OPT3 Is a Phloem-Specific Iron Transporter That Is Essential for Systemic Iron Signaling and Redistribution of Iron and Cadmium in Arabidopsis.

Iron is essential for both plant growth and human health and nutrition. Knowledge of the signaling mechanisms that communicate iron demand from shoots to roots to regulate iron uptake as well as the transport systems mediating iron partitioning into edible plant tissues is critical for the development of crop biofortification strategies. Here, we report that OPT3, previously classified as an oligopeptide transporter, is a plasma membrane transporter capable of transporting transition ions in vitro. Studies in Arabidopsis thaliana show that OPT3 loads iron into the phloem, facilitates iron recirculation from the xylem to the phloem, and regulates both shoot-to-root iron signaling and iron redistribution from mature to developing tissues. We also uncovered an aspect of crosstalk between iron homeostasis and cadmium partitioning that is mediated by OPT3. Together, these discoveries provide promising avenues for targeted strategies directed at increasing iron while decreasing cadmium density in the edible portions of crops and improving agricultural productivity in iron deficient soils.

Nutrient variability in phloem: examining changes in K, Mg, Zn and Fe concentration during grain loading in common wheat (Triticum aestivum).

In wheat, nutrients are transported to seeds via the phloem yet access to this vascular tissue for exudate collection and quantitative analysis of elemental composition is difficult. The purest phloem is collected through the use of aphid stylectomy with volumes of exudate collected normally in the range of 20-500 nl. In this work a new method using inductively coupled plasma mass spectroscopy (ICP-MS) was developed to measure the concentration of K, Mg, Zn and Fe in volumes of wheat (Triticum aestivum, genotype Samnyt 16) phloem as small as 15.5 nl. This improved method was used to observe changes in phloem nutrient concentration during the grain loading period. There were statistically significant increases in phloem Mg and Zn concentration and a significant decrease in K over the period from 1-2 days after anthesis (DAA) to 9-12 DAA. During this period, there was no statistically significant change in phloem Fe concentration.

COPT6 is a plasma membrane transporter that functions in copper homeostasis in Arabidopsis and is a novel target of SQUAMOSA promoter-binding protein-like 7.

Among the mechanisms controlling copper homeostasis in plants is the regulation of its uptake and tissue partitioning. Here we characterized a newly identified member of the conserved CTR/COPT family of copper transporters in Arabidopsis thaliana, COPT6. We showed that COPT6 resides at the plasma membrane and mediates copper accumulation when expressed in the Saccharomyces cerevisiae copper uptake mutant. Although the primary sequence of COPT6 contains the family conserved domains, including methionine-rich motifs in the extracellular N-terminal domain and a second transmembrane helix (TM2), it is different from the founding family member, S. cerevisiae Ctr1p. This conclusion was based on the finding that although the positionally conserved Met(106) residue in the TM2 of COPT6 is functionally essential, the conserved Met(27) in the N-terminal domain is not. Structure-function studies revealed that the N-terminal domain is dispensable for COPT6 function in copper-replete conditions but is important under copper-limiting conditions. In addition, COPT6 interacts with itself and with its homolog, COPT1, unlike Ctr1p, which interacts only with itself. Analyses of the expression pattern showed that although COPT6 is expressed in different cell types of different plant organs, the bulk of its expression is located in the vasculature. We also show that COPT6 expression is regulated by copper availability that, in part, is controlled by a master regulator of copper homeostasis, SPL7. Finally, studies using the A. thaliana copt6-1 mutant and plants overexpressing COPT6 revealed its essential role during copper limitation and excess.

Selenium accumulation in lettuce germplasm.

Selenium (Se) is an essential micronutrient for animals and humans. Increasing Se content in food crops offers an effective approach to reduce the widespread selenium deficiency problem in many parts of the world. In this study, we evaluated 30 diverse accessions of lettuce (Lactuca sativa L.) for their capacity to accumulate Se and their responses to different forms of Se in terms of plant growth, nutritional characteristics, and gene expression. Lettuce accessions responded differently to selenate and selenite treatment, and selenate is superior to selenite in inducing total Se accumulation. At least over twofold change in total Se levels between cultivars with high and low Se content was found. Synergistic relationship between Se and sulfur accumulation was observed in nearly all accessions at the selenate dosage applied. The change in shoot biomass varied between lettuce accessions and the forms of Se used. The growth-stimulated effect by selenate and the growth-inhibited effect by selenite were found to be correlated with the alteration of antioxidant enzyme activities. The different ability of lettuce accessions to accumulate Se following selenate treatment appeared to be associated with an altered expression of genes involved in Se/S uptake and assimilation. Our results provide important information for the effects of different forms of Se on plant growth and metabolism. They will also be of help in selecting and developing better cultivars for Se biofortification in lettuce.

