OsYSL13 Is Involved in Iron Distribution in Rice

Metal Transport Systems in Plants

Plants take up metals, including essential micronutrients [iron (Fe), copper (Cu), zinc (Zn), and manganese (Mn)] and the toxic heavy metal cadmium (Cd), from soil and accumulate these metals in their edible parts, which are direct and indirect intake sources for humans. Multiple transporters belonging to different families are required to transport a metal from the soil to different organs and tissues, but only a few of them have been fully functionally characterized. The transport systems (the transporters required for uptake, translocation, distribution, redistribution, and their regulation) differ with metals and plant species, depending on the physiological roles, requirements of each metal, and anatomies of different organs and tissues. To maintain metal homeostasis in response to spatiotemporal fluctuations of metals in soil, plants have developed sophisticated and tightly regulated mechanisms through the regulation of transporters at the transcriptional and/or posttranscriptional levels. The manipulation of some transporters has succeeded in generating crops rich in essential metals but low in Cd accumulation. A better understanding of metal transport systems will contribute to better and safer crop production.

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.

Deciphering the functional roles of transporter proteins in subcellular metal transportation of plants

MAIN CONCLUSION

The role of transporters in subcellular metal transport is of great significance for plants in coping with heavy metal stress and maintaining their proper growth and development. Heavy metal toxicity is a serious long-term threat to plant growth and agricultural production, becoming a global environmental concern. Excessive heavy metal accumulation not only damages the biochemical and physiological functions of plants but also causes chronic health hazard to human beings through the food chain. To deal with heavy metal stress, plants have evolved a series of elaborate mechanisms, especially a variety of spatially distributed transporters, to strictly regulate heavy metal uptake and distribution. Deciphering the subcellular role of transporter proteins in controlling metal absorption, transport and separation is of great significance for understanding how plants cope with heavy metal stress and improving their adaptability to environmental changes. Hence, we herein introduce the detrimental effects of excessive common essential and non-essential heavy metals on plant growth, and describe the structural and functional characteristics of transporter family members, with a particular emphasis on their roles in maintaining heavy metal homeostasis in various organelles. Besides, we discuss the potential of controlling transporter gene expression by transgenic approaches in response to heavy metal stress. This review will be valuable to researchers and breeders for enhancing plant tolerance to heavy metal contamination.

A molecular study of Italian ryegrass grown on Martian regolith simulant

In the last decade, the exploration of deep space has become the objective of the national space programs of many countries. The International Space Exploration Coordination Group has set a roadmap whose long-range strategy envisions the expansion of human presence in the solar system to progress with exploration and knowledge and to accelerate innovation. Crewed missions to Mars could be envisaged by 2040. In this scenario, finding ways to use the local resources for the provision of food, construction materials, propellants, pharmaceuticals is needed. Plants are important resources for deep space manned missions because they produce phytochemicals of pharmaceutical relevance, are sources of food and provide oxygen which is crucial in bioregenerative life support systems. Growth analysis and plant biomass yield have been previously evaluated on Martian regolith simulants; however, molecular approaches employing gene expression analysis and proteomics are still missing. The present work aims at filling this gap by providing molecular data on a representative member of the Poaceae, Lolium multiflorum Lam., grown on potting soil and a Martian regolith simulant (MMS-1). The molecular data were complemented with optical microscopy of root/leaf tissues and physico-chemical analyses. The results show that the plants grew for 2 weeks on regolith simulants. The leaves were bent downwards and chlorotic, the roots developed a lacunar aerenchyma and small brownish deposits containing Fe were observed. Gene expression analysis and proteomics revealed changes in transcripts related to the phenylpropanoid pathway, stress response, primary metabolism and proteins involved in translation and DNA methylation. Additionally, the growth of plants slightly but significantly modified the pH of the regolith simulants. The results here presented constitute a useful resource to get a comprehensive understanding of the major factors impacting the growth of plants on MMS-1.

