Iron Biofortification of Staple Crops: Lessons and Challenges in Plant Genetics

Importance of the Rhizosphere Microbiota in Iron Biofortification of Plants

Increasing the iron content of plant products and iron assimilability represents a major issue for human nutrition and health. This is also a major challenge because iron is not readily available for plants in most cultivated soils despite its abundance in the Earth's crust. Iron biofortification is defined as the enhancement of the iron content in edible parts of plants. This biofortification aims to reach the objectives defined by world organizations for human nutrition and health while being environment friendly. A series of options has been proposed to enhance plant iron uptake and fight against hidden hunger, but they all show limitations. The present review addresses the potential of soil microorganisms to promote plant iron nutrition. Increasing knowledge on the plant microbiota and plant-microbe interactions related to the iron dynamics has highlighted a considerable contribution of microorganisms to plant iron uptake and homeostasis. The present overview of the state of the art sheds light on plant iron uptake and homeostasis, and on the contribution of plant-microorganism (plant-microbe and plant-plant-microbe) interactions to plant nutritition. It highlights the effects of microorganisms on the plant iron status and on the co-occurring mechanisms, and shows how this knowledge may be valued through genetic and agronomic approaches. We propose a change of paradigm based on a more holistic approach gathering plant and microbial traits mediating iron uptake. Then, we present the possible applications in plant breeding, based on plant traits mediating plant-microbe interactions involved in plant iron uptake and physiology.

Fighting Iron-Deficiency Anemia: Innovations in Food Fortificants and Biofortification Strategies

Iron deficiency remains one of the main nutritional disorders worldwide and low iron intake and/or bioavailability are currently the major causes of anemia. To fight this public health problem, the scientific challenge is to find an iron form with sufficient bioavailability to increase its levels in humans through food fortification. In turn, biofortification appears as a comparatively advantageous and bearable strategy for the delivery of vitamins and other micronutrients for people without access to a healthy and diverse diet. This approach relies on plant breeding, transgenic techniques, or agronomic practices to obtain a final food product with a higher iron content. It is also known that certain food constituents are able to favor or inhibit iron absorption. The management of these compounds can thus successfully improve the absorption of dietary iron and, ultimately, contribute to fight this disorder present all over the world. This review describes the main causes/manifestations of iron-deficiency anemia, forms of disease prevention and treatment, and the importance of a balanced and preventive diet. A special focus was given to innovative food fortification and biofortification procedures used to improve the iron content in staple food crops.

Dissection of Molecular Processes and Genetic Architecture Underlying Iron and Zinc Homeostasis for Biofortification: From Model Plants to Common Wheat

The micronutrients iron (Fe) and zinc (Zn) are not only essential for plant survival and proliferation but are crucial for human health. Increasing Fe and Zn levels in edible parts of plants, known as biofortification, is seen a sustainable approach to alleviate micronutrient deficiency in humans. Wheat, as one of the leading staple foods worldwide, is recognized as a prioritized choice for Fe and Zn biofortification. However, to date, limited molecular and physiological mechanisms have been elucidated for Fe and Zn homeostasis in wheat. The expanding molecular understanding of Fe and Zn homeostasis in model plants is providing invaluable resources to biofortify wheat. Recent advancements in NGS (next generation sequencing) technologies coupled with improved wheat genome assembly and high-throughput genotyping platforms have initiated a revolution in resources and approaches for wheat genetic investigations and breeding. Here, we summarize molecular processes and genes involved in Fe and Zn homeostasis in the model plants Arabidopsis and rice, identify their orthologs in the wheat genome, and relate them to known wheat Fe/Zn QTL (quantitative trait locus/loci) based on physical positions. The current study provides the first inventory of the genes regulating grain Fe and Zn homeostasis in wheat, which will benefit gene discovery and breeding, and thereby accelerate the release of Fe- and Zn-enriched wheats.

