe acquisition and utilization remains largely unexplored. We report that protein kinase CIPK12 regulates ferric reduction by accumulation of riboflavin and its derivatives in roots of M. truncatula via physiological and molecular characterization of its mutants and over-expressing materials. Mutations in CIPK12 enhance Fe accumulation and improve photosynthetic efficiency, whereas overexpression of CIPK12 shows the opposite phenotypes. The Calcineurin B-like proteins CBL3 and CBL8 interact with CIPK12, which negatively regulates the expression of genes encoding key enzymes in the riboflavin biosynthesis pathway. CIPK12 negatively regulates Fe acquisition by suppressing accumulation of riboflavin and its derivatives in roots, which in turn influences ferric reduction activity by riboflavin-dependent electron transport under Fe deficiency. Our findings uncover a new regulatory mechanism by which CIPK12 regulates riboflavin biosynthesis and Fe-deficiency responses in plants.

Biofortification of Crops to Fight Anemia: Role of Vacuolar Iron Transporters

Plant-based foods provide all the crucial nutrients for human health. Among these, iron (Fe) is one of the essential micronutrients for plants and humans. A lack of Fe is a major limiting factor affecting crop quality, production, and human health. There are people who suffer from various health problems due to the low intake of Fe in their plant-based foods. Anemia has become a serious public health issue due to Fe deficiency. Enhancing Fe content in the edible part of food crops is a major thrust area for scientists worldwide. Recent progress in nutrient transporters has provided an opportunity to resolve Fe deficiency or nutritional problems in plants and humans. Understanding the structure, function, and regulation of Fe transporters is essential to address Fe deficiency in plants and to improve Fe content in staple food crops. In this review, we summarized the role of Fe transporter family members in the uptake, cellular and intercellular movement, and long-distance transport of Fe in plants. We draw insights into the role of vacuolar membrane transporters in the crop for Fe biofortification. We also provide structural and functional insights into cereal crops' vacuolar iron transporters (VITs). This review will help highlight the importance of VITs for improving the Fe biofortification of crops and alleviating Fe deficiency in humans.

The nonpathogenic strain of Fusarium oxysporum FO12 induces Fe deficiency responses in cucumber (Cucumis sativus L.) plants

MAIN CONCLUSION

FO12 strain enhances Fe deficiency responses in cucumber plants, probably through the production of ethylene and NO in the subapical regions of the roots. Rhizosphere microorganisms can elicit induced systemic resistance (ISR) in plants. This type of resistance involves complex mechanisms that confer protection to the plant against pathogen attack. Additionally, it has been reported by several studies that ISR and Fe deficiency responses are modulated by common pathways, involving some phytohormones and signaling molecules, like ethylene and nitric oxide (NO). The aim of this study was to determine whether the nonpathogenic strain of Fusarium oxysporum FO12 can induce Fe deficiency responses in cucumber (Cucumis sativus L.) plants. Our results demonstrate that the root inoculation of cucumber plants with the FO12 strain promotes plant growth after several days of cultivation, as well as rhizosphere acidification and enhancement of ferric reductase activity. Moreover, Fe-related genes, such as FRO1, IRT1 and HA1, are upregulated at certain times after FO12 inoculation either upon Fe-deficiency or Fe-sufficient conditions. Furthermore, it has been found that this fungus colonizes root cortical tissues, promoting the upregulation of ethylene synthesis genes and NO production in the root subapical regions. To better understand the effects of the FO12 strain on field conditions, cucumber plants were inoculated and cultivated in a calcareous soil under greenhouse conditions. The results obtained show a modification of some physiological parameters in the inoculated plants, such as flowering and reduction of tissue necrosis. Overall, the results suggest that the FO12 strain could have a great potential as a Fe biofertilizer and biostimulant.

Phytoaccumulation of zinc and its associated impact on the growth performance and tolerance index of six non-food crop plants grown in Zn-contaminated soil

Crop plant remediation and detoxification of Zn-contaminated soils may pose a significant threat to food safety and, thus, human health. Therefore, the current study was carried out to assess the ability of six non-food crop plants (NFCP); Zea mays L. cultivar 360 (T360), Z. mays cultivar 123 (T123), Helianthus annuus L., Brassica juncea (L.) Czern., Ricinus communis L., and Simmondsia chinensis (Link) C.K. Schneid to remediate and restore Zn-contaminated soils. The investigated plants tolerate 150 mg/kg of Zn content of the soil, where they had tolerance index (TI) > 1 for all growth criteria, except the root dry weight (DW) of S. chinensis. Z. mays T123 and R. communis were the most susceptible plants, while B. juncea and S. chinensis were moderately tolerant, while H. annuus was the most tolerant to high Zn concentrations in a growing medium. Increasing the soil Zn content led to a significant increase (p < 0.05) in Zn concentration in the various tissues of the six NFCPs. The studied NFCP did not translocate Zn to their grains/seeds; consequently, they can be used safely for Zn-contaminated soils. The Zn content in root and shoot was negatively correlated with the TI of their length and weight, while the translocation factor (TF) of Zn from root to shoot was positively correlated to the TI of the root length and weight. The six studied NFCPs were arranged based on their phytoremediation efficiency as follows: B. juncea (31.86%) > Z. mays T123 (31.14%) > Z. mays T360 (27.59%) > H. annuus (20.85%) > S. chinensis (20.29%) > R. communis (15.3%). All tested NFCPs accumulated significant concentrations of Zn in their roots and shoots, a high Zn uptake potential, and biomass at 150-450 mg/kg of Zn treatments, indicating that these plants are good candidates for the implementation of a new strategy of cultivating NFCP for phytoremediation of Zn-contaminated soils.

