Paralogs and mutants show that one DMA synthase functions in iron homeostasis in rice

Nicotianamine: A Key Player in Metal Homeostasis and Hyperaccumulation in Plants

Nicotianamine (NA) is a low-molecular-weight N-containing metal-binding ligand, whose accumulation in plant organs changes under metal deficiency or excess. Although NA biosynthesis can be induced in vivo by various metals, this non-proteinogenic amino acid is mainly involved in the detoxification and transport of iron, zinc, nickel, copper and manganese. This review summarizes the current knowledge on NA biosynthesis and its regulation, considers the mechanisms of NA secretion by plant roots, as well as the mechanisms of intracellular transport of NA and its complexes with metals, and its role in radial and long-distance metal transport. Its role in metal tolerance is also discussed. The NA contents in excluders, storing metals primarily in roots, and in hyperaccumulators, accumulating metals mainly in shoots, are compared. The available data suggest that NA plays an important role in maintaining metal homeostasis and hyperaccumulation mechanisms. The study of metal-binding compounds is of interdisciplinary significance, not only regarding their effects on metal toxicity in plants, but also in connection with the development of biofortification approaches to increase the metal contents, primarily of iron and zinc, in agricultural plants, since the deficiency of these elements in food crops seriously affects human health.

Ferrous iron uptake via IRT1/ZIP evolved at least twice in green plants

Iron (Fe) is essential for virtually all organisms, being irreplaceable because of its electrochemical properties that enable many biochemical processes, including photosynthesis. Besides its abundance, Fe is generally found in the poorly soluble form of ferric iron (Fe³⁺ ), while most plants uptake the soluble form Fe²⁺ . The model angiosperm Arabidopsis thaliana, for example, captures Fe through a mechanism that lowers rhizosphere pH through proton pumping that increases Fe³⁺ solubility, which is then reduced by a membrane-bound reductase and transported into the cell by the zinc-regulated, iron-regulated transporter-like protein (ZIP) family protein AtIRT1. ZIP proteins are transmembrane transporters of divalent metals such as Fe²⁺ , Zn²⁺ , Mn²⁺ , and Cd²⁺ . In this work, we investigated the evolution of functional homologs of IRON-REGULATED TRANSPORTER 1/ZIP in the supergroup Archaeplastida (Viridiplantae + Rhodophyta + Glaucophyta) using 51 genomes of diverse lineages. Our analyses suggest that Fe is acquired through deeply divergent ZIP proteins in land plants and chlorophyte green algae, indicating that Fe²⁺ uptake by ZIP proteins evolved independently at least twice throughout green plant evolution. Our results indicate that the archetypical IRON-REGULATED TRANSPORTER (IRT) proteins from angiosperms likely emerged before the origin of land plants during early streptophyte algae terrestrialization, a process that required the evolution of Fe acquisition in terrestrial subaerial settings.

A tale of two metals: Biofortification of rice grains with iron and zinc

Iron (Fe) and zinc (Zn) are essential micronutrients needed by virtually all living organisms, including plants and humans, for proper growth and development. Due to its capacity to easily exchange electrons, Fe is important for electron transport in mitochondria and chloroplasts. Fe is also necessary for chlorophyll synthesis. Zn is a cofactor for several proteins, including Zn-finger transcription factors and redox metabolism enzymes such as copper/Zn superoxide dismutases. In humans, Fe participates in oxygen transport, electron transport, and cell division whereas Zn is involved in nucleic acid metabolism, apoptosis, immunity, and reproduction. Rice (Oryza sativa L.) is one of the major staple food crops, feeding over half of the world's population. However, Fe and Zn concentrations are low in rice grains, especially in the endosperm, which is consumed as white rice. Populations relying heavily on rice and other cereals are prone to Fe and Zn deficiency. One of the most cost-effective solutions to this problem is biofortification, which increases the nutritional value of crops, mainly in their edible organs, without yield reductions. In recent years, several approaches were applied to enhance the accumulation of Fe and Zn in rice seeds, especially in the endosperm. Here, we summarize these attempts involving transgenics and mutant lines, which resulted in Fe and/or Zn biofortification in rice grains. We review rice plant manipulations using ferritin genes, metal transporters, changes in the nicotianamine/phytosiderophore pathway (including biosynthetic genes and transporters), regulators of Fe deficiency responses, and other mutants/overexpressing lines used in gene characterization that resulted in Fe/Zn concentration changes in seeds. This review also discusses research gaps and proposes possible future directions that could be important to increase the concentration and bioavailability of Fe and Zn in rice seeds without the accumulation of deleterious elements. We also emphasize the need for a better understanding of metal homeostasis in rice, the importance of evaluating yield components of plants containing transgenes/mutations under field conditions, and the potential of identifying genes that can be manipulated by gene editing and other nontransgenic approaches.