The bHLH Transcription Factor OsbHLH057 Regulates Iron Homeostasis in Rice

Many basic Helix-Loop-Helix (bHLH) transcription factors precisely regulate the expression of Fe uptake and translocation genes to control iron (Fe) homeostasis, as both Fe deficiency and toxicity impair plant growth and development. In rice, three clade IVc bHLH transcription factors have been characterised as positively regulating Fe-deficiency response genes. However, the function of OsbHLH057, another clade IVc bHLH transcription factor, in regulating Fe homeostasis is unknown. Here, we report that OsbHLH057 is involved in regulating Fe homeostasis in rice. OsbHLH057 was highly expressed in the leaf blades and lowly expressed in the roots; it was mainly expressed in the stele and highly expressed in the lateral roots. In addition, OsbHLH057 was slightly induced by Fe deficiency in the shoots on the first day but was not affected by Fe availability in the roots. OsbHLH057 localised in the nucleus exhibited transcriptional activation activity. Under Fe-sufficient conditions, OsbHLH057 knockout or overexpression lines increased or decreased the shoot Fe concentration and the expression of several Fe homeostasis-related genes, respectively. Under Fe-deficient conditions, plants with an OsbHLH057 mutation showed susceptibility to Fe deficiency and accumulated lower Fe concentrations in the shoot compared with the wild type. Unexpectedly, the OsbHLH057-overexpressing lines had reduced tolerance to Fe deficiency. These results indicate that OsbHLH057 plays a positive role in regulating Fe homeostasis, at least under Fe-sufficient conditions.

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.

Genetic Approaches for Iron and Zinc Biofortification and Arsenic Decrease in Oryza sativa L. Grains

Rice is the staple diet to half of the world's population, being a major source of carbohydrates, vitamins, and some essential elements. However, rice naturally contains low amounts of essential minerals such as iron (Fe) and zinc (Zn), which are drastically decreased after milling. Thus, populations that consume mostly rice may have micronutrient deficiency, which is associated with different diseases. On the other hand, rice irrigated by flooding has a high ability to accumulate arsenic (As) in the grain. Therefore, when rice is grown in areas with contaminated soil or irrigation water, it represents a risk factor for consumers, since As is associated with cancer and other diseases. Different strategies have been used to mitigate micronutrient deficiencies such as Fe and Zn and to prevent As from entering the food chain. Each strategy has its positive and its negative sides. The development of genetically biofortified rice plants with Fe and Zn and with low As accumulation is one of the most promising strategies, since it does not represent an additional cost for farmers, and gives benefits to consumers as well. Considering the importance of genetic improvement (traditional or molecular) to decrease the impact of micronutrient deficiencies such as Fe and Zn and contamination with As, this review aimed to summarize the major efforts, advances, and challenges for genetic biofortification of Fe and Zn and decrease in As content in rice grains.

An insight into the act of iron to impede arsenic toxicity in paddy agro-system

Surplus research on the widespread arsenic (As) revealed its disturbing role in obstructing the metabolic function of plants. Also, the predilection of As towards rice has been an interesting topic. Contrary to As, iron (Fe) is an essential micronutrient for all life forms. Past findings propound about the enhanced As-resistance in rice plants during Fe supplementation. Thus, considering the severity of As contamination and resulting exposure through rice crops, as well as the studied cross-talks between As and Fe, we found this topic of relevance. Keeping these in view, we bring this review discussing the presence of As-Fe in the paddy environment, the criticality of Fe plaque in As sequestration, and the effectiveness of various Fe forms to overcome As toxicity in rice. This type of interactive analysis for As and Fe is also crucial in the context of the involvement of Fe in cellular redox activities such as oxidative stress. Also, this piece of work highlights Fe biofortification approaches for better rice varieties with optimum intrinsic Fe and limited As. Though elaborated by others, we lastly present the acquisition and transport mechanisms of both As and Fe in rice tissues. Altogether we suggest that Fe supply and Fe plaque might be a prospective agronomical tool against As poisoning and for phytostabilization, respectively.

