Whole-plant mineral partitioning throughout the life cycle in Arabidopsis thaliana ecotypes Columbia, Landsberg erecta, Cape Verde Islands, and the mutant line ysl1ysl3

Spatiotemporal analysis of copper homeostasis in Populus trichocarpa reveals an integrated molecular remodeling for a preferential allocation of copper to plastocyanin in the chloroplasts of developing leaves

Plastocyanin, which requires copper (Cu) as a cofactor, is an electron carrier in the thylakoid lumen and essential for photoautotrophic growth of plants. The Cu microRNAs, which are expressed during Cu deprivation, down-regulate several transcripts that encode for Cu proteins. Since plastocyanin is not targeted by the Cu microRNAs, a cofactor economy model has been proposed in which plants prioritize Cu for use in photosynthetic electron transport. However, defects in photosynthesis are classic symptoms of Cu deprivation, and priorities in Cu cofactor delivery have not been determined experimentally. Using hydroponically grown Populus trichocarpa (clone Nisqually-1), we have established a physiological and molecular baseline for the response to Cu deficiency. An integrated analysis showed that Cu depletion strongly reduces the activity of several Cu proteins including plastocyanin, and consequently, photosynthesis and growth are decreased. Whereas plastocyanin mRNA levels were only mildly affected by Cu depletion, this treatment strongly affected the expression of other Cu proteins via Cu microRNA-mediated transcript down-regulation. Polyphenol oxidase was newly identified as Cu regulated and targeted by a novel Cu microRNA, miR1444. Importantly, a spatiotemporal analysis after Cu resupply to previously depleted plants revealed that this micronutrient is preferentially allocated to developing photosynthetic tissues. Plastocyanin and photosynthetic electron transport efficiency were the first to recover after Cu addition, whereas recovery of the other Cu-dependent activities was delayed. Our findings lend new support to the hypothesis that the Cu microRNAs serve to mediate a prioritization of Cu cofactor use. These studies also highlight poplar as an alternative sequenced model for spatiotemporal analyses of nutritional homeostasis.

Effect of the down-regulation of the high Grain Protein Content (GPC) genes on the wheat transcriptome during monocarpic senescence

BACKGROUND

Increasing the nutrient concentration of wheat grains is important to ameliorate nutritional deficiencies in many parts of the world. Proteins and nutrients in the wheat grain are largely derived from the remobilization of degraded leaf molecules during monocarpic senescence. The down-regulation of the NAC transcription factor Grain Protein Content (GPC) in transgenic wheat plants delays senescence (>3 weeks) and reduces the concentration of protein, Zn and Fe in the grain (>30%), linking senescence and nutrient remobilization.Based on the early and rapid up-regulation of GPC in wheat flag leaves after anthesis, we hypothesized that this transcription factor is an early regulator of monocarpic senescence. To test this hypothesis, we used high-throughput mRNA-seq technologies to characterize the effect of the GPC down-regulation on the wheat flag-leaf transcriptome 12 days after anthesis. At this early stage of senescence GPC transcript levels are significantly lower in transgenic GPC-RNAi plants than in the wild type, but there are still no visible phenotypic differences between genotypes.

RESULTS

We generated 1.4 million 454 reads from early senescing flag leaves (average ~350 nt) and assembled 1.2 million into 30,497 contigs that were used as a reference to map 145 million Illumina reads from three wild type and four GPC-RNAi plants. Following normalization and statistical testing, we identified a set of 691 genes differentially regulated by GPC (431 ≥ 2-fold change). Transcript level ratios between transgenic and wild type plants showed a high correlation (R = 0.83) between qRT-PCR and Illumina results, providing independent validation of the mRNA-seq approach. A set of differentially expressed genes were analyzed across an early senescence time-course.