Iron uptake, signaling, and sensing in plants

Iron (Fe) is an essential micronutrient that affects the growth and development of plants because it participates as a cofactor in numerous physiological and biochemical reactions. As a transition metal, Fe is redox active. Fe often exists in soil in the form of insoluble ferric hydroxides that are not bioavailable to plants. Plants have developed sophisticated mechanisms to ensure an adequate supply of Fe in a fluctuating environment. Plants can sense Fe status and modulate the transcription of Fe uptake-associated genes, finally controlling Fe uptake from soil to root. There is a critical need to understand the molecular mechanisms by which plants maintain Fe homeostasis in response to Fe fluctuations. This review focuses on recent advances in elucidating the functions of Fe signaling components. Taking Arabidopsis thaliana and Oryza sativa as examples, this review begins by discussing the Fe acquisition systems that control Fe uptake from soil, the major components that regulate Fe uptake systems, and the perception of Fe status. Future explorations of Fe signal transduction will pave the way for understanding the regulatory mechanisms that underlie the maintenance of plant Fe homeostasis.

OsIRO3 negatively regulates Fe homeostasis by repressing the expression of OsIRO2

Iron (Fe) is crucial for crop productivity and quality. However, Fe deficiency is prevalent worldwide, particularly in alkaline soil. Plants have evolved sophisticated mechanisms to withstand Fe-deficient conditions. Oryza sativa IRON-RELATED BHLH TRANSCRIPTION FACTOR 3 (OsIRO3/OsbHLH63) has been identified as a negative regulator of Fe deficiency response signaling; however, the underlying mechanism remains unclear. In the present study, we constructed two iro3 mutants, which developed leaves with necrotic lesions under Fe-deficient conditions. Loss-of-function of OsIRO3 caused upregulation of Fe deficiency-associated genes in the root. Fe concentration measurements showed that the iro3 mutants had increased shoot Fe concentration only under Fe-deficient conditions. Further analysis revealed that OsIRO3 directly regulated the expression of IRON-RELATED BHLH TRANSCRIPTION FACTOR 2 (OsIRO2), which encodes a positive regulator of the Fe uptake system. Further investigation demonstrated that OsIRO3 interacted with POSITIVE REGULATOR OF IRON HOMEOSTASIS 1(OsPRI1) and OsPRI2, and. OsIRO3 repressed their transcription activation towards OsIRO2. OsIRO3 contains an EAR motif, which recruits the TOPLESS/TOPLESS-RELATED (OsTPL/OsTPRs) corepressors. Mutation of the EAR motif attenuated the repression ability of OsIRO3. This work sheds light on the molecular mechanism by which OsIRO3 modulates Fe homeostasis in rice.

Strategies and Bottlenecks in Hexaploid Wheat to Mobilize Soil Iron to Grains

Our knowledge of iron (Fe) uptake and mobilization in plants is mainly based on Arabidopsis and rice. Although multiple players of Fe homeostasis have been elucidated, there is a significant gap in our understanding of crop species, such as wheat. It is, therefore, imperative not only to understand the different hurdles for Fe enrichment in tissues but also to address specifically the knowns/unknowns involved in the plausible mechanism of Fe sensing, signaling, transport, and subsequent storage in plants. In the present review, a unique perspective has been described in light of recent knowledge generated in wheat, an economically important crop. The strategies to boost efficient Fe uptake, transcriptional regulation, and long-distance mobilization in grains have been discussed, emphasizing recent biotechnological routes to load Fe in grains. This article also highlights the new elements of physiological and molecular genetics that underpin the mechanistic insight for the identified Fe-related genes and discusses the bottlenecks in unloading the Fe in grains. The information presented here will provide much-needed resources and directions to overcome challenges and design efficient strategies to enhance the Fe density in wheat grains.