Genome-Wide Identification and Transcript Analysis Reveal Potential Roles of Oligopeptide Transporter Genes in Iron Deficiency Induced Cadmium Accumulation in Peanut

The oligopeptide transporter (OPT) family is a group of proton-coupled symporters that play diverse roles, including metal homeostasis. However, little is known about this family of peanuts. To reveal the potential roles of AhOPT genes in Fe/Cd interactions, peanut AhOPT genes were genome-widely identified, and the relationships between gene expression and Cd accumulation were detected in two contrasting peanut cultivars (Fenghua 1 and Silihong) under Fe-sufficient or Fe-deficient conditions. A total of 40 AhOPT genes were identified in peanuts, which were divided into two subfamilies (PT and YS). Most AhOPT genes underwent gene duplication events predominated by whole-genome duplication. Clustered members generally have similar protein structures. However, gene structural divergences occurred in most of the duplicated genes. Transcription analysis revealed that AhOPT3.2/3.4 and AhYSL3.1/3.2 might be responsible for Fe deficiency tolerance, while AhOPT3.1/3.4, AhOPT7.1/7.2, and AhYSL1.1 be involved in Fe/Cd interactions. These genes might be regulated by transcription factors, including ATHB-12, ATHB-6, DIVARICATA, MYB30, NAC02, DOF3.4, IDD7, and LUX. Reduced expressions of AhYSL3.1/3.2 and higher expressions of AhOPT3.4 might contribute to higher Fe-deficiency tolerance in Silihong. Higher expression of AhOPT7.3 and AhOPT6.1 might be responsible for low Cd accumulation in Fenghua 1. Our results confirmed that AhOPT3/6/7 and AhYSL1/3 might be involved in the transport of Fe and/or Cd in peanuts and provided new clues to understanding potential mechanisms of Fe/Cd interactions.

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.

Transcriptomic Analysis Revealed Reactive Oxygen Species Scavenging Mechanisms Associated With Ferrous Iron Toxicity in Aromatic Keteki Joha Rice

Lowland acidic soils with water-logged regions are often affected by ferrous iron (Fe²⁺) toxicity, a major yield-limiting factor of rice production. Under severe Fe²⁺ toxicity, reactive oxygen species (ROS) are crucial, although molecular mechanisms and associated ROS homeostasis genes are still unknown. In this study, a comparative RNA-Seq based transcriptome analysis was conducted to understand the Fe²⁺ toxicity tolerance mechanism in aromatic Keteki Joha. About 69 Fe homeostasis related genes and their homologs were identified, where most of the genes were downregulated. Under severe Fe²⁺ toxicity, the biosynthesis of amino acids, RNA degradation, and glutathione metabolism were induced, whereas phenylpropanoid biosynthesis, photosynthesis, and fatty acid elongation were inhibited. The mitochondrial iron transporter (OsMIT), vacuolar iron transporter 2 (OsVIT2), ferritin (OsFER), vacuolar mugineic acid transporter (OsVMT), phenolic efflux zero1 (OsPEZ1), root meander curling (OsRMC), and nicotianamine synthase (OsNAS3) were upregulated in different tissues, suggesting the importance of Fe retention and sequestration for detoxification. However, several antioxidants, ROS scavenging genes and abiotic stress-responsive transcription factors indicate ROS homeostasis as one of the most important defense mechanisms under severe Fe²⁺ toxicity. Catalase (CAT), glutathione (GSH), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), and glutathione reductase (GR) were upregulated. Moreover, abiotic stress-responsive transcription factors, no apical meristem (NAC), myeloblastosis (MYB), auxin response factor (ARF), basic helix-loop-helix (bZIP), WRKY, and C2H2-zinc finger protein (C2H2-ZFP) were also upregulated. Accordingly, ROS homeostasis has been proposed as an essential defense mechanism under such conditions. Thus, the current study may enrich the understanding of Fe-homeostasis in rice.

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.