Temporal and Spatial Patterns of Zinc and Iron Accumulation during Barley (Hordeum vulgare L.) Grain Development

Breeding and engineering of biofortified crops will benefit from a better understanding of bottlenecks controlling micronutrient loading within the seeds. However, few studies have addressed the changes in micronutrient concentrations, localization, and speciation occurring over time. Therefore, we studied spatial patterns of zinc and iron accumulation during grain development in two barley lines with contrasting grain zinc concentrations. Microparticle-induced-X-ray emission and laser ablation-inductively coupled plasma mass spectrometry were used to determine tissue-specific accumulation of zinc, iron, phosphorus, and sulfur. Differences in zinc accumulation between the lines were most evident in the endosperm and aleurone. A gradual decrease in zinc concentrations from the aleurone to the underlying endosperm was observed, while iron and phosphorus concentrations decreased sharply. Iron co-localized with phosphorus in the aleurone, whereas zinc co-localized with sulfur in the sub-aleurone. We hypothesize that differences in grain zinc are largely explained by the endosperm storage capacity. Engineering attempts should be targeted accordingly.

Genetically Modified Plants: Nutritious, Sustainable, yet Underrated

Combating malnutrition is one of the greatest global health challenges. Plant-based foods offer an assortment of nutrients that are essential for adequate nutrition and can promote good health. Unfortunately, the majority of widely consumed crops are deficient in some of these nutrients. Biofortification is the umbrella term for the process by which the nutritional quality of food crops is enhanced. Traditional agricultural breeding approaches for biofortification are time consuming but can enhance the nutritional value of some foods; however, advances in molecular biology are rapidly being exploited to biofortify various crops. Globally, genetically modified organisms are a controversial topic for consumers and governmental agencies, with a vast majority of people apprehensive about the technology. Golden Rice has been genetically modified to contain elevated β-carotene concentrations and is the bellwether for both the promise and angst of agricultural biotechnology. Although there are numerous other nutritional targets of genetically biofortified crops, here I briefly summarize the work to elevate iron and folate concentrations. In addition, the possibility of using modified foods to affect the gut microbiota is examined. For several decades, plant biotechnology has measured changes in nutrient concentrations; however, the bioavailability of nutrients from many biofortified crops has not been demonstrated.

Discovery and validation of candidate genes for grain iron and zinc metabolism in pearl millet [Pennisetum glaucum (L.) R. Br.]

Pearl millet is an important crop for alleviating micronutrient malnutrition through genomics-assisted breeding for grain Fe (GFeC) and Zn (GZnC) content. In this study, we identified candidate genes related to iron (Fe) and zinc (Zn) metabolism through gene expression analysis and correlated it with known QTL regions for GFeC/GZnC. From a total of 114 Fe and Zn metabolism-related genes that were selected from the related crop species, we studied 29 genes. Different developmental stages exhibited tissue and stage-specific expressions for Fe and Zn metabolism genes in parents contrasting for GFeC and GZnC. Results revealed that PglZIP, PglNRAMP and PglFER gene families were candidates for GFeC and GZnC. Ferritin-like gene, PglFER1 may be the potential candidate gene for GFeC. Promoter analysis revealed Fe and Zn deficiency, hormone, metal-responsive, and salt-regulated elements. Genomic regions underlying GFeC and GZnC were validated by annotating major QTL regions for grain Fe and Zn. Interestingly, PglZIP and PglNRAMP gene families were found common with a previously reported linkage group 7 major QTL region for GFeC and GZnC. The study provides insights into the foundation for functional dissection of different Fe and Zn metabolism genes homologs and their subsequent use in pearl millet molecular breeding programs globally.

