Transporters contributing to iron trafficking in plants

Integrative spatial transcriptomic analysis pinpoints the role of the ferroxidase, TaMCO3, in wheat root tip iron mobilization

Roots play a critical role in the sensing and absorption of essential minerals from the rhizosphere. Iron (Fe) deficiency, for example, triggers a well-known series of physiological and molecular responses within roots that facilitate uptake, which differs between monocots and dicots. In monocots, little is known about the molecular responses that occur within specific root development zones in response to iron deprivation, and how these differences result in overall nutrient uptake. Here, we conducted a transcriptome analysis of wheat root tips under Fe deficiency (-Fe) and performed a comparative transcriptome analysis with the previous datasets generated from the whole root. Gene ontology analysis of differentially expressed genes highlighted the significance of oxidoreductase activity and metal/ion transport in the root tip, which are critical for Fe mobilization. Interestingly, wheat, an allohexaploid species consisting of three different genomes (A, B, and D) displayed varying gene expression levels arising from the three genomes that contributed to similar molecular functions. Detailed analysis of oxidoreductase function at the root tip revealed multiple multicopper oxidase (MCO) proteins, such as Fe-responsive TaMCO3, that likely contribute to the overall ferroxidase activity. Further characterization of TaMCO3 shows that it complements the yeast FET3 mutant and rescues the -Fe sensitivity phenotype of Arabidopsis atmco3 mutants by enhancing vascular Fe loading. Transgenic wheat lines overexpressing TaMCO3 exhibited increased root Fe accumulation and improved tolerance to -Fe by augmenting the expression of Fe-mobilizing genes. Our findings highlight the role of spatially resolved gene expression in -Fe responses, suggesting strategies to reprogram cells for improved nutrient stress tolerance.

Identification, characterization, and function analysis of the VIT family in Phaeodactylum tricornutum

Iron is an essential microelement for all living organisms. The vacuolar iron transporters (VIT) gene family is found in various species, including yeast, fungi, protozoa, and plants, where it plays a crucial role in sequestration, homeostasis, and tolerance of the heavy metals, particularly iron and manganese. However, the presence and function of VIT genes in marine phytoplankton have not been previously reported. The study aims to identify the VIT family within the marine diatom Phaeodactylum tricornutum and to analyze the function of these genes. We conducted a comprehensive analysis of the VIT genes in P. tricornutum genome, examining their phylogenetic relationship, physicochemical properties, gene structures, conserved motifs, domains, expression profile, and cis-acting elements using in silico methods. Function analysis were performed through complementation experiments and the expression of eGFP fusion protein in yeast. Four members of the VIT family were identified in P. tricornutum. All belonging to the VTL (VIT like) group in phylogenetic tree and containing a VIT1 domain. These genes are distributed across chromosomes 2, 4, and 13, with tandem duplication of the PtVTL1 and PtVTL2 contributed to the expansion of this gene family. Expression profile showed that the PtVTL3 is induced to express highly under light condition, others are induced to express highly under dark. PtVTL2 is highly induced to express at low Fe condition, and PtVTL3 is highly induced to express at high Fe condition. Analysis of cis-acting regulatory elements indicated that these genes are primarily involved in responses to environmental stress and phytohormones. Heterologous expression of PtVTL3 successfully rescued the iron-sensitive phenotype in yeast mutant △ccc1. The expression of eGFP-PtVTL3 fusion protein in yeast demonstrated that PtVTL3 is located to the tonoplast. These findings suggest that PtVTL3 function to transport Fe2+ across the tonoplast into the vacuole, thereby maintaining iron homeostasis in yeast. Four PtVTL genes were identified in the genome of P. tricornutum, with PtVTL3 playing a key role in iron transport at the tonoplast, highlighting its potential significance in iron homeostasis in marine diatoms.

Shedding light on iron nutrition: exploring intersections of transcription factor cascades in light and iron deficiency signaling

In the dynamic environment of plants, the interplay between light-dependent growth and iron nutrition is a recurring challenge. Plants respond to low iron levels by adjusting growth and physiology through enhanced iron acquisition from the rhizosphere and internal iron pool reallocation. Iron deficiency response assays and gene co-expression networks aid in documenting physiological reactions and unraveling gene-regulatory cascades, offering insight into the interplay between hormonal and external signaling pathways. However, research directly exploring the significance of light in iron nutrition remains limited. This review provides an overview on iron deficiency regulation and its cross-connection with distinct light signals, focusing on transcription factor cascades and long-distance signaling. The circadian clock and retrograde signaling influence iron uptake and allocation. The light-activated shoot-to-root mobile transcription factor ELONGATED HYPOCOTYL5 (HY5) affects iron homeostasis responses in roots. Blue light triggers the formation of biomolecular condensates containing iron deficiency-induced protein complexes. The potential of exploiting the connection between light and iron signaling remains underutilized. With climate change and soil alkalinity on the rise, there is a need to develop crops with improved nutrient use efficiency and modified light dependencies. More research is needed to understand and leverage the interplay between light signaling and iron nutrition.

NnMTP10 from Nelumbo nucifera acts as a transporter mediating manganese and iron efflux

Deficiency or excess of mineral elements in the environment is a primary factor limiting crop yields and nutritional quality. Lotus (Nelumbo nucifera) is an important aquatic crop in Asia, but the mechanism for accumulating mineral nutrients and coping with nutrient deficiency/excess is still largely unknown. Here, we identified NnMTP10, a member of the cation diffusion facilitator family, by screening the cDNA library of lotus. Subcellular localization to the plasma membrane, increased manganese (Mn) and iron (Fe) tolerance and reduced metal accumulation in yeast transformants demonstrated that the protein functions as an exporter to mediate the efflux of Mn and Fe. Arabidopsis overexpressing NnMTP10 exhibited less Mn accumulation in roots, increased long-distance transport to shoots, and increased tolerance to Mn stress under high-Mn conditions. However, the accumulation and tolerance of Fe in Arabidopsis transformants are opposite to those of Mn. Further analysis revealed that excessive Fe in the root apoplast exported by NnMTP10 was sequestrated by the cell wall, thereby reducing the transport of Fe to the shoot. Correspondingly, the expression of NnMTP10 in the roots of lotus was increased under the high-Mn treatment but decreased under the high-Fe treatment. These results suggest that NnMTP10 is involved in the long-distance transport of Mn and Fe in lotus and may play a role in coordinating the adaptation to stresses caused by excessive Mn and Fe.

Inorganic nitrogen inhibits symbiotic nitrogen fixation through blocking NRAMP2-mediated iron delivery in soybean nodules

Symbiotic nitrogen fixation (SNF) in legume-rhizobia serves as a sustainable source of nitrogen (N) in agriculture. However, the addition of inorganic N fertilizers significantly inhibits SNF, and the underlying mechanisms remain not-well understood. Here, we report that inorganic N disrupts iron (Fe) homeostasis in soybean nodules, leading to a decrease in SNF efficiency. This disruption is attributed to the inhibition of the Fe transporter genes Natural Resistance-Associated Macrophage Protein 2a and 2b (GmNRAMP2a&2b) by inorganic N. GmNRAMP2a&2b are predominantly localized at the tonoplast of uninfected nodule tissues, affecting Fe transfer to infected cells and consequently, modulating SNF efficiency. In addition, we identified a pair of N-signal regulators, nitrogen-regulated GARP-type transcription factors 1a and 1b (GmNIGT1a&1b), that negatively regulate the expression of GmNRAMP2a&2b, which establishes a link between N signaling and Fe homeostasis in nodules. Our findings reveal a plausible mechanism by which soybean adjusts SNF efficiency through Fe allocation in response to fluctuating inorganic N conditions, offering valuable insights for optimizing N and Fe management in legume-based agricultural systems.

