Summary
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, β-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.

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

Toxic metal accumulation, responses to exposure and mechanisms of tolerance in plants

Iron is essential for the survival and proliferation of all plants. Higher plants have developed two distinct strategies to acquire iron, which is only slightly soluble, from the rhizosphere: the reduction strategy of nongraminaceous plants and the chelation strategy of graminaceous plants. Key molecular components—including transporters, enzymes, and chelators—have been clarified for both strategies, and many of these components are now thought to also function inside the plant to facilitate internal iron transport. Transporters for intracellular iron trafficking are also being clarified. A majority of genes encoding these components are transcriptionally regulated in response to iron availability. Recent research has uncovered central transcription factors, cis-acting elements, and molecular mechanisms regulating these genes. Manipulation of these molecular components has produced transgenic crops with enhanced tolerance to iron deficiency or with increased iron content in the edible parts.

https://www.uv.mx/personal/tcarmona/files/2010/08/Kobayashi-Y-Nishizawa-2012.pdf

Iron Uptake, Translocation, and Regulation in Higher Plants

Paddy rice (Oryza sativa) is able to accumulate high concentrations of Mn without showing toxicity; however, the molecular mechanisms underlying Mn uptake are unknown. Here, we report that a member of the Nramp (for the Natural Resistance-Associated Macrophage Protein) family, Nramp5, is involved in Mn uptake and subsequently the accumulation of high concentrations of Mn in rice. Nramp5 was constitutively expressed in the roots and encodes a plasma membrane–localized protein. Nramp5 was polarly localized at the distal side of both exodermis and endodermis cells. Knockout of Nramp5 resulted in a significant reduction in growth and grain yield, especially when grown at low Mn concentrations. This growth reduction could be partially rescued by supplying high concentrations of Mn but not by the addition of Fe. Mineral analysis showed that the concentration of Mn and Cd in both the roots and shoots was lower in the knockout line than in wild-type rice. A short-term uptake experiment revealed that the knockout line lost the ability to take up Mn and Cd. Taken together, Nramp5 is a major transporter of Mn and Cd and is responsible for the transport of Mn and Cd from the external solution to root cells.

/pdf/nramp5-is-a-major-transporter-responsible-for-manganese-and-3g4zm2duk0.pdf

Nramp5 Is a Major Transporter Responsible for Manganese and Cadmium Uptake in Rice

Heavy metal contamination of soil and water causing toxicity/stress has become one important constraint to crop productivity and quality. This situation has further worsened by the increasing population growth and inherent food demand. It have been reported in several studies that counterbalancing toxicity, due to heavy metal requires complex mechanisms at molecular, biochemical, physiological, cellular, tissue and whole plant level, which might manifest in terms of improved crop productivity. Recent advances in various disciplines of biological sciences such as metabolomics, transcriptomics, proteomics etc. have assisted in the characterization of metabolites, transcription factors, stress-inducible proteins involved in heavy metal tolerance, which in turn can be utilized for generating heavy metal tolerant crops. This review summarizes various tolerance strategies of plants under heavy metal toxicity, covering the role of metabolites (metabolomics), trace elements (ionomics), transcription factors (transcriptomics), various stress-inducible proteins (proteomics) as well as the role of plant hormones. We also provide a glance at strategies adopted by metal accumulating plants also known as “metallophytes”.

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Heavy Metal Tolerance in Plants: Role of Transcriptomics, Proteomics, Metabolomics, and Ionomics.

Iron (Fe) is necessary for all living cells, but its bioavailability is often limited. Fe deficiency limits agriculture in many areas and affects over a billion human beings worldwide. In mammals, NRAMP2/DMT1/DCT1 was identified as a major pathway for Fe acquisition and recycling. In plants, AtNRAMP3 and AtNRAMP4 are induced under Fe deficiency. The similitude of AtNRAMP3 and AtNRAMP4 expression patterns and their common targeting to the vacuole, together with the lack of obvious phenotype in nramp3-1 and nramp4-1 single knockout mutants, suggested a functional redundancy. Indeed, the germination of nramp3 nramp4 double mutants is arrested under low Fe nutrition and fully rescued by high Fe supply. Mutant seeds have wild type Fe content, but fail to retrieve Fe from the vacuolar globoids. Our work thus identifies for the first time the vacuole as an essential compartment for Fe storage in seeds. Our data indicate that mobilization of vacuolar Fe stores by AtNRAMP3 and AtNRAMP4 is crucial to support Arabidopsis early development until efficient systems for Fe acquisition from the soil take over.

/pdf/mobilization-of-vacuolar-iron-by-atnramp3-and-atnramp4-is-3sfg9gduab.pdf

Mobilization of vacuolar iron by AtNRAMP3 and AtNRAMP4 is essential for seed germination on low iron

The LEAFY (LFY) protein is a key regulator of flower development in angiosperms. Its gradually increased expression governs the sharp floral transition, and LFY subsequently controls the patterning of flower meristems by inducing the expression of floral homeotic genes. Despite a wealth of genetic data, how LFY functions at the molecular level is poorly understood. Here, we report crystal structures for the DNA-binding domain of Arabidopsis thaliana LFY bound to two target promoter elements. LFY adopts a novel seven-helix fold that binds DNA as a cooperative dimer, forming base-specific contacts in both the major and minor grooves. Cooperativity is mediated by two basic residues and plausibly accounts for LFY's effectiveness in triggering sharp developmental transitions. Our structure reveals an unexpected similarity between LFY and helix-turn-helix proteins, including homeodomain proteins known to regulate morphogenesis in higher eukaryotes. The appearance of flowering plants has been linked to the molecular evolution of LFY. Our study provides a unique framework to elucidate the molecular mechanisms underlying floral development and the evolutionary history of flowering plants.

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Structural basis for LEAFY floral switch function and similarity with helix-turn-helix proteins

We demonstrate that the biophysical technique of surface plasmon resonance (SPR) analysis, which has previously been used to measure transcription factor binding to short DNA molecules, can also be used to characterize interactions involving entire gene promoters. This discovery has two main implications that relate, respectively, to novel qualitative and quantitative uses of the SPR technique. Firstly, SPR analysis can be used qualitatively to test the capacity of any transcription factor to interact physically with its putative target genes. This application should prove particularly useful for the confirmation of predicted transcriptional interactions in model species, and for comparative studies of non-model species in which transcriptional interactions are not amenable to study by other methods. Secondly, SPR may be used quantitatively to characterize interactions between transcription factors and gene promoters containing multiple cis-acting sites. This application should prove useful for the detailed dissection of promoter function in known target genes. The qualitative and quantitative applications of the SPR analysis of whole promoters combine to make this a uniquely powerful technique, which should prove particularly useful in systems biology, evolutionary developmental biology and various branches of applied biology.

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Cécile Hamès

Papers

Mobilization of vacuolar iron by AtNRAMP3 and AtNRAMP4 is essential for seed germination on low iron

Structural basis for LEAFY floral switch function and similarity with helix-turn-helix proteins

The analysis of entire gene promoters by surface plasmon resonance