Speciation of essential nutrient trace elements in coconut water

Coconut water (Cocos Nucifera) is shown to be a source of essential elements present in the form of low-molecular weight stable complexes known for their bio-availability. The total element concentrations were in the range of 0.2-2.7, 0.3-1, 3-14 and 0.5-2 ppm for Fe, Cu, Mn, and Zn, respectively, and varied as a function of the origin of the nut and its maturity. Speciation was investigated by size-exclusion chromatography - inductively coupled plasma mass spectrometry (ICPMS), and hydrophilic interaction liquid chromatography (HILIC) - electrospray-OrbitrapMS. The metal species identified included: iron complexes with citrate and malate: Fe^III(Cit)₃(Mal), Fe^III(Cit)₂(Mal)₂, Fe^III(Mal)₂, glutamine: Fe^III(Glu)₂ and nicotianamine: Fe^II(NA); copper complexes with phenylanine: Cu^II(Phe)₂ and Cu^II(Phe)₃ and nicotianamine: Cu^II(NA); zinc complexes with citrate: Zn^II(Cit)₂ and nicotianamine Zn^II(NA) and manganese complex with asparagine Mn^II(Asp)₂. The contributions of the individual species to the total elements concentrations could be estimated by HILIC - ICP MS.

Iron homeostasis and plant immune responses: Recent insights and translational implications

Iron metabolism and the plant immune system are both critical for plant vigor in natural ecosystems and for reliable agricultural productivity. Mechanistic studies of plant iron home-ostasis and plant immunity have traditionally been carried out in isolation from each other; however, our growing understanding of both processes has uncovered significant connections. For example, iron plays a critical role in the generation of reactive oxygen intermediates during immunity and has been recently implicated as a critical factor for immune-initiated cell death via ferroptosis. Moreover, plant iron stress triggers immune activation, suggesting that sensing of iron depletion is a mechanism by which plants recognize a pathogen threat. The iron deficiency response engages hormone signaling sectors that are also utilized for plant immune signaling, providing a probable explanation for iron-immunity cross-talk. Finally, interference with iron acquisition by pathogens might be a critical component of the immune response. Efforts to address the global burden of iron deficiency-related anemia have focused on classical breeding and transgenic approaches to develop crops biofortified for iron content. However, our improved mechanistic understanding of plant iron metabolism suggests that such alterations could promote or impede plant immunity, depending on the nature of the alteration and the virulence strategy of the pathogen. Effects of iron biofortification on disease resistance should be evaluated while developing plants for iron biofortification.

The Central Role of Iron in Human Nutrition: From Folk to Contemporary Medicine

Iron is a fundamental element in human history, from the dawn of civilization to contemporary days. The ancients used the metal to shape tools, to forge weapons, and even as a dietary supplement. This last indication has been handed down until today, when martial therapy is considered fundamental to correct deficiency states of anemia. The improvement of the martial status is mainly targeted with dietary supplements that often couple diverse co-factors, but other methods are available, such as parenteral preparations, dietary interventions, or real-world approaches. The oral absorption of this metal occurs in the duodenum and is highly dependent upon its oxidation state, with many absorption influencers possibly interfering with the intestinal uptake. Bone marrow and spleen represent the initial and ultimate step of iron metabolism, respectively, and the most part of body iron circulates bound to specific proteins and mainly serves to synthesize hemoglobin for new red blood cells. Whatever the martial status is, today’s knowledge about iron biochemistry allows us to embrace exceedingly personalized interventions, which however owe their success to the mythical and historical events that always accompanied this metal.

Applying genomic resources to accelerate wheat biofortification

Wheat has low levels of the micronutrients iron and zinc in the grain, which contributes to 2 billion people suffering from micronutrient deficiency globally. While wheat flour is commonly fortified during processing, an attractive and more sustainable solution is biofortification, which could improve micronutrient content in the human diet, without the sustainability issues and costs associated with conventional fortification. Although many studies have used quantitative trait loci mapping and genome-wide association to identify genetic loci to improve micronutrient contents, recent developments in genomics offer an opportunity to accelerate marker discovery and use gene-focussed approaches to engineer improved micronutrient content in wheat. The recent publication of a high-quality wheat genome sequence, alongside gene expression atlases, variation datasets and sequenced mutant populations, provides a foundation to identify genetic loci and genes controlling micronutrient content in wheat. We discuss how novel genomic resources can identify candidate genes for biofortification, integrating knowledge from other cereal crops, and how these genes can be tested using gene editing, transgenic and TILLING approaches. Finally, we highlight key challenges remaining to develop wheat cultivars with high levels of iron and zinc.

