Biofortification and phytoremediation

Genetic Basis and Breeding Perspectives of Grain Iron and Zinc Enrichment in Cereals

Micronutrient deficiency, also known as "hidden hunger," is an increasingly serious global challenge to humankind. Among the mineral elements, Fe (Iron) and Zn (Zinc) have earned recognition as micronutrients of outstanding and diverse biological relevance, as well as of clinical importance to global public health. The inherently low Fe and Zn content and poor bioavailability in cereal grains seems to be at the root of these mineral nutrient deficiencies, especially in the developing world where cereal-based diets are the most important sources of calories. The emerging physiological and molecular understanding of the uptake of Fe and Zn and their translocation in cereal grains regrettably also indicates accumulation of other toxic metals, with chemically similar properties, together with these mineral elements. This review article emphasizes breeding to develop bioavailable Fe- and Zn-efficient cereal cultivars to overcome malnutrition while minimizing the risks of toxic metals. We attempt to critically examine the genetic diversity regarding these nutritionally important traits as well as the progress in terms of quantitative genetics. We sought to integrate findings from the rhizosphere with Fe and Zn accumulation in grain, and to discuss the promoters as well as the anti-nutritional factors affecting Fe and Zn bioavailability in humans while restricting the content of toxic metals.

How Could Agronomic Biofortification of Rice Be an Alternative Strategy with Higher Cost-Effectiveness for Human Iron and Zinc Deficiency in China?

Background:

Iron and zinc deficiencies affect human health globally, especially in developing countries. Agronomic biofortification, as a strategy for alleviating these issues, has been focused on small-scale field studies, and not widely applied while lacking of cost-effectiveness analysis (CEA).

Objective:

We conducted the CEA of agronomic biofortification, expressed as USD per disability-adjusted life years (DALYs) saved, to recommend a cost-effectiveness strategy that can be widely applied.

Methods:

The DALYs were applied to quantify the health burden due to Fe and/or Zn deficiency and health cost of agronomic biofortification via a single, dual, or triple foliar spray of Fe, Zn, and/or pesticide in 4 (northeast, central China, southeast, and southwest) major Chinese rice-based regions.

Results:

The current health burden by Fe or Zn malnutrition was 0.45 to 1.45 or 0.14 to 0.84 million DALYs for these 4 regions. Compared to traditional rice diets, the daily Fe and/or Zn intake from Fe and/or Zn-biofortified rice increased, and the health burden of Fe and/or Zn deficiency decreased by 28% and 48%, respectively. The cost of saving 1 DALYs ranged from US$376 to US$4989, US$194 to US$2730, and US$37.6 to US$530.1 for the single, dual, and triple foliar Fe, Zn, and/or pesticide application, respectively, due to a substantial decrease in labor costs by the latter 2 applications.

Conclusions:

Agronomic biofortification of rice with the triple foliar spray of Fe, Zn, and pesticide is a rapidly effective and cost-effectiveness pathway to alleviate Fe and Zn deficiency for rice-based dietary populations.

Variations in nutrient and trace element composition of rice in an organic rice-frog coculture system

Introducing frogs into paddy fields can control pests and diseases, and organic farming can improve soil fertility and rice growth. The aim of this 2-year field study was compare the yield and elemental composition of rice between an organic farming system including frogs (ORF) and a conventional rice culture system (CR). The grain yields were almost the same in the ORF system and the CR system. The ORF significantly increased the contents of phosphorus (P), ion (Fe), zinc (Zn), molybdenum (Mo) and selenium (Se) in rice grain at one or both years. However, the ORF system decreased the calcium (Ca) content in grice grains, and increased the concentration of cadmium, which is potentially toxic. A principal components analysis showed the main impacts of ORF agro-ecosystem on the rice grain ionome was to increase the concentration of P and trace metal(loid)s. The results showed that the ORF system is an ecologically, friendly strategy to avoid excessive use of chemical fertilizers, herbicides and pesticides without decreasing yields, and to improve the nutritional status of rice by increasing the micronutrient contents. The potential risks of increasing Cd contents in rice grain should be addressed if this cultivation pattern is used in the long term.

Selenoamino Acid-Enriched Green Pea as a Value-Added Plant Protein Source for Humans and Livestock

Selenium deficiency in various degrees affects around 15% of the world's population, contributing to a variety of health problems. In this study, we examined the accumulation and biotransformation of soil applied Se-supplementation (sodium selenite and sodium selenate forms) at different concentrations, along with growth and yield formation of green pea, in a greenhouse experiment. Biotransformation of inorganic Se was evaluated using HPLC-ICP-MS for Se-species separation in the above ground parts of green pea. Results showed 3 mg kg-1 SeIV increased green pea growth biomarkers and also caused an increase in protein content in leaves by 17%. Selenomethionine represented 65% of the total selenium content in shoots, but was lower in pods and seeds (54 and 38%, respectively). Selenomethionine was the major species in all plant parts and the only organic selenium form in the lower SeIV concentration range. Elevating the dose of SeIV (≥30 mg kg-1) triggered detrimental effects on growth and protein content and caused higher accumulation of inorganic Se in forms of SeVI and SeIV. Selenocysteine, another organic form of proteinogenic amino acid, was determined when SeIV (≥10 mg kg-1) was applied in higher concentrations. Thus, agronomic biofortification using the appropriate chemical form and concentration of Se will have positive effects on green pea growth and its enriched shoots and seeds provide a value-added protein source for livestock and humans with significant increased selenomethionine.

Selenium speciation in seleniferous agricultural soils under different cropping systems using sequential extraction and X-ray absorption spectroscopy

Selenium (Se) speciation in soil is critically important for understanding the solubility, mobility, bioavailability, and toxicity of Se in the environment. In this study, Se fractionation and chemical speciation in agricultural soils from seleniferous areas were investigated using the elaborate sequential extraction and X-ray absorption near-edge structure (XANES) spectroscopy. The speciation results quantified by XANES technique generally agreed with those obtained by sequential extraction, and the combination of both approaches can reliably characterize Se speciation in soils. Results showed that dominant organic Se (56-81% of the total Se) and lesser Se(IV) (19-44%) were observed in seleniferous agricultural soils. A significant decrease in the proportion of organic Se to the total Se was found in different types of soil, i.e., paddy soil (81%) > uncultivated soil (69-73%) > upland soil (56-63%), while that of Se(IV) presented an inverse tendency. This suggests that Se speciation in agricultural soils can be significantly influenced by different cropping systems. Organic Se in seleniferous agricultural soils was probably derived from plant litter, which provides a significant insight for phytoremediation in Se-laden ecosystems and biofortification in Se-deficient areas. Furthermore, elevated organic Se in soils could result in higher Se accumulation in crops and further potential chronic Se toxicity to local residents in seleniferous areas.