CONCLUSIONS

Monocarpic senescence is an active process characterized by large-scale changes in gene expression which begins considerably before the appearance of visual symptoms of senescence. The mRNA-seq approach used here was able to detect small differences in transcript levels during the early stages of senescence. This resulted in an extensive list of GPC-regulated genes, which includes transporters, hormone regulated genes, and transcription factors. These GPC-regulated genes, particularly those up-regulated during senescence, provide valuable entry points to dissect the early stages of monocarpic senescence and nutrient remobilization in wheat.

The tonoplast copper transporter COPT5 acts as an exporter and is required for interorgan allocation of copper in Arabidopsis thaliana

Copper is an essential micronutrient for all organisms because it serves as a cofactor of several proteins involved in electron transfer. Elevated copper concentrations can cause toxic effects and organisms have established suitable mechanisms to regulate the uptake and internal distribution of copper to balance the content at an optimal concentration. In recent studies, a family of copper transporters (COPT) with high homology to other eukaryotic copper transporters (Ctr) has been identified in Arabidopsis thaliana. In this study we clarified the physiological function of COPT5. This carrier is located in the tonoplast and functions as a vacuolar copper exporter. Mutants lacking this transporter have altered copper contents in different organs when compared with wild-type plants. We were able to detect copper accumulation in the root and a decreased copper content in siliques and seeds when the COPT5 gene is mutated by T-DNA insertion. Vacuoles purified from copt5 T-DNA-insertion mutants show remarkably increased copper concentrations compared with wild-type organelles. We assume that on the cellular level COPT5 is important for copper export from the vacuole and on the level of the whole plant it is involved in the interorgan reallocation of copper ions from the root to reproductive organs.

The role of transition metal homeostasis in plant seed development

For human health, transition metal accumulation in edible seeds like cereal grains is of worldwide importance, since Fe and Zn deficiencies are among the most prevalent human nutritional disorders in the world. There have been many recent developments in our understanding of the patterns in which transition metals accumulate in the seeds, the identity of some specific transporters that are required for efficient seed metal accumulation, and the central role played by the ubiquitous plant metal chelator nicotianamine (NA). These and other recent discoveries will be reviewed here.

Transporters contributing to iron trafficking in plants

This review will discuss recent progress in understanding the many roles of transporters in the whole-plant physiological processes that maintain iron (Fe) homeostasis. These processes include uptake from the soil via roots, control of transport from roots to above-ground parts of the plant, unloading of Fe from the xylem in above-ground parts, loading of Fe into mitochondria and plastids, transport of Fe to reproductive parts of the plant, and Fe mobilization during seed germination. In addition, we will discuss the mechanisms that plants use to cope with an apparently unintended consequence of Fe acquisition: the uptake of toxic heavy metals via Fe transporters. Rapid progress has been made in understanding the transport processes involved in each of these areas in the last 5 years and this review will focus on this recent progress. We will also highlight the key questions regarding transport steps that remain to be elucidated.

Characterization of (68)Zn uptake, translocation, and accumulation into developing grains and young leaves of high Zn-density rice genotype

Zinc (Zn) is an essential micronutrient for humans, but Zn deficiency has become serious as equally as iron (Fe) and vitamin A deficiencies nowadays. Selection and breeding of high Zn-density crops is a suitable, cost-effective, and sustainable way to improve human health. However, the mechanism of high Zn density in rice grain is not fully understood, especially how Zn transports from soil to grains. Hydroponics experiments were carried out to compare Zn uptake and distribution in two different Zn-density rice genotypes using stable isotope technique. At seedling stage, IR68144 showed higher (68)Zn uptake and transport rate to the shoot for the short-term, but no significant difference was observed in both genotypes for the long-term. Zn in xylem sap of IR68144 was consistently higher, and IR68144 exhibited higher Zn absorption ratio than IR64 at sufficient (2.0 µmol/L) or surplus (8.0 µmol/L) Zn supply level. IR64 and IR68144 showed similar patterns of (68)Zn accumulation in new leaves at seedling stage and in developing grains at ripening stage, whereas (68)Zn in new leaves and grains of IR68144 was consistently higher. These results suggested that a rapid root-to-shoot translocation and enhanced xylem loading capacity may be the crucial processes for high Zn density in rice grains.