Hydrogen sulfide and nitric oxide regulate the adaptation to iron deficiency through affecting Fe homeostasis and thiol redox modification in Glycine max seedlings

Iron (Fe) is a vital microelement required for the growth and development of plants. Hydrogen sulfide (H₂S) and nitric oxide (NO), as messenger molecules, participated in the regulation of plant physiological processes. Here, we studied the interaction effects of H₂S and NO on the adaptation to Fe deficiency in Glycine max L. Physiological, biochemical and molecular approaches were conducted to analyze the role of H₂S and NO in regulating the adaptation to Fe deficiency in soybean. We found that H₂S and NO had obvious rescuing function on the Fe deficiency-induced the plant growth inhibition, which was significantly correlated with the increase in Fe content in the leaves, stems, and roots of soybean. Meanwhile, H⁺-flux, ferric chelate reductase (FCR) activity, and root apoplast Fe content were significantly affected by H₂S and NO. Under Fe deficiency conditions NO and H₂S regulated the expression of genes related to Fe homeostasis. Moreover, photosynthesis (Pn) and photosystem II (PSII) efficiency were enhanced by H₂S and NO, and thiol redox modification was important for regulating the adaptation of Fe deficiency. The aforementioned affirmative influences caused by H₂S and NO were also totally reversed by cPTIO (a NO scavenger). Our results suggested that H₂S might act upstream of NO in response to Fe deficiency by affecting the Fe homeostasis enzyme activities and gene expression, and by promoting Fe accumulation in plant tissues as well as by enhancing thiol redox modification and photosynthesis in soybean plants.

The Role of Membrane Transporters in the Biofortification of Zinc and Iron in Plants

Over three billion people suffer from various health issues due to the low supply of zinc (Zn) and iron (Fe) in their food. Low supply of micronutrients is the main cause of malnutrition and biofortification could help to solve this issue. Understanding the molecular mechanisms of biofortification is challenging. The membrane transporters are involved in the uptake, transport, storage, and redistribution of Zn and Fe in plants. These transporters are also involved in biofortification and help to load the Zn and Fe into the endosperm of the seeds. Very little knowledge is available on the role and functions of membrane transporters involved in seed biofortification. Understanding the mechanism and role of membrane transporters could be helpful to improve biofortification. In this review, we provide the details on membrane transporters involved in the uptake, transport, storage, and redistribution of Zn and Fe. We also discuss available information on transporters involved in seed biofortification. This review will help plant breeders and molecular biologists understand the importance and implications of membrane transporters for seed biofortification.

Apoplast utilisation of nanohaematite initiates parallel suppression of RIBA1 and FRO1&3 in Cucumis sativus

Nanoscale Fe containing particles can penetrate the root apoplast. Nevertheless, cell wall size exclusion questions that for Fe mobilisation, a close contact between the membrane integrating FERRIC REDUCTASE OXIDASE (FRO) enzymes and Fe containing particles is required. Haematite nanoparticle suspension, size of 10-20 nm, characterized by ⁵⁷Fe Mössbauer spectroscopy, TEM, ICP and SAED was subjected to Fe utilisation by the flavin secreting model plant cucumber (Cucumis sativus). Alterations in the structure and distribution of the particles were revealed by ⁵⁷Fe Mössbauer spectroscopy, HRTEM and EDS element mapping. Biological utilisation of Fe resulted in a suppression of Fe deficiency responses (expression of CsFRO 1, 2 & 3 and RIBOFLAVIN A1; CsRIBA1 genes and root ferric chelate reductase activity). Haematite nanoparticles were stacked in the middle lamella of the apoplast. Fe mobilisation is evidenced by the reduction in the particle size. Fe release from nanoparticles does not require a contact with the plasma membrane. Parallel suppression in the CsFRO 1&3 and CsRIBA1 transcript amounts support that flavin biosynthesis is an inclusive Fe deficiency response involved in the reduction-based Fe utilisation of Cucumis sativus roots. CsFRO2 is suggested to play a role in the intracellular Fe homeostasis.

Iron Availability and Homeostasis in Plants: A Review of Responses, Adaptive Mechanisms, and Signaling

Iron is an essential element for all living organisms, playing a major role in plant biochemistry as a redox catalyst based on iron redox properties. Iron is the fourth most abundant element of the Earth's crust, but its uptake by plants is complex because it is often in insoluble forms that are not easily accessible for plants to use. The physical and chemical speciation of iron, as well as rhizosphere activity, are key factors controlling the bioavailability of Fe. Iron can be under reduced (Fe²⁺) or oxidized (Fe³⁺) ionic forms, adsorbed onto mineral surfaces, forming complexes with organic molecules, precipitated to form poorly crystalline hydroxides to highly crystalline iron oxides, or included in crystalline Fe-rich mineral phases. Plants must thus adapt to a complex and changing iron environment, and their response is finely regulated by multiple signaling pathways initiated by a diversity of stimulus perceptions. Higher plants possess two separate strategies to uptake iron from rhizosphere soil: the chelation strategy and the reduction strategy in grass and non-grass plants, respectively. Molecular actors involved in iron uptake and mobilization through the plant have been characterized for both strategies. All these processes that contribute to iron homeostasis in plants are highly regulated in response to iron availability by downstream signaling responses, some of which are characteristic signaling signatures of iron dynamics, while others are shared with other environmental stimuli. Recent research has thus revealed key transcription factors, cis-acting elements, post-translational regulators, and other molecular mechanisms controlling these genes or their encoded proteins in response to iron availability. In addition, the most recent research is increasingly highlighting the crosstalk between iron homeostasis and nutrient response regulation. These regulatory processes help to avoid plant iron concentrations building up to potential cell functioning disruptions that could adversely affect plant fitness. Indeed, when iron is in excess in the plant, it can lead to the production and accumulation of dangerous reactive oxygen species and free radicals (H₂O₂, HO^•, O₂^•-, HO^•₂) that can cause considerable damages to most cellular components. To cope with iron oxidative stress, plants have developed defense systems involving the complementary action of antioxidant enzymes and molecular antioxidants, safe iron-storage mechanisms, and appropriate morphological adaptations.