Uptake, sequestration and tolerance of cadmium at cellular levels in the hyperaccumulator plant species Sedum alfredii

Sedum alfredii is one of a few plant species known to hyperaccumulate cadmium (Cd). Uptake, localization, and tolerance of Cd at cellular levels in shoots were compared in hyperaccumulating (HE) and non-hyperaccumulating (NHE) ecotypes of Sedum alfredii. X-ray fluorescence images of Cd in stems and leaves showed only a slight Cd signal restricted within vascular bundles in the NHEs, while enhanced localization of Cd, with significant tissue- and age-dependent variations, was detected in HEs. In contrast to the vascular-enriched Cd in young stems, parenchyma cells in leaf mesophyll, stem pith and cortex tissues served as terminal storage sites for Cd sequestration in HEs. Kinetics of Cd transport into individual leaf protoplasts of the two ecotypes showed little difference in Cd accumulation. However, far more efficient storage of Cd in vacuoles was apparent in HEs. Subsequent analysis of cell viability and hydrogen peroxide levels suggested that HE protoplasts exhibited higher resistance to Cd than those of NHE protoplasts. These results suggest that efficient sequestration into vacuoles, as opposed to rapid transport into parenchyma cells, is a pivotal process in Cd accumulation and homeostasis in shoots of HE S. alfredii. This is in addition to its efficient root-to-shoot translocation of Cd.

Biofortification of Wheat Cultivars to Combat Zinc Deficiency

Zinc (Zn) deficiency caused by inadequate dietary intake is a global nutritional problem, particularly in developing countries. Therefore, zinc biofortification of wheat and other cereal crops is being urgently addressed and highly prioritized as a research topic. A field study was planned to evaluate the influence of zinc application on grain yield, grain zinc content, and grain phytic acid concentrations of wheat cultivars, and the relationships between these parameters. Three wheat cultivars, C1 = Faisalabad-2008, C2 = Punjab-2011, and C3 = Millet-2011 were tested with five different methods of zinc application: T1 = control, T2 = seed priming, T3 = soil application, T4 = foliar application, and T5 = soil + foliar application. It was found that grain yield and grain zinc were positively correlated, whereas, grain phytic acid and grain zinc were significantly negatively correlated. Results also revealed that T5, T3, and T4 considerably increased grain yield; however, T2 only slightly enhanced grain yield. Grain zinc concentration increased from 33.1 and 33.7 mg kg-1 in T1 to 62.3 and 63.1 mg kg-1 in T5 in 2013-2014 and 2014-2015, respectively. In particular, T5 markedly decreased grain phytic acid content; however, maximum concentration was recorded in T1. Moreover, all the tested cultivars exhibited considerable variation in grain yield, grain zinc, and grain phytic acid content. In conclusion, T5 was found to be most suitable for both optimum grain yield and grain biofortification of wheat.

Foliar heavy metal uptake, toxicity and detoxification in plants: A comparison of foliar and root metal uptake

Anthropologic activities have transformed global biogeochemical cycling of heavy metals by emitting considerable quantities of these metals into the atmosphere from diverse sources. In spite of substantial and progressive developments in industrial processes and techniques to reduce environmental emissions, atmospheric contamination by toxic heavy metals and associated ecological and health risks are still newsworthy. Atmospheric heavy metals may be absorbed via foliar organs of plants after wet or dry deposition of atmospheric fallouts on plant canopy. Unlike root metal transfer, which has been largely studied, little is known about heavy metal uptake by plant leaves from the atmosphere. To the best of our understanding, significant research gaps exist regarding foliar heavy metal uptake. This is the first review regarding biogeochemical behaviour of heavy metals in atmosphere-plant system. The review summarizes the mechanisms involved in foliar heavy metal uptake, transfer, compartmentation, toxicity and in plant detoxification. We have described the biological and environmental factors that affect foliar uptake of heavy metals and compared the biogeochemical behaviour (uptake, translocation, compartmentation, toxicity and detoxification) of heavy metals for root and foliar uptake. The possible health risks associated with the consumption of heavy metal-laced food are also discussed.

The fascinating facets of plant selenium accumulation - biochemistry, physiology, evolution and ecology

Contents 1582 I. 1582 II. 1583 III. 1588 IV. 1590 V. 1592 1592 References 1592 SUMMARY: The importance of selenium (Se) for medicine, industry and the environment is increasingly apparent. Se is essential for many species, including humans, but toxic at elevated concentrations. Plant Se accumulation and volatilization may be applied in crop biofortification and phytoremediation. Topics covered here include beneficial and toxic effects of Se on plants, mechanisms of Se accumulation and tolerance in plants and algae, Se hyperaccumulation, and ecological and evolutionary aspects of these processes. Plant species differ in the concentration and forms of Se accumulated, Se partitioning at the whole-plant and tissue levels, and the capacity to distinguish Se from sulfur. Mechanisms of Se hyperaccumulation and its adaptive significance appear to involve constitutive up-regulation of sulfate/selenate uptake and assimilation, associated with elevated concentrations of defense-related hormones. Hyperaccumulation has evolved independently in at least three plant families, probably as an elemental defense mechanism and perhaps mediating elemental allelopathy. Elevated plant Se protects plants from generalist herbivores and pathogens, but also gives rise to the evolution of Se-resistant specialists. Plant Se accumulation affects ecological interactions with herbivores, pollinators, neighboring plants, and microbes. Hyperaccumulation tends to negatively affect Se-sensitive ecological partners while facilitating Se-resistant partners, potentially affecting species composition and Se cycling in seleniferous ecosystems.

Biofortification in Millets: A Sustainable Approach for Nutritional Security

Nutritional insecurity is a major threat to the world's population that is highly dependent on cereals-based diet, deficient in micronutrients. Next to cereals, millets are the primary sources of energy in the semi-arid tropics and drought-prone regions of Asia and Africa. Millets are nutritionally superior as their grains contain high amount of proteins, essential amino acids, minerals, and vitamins. Biofortification of staple crops is proved to be an economically feasible approach to combat micronutrient malnutrition. HarvestPlus group realized the importance of millet biofortification and released conventionally bred high iron pearl millet in India to tackle iron deficiency. Molecular basis of waxy starch has been identified in foxtail millet, proso millet, and barnyard millet to facilitate their use in infant foods. With close genetic-relatedness to cereals, comparative genomics has helped in deciphering quantitative trait loci and genes linked to protein quality in finger millet. Recently, transgenic expression of zinc transporters resulted in the development of high grain zinc while transcriptomics revealed various calcium sensor genes involved in uptake, translocation, and accumulation of calcium in finger millet. Biofortification in millets is still limited by the presence of antinutrients like phytic acid, polyphenols, and tannins. RNA interference and genome editing tools [zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and clustered regularly interspaced short palindromic repeats (CRISPR)] needs to be employed to reduce these antinutrients. In this review paper, we discuss the strategies to accelerate biofortification in millets by summarizing the opportunities and challenges to increase the bioavailability of macro and micronutrients.