Moving micronutrients from the soil to the seeds: genes and physiological processes from a biofortification perspective

The micronutrients iron (Fe), zinc (Zn), and copper (Cu) are essential for plants and the humans and animals that consume plants. Increasing the micronutrient density of staple crops, or biofortification, will greatly improve human nutrition on a global scale. This review discusses the processes and genes needed to translocate micronutrients through the plant to the developing seeds, and potential strategies for developing biofortified crops.

Successful reproduction requires the function of Arabidopsis Yellow Stripe-Like1 and Yellow Stripe-Like3 metal-nicotianamine transporters in both vegetative and reproductive structures

Several members of the Yellow Stripe-Like (YSL) family of proteins are transporters of metals that are bound to the metal chelator nicotianamine or the related set of mugineic acid family chelators known as phytosiderophores. Here, we examine the physiological functions of three closely related Arabidopsis (Arabidopsis thaliana) YSL family members, AtYSL1, AtYSL2, and AtYSL3, to elucidate their role(s) in the allocation of metals into various organs of Arabidopsis. We show that AtYSL3 and AtYSL1 are localized to the plasma membrane and function as iron transporters in yeast functional complementation assays. By using inflorescence grafting, we show that AtYSL1 and AtYSL3 have dual roles in reproduction: their activity in the leaves is required for normal fertility and normal seed development, while activity in the inflorescences themselves is required for proper loading of metals into the seeds. We further demonstrate that the AtYSL1 and AtYSL2 proteins, when expressed from the AtYSL3 promoter, can only partially rescue the phenotypes of a ysl1ysl3 double mutant, suggesting that although these three YSL transporters are closely related and have similar patterns of expression, they have distinct activities in planta. In particular, neither AtYSL1 nor AtYSL2 is able to functionally complement the reproductive defects exhibited by ysl1ysl3 double mutant plants.

Biofortification and localization of zinc in wheat grain

Zinc (Zn) deficiency associated with low dietary intake is a well-documented public health problem, resulting in serious health and socioeconomic problems. Field experiments were conducted with wheat to test the role of both soil and foliar application of ZnSO4 in Zn concentration of whole grain and grain fractions (e.g., bran, embryo and endosperm) in 3 locations. Foliar application of ZnSO4 was realized at different growth stages (e.g., stem elongation, boot, milk, dough stages) to study the effect of timing of foliar Zn application on grain Zn concentration. The rate of foliar Zn application at each growth stage was 4 kg of ZnSO4·7H2O ha(-1). Laser ablation (LA)-ICP-MS was used to follow the localization of Zn within grain. Soil Zn application at a rate of 50 kg of ZnSO4·7H2O ha(-1) was effective in increasing grain Zn concentration in the Zn-deficient location, but not in the locations without soil Zn deficiency. In all locations, foliar application of Zn significantly increased Zn concentration in whole grain and in each grain fraction, particularly in the case of high soil N fertilization. In Zn-deficient location, grain Zn concentration increased from 11 mg kg(-1) to 22 mg kg(-1) with foliar Zn application and to 27 mg kg(-1) with a combined application of ZnSO4 to soil and foliar. In locations without soil Zn deficiency, combination of high N application with two times foliar Zn application (e.g., at the booting and milk stages) increased grain Zn concentration, on average, from 28 mg kg(-1) to 58 mg kg(-1). Both ICP-OES and LA-ICP-MS data showed that the increase in Zn concentration of whole grain and grain fractions was pronounced when Zn was sprayed at the late growth stage (e.g., milk and dough). LA-ICP-MS data also indicated that Zn was transported into endosperm through the crease phloem. To our knowledge, this is the first study to show that the timing of foliar Zn application is of great importance in increasing grain Zn in wheat, especially in the endosperm part that is the predominant grain fraction consumed in many countries. Providing a large pool of Zn in vegetative tissues during the grain filling (e.g., via foliar Zn spray) is an important practice to increase grain Zn and contribute to human nutrition.