Phytosiderophore pathway response in barley exposed to iron, zinc or copper starvation

Efficient micronutrient acquisition is a critical factor in selecting micronutrient dense crops for human consumption. Enhanced exudation and re-uptake of metal chelators, so-called phytosiderophores, by roots of graminaceous plants has been implicated in efficient micronutrient acquisition. We compared PS biosynthesis and exudation as a response mechanism to either Fe, Zn or Cu starvation. Two barley (Hordeum vulgare L.) lines with contrasting micronutrient grain yields were grown hydroponically and PS exudation (LC-MS) and root gene expression (RNAseq) were determined after either Fe, Zn, or Cu starvation. The response strength of the PS pathway was micronutrient dependent and decreased in the order Fe > Zn > Cu deficiency. We observed a stronger expression of PS pathway genes and greater PS exudation in the barley line with large micronutrient grain yield suggesting that a highly expressed PS pathway might be an important trait involved in high micronutrient accumulation. In addition to several metal specific transporters, we also found that the expression of IRO2 and bHLH156 transcription factors was not only induced under Fe but also under Zn and Cu deficiency. Our study delivers important insights into the role of the PS pathway in the acquisition of different micronutrients.

Calcium-dependent protein kinases CPK21 and CPK23 phosphorylate and activate the iron-regulated transporter IRT1 to regulate iron deficiency in Arabidopsis

Iron (Fe) is an essential micronutrient for all organisms. Fe availability in the soil is usually much lower than that required for plant growth, and Fe deficiencies seriously restrict crop growth and yield. Calcium (Ca2+) is a second messenger in all eukaryotes; however, it remains largely unknown how Ca2+ regulates Fe deficiency. In this study, mutations in CPK21 and CPK23, which are two highly homologous calcium-dependent protein kinases, conferredimpaired growth and rootdevelopment under Fe-deficient conditions, whereas constitutively active CPK21 and CPK23 enhanced plant tolerance to Fe-deficient conditions. Furthermore, we found that CPK21 and CPK23 interacted with and phosphorylated the Fe transporter IRON-REGULATED TRANSPORTER1 (IRT1) at the Ser149 residue. Biochemical analyses and complementation of Fe transport in yeast and plants indicated that IRT1 Ser149 is critical for IRT1 transport activity. Taken together, these findings suggest that the CPK21/23-IRT1 signaling pathway is critical for Fe homeostasis in plants and provides targets for improving Fe-deficient environments and breeding crops resistant to Fe-deficient conditions.

Forging a symbiosis: transition metal delivery in symbiotic nitrogen fixation

Symbiotic nitrogen fixation carried out by the interaction between legumes and rhizobia is the main source of nitrogen in natural ecosystems and in sustainable agriculture. For the symbiosis to be viable, nutrient exchange between the partners is essential. Transition metals are among the nutrients delivered to the nitrogen-fixing bacteria within the legume root nodule cells. These elements are used as cofactors for many of the enzymes controlling nodule development and function, including nitrogenase, the only known enzyme able to convert N2 into NH3 . In this review, we discuss the current knowledge on how iron, zinc, copper, and molybdenum reach the nodules, how they are delivered to nodule cells, and how they are transferred to nitrogen-fixing bacteria within.

The vacuolar iron transporter mediates iron detoxification in Toxoplasma gondii

Iron is essential to cells as a cofactor in enzymes of respiration and replication, however without correct storage, iron leads to the formation of dangerous oxygen radicals. In yeast and plants, iron is transported into a membrane-bound vacuole by the vacuolar iron transporter (VIT). This transporter is conserved in the apicomplexan family of obligate intracellular parasites, including in Toxoplasma gondii. Here, we assess the role of VIT and iron storage in T. gondii. By deleting VIT, we find a slight growth defect in vitro, and iron hypersensitivity, confirming its essential role in parasite iron detoxification, which can be rescued by scavenging of oxygen radicals. We show VIT expression is regulated by iron at transcript and protein levels, and by altering VIT localization. In the absence of VIT, T. gondii responds by altering expression of iron metabolism genes and by increasing antioxidant protein catalase activity. We also show that iron detoxification has an important role both in parasite survival within macrophages and in virulence in a mouse model. Together, by demonstrating a critical role for VIT during iron detoxification in T. gondii, we reveal the importance of iron storage in the parasite and provide the first insight into the machinery involved.

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.

Distribution of Micronutrients in Arborg Oat (Avena sativa L.) Using Synchrotron X-ray Fluorescence Imaging

It is important to know the mineral distribution in cereal grains for nutritional improvement or genetic biofortification. Distributions and intensities of micro-elements (Mn, Fe, Cu, and Zn) and macro-elements (P, S, K and Ca) in Arborg oat were investigated using synchrotron-based on X-ray fluorescence imaging (XFI). Arborg oat provided by the Crop Development Center (CDC, Aaron Beattie) of the University of Saskatchewan for 2D X-ray fluorescence scans were measured at the BioXAS-Imaging beamline at the Canadian Light Source. The results show that the Ca and Mn were mainly localized in the aleurone layer and scutellum. P, K, Fe, Cu, and Zn were mainly accumulated in the aleurone layer and embryo. Particularly the intensities of P, K, Cu, and Zn in the scutellum were higher compared to other areas. S was also distributed in each tissue and its abundance in the sub-aleurone was the highest. In addition, the intensities of S and Cu were highest in the nucellar projection of the crease region. All these elements were also found in the pericarp but they were at lower levels than other tissues. Overall, the details of these experimental results can provide important information for micronutrient biofortification and processing strategies on oat through elemental mapping in Arborg oat.

Zinc supply influenced the growth, yield, zinc content, and expression of ZIP family transporters in sorghum

Zinc deficiency altered shoot and root growth, plant biomass, yield, and ZIP family transporter gene expression in sorghum. Zinc (Zn) deficiency affects several crop plants' growth and yield, including sorghum. We have evaluated the sorghum under various concentrations of Zn supply for phenotypic changes, Zn content, and expression of Zn-regulated, iron-regulated transporter-like proteins (ZIP) family genes. Zn deficiency reduced the shoot and root growth, plant biomass, and yield by > 50%. The length and number of lateral roots were increased by more than 50% under deficient Zn compared to sufficient Zn. Ten SbZIP family transporter genes showed dynamic expression in shoot and root tissues of sorghum under deficient and sufficient Zn. SbZIP2, 5, 6, 7, and 8 were expressed in all tissues under deficient and sufficient Zn. SbZIP2, 4, 5, 6, 7, 8, and 10 were highly induced in shoot tissues by deficient Zn. The expression level of SbZIP6, 7, 8, and 9 was higher in root tissues under deficient Zn. The phylogenetic analysis revealed that most SbZIP family proteins are closely associated with the ZmZIP family of maize. The functional residues His177 and Gly182 are fully conserved in all SbZIP family transporters, as revealed by homology modeling and multiple sequence alignment. This study may provide a foundation for improving the Zn-use efficiency of sorghum.

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.