Watering regime influences Cd concentrations in cultivated spinach

In washed spinach, a maximum Cd concentration of 0.20 mg/kg fresh weight (FW) is allowed according to European regulations. Producers experience that this concentration can sometimes be exceeded even on soils with baseline Cd concentrations. There is a growing need to quantify the factors determining Cd uptake in the crop in order to anticipate the risk of exceedance when selecting a field for cultivation. Interseasonal variation in precipitation may be one of the factors influencing Cd uptake by crops. A pot experiment was set up where spinach plants were subject to different watering regimes. Treatment with more limited water supply during periods of high demand resulted in significantly higher accumulated Cd concentrations (0.25-0.31 versus 0.17-0.23 mg/kg FW). Concentrations at or above the maximum allowed limit were of concern, considering that the soil used in the experiment originated from a typical field in an agricultural region without any specific contamination. Probabilities to exceed maximum concentrations in the different watering regimes were estimated using Monte Carlo simulation. Results suggested that the watering regimes significantly determine the effective risk of exceeding the maximum concentrations. Their effects may be of high practical importance in the field.

An Overview of Selenium Uptake, Metabolism, and Toxicity in Plants

Selenium (Se) is an essential micronutrient for humans and animals, but lead to toxicity when taken in excessive amounts. Plants are the main source of dietary Se, but essentiality of Se for plants is still controversial. However, Se at low doses protects the plants from variety of abiotic stresses such as cold, drought, desiccation, and metal stress. In animals, Se acts as an antioxidant and helps in reproduction, immune responses, thyroid hormone metabolism. Selenium is chemically similar to sulfur, hence taken up inside the plants via sulfur transporters present inside root plasma membrane, metabolized via sulfur assimilatory pathway, and volatilized into atmosphere. Selenium induced oxidative stress, distorted protein structure and function, are the main causes of Se toxicity in plants at high doses. Plants can play vital role in overcoming Se deficiency and Se toxicity in different regions of the world, hence, detailed mechanism of Se metabolism inside the plants is necessary for designing effective Se phytoremediation and biofortification strategies.

Selenium Biofortification and Phytoremediation Phytotechnologies: A Review

The element selenium (Se) is both essential and toxic for most life forms, with a narrow margin between deficiency and toxicity. Phytotechnologies using plants and their associated microbes can address both of these problems. To prevent Se toxicity due to excess environmental Se, plants may be used to phytoremediate Se from soil or water. To alleviate Se deficiency in humans or livestock, crops may be biofortified with Se. These two technologies may also be combined: Se-enriched plant material from phytoremediation could be used as green fertilizer or as fortified food. Plants may also be used to "mine" Se from seleniferous soils. The efficiency of Se phytoremediation and biofortification may be further optimized. Research in the past decades has provided a wealth of knowledge regarding the mechanisms by which plants take up, metabolize, accumulate, and volatilize Se and the role plant-associated microbes play in these processes. Furthermore, ecological studies have revealed important effects of plant Se on interactions with herbivores, detrivores, pollinators, neighboring vegetation, and the plant microbiome. All this knowledge can be exploited in phytotechnology programs to optimize plant Se accumulation, transformation, volatilization, and/or tolerance via plant breeding, genetic engineering, and tailored agronomic practices.

The Challenges and Opportunities Associated with Biofortification of Pearl Millet (Pennisetum glaucum) with Elevated Levels of Grain Iron and Zinc

Deficiencies of essential micronutrients such as iron and zinc are the cause of extensive health problems in developing countries. They adversely affect performance, productivity and are a major hindrance to economic development. Since many people who suffer from micronutrient deficiencies are dependent on staple crops to meet their dietary requirements, the development of crop cultivars with increased levels of micronutrients in their edible parts is becoming increasingly recognized as a sustainable solution. This is largely facilitated by genetics and genomic platforms. The cereal crop pearl millet (Pennisetum glaucum), is an excellent candidate for genetic improvement due to its ability to thrive in dry, semi-arid regions, where farming conditions are often unfavorable. Not only does pearl millet grow in areas where other crops such as maize and wheat do not survive, it contains naturally high levels of micronutrients, proteins and a myriad of other health benefitting characteristics. This review discusses the current status of iron and zinc deficiencies and reasons why interventions such as fortification, supplementation, and soil management are neither practicable nor affordable in poverty stricken areas. We argue that the most cost effective, sustainable intervention strategy is to biofortify pearl millet with enhanced levels of bioavailable iron and zinc. We discuss how naturally occurring genetic variations present in germplasm collections can be incorporated into elite, micronutrient rich varieties and what platforms are available to drive this research. We also consider the logistics of transgenic methods that could facilitate the improvement of the pearl millet gene pool.

Calcium Deficiency Triggers Phloem Remobilization of Cadmium in a Hyperaccumulating Species

Understanding cadmium (Cd) accumulation in plants is critical for the development of plant-based strategies for soil remediation and crop safety. Sedum alfredii is a nonbrassica plant species known to hyperaccumulate Cd. The characteristics of Cd uptake, distribution, and retranslocation affected by the Ca status were investigated at cellular levels in S. alfredii Low Ca supply significantly increased Cd contents in shoots of S. alfredii, particularly in the young leaves. Micro x-ray fluorescence images confirmed that sequestration of Cd was greatly enhanced in the young leaves under Ca deficiency stress, with a significant amount of Cd localized in mesophyll cells, compared to the young leaves supplied with high Ca levels. Cd influx into protoplasts isolated from young leaves was significantly inhibited by the addition of Ca channel inhibitors, but not by pre-exposure to Ca deficiency. In stems, the Cd signal in vascular systems under low Ca levels was 10-fold higher than in those treated with higher Ca levels. A detailed investigation of vascular bundles revealed that an extremely high Cd signal induced by low Ca supply occurred in the phloem tissues, but not in the xylem tissues. Transfer of Cd pretreated plants to nutrient solutions at different Ca levels confirmed that a much higher amount of Cd was reallocated to the new growth tissues under low Ca stress compared to plants supplied with sufficient Ca. These results suggest that Ca deficiency triggered a highly efficient phloem remobilization of Cd in S. alfredii and subsequently enhanced Cd accumulation in its young leaves.