Advances in Iron Retrograde Signaling Mechanisms and Uptake Regulation in Photosynthetic Organisms

Iron (Fe) is an essential metal for the growth and development of different organisms, including plants and algae. This metal participates in different biological processes, among which are cellular respiration and photosynthesis. Fe is found associated with heme groups and as part of inorganic Fe-S groups as cofactors of numerous cellular proteins. Although Fe is abundant in soils, it is often not bioavailable due to soil pH. For this reason, photosynthetic organisms have developed different strategies for the uptake, the sensing of Fe intracellular levels but also different mechanisms that maintain and regulate adequate concentrations of this metal in response to physiological needs. This work focuses on discussing recent advances in the characterization of the mechanisms of Fe homeostasis and Fe retrograde signaling in photosynthetic organisms.

Comprehensive Survey of ChIP-Seq Datasets to Identify Candidate Iron Homeostasis Genes Regulated by Chromatin Modifications

Vital biochemical reactions including photosynthesis to respiration require iron, which should be tightly regulated. Although increasing evidence reveals the importance of epigenetic regulation in gene expression and signaling, the role of histone modifications and chromatin remodeling in plant iron homeostasis is not well understood. In this study, we surveyed publicly available ChIP-seq datasets of Arabidopsis wild-type and mutants defective in key enzymes of histone modification and chromatin remodeling and compared the deposition of epigenetic marks on loci of genes involved in iron regulation. Based on the analysis, we compiled a comprehensive list of iron homeostasis genes with differential enrichment of various histone modifications. This report will provide a resource for future studies to investigate epigenetic regulatory mechanisms of iron homeostasis in plants.

Comparative physiological and transcriptomics analysis revealed crucial mechanisms of silicon-mediated tolerance to iron deficiency in tomato

Iron (Fe) deficiency is a common abiotic stress in plants grown in alkaline soil that causes leaf chlorosis and affects root development due to low plant-available Fe concentration. Silicon (Si) is a beneficial element for plant growth and can also improve plant tolerance to abiotic stress. However, the effect of Si and regulatory mechanisms on tomato plant growth under Fe deficiency remain largely unclear. Here, we examined the effect of Si application on the photosynthetic capacity, antioxidant defense, sugar metabolism, and organic acid contents under Fe deficiency in tomato plants. The results showed that Si application promoted plant growth by increasing photosynthetic capacity, strengthening antioxidant defense, and reprogramming sugar metabolism. Transcriptomics analysis (RNA-seq) showed that Si application under Fe deficiency up-regulated the expression of genes related to antioxidant defense, carbohydrate metabolism and organic acid synthesis. In addition, Si application under Fe deficiency increased Fe distribution to leaves and roots. Combined with physiological assessment and molecular analysis, these findings suggest that Si application can effectively increase plant tolerance to low Fe stress and thus can be implicated in agronomic management of Fe deficiency for sustainable crop production. Moreover, these findings provide important information for further exploring the genes and underlying regulatory mechanisms of Si-mediated low Fe stress tolerance in crop plants.

FE UPTAKE-INDUCING PEPTIDE1 maintains Fe translocation by controlling Fe deficiency response genes in the vascular tissue of Arabidopsis

FE UPTAKE-INDUCING PEPTIDE1 (FEP1), also named IRON MAN3 (IMA3) is a short peptide involved in the iron deficiency response in Arabidopsis thaliana. Recent studies uncovered its molecular function, but its physiological function in the systemic Fe response is not fully understood. To explore the physiological function of FEP1 in iron homoeostasis, we performed a transcriptome analysis using the FEP1 loss-of-function mutant fep1-1 and a transgenic line with oestrogen-inducible expression of FEP1. We determined that FEP1 specifically regulates several iron deficiency-responsive genes, indicating that FEP1 participates in iron translocation rather than iron uptake in roots. The iron concentration in xylem sap under iron-deficient conditions was lower in the fep1-1 mutant and higher in FEP1-induced transgenic plants compared with the wild type (WT). Perls staining revealed a greater accumulation of iron in the cortex of fep1-1 roots than in the WT root cortex, although total iron levels in roots were comparable in the two genotypes. Moreover, the fep1-1 mutation partially suppressed the iron overaccumulation phenotype in the leaves of the oligopeptide transporter3-2 (opt3-2) mutant. These data suggest that FEP1 plays a pivotal role in iron movement and in maintaining the iron quota in vascular tissues in Arabidopsis.

Comparative zinc tolerance and phytoremediation potential of four biofuel plant species

Soil pollution has become a serious environmental problem worldwide due to rapid industrialization and urbanization. Zinc (Zn) contamination has raised concerns about potential effects on plants and human health. This study was conducted to assess the capability of four biofuel plants: Abelmoschus esculentus, Avena sativa, Guizotia abyssinica, and Glycine max to remediate and restore Zn contaminated soil. Selected plants were grown in soil exposed to different Zn treatments (50, 100, 200, 300, 400, 600, 800 and 1000 mg Zn kg^-1) for 12 weeks. Soil without spike taken as control. Zn induced toxicity significantly (p < 0.05) reduced seed germination and inhibited plant growth and leaf chlorophyll content. The investigated plants can tolerate a soil content of 800 mg Zn kg^-1 with the exception of A. sativa, which was most tolerant to high Zn concentrations (1000 mg Zn kg^-1) for all growth criteria. Moreover, increasing Zn content in soil resulted in a significant (p < 0.05) increase in Zn accumulation in various tissues of the four biofuel plants. According to phytoremediation efficiency, the four biofuel plants studied were arranged as follows: A. sativa (5.05%) > A. esculentus (4.15%) > G. max (2.31%) > G. abyssinica (1.17%). This study concluded that all tested biofuel plants species, especially A. sativa exhibited high Zn concentrations in roots and shoots, high Zn uptake capability, high tolerance, and high biomass at 50-800 mg Zn kg^-1 treatments. Consequently, these biofuel plants are excellent candidates for phytoremediation in Zn contaminated soils.