The active Fe chelator proline-2'-deoxymugineic acid enhances peanut yield by improving soil Fe availability and plant Fe status

Iron (Fe) deficiency restricts crop yields in calcareous soil. Thus, a novel Fe chelator, proline-2'-deoxymugineic acid (PDMA), based on the natural phytosiderophore 2'-deoxymugineic acid (DMA), was developed to solve the Fe deficiency problem. However, the effects and mechanisms of PDMA relevant to the Fe nutrition and yield of dicots grown under field conditions require further exploration. In this study, pot and field experiments with calcareous soil were conducted to investigate the effects of PDMA on the Fe nutrition and yield of peanuts. The results demonstrated that PDMA could dissolve insoluble Fe in the rhizosphere and up-regulate the expression of the yellow stripe-like family gene AhYSL1 to improve the Fe nutrition of peanut plants. Moreover, the chlorosis and growth inhibition caused by Fe deficiency were significantly diminished. Notably, under field conditions, the peanut yield and kernel micronutrient contents were promoted by PDMA application. Our results indicate that PDMA promotes the dissolution of insoluble Fe and a rich supply of Fe in the rhizosphere, increasing yields through integrated improvements in soil-plant Fe nutrition at the molecular and ecological levels. In conclusion, the efficacy of PDMA for improving the Fe nutrition and yield of peanut indicates its outstanding potential for agricultural applications.

The transcription factor WRKY12 negatively regulates iron entry into seeds in Arabidopsis

Yellow Stripe 1-Like 1 (YSL1) and Yellow Stripe 1-Like 3 (YSL3) transport metal-nicotianamine (NA) complexes to leaves, pollen, and developing seeds and play an important role in regulating iron (Fe) accumulation during the seed development and maturation stages; however, how their gene transcript levels are regulated remains unknown. In this study, we used yeast one-hybrid screening to identify a transcription factor, WRKY12, in Arabidopsis that directly regulates the transcription levels of YSL1 and YSL3 genes. WRKY12 has opposite expression patterns to YSL1 and YSL3. wrky12 mutants are tolerant to Fe deficiency, whereas WRKY12 overexpression lines are sensitive to Fe deficiency. During the development and maturation of seeds, WRKY12 can directly bind to the promoters of YSL1 and YSL3 and inhibit their expression. Genetic analysis showed that WRKY12 functions upstream of YSL1 and YSL3 in Fe intake during the seed development and maturation stages. Together, our results suggest that WRKY12 negatively regulates the iron intake in plant seeds by inhibiting the expression of YSL1 and YSL3.

Iron counteracts zinc-induced toxicity in soybeans

Zinc (Zn) and iron (Fe) are essential micronutrients for all living organisms and the major targets for crop biofortification. However, when acquired in excess quantities, Zn and Fe can be toxic to plants. In this study, we examined the interaction between Zn and Fe in soybean plants under various Zn and Fe treatments. While the level of Zn accumulation increased with increasing Zn supplies, Zn content greatly decreased with rising Fe supplies. Moreover, Zn uptake rates were negatively correlated with Fe supplies. However, Fe accumulation was not greatly affected by elevating Zn supplies. Excess Zn supplies were found to induce typical Fe deficiency symptoms under low Fe conditions, which can be counteracted by increasing Fe supplies. Interestingly, leaf chlorosis caused by excess Zn and low Fe supplies was not directly associated with reduced total Fe content but likely associated with deleterious effects of excess Zn. The combination of high Zn and low Fe greatly activates FRO2 and FIT1 gene expression in soybean roots. Besides, Zn-Fe interaction influences the activities of antioxidative enzymes as well as the uptake, accumulation, and homeostasis of other essential micronutrients, such as copper and manganese in soybean plants. These findings provide new perspectives on Zn and Fe interaction and on heavy metal-induced Fe deficiency-like symptoms.

Multi-Omics Uncover the Mechanism of Wheat under Heavy Metal Stress

Environmental pollution of heavy metals has received growing attention in recent years. Heavy metals such as cadmium, lead and mercury can cause physiological and morphological disturbances which adversely affect the growth and quality of crops. Wheat (Triticum aestivum L.) can accumulate high contents of heavy metals in its edible parts. Understanding wheat response to heavy metal stress and its management in decreasing heavy metal uptake and accumulation may help to improve its growth and grain quality. Very recently, emerging advances in heavy metal toxicity and phytoremediation methods to reduce heavy metal pollution have been made in wheat. Especially, the molecular mechanisms of wheat under heavy metal stress are increasingly being recognized. In this review, we focus on the recently described epigenomics, transcriptomics, proteomics, metabolomics, ionomics and multi-omics combination, as well as functional genes uncovering heavy metal stress in wheat. The findings in this review provide some insights into challenges and future recommendations for wheat under heavy metal stress.

Insight into the role of magnetic nutrient solution on leaf morphology and biochemical attributes of Rasha grapevine (Vitis vinifera L.)

The growth, development, and morphology of plants are extremely affected by many internal and external factors. In this regard, plant nourishing solutions take the most impact. Nowadays, the magnetization of nutrient solutions has been recommended as a promising eco-friendly approach for improving the growth and development of plants. This study was designed to explore the potential of magnetic nutrient solutions in altering morphometric characteristics as well as some physiological and nutritional attributes of Rasha grapevines. Magnetic treatments included magnetized nutrient solution (MagS) and pre-magnetized water completed with nutrients (MagW + S) at magnetic field intensities (0.1 and 0.2 T). According to the results, the most considerable changes in leaf shape and size as well as fresh and dry weights were observed in the plants treated with MagS at 0.2 T. Also, MagS 0.2 had a significant effect on increasing photosynthetic pigments, content of total soluble carbohydrates and protein, and activity of antioxidant enzymes. The content of TNK, K, P, Fe, and Cu was considerably amplified by MagW + S 0.2. Overall, the magnetic solutions had favorable influences on physiological, nutritional state, and leaf morphology of grapevines possibly through alerting water and solution properties, mineral solubility, and phytohormones signalling.

Long-distance mobile mRNA CAX3 modulates iron uptake and zinc compartmentalization

Iron deficiency in plants can lead to excessive absorption of zinc; however, important details of this mechanism have yet to be elucidated. Here, we report that MdCAX3 mRNA is transported from the leaf to the root, and that MdCAX3 is then activated by MdCXIP1. Suppression of MdCAX3 expression leads to an increase in the root apoplastic pH, which is associated with the iron deficiency response. Notably, overexpression of MdCAX3 does not affect the apoplastic pH in a MdCXIP1 loss-of-function Malus baccata (Mb) mutant that has a deletion in the MdCXIP1 promoter. This deletion in Mb weakens MdCXIP1 expression. Co-expression of MdCAX3 and MdCXIP1 in Mb causes a decrease in the root apoplastic pH. Furthermore, suppressing MdCAX3 in Malus significantly reduces zinc vacuole compartmentalization. We also show that MdCAX3 activated by MdCXIP1 is not only involved in iron uptake, but also in regulating zinc detoxification by compartmentalizing zinc in vacuoles to avoid iron starvation-induced zinc toxicity. Thus, mobile MdCAX3 mRNA is involved in the regulation of iron and zinc homeostasis in response to iron starvation.

Transcriptional regulation of metal metabolism- and nutrient absorption-related genes in Eucalyptus grandis by arbuscular mycorrhizal fungi at different zinc concentrations

BACKGROUND

Eucalyptus spp. are candidates for phytoremediation in heavy metal (HM)-polluted soils as they can adapt to harsh environments, grow rapidly, and have good economic value. Arbuscular mycorrhizal fungi (AMF) are the most widely distributed plant symbiotic fungi in nature, and they play an important role in promoting the phytoremediation of HM-polluted soils. However, few studies have evaluated the HM detoxification mechanism of E. spp. in symbiosis with AMF, and thus, the molecular mechanism remains unclear.