Maize Interveinal Chlorosis 1 links the Yang Cycle and Fe homeostasis through Nicotianamine biosynthesis

The Yang cycle is involved in many essential metabolic pathways in plant growth and development. As extended products of the Yang cycle, the function and regulation network of ethylene and polyamines are well characterized. Nicotianamine (NA) is also a product of this cycle and works as a key metal chelator for iron (Fe) homeostasis in plants. However, interactions between the Yang cycle and NA biosynthesis remain unclear. Here, we cloned maize interveinal chlorosis 1 (mic1), encoding a 5'-methylthioadenosine nucleosidase (MTN), that is essential for 5'-methylthioadenosine (MTA) salvage and NA biosynthesis in maize (Zea mays). A single base G-A transition in the fourth exon of mic1 causes a Gly to Asp change, resulting in increased MTA, reduced Fe distribution, and growth retardation of seedlings. Knockout of ZmMIC1 but not its paralog ZmMTN2 by CRISPR/Cas9 causes interveinal chlorosis, indicating ZmMIC1 is mainly responsible for MTN activity in maize. Transcriptome analysis showed a typical response of Fe deficiency. However, metabolic analysis revealed dramatically reduced NA content in mic1, suggesting NA biosynthesis was impaired in the mutant. Exogenous application of NA transiently reversed the interveinal chlorosis phenotype of mic1 seedlings. Moreover, the mic1 mutant overexpressing a NA synthase gene not only recovered from interveinal chlorosis and growth retardation but was also fertile. These findings provide a link between the Yang cycle and NA biosynthesis, which highlights an aspect of Fe homeostasis regulation in maize.

Genome-wide analysis of the NAAT, DMAS, TOM, and ENA gene families in maize suggests their roles in mediating iron homeostasis

BACKGROUND

Nicotianamine (NA), 2'-deoxymugineic acid (DMA), and mugineic acid (MA) are chelators required for iron uptake and transport in plants. Nicotianamine aminotransferase (NAAT), 2'-deoxymugineic acid synthase (DMAS), transporter of MAs (TOM), and efflux transporter of NA (ENA) are involved in iron uptake and transport in rice (Oryza sativa), wheat (Triticum aestivum), and barley (Hordeum vulgare); however, these families have not been fully identified and comprehensively analyzed in maize (Zea mays L.).

RESULTS

Here, we identified 5 ZmNAAT, 9 ZmDMAS, 11 ZmTOM, and 2 ZmENA genes by genome mining. RNA-sequencing and quantitative real-time PCR analysis revealed that these genes are expressed in various tissues and respond differently to high and low iron conditions. In particular, iron deficiency stimulated the expression of ZmDMAS1, ZmTOM1, ZmTOM3, and ZmENA1. Furthermore, we determined protein subcellular localization by transient expression of green fluorescent protein fusions in maize mesophyll protoplasts. ZmNAAT1, ZmNAAT-L4, ZmDMAS1, and ZmDMAS-L1 localized in the cytoplasm, whereas ZmTOMs and ZmENAs targeted to plasma and tonoplast membranes, endomembranes, and vesicles.

CONCLUSIONS

Our results suggest that the different gene expression profiles and subcellular localizations of ZmNAAT, ZmDMAS, ZmTOM, and ZmENA family members may enable specific regulation of phytosiderophore metabolism in different tissues and under different external conditions, shedding light on iron homeostasis in maize and providing candidate genes for breeding iron-rich maize varieties.

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.

Challenges and opportunities to regulate mineral transport in rice

Iron (Fe) is an essential mineral for plants, and its deficiency as well as toxicity severely affects plant growth and development. Although Fe is ubiquitous in mineral soils, its acquisition by plants is difficult to regulate particularly in acidic and alkaline soils. Under alkaline conditions, where lime is abundant, Fe and other mineral elements are sparingly soluble. In contrast, under low pH conditions, especially in paddy fields, Fe toxicity could occur. Fe uptake is complicated and could be integrated with copper (Cu), manganese (Mn), zinc (Zn), and cadmium (Cd) uptake. Plants have developed sophisticated mechanisms to regulate the Fe uptake from soil and its transport to root and above-ground parts. Here, we review recent developments in understanding metal transport and discuss strategies to effectively regulate metal transport in plants with a particular focus on rice.