Acclimatory gene expression of primed clams enhances robustness to elevated pCO2

Sub-lethal exposure to environmental challenges may enhance ability to cope with chronic or repeated change, a process known as priming. In a previous study, pre-exposure to seawater enriched with pCO₂ improved growth and reduced antioxidant capacity of juvenile Pacific geoduck Panopea generosa, suggesting that transcriptional shifts may drive phenotypic modifications post-priming. To this end, juvenile clams were sampled and TagSeq gene expression data analyzed after 1) a 110-day acclimation under ambient (921 μatm, naïve) and moderately-elevated pCO₂ (2870 μatm, pre-exposed); then following 2) a second 7-day exposure to three pCO₂ treatments (ambient: 754 μatm; moderately-elevated: 2750 μatm; severely-elevated: 4940 μatm), a 7-day return to ambient pCO₂ , and a third 7-day exposure to two pCO₂ treatments (ambient: 967 μatm; moderately-elevated: 3030 μatm). Pre-exposed geoducks frontloaded genes for stress and apoptosis/innate immune response, homeostatic processes, protein degradation, and transcriptional modifiers. Pre-exposed geoducks were also responsive to subsequent encounters, with gene sets enriched for mitochondrial recycling and immune defense under elevated pCO₂ and energy metabolism and biosynthesis under ambient recovery. In contrast, gene sets with higher expression in naïve clams were enriched for fatty-acid degradation and glutathione components, suggesting naïve clams could be depleting endogenous fuels, with unsustainable energetic requirements if changes in carbonate chemistry persist. Collectively, our transcriptomic data indicates pCO₂ priming during post-larval periods could, via gene expression regulation, enhance robustness in bivalves to environmental change. Such priming approaches may be beneficial for aquaculture, as seafood demand intensifies concurrent with increasing climate change in marine systems.

Mitochondrial iron transporter (MIT) gene in potato (Solanum tuberosum): comparative bioinformatics, physiological and expression analyses in response to drought and salinity

Mitochondrial iron transporter (MIT) genes are essential for mitochondrial acquisition/import of iron and vital to proper functioning of mitochondria. Unlike other organisms, research on the MITs in plants is limited. The present study provides comparative bioinformatics assays for the potato MIT gene (StMIT) as well as gene expression analyses. The phylogenetic analyses revealed monocots-dicot divergence in MIT proteins and it was also found clade specific motif diversity. In addition, docking analyses indicated that Asp172 and Gly100 residues to be identified as the closest residues binding to ferrous iron. The percentage of structure overlap of the StMIT 3D protein model with Arabidopsis, maize and rice MIT proteins was found between 80.18% and 85.71%. The transcript analyses exhibited that the expression of StMIT was triggered under drought and salinity stresses. The findings of the present study would provide valuable leads for further studies targeting specifically the MIT gene and generally the plant iron metabolism.

Carbonate-Induced Chemical Reductants Are Responsible for Iron Acquisition in Strategy I Wild Herbaceous Plants Native to Calcareous Grasslands

Significant progress has been made in understanding Strategy I iron (Fe) acquisition using crop/model plants under controlled conditions in laboratories. However, plant species native to calcareous soils may have evolved unique strategies for adaptation to high carbonate/pH-induced Fe deficiency. Until now, little information is available on the Fe acquisition mechanisms in these plants. Here, we explored the Fe acquisition mechanisms in wild dicot species native to calcareous grasslands, by monitoring the Fe nutrition-related rhizosphere processes in field and greenhouse conditions. Most of these wild species displayed comparable shoot Fe concentration to those of crops, and some dicots actually accumulated very high shoot Fe. However, these species did not exhibit ferric reductase oxidase (FRO)-dependent Strategy I responses to Fe deficiency, including visual rhizosphere acidification and increased Fe3+ reduction. In contrast, chemical reductants exuded by roots of dicots were responsible for Fe3+ reduction in these wild plants. These features were not observed in the FRO-dependent Strategy I crop plant cucumber. Neither leaf chlorophyll nor shoot/root Fe was depressed by 10% CaCO3 application in all the examined wild species. Furthermore, their root exudation was significantly activated by CaCO3, leading to an increased Fe3+ reduction. We show that chemical reductant-mediated Fe3+ reduction occurs preferentially in these wild dicots and that these mechanisms are not sensitive to high soil carbonate/pH. Our findings support that Fe acquisition in Strategy I wild plants native to calcareous soils is substantially different from the enzyme-dependent system of Strategy I plants.