TpIRT1 from Polish wheat (Triticum polonicum L.) enhances the accumulation of Fe, Mn, Co, and Cd in Arabidopsis

Uptake and internal transport of micronutrients are essential for plant growth, development, and yield. In this regard, Iron Regulated Transporters (IRTs) from the Zinc Regulated Transporter (ZRT)/IRT-related protein (ZIP) family play an important role in transition metal uptake. Most studies have been focused on IRT1-like proteins in diploid species. Information on IRT1-like proteins in polyploids is limited. Here, we studied the function of TpIRT1A and TpIRT1B homoeologs in a tetraploid crop, Polish wheat (Triticum polonicum L.). Our results highlighted the importance of TpIRT1 in mediating the uptake and translocation of Fe, Mn, Co, and Cd with direct implications for wheat yield potential. Both TpIRT1A and TpIRT1B were located at the plasma membrane and internal vesicle-like organelle in protoplasts of Arabidopsis thaliana L. and increased Cd and Co sensitivity in yeast. The over-expression of TpIRT1B in A. thaliana increased Fe, Mn, Co, and Cd concentration in its tissues and improved plant growth under Fe, Mn, and Co deficiencies, while increased the sensitivity to Cd compared to wild type. Functional analysis of IRT1 homoeologs from tetraploid and diploid ancestral wheat species in yeast disclosed four distinct amino acid residues in TdiIRT1B (T. dicoccum L. (Schrank)) and TtuIRT1B (T. turgidum L.). Together, our results increase the knowledge of IRT1 function in a globally important crop, wheat.

Transcriptome Profiling of Cu Stressed Petunia Petals Reveals Candidate Genes Involved in Fe and Cu Crosstalk

Copper (Cu) is an essential element for most living plants, but it is toxic for plants when present in excess. To better understand the response mechanism under excess Cu in plants, especially in flowers, transcriptome sequencing on petunia buds and opened flowers under excess Cu was performed. Interestingly, the transcript level of FIT-independent Fe deficiency response genes was significantly affected in Cu stressed petals, probably regulated by basic-helix-loop-helix 121 (bHLH121), while no difference was found in Fe content. Notably, the expression level of bHLH121 was significantly down-regulated in petals under excess Cu. In addition, the expression level of genes related to photosystem II (PSII), photosystem I (PSI), cytochrome b₆/f complex, the light-harvesting chlorophyll II complex and electron carriers showed disordered expression profiles in petals under excess Cu, thus photosynthesis parameters, including the maximum PSII efficiency (F_V/F_M), nonphotochemical quenching (NPQ), quantum yield of the PSII (ΦPS(II)) and photochemical quenching coefficient (qP), were reduced in Cu stressed petals. Moreover, the chlorophyll a content was significantly reduced, while the chlorophyll b content was not affected, probably caused by the increased expression of chlorophyllide a oxygenase (CAO). Together, we provide new insight into excess Cu response and the Cu–Fe crosstalk in flowers.

Heavy metal stress in rice: Uptake, transport, signaling, and tolerance mechanisms

Heavy metal contamination of agricultural fields has become a global concern as it causes a direct impact on human health. Rice is the major food crop for almost half of the world population and is grown under diverse environmental conditions, including heavy metal-contaminated soil. In recent years, the impact of heavy metal contamination on rice yield and grain quality has been shown through multiple approaches. In this review article, different aspects of heavy metal stress, that is uptake, transport, signaling and tolerance mechanisms, are comprehensively discussed with special emphasis on rice. For uptake, some of the transporters have specificity to one or two metal ions, whereas many other transporters are able to transport many different ions. After uptake, the intercellular signaling is mediated through different signaling pathways involving the regulation of various hormones, alteration of calcium levels, and the activation of mitogen-activated protein kinases. Heavy metal stress signals from various intermediate molecules activate various transcription factors, which triggers the expression of various antioxidant enzymes. Activated antioxidant enzymes then scavenge various reactive oxygen species, which eventually leads to stress tolerance in plants. Non-enzymatic antioxidants, such as ascorbate, metalloids, and even metal-binding peptides (metallothionein and phytochelatin) can also help to reduce metal toxicity in plants. Genetic engineering has been successfully used in rice and many other crops to increase metal tolerance and reduce heavy metals accumulation. A comprehensive understanding of uptake, transport, signaling, and tolerance mechanisms will help to grow rice plants in agricultural fields with less heavy metal accumulation in grains.