Natural genetic variation in selected populations of Arabidopsis thaliana is associated with ionomic differences

Controlling elemental composition is critical for plant growth and development as well as the nutrition of humans who utilize plants for food. Uncovering the genetic architecture underlying mineral ion homeostasis in plants is a critical first step towards understanding the biochemical networks that regulate a plant's elemental composition (ionome). Natural accessions of Arabidopsis thaliana provide a rich source of genetic diversity that leads to phenotypic differences. We analyzed the concentrations of 17 different elements in 12 A. thaliana accessions and three recombinant inbred line (RIL) populations grown in several different environments using high-throughput inductively coupled plasma- mass spectroscopy (ICP-MS). Significant differences were detected between the accessions for most elements and we identified over a hundred QTLs for elemental accumulation in the RIL populations. Altering the environment the plants were grown in had a strong effect on the correlations between different elements and the QTLs controlling elemental accumulation. All ionomic data presented is publicly available at www.ionomicshub.org.

Quantitative trait loci affecting seed mineral concentrations in Brassica napus grown with contrasting phosphorus supplies

BACKGROUND AND AIMS

Phosphorus (P) deficiency is one of the major limitations for crop production. A significant relationship exists between plant P uptake from soils and the accumulation of P and other mineral elements in seeds. The aims of this study were to identify and characterize genetic loci (QTLs) controlling the accumulation of mineral elements in seeds of Brassica napus grown with contrasting P availabilities.

METHODS

A population of 124 recombinant inbred lines derived from a cross between P-inefficient 'B104-2' and P-efficient 'Eyou Changjia' was used for phenotypic investigation and QTL analysis. Two-year field trials were conducted with two P treatments. Concentrations of mineral elements (P, Ca, Mg, Fe, Zn, Cu and Mn) in seeds were determined and QTLs were identified by composite interval mapping.

KEY RESULTS

There was significant genetic variation in seed concentrations of all mineral elements, and correlations between most elements were observed. A total of 78 putative QTLs (45 under the normal-P conditions and 33 under the low-P conditions) were detected, covering 17 linkage groups and accounting for 7.84-18.1 % of the phenotypic variation. Sixteen of these QTLs were identified in at least two environments, and co-location of QTLs for different mineral traits was found on several linkage groups. By in silico mapping, 21 genes involved in ion homeostasis in Arabidopsis were mapped to the QTL intervals identified in B. napus.

CONCLUSIONS

The accumulation of mineral elements in seeds is controlled by multiple genes. Common physiological and molecular mechanisms could be involved in the accumulation of several mineral elements, and genes involved in these processes in B. napus are suggested. These results offer insights to the genetic basis of seed mineral accumulation across different P levels in B. napus.

Rice metal-nicotianamine transporter, OsYSL2, is required for the long-distance transport of iron and manganese

Rice (Oryza sativa) is indispensable in the diet of most of the world's population. Thus, it is an important target in which to alter iron (Fe) uptake and homeostasis, so as to increase Fe accumulation in the grain. We previously isolated OsYSL2, a functional iron [Fe(II)]- and manganese [Mn(II)]-nicotianamine complex transporter that is expressed in phloem cells and developing seeds. We produced RNAi (OsYSL2i) and overexpression lines (OXOsYSL2) of OsYSL2. At the vegetative stage in an OsYSL2i line, the Fe and Mn concentrations were decreased in the shoots, and the Fe concentration was increased in the roots. At the reproductive stage, positron-emitting tracer imaging system analysis revealed that Fe translocation to the shoots and seeds was suppressed in OsYSL2i. The Fe and Mn concentrations were decreased in the seeds of OsYSL2i, especially in the endosperm. Moreover, the Fe concentration in OXOsYSL2 was lower in the seeds and shoots, but higher in the roots, compared with the wild type. Furthermore, when OsYSL2 expression was driven by the sucrose transporter promoter, the Fe concentration in the polished rice was up to 4.4-fold higher compared with the wild type. These results indicate that the altered expression of OsYSL2 changes the localization of Fe, and that OsYSL2 is a critical Fe-nicotianamine transporter important for Fe translocation, especially in the shoots and endosperm.