Crosstalk Between Iron and Sulfur Homeostasis Networks in Arabidopsis

The widespread deficiency of iron (Fe) and sulfur (S) is becoming a global concern. The underlying mechanisms regulating Fe and S sensing and signaling have not been well understood. We investigated the crosstalk between Fe and S using mutants impaired in Fe homeostasis, sulfate assimilation, and glutathione (GSH) biosynthesis. We showed that chlorosis symptoms induced by Fe deficiency were not directly related to the endogenous GSH levels. We found dynamic crosstalk between Fe and S networks and more interestingly observed that the upregulated expression of IRT1 and FRO2 under S deficiency in Col-0 was missing in the cad2-1 mutant background, which suggests that under S deficiency, the expression of IRT1 and FRO2 was directly or indirectly dependent on GSH. Interestingly, the bottleneck in sulfite reduction led to a constitutively higher IRT1 expression in the sir1-1 mutant. While the high-affinity sulfate transporter (Sultr1;2) was upregulated under Fe deficiency in the roots, the low-affinity sulfate transporters (Sultr2;1, and Sultr2;2) were down-regulated in the shoots of Col-0 seedlings. Moreover, the expression analysis of some of the key players in the Fe-S cluster assembly revealed that the expression of the so-called Fe donor in mitochondria (AtFH) and S mobilizer of group II cysteine desulfurase in plastids (AtNFS2) were upregulated under Fe deficiency in Col-0. Our qPCR data and ChIP-qPCR experiments suggested that the expression of AtFH is likely under the transcriptional regulation of the central transcription factor FIT.

MIR164b represses iron uptake by regulating the NAC domain transcription factor5-Nuclear Factor Y, Subunit A8 module in Arabidopsis

Recent findings have revealed the important roles of microRNAs (miRNAs) in the secondary responses to oxidative damage caused by iron (Fe) excess. However, the functional importance of miRNAs in plant responses to Fe deficiency remains to be explored. Here, we show that the expression level of miR164 in Arabidopsis (Arabidopsis thaliana) roots was repressed by Fe deficiency. Primary root length, lateral root number, ferric reductase activity, and mRNA abundance of IRON-REGULATED TRANSPORTER1 (IRT1) and FERRIC REDUCTION OXIDASE2 (FRO2) were higher in the mir164b mutant than in the wild-type (WT) under Fe-deficient conditions. Analysis of the Fe concentrations and ferric reductase activities in the roots of miR164 knockdown transgenic plants showed that members of the miR164 family had different functions in Fe-deficiency responses. Promoter::GUS analysis showed that NAM/ATAF/CUC (NAC) domain transcription factor5 (NAC5) is regulated at both transcriptional and posttranscriptional levels under Fe-deficient conditions. Transgenic Arabidopsis plants overexpressing NAC5 were more tolerant of Fe deficiency than the WT. NAC5 has transactivation activity and directly transactivates the expression of Nuclear Factor Y, Subunit A8 (NFYA8), as demonstrated by chromatin immunoprecipitation followed by quantitative polymerase chain reaction, electrophoretic mobility shift assay (EMSA), and dual-luciferase reporter assay. Like overexpression of NAC5, overexpression of NFYA8 increases primary root length, lateral root number, ferric reductase activity, and mRNA abundance of IRT1 and FRO2 under Fe-deficient conditions. Thus, MIR164b is important for Fe-deficiency responses by its regulation of the NAC5-NFYA8 module.

Endophytic Fungus Phomopsis liquidambaris Enhances Fe Absorption in Peanuts by Reducing Hydrogen Peroxide

Iron (Fe) deficiency in alkaline calcium soil is a problem that needs to be solved urgently as Fe is an essential and commonly limiting nutrient for plants. Endophytic fungus, Phomopsis liquidambaris (P. liquidambaris), has been reported to promote Fe absorption in peanuts (Arachis hypogaea L.), however, the mechanisms remain unclear. Under prolonged Fe deficiency, an increase in hydrogen peroxide (H₂O₂) often triggers a series of signaling events and leads to the inhibition of Fe acquisition. The main purpose of this study was to explore whether and how the endophytic fungus P. liquidambaris promote Fe absorption in peanut through regulating H₂O₂ and assisting in resisting oxidative stress. In this study, we detected the Fe deficiency-induced transcription factor (FIT), Fe²⁺ transporter (IRT1), and ferric reduction oxidase 2 (FRO2) of peanuts, and confirmed that they were negatively related to Fe concentration. Similarly, FIT, IRT1, and FRO2 were also inhibited by H₂O₂. The addition of P. liquidambaris reduces H₂O₂ under Fe-deficiency with an increase in Fe content, while the exogenous addition of H₂O₂ further decreases it, and the addition of catalase (CAT) under Fe-deficiency reverses this phenomenon. Through transcriptome analysis, we proved that the expression of FIT, IRT1, FRO2 and CAT are consistent with our hypothesis, and P. liquidambaris has a stress-mitigating effect on peanuts mainly via CAT, glutathione peroxidase, and malondialdehyde. Our study proved the Fe-absorption promoting effect and stress mitigation effect of P. liquidambaris under Fe-deficiency in peanuts, and their combined usage may help peanuts grow better.

Effects of Fe and Mn Deficiencies on the Root Protein Profiles of Tomato (Solanum lycopersicum) Using Two-Dimensional Electrophoresis and Label-Free Shotgun Analyses

Iron (Fe) and manganese (Mn) are two essential elements for plants that compete for the same uptake transporters and show conflicting interactions at the regulatory level. In order to understand the differential response to both metal deficiencies in plants, two proteomic techniques (two-dimensional gel electrophoresis and label-free shotgun) were used to study the proteome profiles of roots from tomato plants grown under Fe or Mn deficiency. A total of 119 proteins changing in relative abundance were confidently quantified and identified, including 35 and 91 in the cases of Fe deficiency and Mn deficiency, respectively, with 7 of them changing in both deficiencies. The identified proteins were categorized according to function, and GO-enrichment analysis was performed. Data showed that both deficiencies provoked a common and intense cell wall remodelling. However, the response observed for Fe and Mn deficiencies differed greatly in relation to oxidative stress, coumarin production, protein, nitrogen, and energy metabolism.