Iron oxide nanoparticles alleviate arsenic phytotoxicity in rice by improving iron uptake, oxidative stress tolerance and diminishing arsenic accumulation

The food chain contaminated with arsenic (As) has developed a hazardous threat to the growth and development of plants, animals and humans. The present study was conducted to examine the application of iron oxide nanoparticles (FeNPs) on biochemical and molecular traits of roots and leaves of rice plants under As phytotoxicity. The results showed that As reduced the accumulation of Fe in roots and leaves and thus reduced photosynthetic pigments and growth of rice plants. As stress enhanced the accumulation of hydrogen peroxide, superoxide anion and methylglyoxal by increasing the accumulation of As in roots and leaves, resulting in damage to membrane lipids and raised electrolyte leakage (EL). However, FeNPs strengthen the glyoxalase system and antioxidant enzymes, thereby alleviating oxidative stress and reducing EL. FeNPs protected plant cells from As phytotoxicity by enhancing the accumulation of chelating agents (proline, glutathione and phytochelatins) and the sequestration and immobilization of As in the vacuoles and the cell walls. FeNPs downregulated the expression of genes involved in As uptake and translocation (Lsi1 and Lsi2) and, consequently, reduced As accumulation in the roots and leaves of As-stressed plants. FeNPs also improved the accumulation of Fe in the roots and leaves by modulating the expression of genes that regulate Fe uptake and its transport to leaves (IRT1, IRT2, YSL2, YSL13, FRDL1, DMAS1, NAS2 and NAS3), resulting in the restoration of photosynthetic pigments and the growth of As-stressed plants. Our findings authenticate the role of FeNPs in diminishing As phytotoxicity on rice.

Iron Biofortification in Rice: An Update on Quantitative Trait Loci and Candidate Genes

Rice is the most versatile model for cereals and also an economically relevant food crop; as a result, it is the most suitable species for molecular characterization of Fe homeostasis and biofortification. Recently there have been significant efforts to dissect genes and quantitative trait loci (QTL) associated with Fe translocation into rice grains; such information is highly useful for Fe biofortification of cereals but very limited in other species, such as maize (Zea mays) and wheat (Triticum aestivum). Given rice's centrality as a model for Poaceae species, we review the current knowledge on genes playing important roles in Fe transport, accumulation, and distribution in rice grains and QTLs that might explain the variability in Fe concentrations observed in different genotypes. More than 90 Fe QTLs have been identified over the 12 rice chromosomes. From these, 17 were recorded as stable, and 25 harbored Fe-related genes nearby or within the QTL. Among the candidate genes associated with Fe uptake, translocation, and loading into rice grains, we highlight the function of transporters from the YSL and ZIP families; transporters from metal-binding molecules, such as nicotianamine and deoxymugineic acid; vacuolar iron transporters; citrate efflux transporters; and others that were shown to play a role in steps leading to Fe delivery to seeds. Finally, we discuss the application of these QTLs and genes in genomics assisted breeding for fast-tracking Fe biofortification in rice and other cereals in the near future.

Adaptation Strategies of Halophytic Barley Hordeum marinum ssp. marinum to High Salinity and Osmotic Stress

The adaptation strategies of halophytic seaside barley Hordeum marinum to high salinity and osmotic stress were investigated by nuclear magnetic resonance imaging, as well as ionomic, metabolomic, and transcriptomic approaches. When compared with cultivated barley, seaside barley exhibited a better plant growth rate, higher relative plant water content, lower osmotic pressure, and sustained photosynthetic activity under high salinity, but not under osmotic stress. As seaside barley is capable of controlling Na⁺ and Cl⁻ concentrations in leaves at high salinity, the roots appear to play the central role in salinity adaptation, ensured by the development of thinner and likely lignified roots, as well as fine-tuning of membrane transport for effective management of restriction of ion entry and sequestration, accumulation of osmolytes, and minimization of energy costs. By contrast, more resources and energy are required to overcome the consequences of osmotic stress, particularly the severity of reactive oxygen species production and nutritional disbalance which affect plant growth. Our results have identified specific mechanisms for adaptation to salinity in seaside barley which differ from those activated in response to osmotic stress. Increased knowledge around salt tolerance in halophytic wild relatives will provide a basis for improved breeding of salt-tolerant crops.