RESULTS

The gene transcription and metabolic pathways of E. grandis were studied with and without inoculation with AMF and at different zinc (Zn) concentrations. Here, we focused on the transcript level of six HM-related gene families (ZNT, COPT/Ctr, YSL, ZIFL and CE). Under high-Zn conditions, thirteen genes (ZNT:2, COPT/Ctr:5, YSL:3, ZIFL:1, CE:2) were upregulated, whereas ten genes (ZNT:3, COPT/Ctr:2, YSL:3, ZIFL:1, CE:1) were downregulated. With AMF symbiosis under high-Zn conditions, ten genes (ZNT:4, COPT/Ctr:2, YSL:3, CE:1) were upregulated, whereas nineteen genes (ZNT:9, COPT/Ctr:2, YSL:3, ZIFL:4, CE:1) were downregulated. Under high-Zn conditions, genes of three potassium-related transporters, six phosphate transporters (PHTs), and two nitrate transporters (NRTs) were upregulated, whereas genes of four potassium-related transporters,four PHTs, and four nitrogen-related transporters were downregulated. With AMF symbiosis under high-Zn conditions, genes of two potassium-related transporters, six ammonium transporters (AMTs) and five PHTs were upregulated, whereas genes of six potassium-related transporters, two AMTs and five PHTs were downregulated.

CONCLUSIONS

Our results indicates that AMF increases the resistance of E. grandis to high-Zn stress by improving nutrients uptake and regulating Zn uptake at the gene transcription level. Meanwhile, our findings provide a genome-level resource for the functional assignments of key genes regulated by Zn treatment and AM symbiosis in six HM-associated gene families and macromineral nutrient-related gene families of E. grandis. This may contribute to the elucidation of the molecular mechanisms of the response to Zn stress in E. grandis with AM symbiosis at the aspect of the interaction between HM tolerance and nutrient acquisition.

Assessment of Biofortification Approaches Used to Improve Micronutrient-Dense Plants That Are a Sustainable Solution to Combat Hidden Hunger

Malnutrition causes diseases, immune system disorders, deterioration in physical growth, mental development, and learning capacity worldwide. Micronutrient deficiency, known as hidden hunger, is a serious global problem. Biofortification is a cost-effective and sustainable agricultural strategy for increasing the concentrations or bioavailability of essential elements in the edible parts of plants, minimizing the risks of toxic metals, and thus reducing malnutrition. It has the advantage of delivering micronutrient-dense food crops to a large part of the global population, especially poor populations. Agronomic biofortification and biofertilization, traditional plant breeding, and optimized fertilizer applications are more globally accepted methods today; however, genetic biofortification based on genetic engineering such as increasing or manipulating (such as CRISPR-Cas9) the expression of genes that affect the regulation of metal homeostasis and carrier proteins that serve to increase the micronutrient content for higher nutrient concentration and greater productivity or that affect bioavailability is also seen as a promising high-potential strategy in solving this micronutrient deficiency problem. Data that micronutrients can help strengthen the immune system against the COVID-19 pandemic and other diseases has highlighted the importance of tackling micronutrient deficiencies. In this study, biofortification approaches such as plant breeding, agronomic techniques, microbial fertilization, and some genetic and nanotechnological methods used in the fight against micronutrient deficiency worldwide were compiled.

Eliciting effects of magnetized solution on physiological and biochemical characteristics and elemental uptake in hydroponically grown grape (Vitis vinifera L. cv. Thompson Seedless)

Hydroponic systems are used as an instrument to solve a wide range of environmental concerns such as climate change, drought, and soil fertility. Effective growing media are vital to the hydroponically plant cultivation system. Plant reaction to the growing media treated by magnetic fields, as an innovative technology, should be investigated since the magnetic field can constitute a non-toxic and non-residual stimulus. In the current study, some physiological and biochemical responses (leaf fresh and dry weight, biomass, leaf area, plant height, stem diameter, photosynthesis pigments, carbohydrates, protein, and proline contents and peroxidase activity (POD)) and elemental uptake of hydroponically grown grapevines to the magnetic solutions (with 0.1 and 0.2 T intensities) were evaluated. The solutions were magnetized in two ways: 1) solutions magnetized after preparing (S 0.1 and S 0.2), and 2) salts were added to the pre-magnetized waters (W + S 0.1 and W + S 0.2). The results revealed that magnetic treatments had effect on increasing of leaf elements uptake including N⁺, P⁺, K⁺, Ca²⁺, Fe²⁺, and Zn²⁺. Magnetic treatments also stimulated chlorophyll content, leaf extension, leaf fresh and dry weight, and biomass accumulation. Carbohydrates content and POD were promoted in grapes treated by S 0.1 and W + S 0.1 than control. W + S 0.1 and S 0.2 increased the proline and protein content, respectively. The findings proposed that solution magnetization application could act as an elicitor agent to mimic the stress condition, but at the lesser level resulting in activation of grape physiological and biochemical processes and elemental state through stress-related responses.

Identification of genomic loci regulating grain iron content in aus rice under two irrigation management systems

Iron (Fe) deficiency is one of the common causes of anaemia in humans. Improving grain Fe in rice, therefore, could have a positive impact for humans worldwide, especially for those people who consume rice as a staple food. In this study, 225–269 accessions of the Bengal and Assam Aus Panel (BAAP) were investigated for their accumulation of grain Fe in two consecutive years in a field experiment under alternative wetting and drying (AWD) and continuous flooded (CF) irrigation. AWD reduced straw Fe by 40% and grain Fe by 5.5–13%. Genotype differences accounted for 35% of the variation in grain Fe, while genotype by irrigation interaction accounted for 12% of the variation in straw and grain Fe in year 1, with no significant interactions detected in year 2. Twelve rice accessions were identified as having high grain Fe for both years regardless of irrigation treatment, half of which were from BAAP aus subgroup 3 which prominently comes from Bangladesh. On average, subgroup 3 had higher grain Fe than the other four subgroups of aus. Genome‐wide association mapping identified 6 genomic loci controlling natural variation of grain Fe concentration in plants grown under AWD. For one QTL, nicotianamine synthase OsNAS3 is proposed as candidate for controlling natural variation of grain Fe in rice. The BAAP contains three haplotypes of OsNAS3 where one haplotype (detected in 31% of the individuals) increased grain Fe up to 11%. Haplotype analysis of this gene in rice suggests that the ability to detect the QTL is enhanced in the BAAP because the high Fe allele is balanced in aus, unlike indica and japonica subgroups.

Roles of subcellular metal homeostasis in crop improvement

Improvement of crop production in response to rapidly changing environmental conditions is a serious challenge facing plant breeders and biotechnologists. Iron (Fe), zinc (Zn), manganese (Mn), and copper (Cu) are essential micronutrients for plant growth and reproduction. These minerals are critical to several cellular processes including metabolism, photosynthesis, and cellular respiration. Regulating the uptake and distribution of these minerals could significantly improve plant growth and development, ultimately leading to increased crop production. Plant growth is limited by mineral deficiency, but on the other hand, excess Fe, Mn, Cu, and Zn can be toxic to plants; therefore, their uptake and distribution must be strictly regulated. Moreover, the distribution of these metals among subcellular organelles is extremely important for maintaining optimal cellular metabolism. Understanding the mechanisms controlling subcellular metal distribution and availability would enable development of crop plants that are better adapted to challenging and rapidly changing environmental conditions. Here, we describe advances in understanding of subcellular metal homeostasis, with a particular emphasis on cellular Fe homeostasis in Arabidopsis and rice, and discuss strategies for regulating cellular metabolism to improve plant production.