Root-to-shoot iron partitioning in Arabidopsis requires IRON-REGULATED TRANSPORTER1 (IRT1) protein but not its iron(II) transport function

IRON-REGULATED TRANSPORTER1 (IRT1) is the root high-affinity ferrous iron (Fe) uptake system and indispensable for the completion of the life cycle of Arabidopsis thaliana without vigorous Fe supplementation. Here we provide evidence supporting a second role of IRT1 in root-to-shoot partitioning of Fe. We show that irt1 mutants overaccumulate Fe in roots, most prominently in the cortex of the differentiation zone in irt1-2, compared to the wild type. Shoots of irt1-2 are severely Fe-deficient according to Fe content and marker transcripts, as expected. We generated irt1-2 lines producing IRT1 mutant variants carrying single amino-acid substitutions of key residues in transmembrane helices IV and V, Ser206 and His232, which are required for transport activity in yeast. Root short-term ⁵⁵ Fe uptake rates were uninformative concerning IRT1-mediated transport. Overall irt1-like concentrations of the secondary substrate Mn suggested that the transgenic Arabidopsis lines also remain incapable of IRT1-mediated root Fe uptake. Yet, IRT1_S206A partially complements rosette dwarfing and leaf chlorosis of irt1-2, as well as root-to-shoot Fe partitioning and gene expression defects of irt1-2, all of which are fully complemented by wild-type IRT1. Taken together, these results suggest a regulatory function for IRT1 in root-to-shoot Fe partitioning that does not require Fe transport activity of IRT1. Among the genes of which transcript levels are partially dependent on IRT1, we identify MYB DOMAIN PROTEIN10, MYB DOMAIN PROTEIN72 and NICOTIANAMINE SYNTHASE4 as candidates for effecting IRT1-dependent Fe mobilization in roots. Understanding the biological functions of IRT1 will help to improve Fe nutrition and the nutritional quality of agricultural crops.

Root-to-shoot iron partitioning in Arabidopsis requires IRON-REGULATED TRANSPORTER1 (IRT1) protein but not its iron(II) transport function

IRON-REGULATED TRANSPORTER1 (IRT1) is the root high-affinity ferrous iron (Fe) uptake system and indispensable for the completion of the life cycle of Arabidopsis thaliana without vigorous Fe supplementation. Here we provide evidence supporting a second role of IRT1 in root-to-shoot partitioning of Fe. We show that irt1 mutants overaccumulate Fe in roots, most prominently in the cortex of the differentiation zone in irt1-2, compared to the wild type. Shoots of irt1-2 are severely Fe-deficient according to Fe content and marker transcripts, as expected. We generated irt1-2 lines producing IRT1 mutant variants carrying single amino-acid substitutions of key residues in transmembrane helices IV and V, Ser206 and His232, which are required for transport activity in yeast. Root short-term ⁵⁵ Fe uptake rates were uninformative concerning IRT1-mediated transport. Overall irt1-like concentrations of the secondary substrate Mn suggested that the transgenic Arabidopsis lines also remain incapable of IRT1-mediated root Fe uptake. Yet, IRT1_S206A partially complements rosette dwarfing and leaf chlorosis of irt1-2, as well as root-to-shoot Fe partitioning and gene expression defects of irt1-2, all of which are fully complemented by wild-type IRT1. Taken together, these results suggest a regulatory function for IRT1 in root-to-shoot Fe partitioning that does not require Fe transport activity of IRT1. Among the genes of which transcript levels are partially dependent on IRT1, we identify MYB DOMAIN PROTEIN10, MYB DOMAIN PROTEIN72 and NICOTIANAMINE SYNTHASE4 as candidates for effecting IRT1-dependent Fe mobilization in roots. Understanding the biological functions of IRT1 will help to improve Fe nutrition and the nutritional quality of agricultural crops.

Overexpression of METAL TOLERANCE PROTEIN8 reveals new aspects of metal transport in Arabidopsis thaliana seeds

METAL TOLERANCE PROTEIN8 (MTP8) of Arabidopsis thaliana is a member of the CATION DIFFUSION FACILITATOR (CDF) family of proteins that transports primarily manganese (Mn), but also iron (Fe). MTP8 mediates Mn allocation to specific cell types in the developing embryo, and Fe re-allocation as well as Mn tolerance during imbibition. We analysed if an overexpression of MTP8 driven by the CaMV 35S promoter has an effect on Mn tolerance during imbibition and on Mn and Fe storage in seeds, which would render it a biofortification target. Fe, Mn and Zn concentrations in MTP8-overexpressing lines in wild type and vit1-1 backgrounds were analysed by ICP-MS. Distribution of metals in intact seeds was determined by synchrotron µXRF tomography. MTP8 overexpression led to a strongly increased Mn tolerance of seeds during imbibition, supporting its effectiveness in loading excess Mn into the vacuole. In mature seeds, MTP8 overexpression did not cause a consistent increase in Mn and Fe accumulation, and it did not change the allocation pattern of these metals. Zn concentrations were consistently increased in bulk samples. The results demonstrate that Mn and Fe allocation is not determined primarily by the MTP8 expression pattern, suggesting either a cell type-specific provision of metals for vacuolar sequestration by upstream transport processes, or the determination of MTP8 activity by post-translational regulation.

Fe and Zn stress induced gene expression analysis unraveled mechanisms of mineral homeostasis in common bean (Phaseolus vulgaris L.)