Improving nutritional quality of rice for human health

This review surveys rice nutritional value, mainly focusing on breeding achievements via adoption of both genetic engineering and non-transgenic strategies to improve key nutrients associated with human health. Rice (Oryza sativa) is an essential component of the diets and livelihoods of over 3.5 billion people. Polished rice is mostly consumed as staple food, fulfilling daily energy demands and part of the protein requirement. Brown rice is comparatively more nutritious, containing more lipids, minerals, vitamins, dietary fiber, micronutrients, and bioactive compounds. In this article, we review the nutritional facts about rice including the level of γ-aminobutyric acid, resistant starch, lysine, iron, zinc, β-carotene, folate, anthocyanin, various carotenoids, and flavonoids, focusing on their synthesis and metabolism and the advances in their biofortification via adoption of both conventional and genetic engineering strategies. We conclude that besides representing a staple food, rice has the potential to become a source of various essential nutrients or bioactive compounds through appropriate genetic improvements to benefit human health and prevent certain chronic diseases. Finally, we discuss the available, non-genetically engineering strategies for the nutritional improvement of rice, including their main strengths and constraints.

Potential Implications of Interactions between Fe and S on Cereal Fe Biofortification

Iron (Fe) and sulfur (S) are two essential elements for plants, whose interrelation is indispensable for numerous physiological processes. In particular, Fe homeostasis in cereal species is profoundly connected to S nutrition because phytosiderophores, which are the metal chelators required for Fe uptake and translocation in cereals, are derived from a S-containing amino acid, methionine. To date, various biotechnological cereal Fe biofortification strategies involving modulation of genes underlying Fe homeostasis have been reported. Meanwhile, the resultant Fe-biofortified crops have been minimally characterized from the perspective of interaction between Fe and S, in spite of the significance of the crosstalk between the two elements in cereals. Here, we intend to highlight the relevance of Fe and S interrelation in cereal Fe homeostasis and illustrate the potential implications it has to offer for future cereal Fe biofortification studies.

Gene Expression Pattern of Vacuolar-Iron Transporter-Like (VTL) Genes in Hexaploid Wheat during Metal Stress

Iron is one of the important micronutrients that is required for crop productivity and yield-related traits. To address the Fe homeostasis in crop plants, multiple transporters belonging to the category of major facilitator superfamily are being explored. In this direction, earlier vacuolar iron transporters (VITs) have been reported and characterized functionally to address biofortification in cereal crops. In the present study, the identification and characterization of new members of vacuolar iron transporter-like proteins (VTL) was performed in wheat. Phylogenetic distribution demonstrated distinct clustering of the identified VTL genes from the previously known VIT genes. Our analysis identifies multiple VTL genes from hexaploid wheat with the highest number genes localized on chromosome 2. Quantitative expression analysis suggests that most of the VTL genes are induced mostly during the Fe surplus condition, thereby reinforcing their role in metal homeostasis. Interestingly, most of the wheat VTL genes were also significantly up-regulated in a tissue-specific manner under Zn, Mn and Cu deficiency. Although, no significant changes in expression of wheat VTL genes were observed in roots under heavy metals, but TaVTL2, TaVTL3 and TaVTL5 were upregulated in the presence of cobalt stress. Overall, this work deals with the detailed characterization of wheat VTL genes that could provide an important genetic framework for addressing metal homeostasis in bread wheat.

Alleviation of Salinity Induced Oxidative Stress in Chenopodium quinoa by Fe Biofortification and Biochar—Endophyte Interaction