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 Mechanisms and Biochemical Pathways for Micronutrient Acquisition and Storage in Legumes to Support Biofortification for Nutritional Security

The world faces a grave situation of nutrient deficiency as a consequence of increased uptake of calorie-rich food that threaten nutritional security. More than half the world's population is affected by different forms of malnutrition. Unhealthy diets associated with poor nutrition carry a significant risk of developing non-communicable diseases, leading to a high mortality rate. Although considerable efforts have been made in agriculture to increase nutrient content in cereals, the successes are insufficient. The number of people affected by different forms of malnutrition has not decreased much in the recent past. While legumes are an integral part of the food system and widely grown in sub-Saharan Africa and South Asia, only limited efforts have been made to increase their nutrient content in these regions. Genetic variation for a majority of nutritional traits that ensure nutritional security in adverse conditions exists in the germplasm pool of legume crops. This diversity can be utilized by selective breeding for increased nutrients in seeds. The targeted identification of precise factors related to nutritional traits and their utilization in a breeding program can help mitigate malnutrition. The principal objective of this review is to present the molecular mechanisms of nutrient acquisition, transport and metabolism to support a biofortification strategy in legume crops to contribute to addressing malnutrition.

Cobalt: An Essential Micronutrient for Plant Growth?

Cobalt is a transition metal located in the fourth row of the periodic table and is a neighbor of iron and nickel. It has been considered an essential element for prokaryotes, human beings, and other mammals, but its essentiality for plants remains obscure. In this article, we proposed that cobalt (Co) is a potentially essential micronutrient of plants. Co is essential for the growth of many lower plants, such as marine algal species including diatoms, chrysophytes, and dinoflagellates, as well as for higher plants in the family Fabaceae or Leguminosae. The essentiality to leguminous plants is attributed to its role in nitrogen (N) fixation by symbiotic microbes, primarily rhizobia. Co is an integral component of cobalamin or vitamin B₁₂, which is required by several enzymes involved in N₂ fixation. In addition to symbiosis, a group of N₂ fixing bacteria known as diazotrophs is able to situate in plant tissue as endophytes or closely associated with roots of plants including economically important crops, such as barley, corn, rice, sugarcane, and wheat. Their action in N₂ fixation provides crops with the macronutrient of N. Co is a component of several enzymes and proteins, participating in plant metabolism. Plants may exhibit Co deficiency if there is a severe limitation in Co supply. Conversely, Co is toxic to plants at higher concentrations. High levels of Co result in pale-colored leaves, discolored veins, and the loss of leaves and can also cause iron deficiency in plants. It is anticipated that with the advance of omics, Co as a constitute of enzymes and proteins and its specific role in plant metabolism will be exclusively revealed. The confirmation of Co as an essential micronutrient will enrich our understanding of plant mineral nutrition and improve our practice in crop production.

The Medicago truncatula Yellow Stripe1-Like3 gene is involved in vascular delivery of transition metals to root nodules

Symbiotic nitrogen fixation carried out in legume root nodules requires transition metals. These nutrients are delivered by the host plant to the endosymbiotic nitrogen-fixing bacteria living within the nodule cells, a process in which vascular transport is essential. As members of the Yellow Stripe-Like (YSL) family of metal transporters are involved in root to shoot transport, they should also be required for root to nodule metal delivery. The genome of the model legume Medicago truncatula encodes eight YSL proteins, four of them with a high degree of similarity to Arabidopsis thaliana YSLs involved in long-distance metal trafficking. Among them, MtYSL3 is a plasma membrane protein expressed by vascular cells in roots and nodules and by cortical nodule cells. Reducing the expression level of this gene had no major effect on plant physiology when assimilable nitrogen was provided in the nutrient solution. However, nodule functioning was severely impaired, with a significant reduction of nitrogen fixation capabilities. Further, iron and zinc accumulation and distribution changed. Iron was retained in the apical region of the nodule, while zinc became strongly accumulated in the nodule veins in the ysl3 mutant. These data suggest a role for MtYSL3 in vascular delivery of iron and zinc to symbiotic nitrogen fixation.

Comparative Transcriptome Analysis of Iron and Zinc Deficiency in Maize (Zea mays L.)

Globally, one-third of the population is affected by iron (Fe) and zinc (Zn) deficiency, which is severe in developing and underdeveloped countries where cereal-based diets predominate. The genetic biofortification approach is the most sustainable and one of the cost-effective ways to address Fe and Zn malnutrition. Maize is a major source of nutrition in sub-Saharan Africa, South Asia and Latin America. Understanding systems’ biology and the identification of genes involved in Fe and Zn homeostasis facilitate the development of Fe- and Zn-enriched maize. We conducted a genome-wide transcriptome assay in maize inbred SKV616, under –Zn, –Fe and –Fe–Zn stresses. The results revealed the differential expression of several genes related to the mugineic acid pathway, metal transporters, photosynthesis, phytohormone and carbohydrate metabolism. We report here Fe and Zn deficiency-mediated changes in the transcriptome, root length, stomatal conductance, transpiration rate and reduced rate of photosynthesis. Furthermore, the presence of multiple regulatory elements and/or the co-factor nature of Fe and Zn in enzymes indicate their association with the differential expression and opposite regulation of several key gene(s). The differentially expressed candidate genes in the present investigation would help in breeding for Fe and Zn efficient and kernel Fe- and Zn-rich maize cultivars through gene editing, transgenics and molecular breeding.

Molecular mechanisms of plant adaptive responses to heavy metals stress

Heavy metals (HMs) are among the main environmental pollutants that can enter the soil, water bodies, and the atmosphere as a result of natural processes (weathering of rocks, volcanic activity), and also as a result of human activities (mining, metallurgical and chemical industries, transport, application of mineral fertilizers). Plants counteract the HMs stresses through morphological and physiological adaptations, which are imparted through well-coordinated molecular mechanisms. New approaches, which include transcriptomics, genomics, proteomics, and metabolomics analyses, have opened the paths to understand such complex networks. This review sheds light on molecular mechanisms included in plant adaptive and defense responses during metal stress. It is focused on the entry of HMs into plants, its transport and accumulation, effects on the main physiological processes, gene expressions included in plant adaptive and defense responses during HM stress. Analysis of new data allowed the authors to conclude that the most important mechanism of HM tolerance is extracellular and intracellular HM sequestration. Organic anions (malate, oxalate, etc.) provide extracellular sequestration of HM ions. Intracellular HM sequestration depends not only on a direct binding mechanism with different polymers (pectin, lignin, cellulose, hemicellulose, etc.) or organic anions but also on the action of cellular receptors and transmembrane transporters. We focused on the functioning chloroplasts, mitochondria, and the Golgi complex under HM stress. The currently known molecular mechanisms of plant tolerance to the toxic effects of HMs are analyzed.

Structure, Function, Regulation and Phylogenetic Relationship of ZIP Family Transporters of Plants

Zinc (Zn) is an essential micronutrient for plants and humans. Nearly 50% of the agriculture soils of world are Zn-deficient. The low availability of Zn reduces the yield and quality of the crops. The zinc-regulated, iron-regulated transporter-like proteins (ZIP) family and iron-regulated transporters (IRTs) are involved in cellular uptake of Zn, its intracellular trafficking and detoxification in plants. In addition to Zn, ZIP family transporters also transport other divalent metal cations (such as Cd2+, Fe2+, and Cu2+). ZIP transporters play a crucial role in biofortification of grains with Zn. Only a very limited information is available on structural features and mechanism of Zn transport of plant ZIP family transporters. In this article, we present a detailed account on structure, function, regulations and phylogenetic relationships of plant ZIP transporters. We give an insight to structure of plant ZIPs through homology modeling and multiple sequence alignment with Bordetella bronchiseptica ZIP (BbZIP) protein whose crystal structure has been solved recently. We also provide details on ZIP transporter genes identified and characterized in rice and other plants till date. Functional characterization of plant ZIP transporters will help for the better crop yield and human health in future.