Low-molecular-weight ligands in plants: role in metal homeostasis and hyperaccumulation

Mineral nutrition is one of the key factors determining plant productivity. In plants, metal homeostasis is achieved through the functioning of a complex system governing metal uptake, translocation, distribution, and sequestration, leading to the maintenance of a regulated delivery of micronutrients to metal-requiring processes as well as detoxification of excess or non-essential metals. Low-molecular-weight ligands, such as nicotianamine, histidine, phytochelatins, phytosiderophores, and organic acids, play an important role in metal transport and detoxification in plants. Nicotianamine and histidine are also involved in metal hyperaccumulation, which determines the ability of some plant species to accumulate a large amount of metals in their shoots. In this review we extensively summarize and discuss the current knowledge of the main pathways for the biosynthesis of these ligands, their involvement in metal uptake, radial and long-distance transport, as well as metal influx, isolation and sequestration in plant tissues and cell compartments. It is analyzed how diverse endogenous ligand levels in plants can determine their different tolerance to metal toxic effects. This review focuses on recent advances in understanding the physiological role of these compounds in metal homeostasis, which is an essential task of modern ionomics and plant physiology. It is of key importance in studying the influence of metal deficiency or excess on various physiological processes, which is a prerequisite to the improvement of micronutrient uptake efficiency and crop productivity and to the development of a variety of applications in phytoremediation, phytomining, biofortification, and nutritional crop safety.

Foliar Application of Nano, Chelated, and Conventional Iron Forms Enhanced Growth, Nutritional Status, Fruiting Aspects, and Fruit Quality of Washington Navel Orange Trees (Citrus sinensis L. Osbeck)

Iron (Fe) is required for most metabolic processes, including DNA synthesis, respiration, photosynthesis, and chlorophyll biosynthesis; however, Fe deficiency is common in arid regions, necessitating additional research to determine the most efficient form of absorbance. Nano-fertilizers have characteristics that are not found in their traditional equivalents. This research was implemented on Washington navel orange trees (Citrus sinensis L. Osbeck) to investigate the effect of three iron forms-nano (Fe-NPs), sulfate (FeSO₄), and chelated (Fe-chelated)-as a foliar spray on the growth, fruiting aspects, and nutritional status of these trees compared to control. The highest values of the tested parameters were reported when the highest Fe-NPs level and the highest Fe-chelated (EDTA) rate were used. Results obtained here showed that the spraying of the Washington navel orange trees grown under similar environmental conditions and horticulture practices adopted in the current experiment with Fe-NPs (nanoform) and/or Fe-chelated (EDTA) at 0.1% is a beneficial application for enhancing vegetative growth, flower set, tree nutritional status, and fruit production and quality. Application of Fe-NPs and Fe-chelated (EDTA, 0.1%) increased yield by 32.0% and 25% and total soluble solids (TSS) by 18.5% and 17.0%, respectively, compared with control. Spraying Washington navel orange trees with nano and chelated iron could be considered a significant way to improve vegetative growth, fruit production, quality, and nutritional status while also being environmentally preferred in the arid regions.

IRONing out stress problems in crops: a homeostatic perspective

Iron (Fe) is essential for plant growth and therefore plays a key role in influencing crop productivity worldwide. Apart from its central role in chlorophyll biosynthesis and oxidative phosphorylation (electron transfer), it is an important constituent of many enzymes involved in primary metabolism. Fe has different accessibilities to the roots in the rhizosphere depending upon whether it is ferrous (soluble) or ferric (insoluble) oxidation stages, which in turn, determine two kinds of Fe uptake strategies employed by the plants. The reduction strategy is exclusively found in non-graminaceous plants wherein the ferrous Fe²⁺ is absorbed and translocated from the soil through specialized transporters. In contrast, the chelation strategy (widespread in graminaceous plants) relies on the formation of Fe (III)-chelate complex as the necessary requirement of Fe uptake. Once inside the cell, Fe is translocated, compartmentalized and stored through a common set of physiological processes involving many transporters and enzymes whose functions are controlled by underlying genetic components, so that a fine balance of Fe homeostasis is maintained. Recently, molecular and mechanistic aspects of the process involving the role of transcription factors, signaling components, and cis-acting elements have been obtained, which has enabled a much better understanding of its ecophysiology. This mini-review summarizes recent developments in our understanding of Fe transport in higher plants with particular emphasis on crops in the context of major agronomically important abiotic stresses. It also highlights outstanding questions on the regulation of Fe homeostasis and lists potentially useful genes/regulatory pathways that may be useful for subsequent crop improvement under the stresses discussed through either conventional or transgenic approaches.