Iron-biofortification is a sustainable food-based approach to combat iron deficiency by increasing iron content and bioavailability in agronomic crops. Siderophore producing microbes offer a sustainable and low-cost way to increase iron supply in crops. Also, certain substances released from organic amendments act as iron-chelators which increase the solubility as well as the availability of iron to plants. Present study investigated the role of siderophore-producing endophytic bacteria and biochar on iron-fortification of a novel crop quinoa in iron-limited saline conditions. The surface-disinfected seeds of quinoa were inoculated with Burkholderia phytofirmans PsJN (CFU = 109) and sown in saline soil (EC 20 dS m−1) amended with biochar (1% w/w). Results revealed that biochar and PsJN particularly when applied together significantly enhanced plant growth, grain yield, and grain nutrient contents of quinoa. Strikingly, iron concentration in quinoa grains was increased up to 71% by the combined application of biochar and PsJN. Moreover, plant physiological parameters were also improved significantly by the integrated application. However, enzymatic/non-enzymatic antioxidants activities were decreased by integrated treatment thus ameliorated salinity stress. Our study suggests that integrated application of siderophore-producing bacteria and biochar could be a promising, sustainable and cost-effective strategy which is easily integratable into the existing farming practices to achieve food fortification with micronutrients in developing countries.

Handing off iron to the next generation: how does it get into seeds and what for?

To ensure the success of the new generation in annual species, the mother plant transfers a large proportion of the nutrients it has accumulated during its vegetative life to the next generation through its seeds. Iron (Fe) is required in large amounts to provide the energy and redox power to sustain seedling growth. However, free Fe is highly toxic as it leads to the generation of reactive oxygen species. Fe must, therefore, be tightly bound to chelating molecules to allow seed survival for long periods of time without oxidative damage. Nevertheless, when conditions are favorable, the seed's Fe stores have to be readily remobilized to achieve the transition toward active photosynthesis before the seedling becomes able to take up Fe from the environment. This is likely critical for the vigor of the young plant. Seeds constitute an important dietary source of Fe, which is essential for human health. Understanding the mechanisms of Fe storage in seeds is a key to improve their Fe content and availability in order to fight Fe deficiency. Seed longevity, germination efficiency and seedling vigor are also important traits that may be affected by the chemical form under which Fe is stored. In this review, we summarize the current knowledge on seed Fe loading during development, long-term storage and remobilization upon germination. We highlight how this knowledge may help seed Fe biofortification and discuss how Fe storage may affect the seed quality and germination efficiency.

Mineral Element Composition in Grain of Awned and Awnletted Wheat (Triticum aestivum L.) Cultivars: Tissue-Specific Iron Speciation and Phytate and Non-Phytate Ligand Ratio

In wheat (Triticum aestivum L.), the awns—the bristle-like structures extending from lemmas—are photosynthetically active. Compared to awned cultivars, awnletted cultivars produce more grains per unit area and per spike, resulting in significant reduction in grain size, but their mineral element composition remains unstudied. Nine awned and 11 awnletted cultivars were grown simultaneously in the field. With no difference in 1000-grain weight, a larger calcium and manganese—but smaller iron (Fe) concentrations—were found in whole grain of awned than in awnletted cultivars. Micro X-ray absorption near edge structure analysis of different tissues of frozen-hydrated grain cross-sections revealed that differences in total Fe concentration were not accompanied by differences in Fe speciation (64% of Fe existed as ferric and 36% as ferrous species) or Fe ligands (53% were phytate and 47% were non-phytate ligands). In contrast, there was a distinct tissue-specificity with pericarp containing the largest proportion (86%) of ferric species and nucellar projection (49%) the smallest. Phytate ligand was predominant in aleurone, scutellum and embryo (72%, 70%, and 56%, respectively), while nucellar projection and pericarp contained only non-phytate ligands. Assuming Fe bioavailability depends on Fe ligands, we conclude that Fe bioavailability from wheat grain is tissue specific.

The Yin and Yang of Iron in Plants and Beyond: 19th International Symposium on Iron Nutrition and Interactions in Plants (ISINIP) in Taiwan

The19th edition of the International Symposium on Iron Nutrition and Interactions in Plants, a biannual meeting initiated in 1981, took place in Taiwan for the first time. The five-day event was held at the Academia Sinica campus in the Southeast of Taiwan's capital city Taipei, and hosted around 200 scientists from around the world. The meeting covered a diverse array of topics centered around iron nutrition, including but not limited to soil processes, biofortification, transport, signaling and molecular processes regulating the cellular homeostasis of iron. Here, I review the research foci highlighted during the meeting by oral and poster presentations.