Mimosine facilitates metallic cation uptake by plants through formation of mimosine-cation complexes

Iron deficiency conditions as well as iron supplied as a Fe(III)-mimosine complex induced a number of strategy I and strategy II genes for iron uptake in leucaena. Leucaena leucocephala (leucaena) is a tree-legume that can grow in alkaline soils, where metal-cofactors like Fe(III) are sparingly available. Mimosine, a known chelator of Fe(III), may facilitate Fe(III) uptake in leucaena by serving as a phytosiderophore. To test if mimosine can serve as a phytosiderophore, three sets of experiments were carried out. First, the binding properties and solubility of metal-mimosine complexes were assessed through spectrophotometry. Second, to study mimosine uptake in plants, pole bean, common bean, and tomato plants were supplied with mimosine alone and metal-mimosine complexes. Third, the expression of strategy I (S1) and strategy II (S2) genes for iron uptake from the soil was studied in leucaena plants exposed to different Fe(III) complexes. The results of this study show that (i) mimosine has high binding affinity for metallic cations at alkaline pH, Fe(III)-mimosine complexes are water soluble at alkaline pH, and that mimosine can bind soil iron under alkaline pH; (ii) pole bean, common bean, and tomato plants can uptake mimosine and transport it throughout the plant; and (iii) a number of S1 and S2 genes were upregulated in leucaena under iron-deficiency condition or when Fe(III) was supplied as a Fe(III)-mimosine complex. These findings suggest that leucaena may utilize both S1 and S2 strategies for iron uptake; and mimosine may play an important role in both strategies.

Phenazine-1-Carboxylic Acid-Producing Bacteria Enhance the Reactivity of Iron Minerals in Dryland and Irrigated Wheat Rhizospheres

Phenazine-1-carboxylic acid (PCA) is a broad-spectrum antibiotic produced by rhizobacteria in the dryland wheat fields of the Columbia Plateau. PCA and other phenazines reductively dissolve Fe and Mn oxyhydroxides in bacterial culture systems, but the impact of PCA upon Fe and Mn cycling in the rhizosphere is unknown. Here, concentrations of dithionite-extractable and poorly crystalline Fe were approximately 10% and 30-40% higher, respectively, in dryland and irrigated rhizospheres inoculated with the PCA-producing (PCA⁺) strain Pseudomonas synxantha 2-79 than in rhizospheres inoculated with a PCA-deficient mutant. However, rhizosphere concentrations of Fe(II) and Mn did not differ significantly, indicating that PCA-mediated redox transformations of Fe and Mn were transient or were masked by competing processes. Total Fe and Mn uptake into wheat biomass also did not differ significantly, but the PCA⁺ strain significantly altered Fe translocation into shoots. X-ray absorption near edge spectroscopy revealed an abundance of Fe-bearing oxyhydroxides and phyllosilicates in all rhizospheres. These results indicate that the PCA⁺ strain enhanced the reactivity and mobility of Fe derived from soil minerals without producing parallel changes in plant Fe uptake. This is the first report that directly links significant alterations of Fe-bearing minerals in the rhizosphere to a single bacterial trait.

Evaluation of antioxidant activities and essential/toxicmetal levels and their health risk assessment in citrus fruits from Pakistan

Fruits are important components of human diet, and their contamination by environmental pollutants is an emerging challenge nowadays. The present study is based on the measurement of selected essential and toxic trace metals including Na, K, Ca, Mg, Fe, Zn, Cu, Mn, Cr, Co, Sr, Li, Ni, Pb, and Cd in commercially available citrus fruits from Pakistan. The samples were digested in HNO₃ and HCLO₄ mixture, and the metal contents were quantified by flame atomic absorption spectrometry. Highest concentration was found for Ca (609.0-3596 mg/kg), followed by relatively higher levels of K (277.6-682.1 mg/kg), Mg (53.65-123.4 mg/kg), Na (1.173-52.14 mg/kg), and Fe (0.236-10.57 mg/kg), while Li, Ni, and Cd showed the lowest contributions in most of the samples. In addition, antioxidant activities such as DPPH radical scavenging assay, hydroxyl radical scavenging activity, ferrous chelating activity, ferric reducing antioxidant power assay, and phosphomolybdenum assay were also evaluated in the fruit samples. Considerably higher antioxidant activities were shown by grapefruit, mandarin, sweet lime, and tangerine. Most of the antioxidant assays were significantly correlated with Na, Mg, Fe, Mn, and Cu levels in the fruits. Human health risk was evaluated in terms of health risk index (HRI), target hazard quotient (THQ), and target cancer risk (TCR) which revealed insignificant health risks; thus, the consumption of these fruits can be considered as safe for human diet.

Interactions Between Phosphorus, Zinc, and Iron Homeostasis in Nonmycorrhizal and Mycorrhizal Plants

Phosphorus (P), zinc (Zn), and iron (Fe) are three essential elements for plant survival, and severe deficiencies in these nutrients lead to growth retardation and crop yield reduction. This review synthesizes recent progress on how plants coordinate the acquisition and signaling of Pi, Zn, and Fe from surrounding environments and which genes are involved in these Pi-Zn-Fe interactions with the aim of better understanding of the cross-talk between these macronutrient and micronutrient homeostasis in plants. In addition, identification of genes important for interactions between Pi, Zn, and/or Fe transport and signaling is a useful target for breeders for improvement in plant nutrient acquisition. Furthermore, to understand these processes in arbuscular mycorrhizal plants, the preliminary examination of interactions between Pi, Zn, and Fe homeostasis in some relevant crop species has been performed at the physiological level and is summarized in this article. In conclusion, the development of integrative study of cross-talks between Pi, Zn, and Fe signaling pathway in mycorrhizal plants will be essential for sustainable agriculture all around the world.

A Review of Plant Vacuoles: Formation, Located Proteins, and Functions

Vacuoles, cellular membrane-bound organelles, are the largest compartments of cells, occupying up to 90% of the volume of plant cells. Vacuoles are formed by the biosynthetic and endocytotic pathways. In plants, the vacuole is crucial for growth and development and has a variety of functions, including storage and transport, intracellular environmental stability, and response to injury. Depending on the cell type and growth conditions, the size of vacuoles is highly dynamic. Different types of cell vacuoles store different substances, such as alkaloids, protein enzymes, inorganic salts, sugars, etc., and play important roles in multiple signaling pathways. Here, we summarize vacuole formation, types, vacuole-located proteins, and functions.

The Adaptive Mechanism of Plants to Iron Deficiency via Iron Uptake, Transport, and Homeostasis

Iron is an essential element for plant growth and development. While abundant in soil, the available Fe in soil is limited. In this regard, plants have evolved a series of mechanisms for efficient iron uptake, allowing plants to better adapt to iron deficient conditions. These mechanisms include iron acquisition from soil, iron transport from roots to shoots, and iron storage in cells. The mobilization of Fe in plants often occurs via chelating with phytosiderophores, citrate, nicotianamine, mugineic acid, or in the form of free iron ions. Recent work further elucidates that these genes’ response to iron deficiency are tightly controlled at transcriptional and posttranscriptional levels to maintain iron homeostasis. Moreover, increasing evidences shed light on certain factors that are identified to be interconnected and integrated to adjust iron deficiency. In this review, we highlight the molecular and physiological bases of iron acquisition from soil to plants and transport mechanisms for tolerating iron deficiency in dicotyledonous plants and rice.