Influence of Se concentrations and species in hydroponic cultures on Se uptake, translocation and assimilation in non-accumulator ryegrass

The success of biofortification and phytoremediation practices, addressing Se deficiency and Se pollution issues, hinges crucially on the fate of selenium in the plant media in response to uptake, translocation and assimilation processes. We investigate the fate of selenium in root and shoot compartments after 3 and 6 weeks of experiment using a total of 128 plants grown in hydroponic solution supplied with 0.2, 2, 5, 20 and 100 mg L^-1 of selenium in the form of selenite, selenate and a mixture of both species. Selenate-treated plants exhibited higher root-to-shoot Se translocation and total Se uptake than selenite-treated plants. Plants took advantage of the selenate mobility and presumably of the storage capacity of leaf vacuoles to circumvent selenium toxicity within the plant. Surprisingly, 28% of selenate was found in shoots of selenite-treated plants, questioning the ability of plants to oxidize selenite into selenate. Selenomethionine and methylated organo-selenium amounted to 30% and 8% respectively in shoots and 35% and 9% in roots of the identified Se, suggesting that selenium metabolization occurred concomitantly in root and shoot plant compartments and demonstrating that non-accumulator plants can synthesize notable quantities of precursor compound for volatilization. The present study demonstrated that non-accumulator plants can develop the same strategies as hyper-accumulator plants to limit selenium toxicity. When both selenate and selenite were supplied together, plants used selenate in a storage pathway and selenite in an assimilation pathway. Plants might thereby benefit from mixed supplies of selenite and selenate by saving enzymes and energy required for selenate reduction.

Physiological responses and tolerance of kenaf (Hibiscus cannabinus L.) exposed to chromium

Selection of kenaf species with chromium (Cr) tolerance and exploring the physiological mechanisms involved in Cr tolerance are crucial for application of these species to phyto-remediation. In the present study, a hydroponic experiment was conducted to investigate the variation in two kenaf cultivars, K39-2 and Zhe50-3 under Cr stress. At the same Cr concentration, the tolerance index (TI) of K39-2 was higher than that of Zhe50-3, indicating that K39-2 may be more tolerant to Cr than Zhe50-3. It was also observed that high concentration of chromium was accumulated both in the shoots and the roots of Hibiscus cannabinus L. The leaves of K39-2 accumulated 4760.28mgkg(-1) of dry weight under 1.50mM Cr stress, and the roots accumulated 11,958.33mgkg(-1). Physiological response shows that the antioxidant enzymes' superoxide dismutase (SOD), catalase activity (CAT) and peroxidase (POD) activities increased in the leaves and decreased in roots of the Cr-stressed plants nearly compared to the control. Moreover, the variation of antioxidant enzymes activities indicated Zhe50-3 was more vulnerable than K39-2, and the contents of the non-protein thiol pool (GSH, NPT and PCs) were higher in K39-2 than Zhe50-3 with the increased Cr concentration. Based on the observations above, it can be concluded that the well-coordinated physiological changes confer a greater Cr tolerance to K39-2 than Zhe50-3 under Cr exposure, and Hibiscus cannabinus L. has a great accumulation capacity for chromium.

Spatially resolved analysis of variation in barley (Hordeum vulgare) grain micronutrient accumulation

Genetic biofortification requires knowledge on natural variation and the underlying mechanisms of micronutrient accumulation. We therefore studied diversity in grain micronutrient concentrations and spatial distribution in barley (Hordeum vulgare), a genetically tractable model cereal and an important crop with widespread cultivation. We assembled a diverse collection of barley cultivars and landraces and analysed grain micronutrient profiles in genebank material and after three independent cultivations. Lines with contrasting grain zinc (Zn) accumulation were selected for in-depth analysis of micronutrient distribution within the grain by micro-proton-induced X-ray emission (μ-PIXE). Also, we addressed association with grain cadmium (Cd) accumulation. The analysis of > 120 lines revealed substantial variation, especially in grain Zn concentrations. A large fraction of this variation is due to genetic differences. Grain dissection and μ-PIXE analysis of contrasting lines showed that differences in grain Zn accumulation apply to all parts of the grain including the endosperm. Cd concentrations exceeded the Codex Alimentarius threshold in most of the representative barley lines after cultivation in a Cd-contaminated agricultural soil. Two important conclusions for biofortification are: first, high-Zn grains contain more Zn also in the consumed parts of the grain; and second, higher micronutrient concentrations are strongly associated with higher Cd accumulation.

Zinc Isotope Fractionation in the Hyperaccumulator Noccaea caerulescens and the Nonaccumulating Plant Thlaspi arvense at Low and High Zn Supply

On the basis of our previous field survey, we postulate that the pattern and degree of zinc (Zn) isotope fractionation in the Zn hyperaccumulator Noccaea caerulescens (J. & C. Presl) F. K. Mey may reflect a relationship between Zn bioavailability and plant uptake strategies. Here, we investigated Zn isotope discrimination during Zn uptake and translocation in N. caerulescens and in a nonaccumulator Thlaspi arvense L. with a contrasting Zn accumulation ability in response to low (Zn-L) and high (Zn-H) Zn supplies. The average isotope fractionations of the N. caerulescens plant as a whole, relative to solution (Δ(66)Znplant-solution), were -0.06 and -0.12‰ at Zn-L-C and Zn-H-C, respectively, indicative of the predominance of a high-affinity (e.g., ZIP transporter proteins) transport across the root cell membrane. For T. arvense, plants were more enriched in light isotopes under Zn-H-A (Δ(66)Znplant-solution = -0.26‰) than under Zn-L-A and N. caerulescens plants, implying that a low-affinity (e.g., ion channel) transport might begin to function in the nonaccumulating plants when external Zn supply increases. Within the root tissues of both species, the apoplast fractions retained up to 30% of Zn mass under Zn-H. Moreover, the highest δ(66)Zn (0.75‰-0.86‰) was found in tightly bound apoplastic Zn, pointing to the strong sequestration in roots (e.g., binding to high-affinity ligands/precipitation with phosphate) when plants suffer from high Zn stress. During translocation, the magnitude of isotope fractionation was significantly greater at Zn-H (Δ(66)Znroot-shoot = 0.79‰) than at Zn-L, indicating that fractionation mechanisms associated with root-shoot translocation might be identical to the two plant species. Hence, we clearly demonstrated that Zn isotope fractionation could provide insight into the internal sequestration mechanisms of roots when plants respond to low and high Zn supplies.

Improving Rice Zinc Biofortification Success Rates Through Genetic and Crop Management Approaches in a Changing Environment

Though rice is the predominant source of energy and micronutrients for more than half of the world population, it does not provide enough zinc (Zn) to match human nutritional requirements. Moreover, climate change, particularly rising atmospheric carbon dioxide concentration, reduces the grain Zn concentration. Therefore, rice biofortification has been recognized as a key target to increase the grain Zn concentration to address global Zn malnutrition. Major bottlenecks for Zn biofortification in rice are identified as low Zn uptake, transport and loading into the grain; however, environmental and genetic contributions to grain Zn accumulation in rice have not been fully explored. In this review, we critically analyze the key genetic, physiological and environmental factors that determine Zn uptake, transport and utilization in rice. We also explore the genetic diversity of rice germplasm to develop new genetic tools for Zn biofortification. Lastly, we discuss the strategic use of Zn fertilizer for developing biofortified rice.