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.

Population genetic structure and association mapping for iron toxicity tolerance in rice

Iron (Fe) toxicity is a major abiotic stress which severely reduces rice yield in many countries of the world. Genetic variation for this stress tolerance exists in rice germplasms. Mapping of gene(s)/QTL controlling the stress tolerance and transfer of the traits into high yielding rice varieties are essential for improvement against the stress. A panel population of 119 genotypes from 352 germplasm lines was constituted for detecting the candidate gene(s)/QTL through association mapping. STRUCTURE, GenAlEx and Darwin softwares were used to classify the population. The marker-trait association was detected by considering both the Generalized Linear Model (GLM) and Mixed Linear Model (MLM) analyses. Wide genetic variation was observed among the genotypes present in the panel population for the stress tolerance. Linkage disequilibrium was detected in the population for iron toxicity tolerance. The population was categorized into three genetic structure groups. Marker-trait association study considering both the Generalized Linear Model (GLM) and Mixed Linear Model (MLM) showed significant association of leaf browning index (LBI) with markers RM471, RM3, RM590 and RM243. Three novel QTL controlling Fe-toxicity tolerance were detected and designated as qFeTox4.3, qFeTox6.1 and qFeTox10.1. A QTL reported earlier in the marker interval of C955-C885 on chromosome 1 is validated using this panel population. The present study showed that QTL controlling Fe-toxicity tolerance to be co-localized with the QTL for Fe-biofortification of rice grain indicating involvement of common pathway for Fe toxicity tolerance and Fe content in rice grain. Fe-toxicity tolerance QTL qFeTox6.1 was co-localized with grain Fe-biofortification QTLs qFe6.1 and qFe6.2 on chromosome 6, whereas qFeTox10.1 was co-localized with qFe10.1 on chromosome 10. The Fe-toxicity tolerance QTL detected from this mapping study will be useful in marker-assisted breeding programs.

Metal ligands in micronutrient acquisition and homeostasis

Acquisition and homeostasis of micronutrients such as iron (Fe) and zinc (Zn) pose specific challenges. Poor solubility and high reactivity require controlled synthesis and supply of ligands to complex these metals extracellularly and intracellularly. Cytosolic labile pools represent only a minute fraction of the total cellular content. Several low-molecular-weight ligands are known in plants, including sulfur ligands (cysteine and peptides), nitrogen/oxygen ligands (S-adenosyl-l-methionine-derived molecules and histidine), and oxygen ligands (phenolics and organic acids). Some ligands are secreted into the extracellular space and influence the phytoavailability of metal ions. A second principal function is the intracellular buffering of micronutrients as well as the facilitation of long-distance transport in xylem and phloem. Furthermore, low-molecular-weight ligands are involved in the storage of metals, predominantly in vacuoles. A detailed molecular understanding is hampered by technical limitations, in particular the difficulty to detect and quantify cellular metal-ligand complexes. More, but still too little, is known about ligand synthesis and the transport across membranes, either with or without a complexed metal. Metal ligands have an immediate impact on human well-being. Engineering metal ligand synthesis and distribution in crops has tremendous potential to improve the nutritional quality of food and to tackle major human health risks.