Iron (Fe) and zinc (Zn) stress significantly affects fundamental metabolic and physiological processes in plants that results in reduction of plant growth and development. In the present study, common bean variety; Shalimar French Bean-1 (SFB-1) was used as an experimental material. Four different MGRL media i.e. normal MGRL medium (Control), media without Fe (0-Fe), media without Zn (0-Zn) and media with excess Zn (300-Zn) were used for growing seeds of SFB-1 under in vitro condition for three weeks under optimum conditions. Three week old shoot and root tissues were harvested from the plants grown in these four different in vitro conditions and were, subjected to Fe and Zn estimation. Further, extraction of total RNA for differential gene expression of ten candidate genes selected based on our in silico investigation and their classification, phylogeny and expression pattern was unraveled. Expression analysis of three candidate genes (OPT3, NRAMP2 and NRAMP3) in roots revealed possible cross talk among Fe/Zn stress that was further confirmed by observing less accumulation of Fe in roots under both these conditions. However, we observed, higher accumulation of Fe in shoots under 0-Fe condition compared to control that suggests precise sensing for priority based compartmentalization and partitioning leading to higher accumulation of Fe in shoots. Furthermore, the expression analysis of IRT1, FRO1 and Ferritin 1 genes under Fe/Zn stress suggested their role in uptake/transport and signaling of Fe and Zn, whereas the expression of ZIP2, NRAMP1, HA2 and GLP1 genes were highly responsive to Zn in Phaseolus vulgaris. The identified genes highly responsive to Fe and Zn stress condition can be potential candidates for overcoming mineral stress in dicot crop plants.

Pangenome of white lupin provides insights into the diversity of the species

White lupin is an old crop with renewed interest due to its seed high protein content and high nutritional value. Despite a long domestication history in the Mediterranean basin, modern breeding efforts have been fairly scarce. Recent sequencing of its genome has provided tools for further description of genetic resources but detailed characterization of genomic diversity is still missing. Here, we report the genome sequencing of 39 accessions that were used to establish a white lupin pangenome. We defined 32 068 core genes that are present in all individuals and 14 822 that are absent in some and may represent a gene pool for breeding for improved productivity, grain quality, and stress adaptation. We used this new pangenome resource to identify candidate genes for alkaloid synthesis, a key grain quality trait. The white lupin pangenome provides a novel genetic resource to better understand how domestication has shaped the genomic variability within this crop. Thus, this pangenome resource is an important step towards the effective and efficient genetic improvement of white lupin to help meet the rapidly growing demand for plant protein sources for human and animal consumption.

Two NPF transporters mediate iron long-distance transport and homeostasis in Arabidopsis

Iron (Fe) transport and reallocation are essential to Fe homeostasis in plants, but it is unclear how Fe homeostasis is regulated, especially under stress. Here we report that NPF5.9 and its close homolog NPF5.8 redundantly regulate Fe transport and reallocation in Arabidopsis. NPF5.9 is highly upregulated in response to Fe deficiency. NPF5.9 expresses preferentially in vasculature tissues and localizes to the trans-Golgi network, and NPF5.8 showed a similar expression pattern. Long-distance Fe transport and allocation into aerial parts was significantly increased in NPF5.9-overexpressing lines. In the double mutant npf5.8 npf5.9, Fe loading in aerial parts and plant growth were decreased, which were partially rescued by Fe supplementation. Further analysis showed that expression of PYE, the negative regulator for Fe homeostasis, and its downstream target NAS4 were significantly altered in the double mutant. NPF5.9 and NPF5.8 were shown to also mediate nitrate uptake and transport, although nitrate and Fe application did not reciprocally affect each other. Our findings uncovered the novel function of NPF5.9 and NPF5.8 in long-distance Fe transport and homeostasis, and further indicated that they possibly mediate nitrate transport and Fe homeostasis independently in Arabidopsis.

A quick journey into the diversity of iron uptake strategies in photosynthetic organisms

Iron (Fe) is involved in multiple processes that contribute to the maintenance of the cellular homeostasis of all living beings. In photosynthetic organisms, Fe is notably required for photosynthesis. Although iron is generally abundant in the environment, it is frequently poorly bioavailable. This review focuses on the molecular strategies that photosynthetic organisms have evolved to optimize iron acquisition, using Arabidopsis thaliana, rice (Oryza sativa), and some unicellular algae as models. Non-graminaceous plants, including Arabidopsis, take up iron from the soil by an acidification-reduction-transport process (strategy I) requiring specific proteins that were recently shown to associate in a dedicated complex. On the other hand, graminaceous plants, such as rice, use the so-called strategy II to acquire iron, which relies on the uptake of Fe³⁺ chelated by phytosiderophores that are secreted by the plant into the rhizosphere. However, apart these main strategies, accessory mechanisms contribute to robust iron uptake in both Arabidopsis and rice. Unicellular algae combine reductive and non-reductive mechanisms for iron uptake and present important specificities compared to land plants. Since the majority of the molecular actors required for iron acquisition in algae are not conserved in land plants, questions arise about the evolution of the Fe uptake processes upon land colonization.

Inactivation of cytosolic FUMARASE2 enhances growth and photosynthesis under simultaneous copper and iron deprivation in Arabidopsis

Copper (Cu) and iron (Fe) are essential for plant growth and are often in short supply under natural conditions. Molecular responses to simultaneous lack of both metals (-Cu-Fe) differ from those seen in the absence of either alone. Metabolome profiling of plant leaves previously revealed that fumarate levels fall under -Cu-Fe conditions. We employed lines lacking cytosolic FUMARASE2 (FUM2) activity to study the impact of constitutive suppression of cytosolic fumarate synthesis on plant growth under Cu and/or Fe deficiency. In fum2 mutants, photosynthesis and growth were less impaired under -Cu-Fe conditions than in wild-type (WT) seedlings. In particular, levels of photosynthetic proteins, chloroplast ultrastructure, amino acid profiles and redox state were less perturbed by simultaneous Cu-Fe deficiency in lines that cannot produce fumarate in the cytosol. Although cytosolic fumarate has been reported to promote acclimation of photosynthesis to low temperatures when metal supplies are adequate, the photosynthetic efficiency of fum2 lines grown under Cu-Fe deficiency in the cold was higher than in WT. Uptake and contents of Cu and Fe are similar in WT and fum2 plants under control and -Cu-Fe conditions, and lack of FUM2 does not alter the ability to sense metal deficiency, as indicated by marker gene expression. Collectively, we propose that reduced levels of cytosolic fumarate synthesis ultimately increase the availability of Fe for incorporation into metalloproteins.