Crop acquisition of phosphorus, iron and zinc from soil in cereal/legume intercropping systems: a critical review

BACKGROUND

Phosphorus (P), iron (Fe) and zinc (Zn) are essential elements for plant growth and development, but their availability in soil is often limited. Intercropping contributes to increased P, Fe and Zn uptake and thereby increases yield and improves grain nutritional quality and ultimately human health. A better understanding of how intercropping leads to increased plant P, Fe and Zn availability will help to improve P-fertilizer-use efficiency and agronomic Fe and Zn biofortification.

SCOPE

This review synthesizes the literature on how intercropping of legumes with cereals increases acquisition of P, Fe and Zn from soil and recapitulates what is known about root-to-shoot nutrient translocation, plant-internal nutrient remobilization and allocation to grains.

CONCLUSIONS

Direct interspecific facilitation in intercropping involves below-ground processes in which cereals increase Fe and Zn bioavailability while companion legumes benefit. This has been demonstrated and verified using isotopic nutrient tracing and molecular analysis. The same methodological approaches and field studies should be used to explore direct interspecific P facilitation. Both niche complementarity and interspecific facilitation contribute to increased P acquisition in intercropping. Niche complementarity may also contribute to increased Fe and Zn acquisition, an aspect poorly understood. Interspecific mobilization and uptake facilitation of sparingly soluble P, Fe and Zn from soil, however, are not the only determinants of the concentrations of P, Fe and Zn in grains. Grain yield and nutrient translocation from roots to shoots further influence the concentrations of these nutrients in grains.

Effect of Selenium on Control of Postharvest Gray Mold of Tomato Fruit and the Possible Mechanisms Involved

Selenium (Se) has important benefits for crop growth and stress tolerance at low concentrations. However, there is very little information on antimicrobial effect of Se against the economically important fungus Botrytis cinerea. In the present study, using sodium selenite as Se source, we investigated the effect of Se salts on spore germination and mycelial growth of the fungal pathogen in vitro and gray mold control in harvested tomato fruit. Se treatment at 24 mg/L significantly inhibited spore germination of the fungal pathogen and effectively controlled gray mold in harvested tomato fruit. Se treatment at 24 mg/L seems to induce the generation of intracellular reactive oxygen species in the fungal spores. The membrane integrity damage was observed with fluorescence microscopy following staining with propidium iodide after treatment of the spores with Se. These results suggest that Se has the potential for controlling gray mold rot of tomato fruits and might be useful in integrated control against gray mold disease of postharvest fruits and vegetables caused by B. cinerea. The mechanisms by which Se decreased gray mold decay of tomato fruit may be directly related to the severe damage to the conidia plasma membrane and loss of cytoplasmic materials from the hyphae.

Biofortification of Carrot (Daucus carota L.) with Iodine and Selenium in a Field Experiment

The low content of iodine (I) and selenium (Se) forms available to plants in soil is one of the main causes of their insufficient transfer in the soil-plant-consumer system. Their deficiency occurs in food in the majority of human and farm animal populations around the world. Both elements are classified as beneficial elements. However, plant response to simultaneous fertilization with I and Se has not been investigated in depth. The study (conducted in 2012-2014) included soil fertilization of carrot cv. "Kazan F1" in the following combinations: (1) Control; (2) KI; (3) KIO3; (4) Na2SeO4; (5) Na2SeO3; (6) KI+Na2SeO4; (7) KIO3+Na2SeO4; (8) KI+Na2SeO3; (9) KIO3+Na2SeO3. I and Se were applied twice: before sowing and as top-dressing in a total dose of 5 kg I⋅ha(-1) and 1 kg Se⋅ha(-1). No negative effects of I and Se fertilization were noted with respect to carrot yield. Higher accumulation and the uptake by leaves and storage roots of I and Se were obtained after the application of KI than KIO3, as well as of Na2SeO4 than Na2SeO3, respectively. Transfer factor values for leaves and roots were about a dozen times higher for Se than for I. Selenomethionine content in carrot was higher after fertilization with Na2SeO4 than with Na2SeO3. However, it was the application of Na2SeO3, KI+Na2SeO3 and KIO3+Na2SeO3 that resulted in greater evenness within the years and a higher share of Se from selenomethionine in total Se in carrot plants. Consumption of 100 g f.w. of carrots fertilized with KI+Na2SeO3 and KIO3+Na2SeO3 can supply approximately or slightly exceed 100% of the Recommended Daily Allowance for I and Se. Moreover, the molar ratio of I and Se content in carrot fertilized with KI+Na2SeO3 and KIO3+Na2SeO3 was the best among the research plots.

Iodine and Selenium Biofortification with Additional Application of Salicylic Acid Affects Yield, Selected Molecular Parameters and Chemical Composition of Lettuce Plants (Lactuca sativa L. var. capitata)

Iodine (I) and selenium (Se) are included in the group of beneficial elements. They both play important roles in humans and other animals, particularly in the regulation of thyroid functioning. A substantial percentage of people around the world suffer from health disorders related to the deficiency of these elements in the diet. Salicylic acid (SA) is a compound similar to phytohormones and is known to improve the efficiency of I biofortification of plants. The influence of SA on Se enrichment of plants has not, however, been recognized together with its effect on simultaneous application of I and Se to plants. Two-year studies (2014-2015) were conducted in a greenhouse with hydroponic cultivation of lettuce in an NFT (nutrient film technique) system. They included the application of I (as KIO₃), Se (as Na₂SeO₃) and SA into the nutrient solution. KIO₃ was used at a dose of 5 mg I⋅dm^-3 (i.e., 39.4 μM I), while Na₂SeO₃ was 0.5 mg Se⋅dm^-3 (i.e., 6.3 μM Se). SA was introduced at three doses: 0.1, 1.0, and 10.0 mg⋅dm^-3 nutrient solutions, equivalent to 0.724, 7.24, and 72.4 μM SA, respectively. The tested combinations were as follows: (1) control, (2) I + Se, (3) I + Se + 0.1 mg SA⋅dm^-3, (4) I + Se + 1.0 mg SA⋅dm^-3 and (5) I + Se + 10.0 mg SA⋅dm^-3. The applied treatments had no significant impact on lettuce biomass (leaves and roots). Depending on the dose, a diverse influence of SA was noted with respect to the efficiency of I and Se biofortification; chemical composition of leaves; and mineral nutrition of lettuce plants, including the content of macro- and microelements and selenocysteine methyltransferase (SMT) gene expression. SA application at all tested doses comparably increased the level of selenomethionine (SeMet) and decreased the content of SA in leaves.