Improvement of nutrient use efficiency in rice: current toolbox and future perspectives

Modern agriculture relies heavily on chemical fertilizers, especially in terms of cereal production. The excess application of fertilizers not only increases production cost, but also causes severe environmental problems. As one of the major cereal crops, rice (Oryza sativa L.) provides the staple food for nearly half of population worldwide, especially in developing countries. Therefore, improving rice yield is always the priority for rice breeding. Macronutrients, especially nitrogen (N) and phosphorus (P), are two most important players for the grain yield of rice. However, with economic development and improved living standard, improving nutritional quality such as micronutrient contents in grains has become a new goal in order to solve the "hidden hunger." Micronutrients, such as iron (Fe), zinc (Zn), and selenium (Se), are critical nutritional elements for human health. Therefore, breeding the rice varieties with improved nutrient use efficiency (NUE) is thought to be one of the most feasible ways to increase both grain yield and nutritional quality with limited fertilizer input. In this review, we summarized the progresses in molecular dissection of genes for NUE by reverse genetics on macronutrients (N and P) and micronutrients (Fe, Zn, and Se), exploring natural variations for improving NUE in rice; and also, the current genetic toolbox and future perspectives for improving rice NUE are discussed.

The Molecular Mechanisms Underlying Iron Deficiency Responses in Rice

Iron (Fe) is an essential element required for plant growth and development. Under Fe-deficientconditions, plants have developed two distinct strategies (designated as strategy I and II) to acquire Fe from soil. As a graminaceous species, rice is not a typical strategy II plant, as it not only synthesizes DMA (2'-deoxymugineic acid) in roots to chelate Fe3+ but also acquires Fe2+ through transporters OsIRT1 and OsIRT2. During the synthesis of DMA in rice, there are three sequential enzymatic reactions catalyzed by enzymes NAS (nicotianamine synthase), NAAT (nicotianamine aminotransferase), and DMAS (deoxymugineic acid synthase). Many transporters required for Fe uptake from the rhizosphere and internal translocation have also been identified in rice. In addition, the signaling networks composed of various transcription factors (such as IDEF1, IDEF2, and members of the bHLH (basic helix-loop-helix) family), phytohormones, and signaling molecules are demonstrated to regulate Fe uptake and translocation. This knowledge greatly contributes to our understanding of the molecular mechanisms underlying iron deficiency responses in rice.

Mutagenesis in Rice: The Basis for Breeding a New Super Plant

The high selection pressure applied in rice breeding since its domestication thousands of years ago has caused a narrowing in its genetic variability. Obtaining new rice cultivars therefore becomes a major challenge for breeders and developing strategies to increase the genetic variability has demanded the attention of several research groups. Understanding mutations and their applications have paved the way for advances in the elucidation of a genetic, physiological, and biochemical basis of rice traits. Creating variability through mutations has therefore grown to be among the most important tools to improve rice. The small genome size of rice has enabled a faster release of higher quality sequence drafts as compared to other crops. The move from structural to functional genomics is possible due to an array of mutant databases, highlighting mutagenesis as an important player in this progress. Furthermore, due to the synteny among the Poaceae, other grasses can also benefit from these findings. Successful gene modifications have been obtained by random and targeted mutations. Furthermore, following mutation induction pathways, techniques have been applied to identify mutations and the molecular control of DNA damage repair mechanisms in the rice genome. This review highlights findings in generating rice genome resources showing strategies applied for variability increasing, detection and genetic mechanisms of DNA damage repair.

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.