Identification of up-regulated genes in flag leaves during rice grain filling and characterization of OsNAC5, a new ABA-dependent transcription factor

Rice is a poor source of micronutrients such as iron and zinc. To help clarify the molecular mechanisms that regulate metal mobilization from leaves to developing seeds, we conducted suppression subtractive hybridization analysis in flag leaves of two rice cultivars. Flag leaves are the major source of remobilized metals for developing seeds. We isolated 78 sequences up-regulated in flag leaves at the grain filling stage relative to the panicle exertion stage. Differential expression of selected genes (encoding 7 transport proteins, the OsNAS3 enzyme and the OsNAC5 transcription factor) was confirmed by quantitative RT-PCR. We show that OsNAC5 expression is up-regulated by natural (aging) and induced senescence processes (dark, ABA application, high salinity, cold and Fe-deficiency) and its expression is not affected in the presence of 6-benzylaminopurine (a senescence inhibitor) under dark-induced senescence. Salt induction of OsNAC5 expression is abolished by nicotinamide, an inhibitor of ABA effects. This result and the presence of cis-acting elements in the promoter region of the OsNAC5 gene suggest an ABA-dependent regulation. Using four different rice cultivars, we show that OsNAC5 up-regulation is higher and earlier in flag leaves and panicles of IR75862 plants, which have higher seed concentrations of Fe, Zn and protein. We suggest that OsNAC5 is a novel senescence-associated ABA-dependent NAC transcription factor and its function could be related to Fe, Zn and amino acids remobilization from green tissues to seeds.

Post-translational regulation of AtFER2 ferritin in response to intracellular iron trafficking during fruit development in Arabidopsis

Ferritins are major players in plant iron homeostasis. Surprisingly, their overexpression in transgenic plants led only to a moderate increase in seed iron content, suggesting the existence of control checkpoints for iron loading and storage in seeds. This work reports the identification of two of these checkpoints. First, measurement of seed metal content during fruit development in Arabidopsis thaliana reveals a similar dynamic of loading for Fe, Mn, Cu, and Zn. The step controlling metal loading into the seed occurs by the regulation of transport from the hull to the seed. Second, metal loading and ferritin abundance were monitored in different genetic backgrounds affected in vacuolar iron transport (AtVIT1, AtNRAMP3, AtNRAMP4) or plastid iron storage (AtFER1 to 4). This approach revealed (1) a post-translational regulation of ferritin accumulation in seeds, and (2) that ferritin stability depends on the balance of iron allocation between vacuoles and plastids. Thus, the success of ferritin overexpression strategies for iron biofortification, a promising approach to reduce iron-deficiency anemia in developing countries, would strongly benefit from the identification and engineering of mechanisms enabling the translocation of high amounts of iron into seed plastids.

Identification of QTL affecting seed mineral concentrations and content in the model legume Medicago truncatula

Increasing the amount of bioavailable micronutrients such as iron and zinc in plant foods for human consumption is an international goal, intended especially for developing countries where micronutrient deficiencies are an ongoing health risk. Legume seeds have the potential to provide the essential nutrients required by humans, but concentrations of several minerals are low when compared to other foods. In order to increase seed mineral concentrations, it is important to understand the genes and processes involved in mineral distribution within the plant. The main objectives of this study were to use a Medicago truncatula recombinant inbred population (Jemalong-6 x DZA 315.16) to determine loci governing seed mineral concentrations, seed mineral content, and average seed weight, and to use these loci to propose candidate genes whose expression might contribute to these traits. Ninety-three lines in 2004 and 169 lines in 2006 were grown for seed harvest and subsequent analysis of seed Ca, Cu, Fe, K, Mg, Mn, P, and Zn concentrations and content. Quantitative trait loci (QTL) cartographer was used to identify QTL using composite interval mapping (CIM). CIM identified 46 QTL for seed mineral concentration, 26 for seed mineral content, and 3 for average seed weight. At least one QTL was detected for each mineral trait, and colocation of QTL for several minerals was found in both years. Results comparing seed weight with seed mineral concentration and content QTL demonstrate that seed size can be an important determinant of seed mineral concentration. The identification, in this model legume, of transgressive segregation for nearly all the minerals suggests that allelic recombination of relevant mineral-related genes in agronomic legumes could be a successful strategy to increase seed mineral concentrations above current levels.