Tolerance of Iron-Deficient and -Toxic Soil Conditions in Rice

Iron (Fe) deficiency and toxicity are the most widely prevalent soil-related micronutrient disorders in rice (Oryza sativa L.). Progress in rice cultivars with improved tolerance has been hampered by a poor understanding of Fe availability in the soil, the transportation mechanism, and associated genetic factors for the tolerance of Fe toxicity soil (FTS) or Fe deficiency soil (FDS) conditions. In the past, through conventional breeding approaches, rice varieties were developed especially suitable for low- and high-pH soils, which indirectly helped the varieties to tolerate FTS and FDS conditions. Rice-Fe interactions in the external environment of soil, internal homeostasis, and transportation have been studied extensively in the past few decades. However, the molecular and physiological mechanisms of Fe uptake and transport need to be characterized in response to the tolerance of morpho-physiological traits under Fe-toxic and -deficient soil conditions, and these traits need to be well integrated into breeding programs. A deeper understanding of the several factors that influence Fe absorption, uptake, and transport from soil to root and above-ground organs under FDS and FTS is needed to develop tolerant rice cultivars with improved grain yield. Therefore, the objective of this review paper is to congregate the different phenotypic screening methodologies for prospecting tolerant rice varieties and their responsible genetic traits, and Fe homeostasis related to all the known quantitative trait loci (QTLs), genes, and transporters, which could offer enormous information to rice breeders and biotechnologists to develop rice cultivars tolerant of Fe toxicity or deficiency. The mechanism of Fe regulation and transport from soil to grain needs to be understood in a systematic manner along with the cascade of metabolomics steps that are involved in the development of rice varieties tolerant of FTS and FDS. Therefore, the integration of breeding with advanced genome sequencing and omics technologies allows for the fine-tuning of tolerant genotypes on the basis of molecular genetics, and the further identification of novel genes and transporters that are related to Fe regulation from FTS and FDS conditions is incredibly important to achieve further success in this aspect.

Genetic Biofortification to Enrich Rice and Wheat Grain Iron: From Genes to Product

The micronutrient iron (Fe) is not only essential for plant survival and proliferation but also crucial for healthy human growth and development. Rice and wheat are the two leading staples globally; unfortunately, popular rice and wheat cultivars only have a minuscule amount of Fe content and mainly present in the outer bran layers. Unavailability of considerable Fe-rich rice and wheat germplasms limits the potential of conventional breeding to develop this micronutrient trait in both staples. Agronomic biofortification, defined as soil and foliar fertilizer application, has potential but remains quite challenging to improve grain Fe to the significant level. In contrast, recent accomplishments in genetic biofortification can help to develop Fe-enriched cereal grains to sustainably address the problem of "hidden hunger" when the roadmap from proof of concept to product and adoption can be achieved. Here, we highlight the different genetic biofortification strategies for rice and wheat and path to develop a product.

POSITIVE REGULATOR OF IRON HOMEOSTASIS1, OsPRI1, Facilitates Iron Homeostasis

Oryza sativa HEMERYTHRIN MOTIF-CONTAINING REALLY INTERESTING NEW GENE AND ZINC-FINGER PROTEIN1 (OsHRZ1) is a putative iron-binding sensor. However, it is unclear how OsHRZ1 transmits signals. In this study, we reveal that POSITIVE REGULATOR OF IRON HOMEOSTASIS1 (OsPRI1) interacts with OsHRZ1. A loss-of-function mutation to OsPRI1 increased the sensitivity of plants to Fe-deficient conditions and down-regulated the expression of Fe-deficiency-responsive genes. Yeast one-hybrid and electrophoretic mobility shift assay results suggested that OsPRI1 binds to the OsIRO2 and OsIRO3 promoters. In vitro ubiquitination experiments indicated that OsPRI1 is ubiquitinated by OsHRZ1. Cell-free degradation assays revealed that the stability of OsPRI1 decreased in wild-type roots but increased in the hrz1-2 mutant, suggesting OsHRZ1 is responsible for the instability of OsPRI1. The hrz1-2 seedlings were insensitive to Fe-deficient conditions. When the pri1-1 mutation was introduced into hrz1-2 mutants, the pri1hrz1 double mutant was more sensitive to Fe deficiency than the hrz1-2 mutant. Additionally, the expression levels of Fe-deficiency-responsive genes were lower in the hrz1pri1 double mutant than in the hrz1-2 mutant. Collectively, these results imply that OsPRI1, which is ubiquitinated by OsHRZ1, mediates rice responses to Fe deficiency by positively regulating OsIRO2 and OsIRO3 expression as part of the OsHRZ1-OsPRI1-OsIRO2/3 signal transduction cascade.