Gomphrena claussenii, a novel metal-hypertolerant bioindicator species, sequesters cadmium, but not zinc, in vacuolar oxalate crystals

Gomphrena claussenii is a recently described zinc (Zn)- and cadmium (Cd)-hypertolerant Amaranthaceae species displaying a metal bioindicator Zn/Cd accumulation response. We investigated the Zn and Cd distribution in stem and leaf tissues of G. claussenii at the cellular level, and determined metabolite profiles to investigate metabolite involvement in Zn and Cd sequestration. Gomphrena claussenii plants exposed to high Zn and Cd supply were analysed by scanning electron microscopy with energy-dispersive X-ray (SEM-EDX) and micro-proton-induced X-ray emission (micro-PIXE). In addition, gas chromatography-time of flight-mass spectrometry (GC-TOF-MS) was used to determine metabolite profiles on high Zn and Cd exposure. Stem and leaf tissues of G. claussenii plants exposed to control and high Cd conditions showed the abundant presence of calcium oxalate (CaOx) crystals, but on high Zn exposure, their abundance was strongly reduced. Ca and Cd co-localized to the CaOx crystals in Cd-exposed plants. Citrate, malate and oxalate levels were all higher in shoot tissues of metal-exposed plants, with oxalate levels induced 2.6-fold on Zn exposure and 6.4-fold on Cd exposure. Sequestration of Cd in vacuolar CaOx crystals of G. claussenii is found to be a novel mechanism to deal with Cd accumulation and tolerance.

Assessment of amendments for the immobilization of Cu in soils containing EDDS leachates

In this study, the effectiveness of six soil amendments (ferrihydrite, manganese dioxide, gibbsite, calcium carbonate, biochar, and organic fertilizer) was investigated to assess the feasibility of minimizing possible environmental contaminant leaching during S,S-ethylenediaminedisuccinic acid (EDDS)-enhanced phytoextraction process based on 0.01-M CaCl2 extraction. Results showed that the application of EDDS could significantly increase Cu concentrations in the leaching solution. Compared with control, incorporation of six amendments (excluding organic fertilizer) significantly decreased CaCl2-extractable Cu concentrations in both soils. When EDDS-containing solutions leached from the soil columns (mimicking the upper soil layers) were added to soils with different amendments (mimicking the subsoil), CaCl2-extractable Cu in the soils amended with ferrihydrite, manganese dioxide, gibbsite, and calcium carbonate was significantly lower than that in the control soil (no amendments) and remained relatively constant during the first 14 days. Incorporation of biochar or organic fertilizer had no positive effect on the immobilization of Cu in EDDS leachates in soils. After 14 days, CaCl2-extractable Cu concentration decreased rapidly in soils incorporated with various amendments. Integrating soil washing with biodegradable chelating agents or chelant-enhanced phytoextraction and immobilization of heavy metals in subsoil could be used to rapidly reduce the concentration of bioavailable metal fractions in the upper soil layers and minimize environmental risks of secondary pollution.

Organoselenium Compounds as Phytochemicals from the Natural Kingdom

Selenium is naturally present in soils but it is also produced by pollution from human activities into the environment. Its incorporation into plants affords organoselenium metabolites that, depending on the nature of the molecules and the plant species, can be incorporated into proteins, stored or eliminated by volatilization. The possibility to use the selenium metabolism of some plants as a method for bioremediation and, at the main time, as a source of selenated phytochemicals is here discussed taking into consideration the growing interest in organic selenium derivatives as new potential therapeutic agents.

Functional analysis of the three HMA4 copies of the metal hyperaccumulator Arabidopsis halleri

In Arabidopsis halleri, the HMA4 gene has an essential function in Zn/Cd hypertolerance and hyperaccumulation by mediating root-to-shoot translocation of metals. Constitutive high expression of AhHMA4 results from a tandem triplication and cis-activation of the promoter of all three copies. The three AhHMA4 copies possess divergent promoter sequences, but highly conserved coding sequences, and display identical expression profiles in the root and shoot vascular system. Here, an AhHMA4::GFP fusion was expressed under the control of each of the three A. halleri HMA4 promoters in a hma2hma4 double mutant of A. thaliana to individually examine the function of each AhHMA4 copy. The protein showed non-polar localization at the plasma membrane of the root pericycle cells of both A. thaliana and A. halleri. The expression of each AhHMA4::GFP copy complemented the severe Zn-deficiency phenotype of the hma2hma4 mutant by restoring root-to-shoot translocation of Zn. However, each copy had a different impact on metal homeostasis in the A. thaliana genetic background: AhHMA4 copies 2 and 3 were more highly expressed and provided higher Zn tolerance in roots and accumulation in shoots than copy 1, and AhHMA4 copy 3 also increased Cd tolerance in roots. These data suggest a certain extent of functional differentiation among the three A. halleri HMA4 copies, stemming from differences in expression levels rather than in expression profile. HMA4 is a key node of the Zn homeostasis network and small changes in expression level can have a major impact on Zn allocation to root or shoot tissues.

Variability of cadmium, lead, and zinc tolerance and accumulation among and between germplasms of the fiber crop Boehmeria nivea with different root-types

Crop germplasms substantially vary in their tolerance for and accumulation of heavy metals, and assessment of this variability plays a significant role in selecting species to use in phytoremediation projects. Here, we examined germplasm-variations in cadmium (Cd), lead (Pb), and zinc (Zn) tolerance and accumulation in ramie (Boehmeria nivea), a fiber crop native to China, which has received little attention. In an 8-week greenhouse test, fourteen germplasms of ramie, among and within deep, middle, and shallow rooted-types, were compared for growth and metal accumulation traits. Results showed that both tolerance and accumulation traits varied across germplasms and rooted-types. The deep rooted-type germplasms produced more biomass and had higher tolerance to metals than the two others. In addition, considerable variations in metal accumulation were observed among plant organs (root, stem, and leaf), rooted-types, germplasms, and metal supply. However, the observed variations in metal tolerance and accumulation among both germplasms and rooted-types were not significant in most cases. In addition to supporting the idea of a certain degree of constitutional metal tolerance for ramie, our results also contribute to deep-rooted germplasms of ramie as a good candidate, rather than middle-/shallow- ones as a least-bad option, for the remediation of multi metal-contaminated soils.

Dietary calcium and zinc deficiency risks are decreasing but remain prevalent

Globally, more than 800 million people are undernourished while >2 billion people have one or more chronic micronutrient deficiencies (MNDs). More than 6% of global mortality and morbidity burdens are associated with undernourishment and MNDs. Here we show that, in 2011, 3.5 and 1.1 billion people were at risk of calcium (Ca) and zinc (Zn) deficiency respectively due to inadequate dietary supply. The global mean dietary supply of Ca and Zn in 2011 was 684 ± 211 and 16 ± 3 mg capita(-1) d(-1) (± SD) respectively. Between 1992 and 2011, global risk of deficiency of Ca and Zn decreased from 76 to 51%, and 22 to 16%, respectively. Approximately 90% of those at risk of Ca and Zn deficiency in 2011 were in Africa and Asia. To our knowledge, these are the first global estimates of dietary Ca deficiency risks based on food supply. We conclude that continuing to reduce Ca and Zn deficiency risks through dietary diversification and food and agricultural interventions including fortification, crop breeding and use of micronutrient fertilisers will remain a significant challenge.