The many facets of protein ubiquitination and degradation in plant root iron-deficiency responses

Organisms need to deal with the absolute requirement for metals and also their possible toxicity. This is achieved through an intricate network of signaling pathways that are integrated to ultimately fine-tune iron uptake and metabolism. The mechanisms by which plants cope with iron limitation and the associated genomic responses are well characterized. On top of this transcriptional cascade is another level of regulation involving the post-translational protein modification and degradation. The ubiquitination and/or degradation of several transcription factors in the iron-deficiency signaling pathways and metal transporters has recently come to light. In this review we discuss the mechanisms and possible roles of protein modification and turnover in the regulation of root iron-deficiency responses. We also highlight the tight coupling between metal sensing by E3 ubiquitin ligases or bifunctional transporters and protein degradation.

Using natural variation to understand plant responses to iron availability

Iron bioavailability varies dramatically between soil types across the globe. This has given rise to high levels of natural variation in plant iron responses, allowing members of even a single species to thrive across a wide range of soil types. In recent years we have seen the use of genome-wide association analysis to identify natural variants underlying plant responses to changes in iron availability in both Arabidopsis and important crop species. These studies have provided insights into which genes have been important in shaping local adaptation to iron availability in different plant species and have allowed the discovery of novel regulators and mechanisms, not previously identified using mutagenesis approaches. Furthermore, these studies have allowed the identification of markers that can be used to accelerate breeding of future elite varieties with increased resilience to iron stress and improved nutritional quality. The studies highlighted here show that, in addition to studying plant responses to iron alone, it is important to consider these responses within the context of plant nutrition more broadly and to also consider iron regulation in relation to additional traits of agronomic importance such as yield and disease resistance.

Molecular characterization of Fe-acquisition genes causing decreased Fe uptake and photosynthetic inefficiency in Fe-deficient sunflower

Iron (Fe) deficiency in plants hinders growth and yield. Thus, this study aims to elucidate the responses and molecular characterization of genes in Fe-deficient sunflower. The study was conducted on 14 days-old sunflower plants cultivated in hydroponic culture under Fe-sufficient and Fe-deficient conditions. The Fe-starved sunflower showed substantial decrease in plant biomass, SPAD score, quantum yield efficiency of PSII (Fv/Fm), photosynthetic performance index (Pi_ABS). Further, Fe shortage reduced Fe and Zn concentrations in roots and shoots, accompanied by a marked decrease of HaNramp1 and HaZIP1 expression in roots, suggesting the association of Zn status contributing to photosynthetic inefficiency in sunflower. The ferric chelate reductase (FCR) activity, along with HaFRO2 and HaIRT1 transcripts, were constitutively expressed, suggesting that sunflower plants can regulate FCR activity, although the lack of bioavailable Fe in the rhizosphere strongly corresponds to the limited Fe uptake in sunflower. The substantial increase of proton extrusion in roots and the localization of Fe-related genes in the plasma membrane are also evident in sunflower as common responses to Fe-deficiency by this Strategy I plant species. Analysis showed that three motifs of Fe-related proteins were linked to the ZIP zinc transporter. The interactome map revealed the close partnership of these Fe-related genes in addition to FRU gene encoding putative transcription factor linked to Fe uptake response. The cis-regulatory analysis of promoter suggested the involvement of auxin, salicylic acid, and methyl jasmonate-responsive elements in the regulatory process in response to Fe deficiency. These findings may be beneficial to develop Fe-efficient sunflower plants through breeding or genome editing approaches.

Comparative Study of Several Fe Deficiency Responses in the Arabidopsis thaliana Ethylene Insensitive Mutants ein2-1 and ein2-5

Iron (Fe) is an essential micronutrient for plants since it participates in essential processes such as photosynthesis, respiration and nitrogen assimilation. Fe is an abundant element in most soils, but its availability for plants is low, especially in calcareous soils. Fe deficiency causes Fe chlorosis, which can affect the productivity of the affected crops. Plants favor Fe acquisition by developing morphological and physiological responses in their roots. Ethylene (ET) and nitric oxide (NO) have been involved in the induction of Fe deficiency responses in dicot (Strategy I) plants, such as Arabidopsis. In this work, we have conducted a comparative study on the development of subapical root hairs, of the expression of the main Fe acquisition genes FRO2 and IRT1, and of the master transcription factor FIT, in two Arabidopsis thaliana ET insensitive mutants, ein2-1 and ein2-5, affected in EIN2, a critical component of the ET transduction pathway. The results obtained show that both mutants do not induce subapical root hairs either under Fe deficiency or upon treatments with the ET precursor 1-aminocyclopropane-1-carboxylate (ACC) and the NO donor S-nitrosoglutathione (GSNO). By contrast, both of them upregulate the Fe acquisition genes FRO2 and IRT1 (and FIT) under Fe deficiency. However, the upregulation was different when the mutants were exposed to ET [ACC and cobalt (Co), an ET synthesis inhibitor] and GSNO treatments. All these results clearly support the participation of ET and NO, through EIN2, in the regulation of subapical root hairs and Fe acquisition genes. The results will be discussed, taking into account the role of both ET and NO in the regulation of Fe deficiency responses.