Changes in iron availability in Arabidopsis are rapidly sensed in the leaf vasculature and impaired sensing leads to opposite transcriptional programs in leaves and roots

The OLIGOPEPTIDE TRANSPORTER 3 (OPT3) has recently been identified as a component of the systemic network mediating iron (Fe) deficiency responses in Arabidopsis. Reduced expression of OPT3 induces an over accumulation of Fe in roots and leaves, due in part by an elevated expression of the IRON-REGULATED TRANSPORTER 1. Here we show however, that opt3 leaves display a transcriptional program consistent with an Fe overload, suggesting that Fe excess is properly sensed in opt3 leaves and that the OPT3-mediated shoot-to-root signaling is critical to prevent a systemic Fe overload. We also took advantage of the tissue-specific localization of OPT3, together with other Fe-responsive genes, to determine the timing and location of early transcriptional events during Fe limitation and resupply. Our results show that the leaf vasculature responds more rapidly than roots to both Fe deprivation and resupply, suggesting that the leaf vasculature is within the first tissues that sense and respond to changes in Fe availability. Our data highlight the importance of the leaf vasculature in Fe homeostasis by sensing changes in apoplastic levels of Fe coming through the xylem and relaying this information back to roots via the phloem to regulate Fe uptake at the root level.

Sucrose is involved in the regulation of iron deficiency responses in rice (Oryza sativa L.)

Sucrose signaling pathways were rapidly induced in response to early iron deficiency in rice plants, and the change of sucrose contents in plants was essential for the activation of iron deficiency responses. Sucrose is the main product of photosynthesis in plants, and it functions not only as an energy metabolite but also a signal molecule. However, a few studies have examined the involvement of sucrose in mediating iron deficiency responses in rice. In this study, we found that the decrease in photosynthesis and total chlorophyll concentration (SPAD values) in leaves occurred at a very early stage under iron deficiency. In addition, the sucrose was increased in leaves but decreased in roots of rice plants under iron deficiency, and also the sucrose transporter (SUT) encoded genes' expression levels in leaves were all inhibited, including OsSUT1, OsSUT2, OsSUT3, OsSUT4, and OsSUT5. The carbohydrate distribution was changed under iron deficiency and sucrose might be involved in the iron deficiency responses of rice plants. Furthermore, exogenous application of sucrose or dark treatment experiments were used to test the hypothesis; we found that the increased endogenous sucrose would cause the repression of iron acquisition-related genes in roots, while further stimulated iron transport-related genes in leaves. Compared to the exogenous application of sucrose, the dark treatment had the opposite effects. All the above results highlighted the important role of sucrose in regulating the responses of rice plants to iron deficiency.

Medicago truncatula copper transporter 1 (MtCOPT1) delivers copper for symbiotic nitrogen fixation

Copper is an essential nutrient for symbiotic nitrogen fixation. This element is delivered by the host plant to the nodule, where membrane copper (Cu) transporter would introduce it into the cell to synthesize cupro-proteins. COPT family members in the model legume Medicago truncatula were identified and their expression determined. Yeast complementation assays, confocal microscopy and phenotypical characterization of a Tnt1 insertional mutant line were carried out in the nodule-specific M. truncatula COPT family member. Medicago truncatula genome encodes eight COPT transporters. MtCOPT1 (Medtr4g019870) is the only nodule-specific COPT gene. It is located in the plasma membrane of the differentiation, interzone and early fixation zones. Loss of MtCOPT1 function results in a Cu-mitigated reduction of biomass production when the plant obtains its nitrogen exclusively from symbiotic nitrogen fixation. Mutation of MtCOPT1 results in diminished nitrogenase activity in nodules, likely an indirect effect from the loss of a Cu-dependent function, such as cytochrome oxidase activity in copt1-1 bacteroids. These data are consistent with a model in which MtCOPT1 transports Cu from the apoplast into nodule cells to provide Cu for essential metabolic processes associated with symbiotic nitrogen fixation.

Ecophysiological responses to excess iron in lowland and upland rice cultivars

Iron (Fe) is an essential nutrient for plants but under high concentrations, such as that found naturally in clay and waterlogged soils, its toxic effect can limit production. This study aimed to investigate the stress tolerance responses exhibited by different rice cultivars. Both lowland and upland cultivars were grown under excess Fe and hypoxic conditions. Lowland cultivars showed higher Fe accumulation in roots compared with upland cultivars suggesting the use of different strategies to tolerate excess Fe. The upland Canastra cultivar displayed a mechanism to limit iron translocation from roots to the shoots, minimizing leaf oxidative stress induced by excess Fe. Conversely, the cultivar Curinga invested in the increase of R1/A, as an alternative drain of electrons. However, the higher iron accumulation in the leaves, was not necessarily related to high toxicity. Nutrient uptake and/or utilization mechanisms in rice plants are in accordance with their needs, which may be defined in relation to crop environments. Alterations in the biochemical parameters of photosynthesis suggest that photosynthesis in rice under excess Fe is primarily limited by biochemical processes rather than by diffusional limitations, particularly in the upland cultivars. The electron transport rate, carboxylation efficiency and electron excess dissipation by photorespiration demonstrate to be good indicators of iron tolerance. Altogether, these chemical and molecular patterns suggests that rice plants grown under excess Fe exhibit gene expression reprogramming in response to the Fe excess per se and in response to changes in photosynthesis and nutrient levels to maintain growth under stress.

Divergent biology of facultative heavy metal plants

Among heavy metal plants (the metallophytes), facultative species can live both in soils contaminated by an excess of heavy metals and in non-affected sites. In contrast, obligate metallophytes are restricted to polluted areas. Metallophytes offer a fascinating biology, due to the fact that species have developed different strategies to cope with the adverse conditions of heavy metal soils. The literature distinguishes between hyperaccumulating, accumulating, tolerant and excluding metallophytes, but the borderline between these categories is blurred. Due to the fact that heavy metal soils are dry, nutrient limited and are not uniform but have a patchy distribution in many instances, drought-tolerant or low nutrient demanding species are often regarded as metallophytes in the literature. In only a few cases, the concentrations of heavy metals in soils are so toxic that only a few specifically adapted plants, the genuine metallophytes, can cope with these adverse soil conditions. Current molecular biological studies focus on the genetically amenable and hyperaccumulating Arabidopsis halleri and Noccaea (Thlaspi) caerulescens of the Brassicaceae. Armeria maritima ssp. halleri utilizes glands for the excretion of heavy metals and is, therefore, a heavy metal excluder. The two endemic zinc violets of Western Europe, Viola lutea ssp. calaminaria of the Aachen-Liège area and Viola lutea ssp. westfalica of the Pb-Cu-ditch of Blankenrode, Eastern Westphalia, as well as Viola tricolor ecotypes of Eastern Europe, keep their cells free of excess heavy metals by arbuscular mycorrhizal fungi which bind heavy metals. The Caryophyllaceae, Silene vulgaris f. humilis and Minuartia verna, apparently discard leaves when overloaded with heavy metals. All Central European metallophytes have close relatives that grow in areas outside of heavy metal soils, mainly in the Alps, and have, therefore, been considered as relicts of the glacial epoch in the past. However, the current literature favours the idea that hyperaccumulation of heavy metals serves plants as deterrent against attack by feeding animals (termed elemental defense hypothesis). The capability to hyperaccumulate heavy metals in A. halleri and N. caerulescens is achieved by duplications and alterations of the cis-regulatory properties of genes coding for heavy metal transporting/excreting proteins. Several metallophytes have developed ecotypes with a varying content of such heavy metal transporters as an adaption to the specific toxicity of a heavy metal site.

Understanding Fe2+ toxicity and P deficiency tolerance in rice for enhancing productivity under acidic soils

Plants experience low phosphorus (P) and high iron (Fe) levels in acidic lowland soils that lead to reduced crop productivity. A better understanding of the relationship between these two stresses at molecular and physiological level will lead to development of suitable strategies to increase crop productivity in such poor soils. Tolerance for most abiotic stresses including P deficiency and Fe toxicity is a quantitative trait in rice. Recent studies in the areas of physiology, genetics, and overall metabolic pathways in response to P deficiency of rice plants have improved our understanding of low P tolerance. Phosphorous uptake and P use efficiency are the two key traits for improving P deficiency tolerance. In the case of Fe toxicity tolerance, QTLs have been reported but the identity and role played by underlying genes is just emerging. Details pertaining to Fe deficiency tolerance in rice are well worked out including genes involved in Fe sensing and uptake. But, how rice copes with Fe toxicity is not clearly understood. This review focuses on the progress made in understanding these key environmental stresses. Finally, an opinion on the key genes which can be targeted for this stress is provided.

