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.

How do plants respond to nutrient shortage by biomass allocation

On a role

Metal ions are essential to many chemical, biological, and environmental processes. In the past two decades, many DNA-based metal sensors have emerged. While the main biological role of DNA is to store genetic information, its chemical structure is ideal for metal binding via both the phosphate backbone and nucleobases. DNA is highly stable, cost-effective, easy to modify, and amenable to combinatorial selection. Two main classes of functional DNA were developed for metal sensing: aptamers and DNAzymes. While a few metal binding aptamers are known, it is generally quite difficult to isolate such aptamers. On the other hand, DNAzymes are powerful tools for metal sensing since they are selected based on catalytic activity, thus bypassing the need for metal immobilization. In the last five years, a new surge of development has been made on isolating new metal-sensing DNA sequences. To date, many important metals can be selectively detected by DNA often down to the low parts-per-billion level. Herein, each me...

Metal Sensing by DNA

Nonselective cation channels (NSCCs) catalyse passive fluxes of cations through plant membranes. NSCCs do not, or only to a small extent, select between monovalent cations, and several are also permeable to divalent cations. Although a number of NSCC genes has been identified in plant genomes, a direct correlation between gene products and in vivo observed currents is still largely absent for most NSCCs. In this review, physiological functions and molecular properties of NSCCs are critically discussed. Recent studies have demonstrated that NSCCs are directly involved in a multitude of stress responses, growth and development, uptake of nutrients and calcium signalling. NSCCs can also function in the perception of external stimuli and as signal transducers for reactive oxygen species, pathogen elicitors, cyclic nucleotides, membrane stretch, amino acids and purines.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.2007.02128.x

Physiological roles of nonselective cation channels in plants: from salt stress to signalling and development

Caesium (Cs + ) is a potentially toxic mineral element that is released into the environment and taken up by plants. Although Cs + is chemically similar to potassium (K + ), and much is known about K + transport mechanisms, it is not clear through which K + transport mechanisms Cs + is taken up by plant roots. In this study, the role of AtHAK5 in high affinity K + and Cs +

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The high affinity K+ transporter AtHAK5 plays a physiological role in planta at very low K+ concentrations and provides a caesium uptake pathway in Arabidopsis

Cesium (Cs) is chemically similar to potassium (K). However, although K is an essential element, Cs is toxic to plants. Two contrasting hypotheses to explain Cs toxicity have been proposed: (1) extracellular Cs+ prevents K+ uptake and, thereby, induces K starvation; and (2) intracellular Cs+ interacts with vital K+-binding sites in proteins, either competitively or noncompetitively, impairing their activities. We tested these hypotheses with Arabidopsis (Arabidopsis thaliana). Increasing the Cs concentration in the agar ([Cs]agar) on which Arabidopsis were grown reduced shoot growth. Increasing the K concentration in the agar ([K]agar) increased the [Cs]agar at which Cs toxicity was observed. However, although increasing [Cs]agar reduced shoot K concentration ([K]shoot), the decrease in shoot growth appeared unrelated to [K]shoot per se. Furthermore, the changes in gene expression in Cs-intoxicated plants differed from those of K-starved plants, suggesting that Cs intoxication was not perceived genetically solely as K starvation. In addition to reducing [K]shoot, increasing [Cs]agar also increased shoot Cs concentration ([Cs]shoot), but shoot growth appeared unrelated to [Cs]shoot per se. The relationship between shoot growth and [Cs]shoot/[K]shoot suggested that, at a nontoxic [Cs]shoot, growth was determined by [K]shoot but that the growth of Cs-intoxicated plants was related to the [Cs]shoot/[K]shoot quotient. This is consistent with Cs intoxication resulting from competition between K+ and Cs+ for K+-binding sites on essential proteins.