Essential transition metal homeostasis in plants

The homeostasis of the essential transition metals copper, iron, manganese and zinc requires balanced activities of transporters that mediate import into the cell, distribution to organelles and export from the cell. Transcriptional control is important for the regulation of cellular homeostasis. In the case of Fe and Cu much progress has been made in uncovering the regulatory networks that mediate homeostasis, and key transcription factors have now been described. A master regulator of Cu homeostasis in Arabidopsis thaliana, AtSPL7, is related to the Chlamydomonas master regulator CCR1, suggesting that the key switch is conserved between the two systems even though different sets of targets are regulated in the two systems.

Copper homeostasis

Copper (Cu) is a cofactor in proteins that are involved in electron transfer reactions and is an essential micronutrient for plants. Copper delivery is accomplished by the concerted action of a set of evolutionarily conserved transporters and metallochaperones. As a result of regulation of transporters in the root and the rarity of natural soils with high Cu levels, very few plants in nature will experience Cu in toxic excess in their tissues. However, low Cu bioavailability can limit plant productivity and plants have an interesting response to impending Cu deficiency, which is regulated by an evolutionarily conserved master switch. When Cu supply is insufficient, systems to increase uptake are activated and the available Cu is utilized with economy. A number of Cu-regulated small RNA molecules, the Cu-microRNAs, are used to downregulate Cu proteins that are seemingly not essential. On low Cu, the Cu-microRNAs are upregulated by the master Cu-responsive transcription factor SPL7, which also activates expression of genes involved in Cu assimilation. This regulation allows the most important proteins, which are required for photo-autotrophic growth, to remain active over a wide range of Cu concentrations and this should broaden the range where plants can thrive.

The analysis of Arabidopsis nicotianamine synthase mutants reveals functions for nicotianamine in seed iron loading and iron deficiency responses

Nicotianamine chelates and transports micronutrient metal ions in plants. It has been speculated that nicotianamine is involved in seed loading with micronutrients. A tomato (Solanum lycopersicum) mutant (chloronerva) and a tobacco (Nicotiana tabacum) transgenic line have been utilized to analyze the effects of nicotianamine loss. These mutants showed early leaf chlorosis and had sterile flowers. Arabidopsis (Arabidopsis thaliana) has four NICOTIANAMINE SYNTHASE (NAS) genes. We constructed two quadruple nas mutants: one had full loss of NAS function, was sterile, and showed a chloronerva-like phenotype (nas4x-2); another mutant, with intermediate phenotype (nas4x-1), developed chlorotic leaves, which became severe upon transition from the vegetative to the reproductive phase and upon iron (Fe) deficiency. Residual nicotianamine levels were sufficient to sustain the life cycle. Therefore, the nas4x-1 mutant enabled us to study late nicotianamine functions. This mutant had no detectable nicotianamine in rosette leaves of the reproductive stage but low nicotianamine levels in vegetative rosette leaves and seeds. Fe accumulated in the rosette leaves, while less Fe was present in flowers and seeds. Leaves, roots, and flowers showed symptoms of Fe deficiency, whereas leaves also showed signs of sufficient Fe supply, as revealed by molecular-physiological analysis. The mutant was not able to fully mobilize Fe to sustain Fe supply of flowers and seeds in the normal way. Thus, nicotianamine is needed for correct supply of seeds with Fe. These results are fundamental for plant manipulation approaches to modify Fe homeostasis regulation through alterations of NAS genes.