Marker-Trait Association Analysis of Iron and Zinc Concentration in Lentil (Lens culinaris Medik.) Seeds

Lentil ( Medik.) seeds are relatively rich in iron (Fe) and zinc (Zn), making lentil a potential crop to aid in the global battle against human micronutrient deficiency. Understanding the genetic basis for uptake of seed Fe and Zn is required to increase sustainable concentrations of these minerals in seeds. The objectives of this study were to characterize genetic variation in seed Fe and Zn concentration and to identify molecular markers associated with these traits across diverse lentil accessions. A set of 138 cultivated lentil accessions from 34 countries were evaluated in four environments (2 sites × 2 yr) in Saskatchewan, Canada. The collection was genotyped using 1150 single-nucleotide polymorphism (SNP) markers that are distributed across the lentil genome. The germplasm tested exhibited a wide range of variation for seed Fe and Zn concentration. The marker-trait association analysis detected two SNP markers tightly linked to seed Fe and one linked to seed Zn concentration (-log10 ≥ 4.36). Additional markers were detected at -log10 ≥ 3.06. A number of putative candidate genes underlying detected loci encode Fe- and Zn-related functions. This study provides insight into the genetics of seed Fe and Zn concentration of lentil and opportunities for marker-assisted selection to improve micronutrient concentration as part of micronutrient biofortification programs.

Arabidopsis Glutaredoxin S17 Contributes to Vegetative Growth, Mineral Accumulation, and Redox Balance during Iron Deficiency

Iron (Fe) is an essential mineral nutrient and a metal cofactor required for many proteins and enzymes involved in the processes of DNA synthesis, respiration, and photosynthesis. Iron limitation can have detrimental effects on plant growth and development. Such effects are mediated, at least in part, through the generation of reactive oxygen species (ROS). Thus, plants have evolved a complex regulatory network to respond to conditions of iron limitations. However, the mechanisms that couple iron deficiency and oxidative stress responses are not fully understood. Here, we report the discovery that an Arabidopsis thaliana monothiol glutaredoxin S17 (AtGRXS17) plays a critical role in the plants ability to respond to iron deficiency stress and maintain redox homeostasis. In a yeast expression assay, AtGRXS17 was able to suppress the iron accumulation in yeast ScGrx3/ScGrx4 mutant cells. Genetic analysis indicated that plants with reduced AtGRXS17 expression were hypersensitive to iron deficiency and showed increased iron concentrations in mature seeds. Disruption of AtGRXS17 caused plant sensitivity to exogenous oxidants and increased ROS production under iron deficiency. Addition of reduced glutathione rescued the growth and alleviates the sensitivity of atgrxs17 mutants to iron deficiency. These findings suggest AtGRXS17 helps integrate redox homeostasis and iron deficiency responses.

Genome-Wide Identification and Expression Analysis of NRAMP Family Genes in Soybean (Glycine Max L.)

The NRAMP (natural resistance-associated macrophage protein) family of genes has been widely characterized in organisms ranging from bacteria to yeast, plants, mice, and humans. This gene family plays vital roles in divalent metal ion transport across cellular membranes. As yet, comprehensive analysis of NRAMP family genes has not been reported for soybean. In this study, bioinformatics analysis was conducted to identify 13 soybean NRAMP genes, along with their gene structures, phylogenetic relationships, and transmembrane domains. Expression analysis suggests that GmNRAMP genes function in numerous tissues and development stages. Moreover, soybean NRAMP genes were differentially regulated by deficiencies of N, P, K, Fe, and S, along with toxicities of Fe, Cu, Cd, and Mn. These results indicate that GmNRAMP genes function in many nutrient stress pathways, and might be involved in crosstalk among nutrient stress pathways. Subcellular localization analysis in Arabidopsis protoplasts confirmed the tonoplast or plasma membrane localization of selected soybean NRMAP proteins. Protein-protein interaction analysis found that the networks of three GmNRAMP proteins which putatively interact with nodulin-like proteins, almost distinct from the network that is common to the other 10 soybean NRAMP proteins. Subsequent qRT-PCR results confirmed that these three GmNRMAP genes exhibited enhanced expression in soybean nodules, suggesting potential functions in the transport of Fe or other metal ions in soybean nodules. Overall, the systematic analysis of the GmNRAMP gene family reported herein provides valuable information for further studies on the biological roles of GmNRAMPs in divalent metal ion transport in various soybean tissues under numerous nutrient stresses and soybean-rhizobia symbiosis.

Arabidopsis Glutaredoxin S17 Contributes to Vegetative Growth, Mineral Accumulation, and Redox Balance during Iron Deficiency

Iron (Fe) is an essential mineral nutrient and a metal cofactor required for many proteins and enzymes involved in the processes of DNA synthesis, respiration, and photosynthesis. Iron limitation can have detrimental effects on plant growth and development. Such effects are mediated, at least in part, through the generation of reactive oxygen species (ROS). Thus, plants have evolved a complex regulatory network to respond to conditions of iron limitations. However, the mechanisms that couple iron deficiency and oxidative stress responses are not fully understood. Here, we report the discovery that an Arabidopsis thaliana monothiol glutaredoxin S17 (AtGRXS17) plays a critical role in the plants ability to respond to iron deficiency stress and maintain redox homeostasis. In a yeast expression assay, AtGRXS17 was able to suppress the iron accumulation in yeast ScGrx3/ScGrx4 mutant cells. Genetic analysis indicated that plants with reduced AtGRXS17 expression were hypersensitive to iron deficiency and showed increased iron concentrations in mature seeds. Disruption of AtGRXS17 caused plant sensitivity to exogenous oxidants and increased ROS production under iron deficiency. Addition of reduced glutathione rescued the growth and alleviates the sensitivity of atgrxs17 mutants to iron deficiency. These findings suggest AtGRXS17 helps integrate redox homeostasis and iron deficiency responses.

Comparison on cellular mechanisms of iron and cadmium accumulation in rice: prospects for cultivating Fe-rich but Cd-free rice

Iron (Fe) is essential for rice growth and humans consuming as their staple food but is often deficient because of insoluble Fe(III) in soil for rice growth and limited assimilation for human bodies, while cadmium (Cd) is non-essential and toxic for rice growth and humans if accumulating at high levels. Over-accumulated Cd can cause damage to human bodies. Selecting and breeding Fe-rich but Cd-free rice cultivars are ambitious, challenging and meaningful tasks for researchers. Although evidences show that the mechanisms of Fe/Cd uptake and accumulation in rice are common to some extent as a result of similar entry routes within rice, an increasing number of researchers have discovered distinct mechanisms between Fe/Cd uptake and accumulation in rice. This comprehensive review systematically elaborates and compares cellular mechanisms of Fe/Cd uptake and accumulation in rice, respectively. Mechanisms for maintaining Fe homeostasis and Cd detoxicification are also elucidated. Then, effects of different fertilizer management on Fe/Cd accumulation in rice are discussed. Finally, this review enumerates various approaches for reducing grain Cd accumulation and enhancing Fe content in rice. In summary, understanding of discrepant cellular mechanisms of Fe/Cd accumulation in rice provides guidance for cultivating Fe-fortified rice and has paved the way to develop rice that are tolerant to Cd stress, aiming at breeding Fe-rich but Cd-free rice.