Improved yield and Zn accumulation for rice grain by Zn fertilization and optimized water management

Zinc (Zn) deficiency and water scarcity are major challenges in rice (Oryza sativa L.) under an intensive rice production system. This study aims to investigate the impact of water-saving management and different Zn fertilization source (ZnSO4 and Zn-EDTA) regimes on grain yield and Zn accumulation in rice grain. Different water managements, continuous flooding (CF), and alternate wetting and drying (AWD) were applied during the rice growing season. Compared with CF, the AWD regime significantly increased grain yield and Zn concentrations in both brown rice and polished rice. Grain yield of genotypes (Nipponbare and Jiaxing27), on the average, was increased by 11.4%, and grain Zn concentration by 3.9% when compared with those under a CF regime. Zn fertilization significantly increased Zn density in polished rice, with a more pronounced effect of ZnSO4 being observed as compared with Zn-EDTA, especially under an AWD regime. Decreased phytic acid content and molar ratio of phytic acid to Zn were also noted in rice grains with Zn fertilization. The above results demonstrated that water management of AWD combined with ZnSO4 fertilization was an effective agricultural practice to elevate grain yield and increase Zn accumulation and bioavailability in rice grains.

Biofortification and phytoremediation of selenium in China

Selenium (Se) is an essential trace element for humans and animals but at high concentrations, Se becomes toxic to organisms due to Se replacing sulfur in proteins. Selenium biofortification is an agricultural process that increases the accumulation of Se in crops, through plant breeding, genetic engineering, or use of Se fertilizers. Selenium phytoremediation is a green biotechnology to clean up Se-contaminated environments, primarily through phytoextraction and phytovolatilization. By integrating Se phytoremediation and biofortification technologies, Se-enriched plant materials harvested from Se phytoremediation can be used as Se-enriched green manures or other supplementary sources of Se for producing Se-biofortified agricultural products. Earlier studies primarily aimed at enhancing efficacy of phytoremediation and biofortification of Se based on natural variation in progenitor or identification of unique plant species. In this review, we discuss promising approaches to improve biofortification and phytoremediation of Se using knowledge acquired from model crops. We also explored the feasibility of applying biotechnologies such as inoculation of microbial strains for improving the efficiency of biofortification and phytoremediation of Se. The key research and practical challenges that remain in improving biofortification and phytoremediation of Se have been highlighted, and the future development and uses of Se-biofortified agricultural products in China has also been discussed.

Speciation and localization of Zn in the hyperaccumulator Sedum alfredii by extended X-ray absorption fine structure and micro-X-ray fluorescence

Differences in metal homeostasis among related plant species can give important information of metal hyperaccumulation mechanisms. Speciation and distribution of Zn were investigated in a hyperaccumulating population of Sedum alfredii by using extended X-ray absorption fine structure and micro-synchrotron X-ray fluorescence (μ-XRF), respectively. The hyperaccumulator uses complexation with oxygen donor ligands for Zn storage in leaves and stems, and variations in the Zn speciation was noted in different tissues. The dominant chemical form of Zn in leaves was most probably a complex with malate, the most prevalent organic acid in S. alfredii leaves. In stems, Zn was mainly associated with malate and cell walls, while Zn-citrate and Zn-cell wall complexes dominated in the roots. Two-dimensional μ-XRF images revealed age-dependent differences in Zn localization in S. alfredii stems and leaves. In old leaves of S. alfredii, Zn was high in the midrib, margin regions and the petiole, whereas distribution of Zn was essentially uniform in young leaves. Zinc was preferentially sequestered by cells near vascular bundles in young stems, but was highly localized to vascular bundles and the outer cortex layer of old stems. The results suggest that tissue- and age-dependent variations of Zn speciation and distribution occurred in the hyperaccumulator S. alfredii, with most of the Zn complexed with malate in the leaves, but a shift to cell wall- and citric acid-Zn complexes during transportation and storage in stems and roots. This implies that biotransformation in Zn complexation occurred during transportation and storage processes in the plants of S. alfredii.

Metallothionein 2 (SaMT2) from Sedum alfredii Hance confers increased Cd tolerance and accumulation in yeast and tobacco

Metallothioneins are cysteine-rich metal-binding proteins. In the present study, SaMT2, a type 2 metallothionein gene, was isolated from Cd/Zn co-hyperaccumulator Sedum alfredii Hance. SaMT2 encodes a putative peptide of 79 amino acid residues including two cysteine-rich domains. The transcript level of SaMT2 was higher in shoots than in roots of S. alfredii, and was significantly induced by Cd and Zn treatments. Yeast expression assay showed SaMT2 significantly enhanced Cd tolerance and accumulation in yeast. Ectopic expression of SaMT2 in tobacco enhanced Cd and Zn tolerance and accumulation in both shoots and roots of the transgenic plants. The transgenic plants had higher antioxidant enzyme activities and accumulated less H2O2 than wild-type plants under Cd and Zn treatment. Thus, SaMT2 could significantly enhance Cd and Zn tolerance and accumulation in transgenic tobacco plants by chelating metals and improving antioxidant system.

Global hunger: a challenge to agricultural, food, and nutritional sciences

Hunger has been a concern for generations and has continued to plague hundreds of millions of people around the world. Although many efforts have been devoted to reduce hunger, challenges such as growing competitions for natural resources, emerging climate changes and natural disasters, poverty, illiteracy, and diseases are posing threats to food security and intensifying the hunger crisis. Concerted efforts of scientists to improve agricultural and food productivity, technology, nutrition, and education are imperative to facilitate appropriate strategies for defeating hunger and malnutrition. This paper provides some aspects of world hunger issues and summarizes the efforts and measures aimed to alleviate food problems from the food and nutritional sciences perspectives. The prospects and constraints of some implemented strategies for alleviating hunger and achieving sustainable food security are also discussed. This comprehensive information source could provide insights into the development of a complementary framework for dealing with the global hunger issue.