Wheat (Triticum aestivum) NAM proteins regulate the translocation of iron, zinc, and nitrogen compounds from vegetative tissues to grain

The NAM-B1 gene is a NAC transcription factor that affects grain nutrient concentrations in wheat (Triticum aestivum). An RNAi line with reduced expression of NAM genes has lower grain protein, iron (Fe), and zinc (Zn) concentrations. To determine whether decreased remobilization, lower plant uptake, or decreased partitioning to grain are responsible for this phenotype, mineral dynamics were quantified in wheat tissues throughout grain development. Control and RNAi wheat were grown in potting mix and hydroponics. Mineral (Ca, Cu, Fe, K, Mg, Mn, P, S, and Zn) and nitrogen (N) contents of organs were determined at regular intervals to quantify the net remobilization from vegetative tissues and the accumulation of nutrients in grain. Total nutrient accumulation was similar between lines, but grain Fe, Zn, and N were at lower concentrations in the NAM knockdown line. In potting mix, net remobilization of N, Fe, and Zn from vegetative tissues was impaired in the RNAi line. In hydroponics with ample nutrients, net remobilization was not observed, but grain Fe and Zn contents and concentrations remained lower in the RNAi line. When Fe or Zn was withheld post-anthesis, both lines demonstrated remobilization. These results suggest that a major effect of the NAM genes is an increased efflux of nutrients from the vegetative tissues and a higher partitioning of nutrients to grain.

Biofortification of crops with seven mineral elements often lacking in human diets--iron, zinc, copper, calcium, magnesium, selenium and iodine

The diets of over two-thirds of the world's population lack one or more essential mineral elements. This can be remedied through dietary diversification, mineral supplementation, food fortification, or increasing the concentrations and/or bioavailability of mineral elements in produce (biofortification). This article reviews aspects of soil science, plant physiology and genetics underpinning crop biofortification strategies, as well as agronomic and genetic approaches currently taken to biofortify food crops with the mineral elements most commonly lacking in human diets: iron (Fe), zinc (Zn), copper (Cu), calcium (Ca), magnesium (Mg), iodine (I) and selenium (Se). Two complementary approaches have been successfully adopted to increase the concentrations of bioavailable mineral elements in food crops. First, agronomic approaches optimizing the application of mineral fertilizers and/or improving the solubilization and mobilization of mineral elements in the soil have been implemented. Secondly, crops have been developed with: increased abilities to acquire mineral elements and accumulate them in edible tissues; increased concentrations of 'promoter' substances, such as ascorbate, beta-carotene and cysteine-rich polypeptides which stimulate the absorption of essential mineral elements by the gut; and reduced concentrations of 'antinutrients', such as oxalate, polyphenolics or phytate, which interfere with their absorption. These approaches are addressing mineral malnutrition in humans globally.

A strong effect of growth medium and organ type on the identification of QTLs for phytate and mineral concentrations in three Arabidopsis thaliana RIL populations

The regulation of mineral accumulation in plants is genetically complex, with several genetic loci involved in the control of one mineral and loci affecting the accumulation of different minerals. To investigate the role of growth medium and organ type on the genetics of mineral accumulation, two existing (LerxKond, LerxAn-1) and one new (LerxEri-1) Arabidopsis thaliana Recombinant Inbred Line populations were raised on soil and hydroponics as substrates. Seeds, roots, and/or rosettes were sampled for the determination of their Ca, Fe, K, Mg, Mn, P or Zn concentrations. For seeds only, the concentration of phytate (IP6), a strong chelator of seed minerals, was determined. Correlations between minerals/IP6, populations, growth conditions, and organs were determined and mineral/IP6 concentration data were used to identify quantitative trait loci (QTLs) for these traits. A striking difference was found between QTLs identified for soil-grown versus hydroponics-grown populations and between QTLs identified for different plant organs. Three common QTLs were identified for several populations, growth conditions, and organs, one of which corresponded to the ERECTA locus, variation of which has a strong effect on plant morphology.