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Cesium Toxicity in Arabidopsis

Summary
• Ingestion of caesium (Cs) radioisotopes poses a health risk to humans. Crop varieties that accumulate less Cs in their edible tissues may provide a useful countermeasure. This study was performed to determine whether quantitative genetics on a model plant (Arabidopsis thaliana) might inform such ‘safe’-crop strategies.
• Arabidopsis accessions and recombinant inbred lines (RILs), from Landsberg erecta (Ler) × Cape Verdi Island (Cvi), Ler × Columbia (Col), and Niederzenz (Nd) × Col mapping populations, were grown on agar supplemented with subtoxic levels of Cs.
• Shoot Cs concentration varied up to three-fold, and shoot f. wt varied up to 25-fold within populations. The heritability of growth and Cs accumulation traits ranged from 0.06 to 0.28. Four quantitative trait loci (QTL) accounted for > 80% of the genetic contribution to the total phenotypic variation in shoot Cs concentration in the Ler × Col population.
• QTL identified in this study, in particular, QTL co-localizing to the top and bottom regions of Chromosomes I and V in two different mapping populations, are amenable to positional cloning and, through collinearity, may inform selection or breeding strategies for the development of ‘safe’ crops.

https://www.jstor.org/stable/pdfplus/1514523.pdf

Natural genetic variation in caesium (Cs) accumulation by Arabidopsis thaliana

Both theoretical models and pharmacological dissection suggest that Cs + influx to arabidopsis root cells occurs through voltage-insensitive cation channels (VICCs), encoded by members of the AtCNGC and AtGLR gene families, and 'high-affinity' K + /H + symporters (KUPs), encoded by members of the AtKUP/AtHAK gene family. When arabidopsis have sufficient K, it is observed that VICCs mediate most Cs + influx to root cells. However, KUPs contribute more to Cs + influx in roots of K-starved plants. This phenomenon has been attributed to an increased expression of AtHAK5 in roots of K-starved plants. Curiously, although arabidopsis mutants lacking some AtCNGCs show reduced Cs accumulation, mutants lacking other AtCNGCs accumulate more Cs in their shoot than wildtype plants. It is hypothesised, therefore, that the expression of genes encoding diverse K + -transporters might be altered to compensate for the absence of AtCNGCs that contribute significantly to cellular K homeostasis. Increased Cs + influx and accumulation could then be explained if the lack of an AtCNGC caused a physiological K-deficiency that increased the expression of AtKUPs. Such observations imply that the consequences of a simple genetic manipulation, such as the mis-expression of a AtCNGC gene, on Cs + influx and accumulation might not be predicted a priori. Finally, since AtCGNCs, AtGLRs and AtKUPs have contrasting Cs + :K + selectivities, and their relative expression is determined by diverse environmental variables, both the Cs:K ratio in plant tissues and the absolute rates of Cs + influx and accumulation will depend critically on environmental conditions. This will impact on strategies for phytoremediation and/or the development of 'safer' crops for radiocaesium-contaminated land.

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Short review: the mechanisms of radiocaesium uptake by Arabidopsis roots

The uptake and accumulation of toxic cations, including radionuclides, by plants growing on contaminated soils can adversely affect the health of humans and livestock. Using natural genetic variation and molecular-based quantitative genetic approaches, it is possible to identify chromosomal loci that underpin genetic variation in plant shoot radionuclide accumulation. Resolving these loci could allow the identification of candidate genes impacting on shoot radionuclide accumulation. Such methods enable gene-based crop selection or improvement strategies to be contemplated to either (1) exclude radionuclides from the food chain to minimize health risks or (2) enhance radionuclide phytoextraction. Using radiocesium (137Cs) as a case study, this chapter provides an overview of how natural genetic variation and quantitative trait loci approaches in a model plant species, Arabidopsis thaliana, can be used to identify candidate genes/genetic loci impacting on radionuclide accumulation by plants.

Corrina R. Hampton

Papers

The high affinity K+ transporter AtHAK5 plays a physiological role in planta at very low K+ concentrations and provides a caesium uptake pathway in Arabidopsis

Cesium Toxicity in Arabidopsis

Natural genetic variation in caesium (Cs) accumulation by Arabidopsis thaliana

Short review: the mechanisms of radiocaesium uptake by Arabidopsis roots

Using Quantitative Trait Loci Analysis to Select Plants for Altered Radionuclide Accumulation