HMA4 expression in tobacco reduces Cd accumulation due to the induction of the apoplastic barrier

Ectopic expression in tobacco (Nicotiana tabacum v. Xanthi) of the export protein AtHMA4 (responsible in Arabidopsis for the control of Zn/Cd root to shoot translocation) resulted in decreased Cd uptake/accumulation in roots and shoots. This study contributes to understanding the mechanisms underlying this Cd-dependent phenotype to help predict the consequences of transgene expression for potential phytoremediation/biofortification-based strategies. Microarray analysis was performed to identify metal homeostasis genes that were differentially expressed in roots of Cd-exposed AtHMA4-expressing tobacco relative to the wild type. It was established that down-regulation of genes known to mediate Cd uptake was not responsible for reduced Cd uptake/accumulation in AtHMA4 transformants. The transcript levels of NtIRT1 and NtZIP1 were higher in transgenic plants, indicating an induction of the Fe and Zn deficiency status due to AtHMA4 expression. Interestingly, upon exposure to Cd, genes involved in cell wall lignification (NtHCT, NtOMET, and NtPrx11a) were up-regulated in transformants. Microscopic analysis of roots demonstrated that expression of AtHMA4 caused an induction of cell wall lignification in the external cell layers that was accompanied by enhanced H2O2 accumulation. Further study showed that the concentration of other elements (B, Co, Cu, Ni, Mo, and Zn) was reduced in AtHMA4 transformants in the presence of Cd. In conclusion, due to ectopic expression of 35S::AtHMA4, the physical apoplastic barrier within the external cell layer developed, which is likely to be responsible for the reduction of Cd uptake/accumulation.

Ectomycorrhizas with Paxillus involutus enhance cadmium uptake and tolerance in Populus × canescens

Ectomycorrhizas (EMs), which are symbiotic organs formed between tree roots and certain fungi, can mediate cadmium (Cd) tolerance of host plants, but the underlying physiological and molecular mechanisms are not fully understood. To investigate EMs mediated Cd tolerance in woody plants, Populus × canescens was inoculated with Paxillus involutus (strain MAJ) to establish mycorrhizal roots. Mycorrhizal poplars and non-mycorrhizal controls were exposed to 0 or 50 μM CdSO4 . EMs displayed higher net Cd(2+) influx than non-mycorrhizal roots. Net Cd(2+) influx was coupled with net H(+) efflux and inactivation of plasma membrane (PM) H(+) -ATPases reduced Cd(2+) uptake of EMs less than of non-mycorrhizal roots. Consistent with higher Cd(2+) uptake in EMs, in most cases, transcript levels of genes involved in Cd(2+) uptake, transport and detoxification processes were increased in EMs compared to non-mycorrhizal roots. Higher CO2 assimilation, improved nutrient and carbohydrate status, and alleviated oxidative stress were found in mycorrhizal compared to non-mycorrhizal poplars despite higher Cd(2+) accumulation. These results indicate that mycorrhizas increase Cd(2+) uptake, probably by an enlarged root volume and overexpression of genes involved in Cd(2+) uptake and transport, and concurrently enhance Po. × canescens Cd tolerance by increased detoxification, improved nutrient and carbohydrate status and defence preparedness.

HvZIP7 mediates zinc accumulation in barley (Hordeum vulgare) at moderately high zinc supply

High expression of zinc (Zn)-regulated, iron-regulated transporter-like protein (ZIP) genes increases root Zn uptake in dicots, leading to high accumulation of Zn in shoots. However, none of the ZIP genes tested previously in monocots could enhance shoot Zn accumulation. In this report, barley (Hordeum vulgare) HvZIP7 was investigated for its functions in Zn transport. The functions of HvZIP7 in planta were studied using in situ hybridization and transient analysis of subcellular localization with a green fluorescent protein (GFP) reporter. Transgenic barley lines overexpressing HvZIP7 were also generated to further understand the functions of HvZIP7 in metal transport. HvZIP7 is strongly induced by Zn deficiency, primarily in vascular tissues of roots and leaves, and its protein was localized in the plasma membrane. These properties are similar to its closely related homologs in dicots. Overexpression of HvZIP7 in barley plants increased Zn uptake when moderately high concentrations of Zn were supplied. Significantly, there was a specific enhancement of shoot Zn accumulation, with no measurable increase in iron (Fe), manganese (Mn), copper (Cu) or cadmium (Cd). HvZIP7 displays characteristics of low-affinity Zn transport. The unique function of HvZIP7 provides new insights into the role of ZIP genes in Zn homeostasis in monocots, and offers opportunities to develop Zn biofortification strategies in cereals.

Methodological approaches for using synchrotron X-ray fluorescence (SXRF) imaging as a tool in ionomics: examples from Arabidopsis thaliana

Here we present approaches for using multi-elemental imaging (specifically synchrotron X-ray fluorescence microscopy, SXRF) in ionomics, with examples using the model plant Arabidopsis thaliana. The complexity of each approach depends on the amount of a priori information available for the gene and/or phenotype being studied. Three approaches are outlined, which apply to experimental situations where a gene of interest has been identified but has an unknown phenotype (phenotyping), an unidentified gene is associated with a known phenotype (gene cloning) and finally, a screening approach, where both gene and phenotype are unknown. These approaches make use of open-access, online databases with which plant molecular genetics researchers working in the model plant Arabidopsis will be familiar, in particular the Ionomics Hub and online transcriptomic databases such as the Arabidopsis eFP browser. The approaches and examples we describe are based on the assumption that altering the expression of ion transporters can result in changes in elemental distribution. We provide methodological details on using elemental imaging to aid or accelerate gene functional characterization by narrowing down the search for candidate genes to the tissues in which elemental distributions are altered. We use synchrotron X-ray microprobes as a technique of choice, which can now be used to image all parts of an Arabidopsis plant in a hydrated state. We present elemental images of leaves, stem, root, siliques and germinating hypocotyls.

Expression of HvHMA2 in tobacco modifies Zn-Fe-Cd homeostasis

HvHMA2 is a plasma membrane P1B-ATPase from barley that functions in Zn/Cd root-to-shoot transport. To assess the usefulness of HvHMA2 for modifying the metal content in aerial plant parts, it was expressed in tobacco under the CaMV35S promoter. Transformation with HvHMA2 did not produce one unique pattern of Zn and Cd accumulation; instead it depended on external metal supply. Thus Zn and Cd root-to-shoot translocation was facilitated, but not at all applied Zn/Cd concentrations. Metal uptake was restricted in HvHMA2-transformed plants and the level in the shoot was not enhanced. It was shown that HvHMA2 localizes to the plasma membrane of tobacco cells, and overloads the apoplast with Zn, which could explain the overall decrease in metal uptake observed. Despite the lower levels in the shoot, HvHMA2 transformants showed increased Zn sensitivity. Moreover, introduction of HvHMA2 into tobacco interfered with Fe metabolism and Fe accumulation was modified in HvHMA2-transformants in a Zn- and Cd-concentration dependent manner. The results indicate that ectopic expression of the export protein HvHMA2 in tobacco interferes with tobacco metal Zn-Cd-Fe cross-homeostasis, inducing internal mechanisms regulating metal uptake and tolerance.