Metal movement within the plant: contribution of nicotianamine and yellow stripe 1-like transporters

Background Since the identification of the genes controlling the root acquisition of iron (Fe), the control of inter- and intracellular distribution has become an important challenge in understanding metal homeostasis. The identification of the yellow stripe-like (YSL) transporter family has paved the way to decipher the mechanisms of long-distance transport of Fe. Scope Once in the plant, Fe will systematically react with organic ligands whose identity is poorly known so far. Among potential ligands, nicotianamine has been identified as an important molecule for the circulation and delivery of metals since it participates in the loading of copper (Cu) and nickel in xylem and prevents Fe precipitation in leaves. Nicotianamine is a precursor of phytosiderophores, which are high-affinity Fe ligands exclusively synthesized by Poaceae species and excreted by roots for the chelation and acquisition of Fe. Maize YS1 is the founding member of a family of membrane transporters called YS1-like (YSL), which functions in root Fe-phytosiderophore uptake from the soil. Next to this well-known Fe acquisition role, most of the other YSL family members are likely to function in plant-wide distribution of metals since (a) they are produced in vascular tissues throughout the plant and (b) they are found in non-Poaceae species that do not synthesize phytosiderophores. The hypothesized activity as Fe-nicotianamine transporters of several YSL members has been demonstrated experimentally by heterologous expression in yeast or by electrophysiology in Xenopus oocytes but, despite numerous attempts, proof of the arabidopsis YSL substrate specificity is still lacking. Reverse genetics, however, has revealed a role for AtYSL members in the remobilization of Cu and zinc from senescing leaves, in the formation of pollen and in the Fe, zinc and Cu loading of seeds. Conclusions Preliminary data on the YSL family of transporters clearly argues in favour of its role in the long-distance transport of metals through and between vascular tissues to eventually support gametogenesis and embryo development.

Zinc biofortification of cereals: problems and solutions

The goal of biofortification is to develop plants that have an increased content of bioavailable nutrients in their edible parts. Cereals serve as the main staple food for a large proportion of the world population but have the shortcoming, from a nutrition perspective, of being low in zinc and other essential nutrients. Major bottlenecks in plant biofortification appear to be the root-shoot barrier and--in cereals--the process of grain filling. New findings demonstrate that the root-shoot distribution of zinc is controlled mainly by heavy metal transporting P1B-ATPases and the metal tolerance protein (MTP) family. A greater understanding of zinc transport is important to improve crop quality and also to help alleviate accumulation of any toxic metals.

Quantitative trait locus mapping for seed mineral concentrations in two Arabidopsis thaliana recombinant inbred populations

Biofortification of foods, achieved by increasing the concentrations of minerals such as iron (Fe) and zinc (Zn), is a goal of plant scientists. Understanding genes that influence seed mineral concentration in a model plant such as Arabidopsis could help in the development of nutritionally enhanced crop cultivars. Quantitative trait locus (QTL) mapping for seed concentrations of calcium (Ca), copper (Cu), Fe, potassium (K), magnesium (Mg), manganese (Mn), phosphorus (P), sulfur (S), and Zn was performed using two recombinant inbred line (RIL) populations, Columbia (Col) x Landsberg erecta (Ler) and Cape Verde Islands (Cvi) x Ler, grown on multiple occasions. QTL mapping was also performed using data from silique hulls and the ratio of seed:hull mineral concentration of the Cvi x Ler population. Over 100 QTLs that affected seed mineral concentration were identified. Twenty-nine seed QTLs were found in more than one experiment, and several QTLs were found for both seed and hull mineral traits. A number of candidate genes affecting seed mineral concentration are discussed. These results indicate that A. thaliana is a suitable and convenient model for discovery of genes that affect seed mineral concentration. Some strong QTLs had no obvious candidate genes, offering the possibility of identifying unknown genes that affect mineral uptake and translocation to seeds.