The heavy metal budget of an urban rooftop farm.

Urban rooftop agriculture is a growing enterprise in the US with the goal of providing high quality, healthy, locally grown produce for city dwellers. However, air pollution abatement and the purification of stormwater are among the ecosystem services emphasized in studies of conventional green roofs. If rooftop farms actually capture pollutants, then accumulation of heavy metals in the soil could pose a problem over time. This study reports the heavy metal concentrations in soil, atmospheric deposition, and drainage output of 8 metals from the Brooklyn Grange Navy Yard Farm, rooftop vegetable farm in New York City, USA. Drainage of Pb and Mn were 6% and 14% of atmospheric bulk deposition, respectively, meaning that the Grange could be a net sink for Pb and Mn. Although there were small scale hotspots in the soil, farm-wide averages for heavy metal concentrations never exceeded guideline levels, and relatively low concentrations of Pb and Ba in the soil suggest that rooftop soils may be less vulnerable to contamination related to traffic and construction. In comparison to the growing seasons, we found relatively high concentrations of Pb and Cr in the soil during fallow periods when the soil was bare. To reduce the atmospheric deposition of heavy metals to soil, it is important to cover the soil with mulch, and discard the used mulch and unmarketable portion of vegetables, instead of recycling them via composting for soil amendments.

Anomalous bioaccumulation of lead in the earthworm Eisenoides lonnbergi (Michaelsen).

Lead concentrations in soil organisms are usually well below those in the associated soil and tend to decrease with each higher trophic level in a food chain. Earthworms of the species Eisenoides lonnbergi provide an exception to this observation, accumulating very high concentrations of lead from acidic soils. Earthworms belonging to this species were collected from strongly to extremely acidic soils at 16 sites on a wildlife refuge in Maryland, USA. A lead concentration as high as 766 mg/kg, dry weight, was detected in depurated E. lonnbergi collected from soil containing only 17 mg/kg of lead. Concentration factors (ratio of lead concentration in earthworms to lead concentration in soil, dry wt) were highly variable at the sites, from 1.0 to 83. As suggested previously, lead absorption by earthworms is enhanced in low-calcium soils. The anomalously high concentrations of lead found in E. lonnbergi are more closely correlated with the uptake of calcium from acidic soils than with bioaccessibility of soil lead. Environ Toxicol Chem 2018;37:914-919. Published 2017 Wiley Periodicals Inc. on behalf of SETAC. This article is a US government work and, as such, is in the public domain in the United States of America.

Evaluation of selected ultra-trace minerals in commercially available dry dog foods.

PURPOSE: To evaluate the concentrations of chromium, nickel, molybdenum, silica, and aluminum in several commercially available dry dog foods and compare these with current World Health Organization's (WHO) mean human daily dietary intake. Conversion of dietary intake per megacalorie (Mcal) for both dog foods and human average intake was performed based on the National Research Council recommendation of a 2,900 kcal diet for comparative purposes to average intake and potential toxic exposure. MATERIALS AND METHODS: Forty-nine over-the-counter dry foods formulated for maintenance of healthy dogs yet listed as all life stage foods were analyzed. Concentrations of the ultra-trace minerals were measured via inductively coupled plasma atomic emission and represented per Mcal for comparative purposes as it relates to common intake in dogs in comparison with humans. RESULTS: Chromium, molybdenum, and aluminum concentrations in all of the dog foods were at levels that would be considered above average human daily consumption on a caloric basis. Nickel and silica calculated intakes per Mcal were comparable with human intake patterns, while both trace minerals displayed outliers exceeding at least twofold of the upper range of human daily intake. CONCLUSION: Overall, ultra-trace minerals found in dog foods were above the expected average daily intake for humans on a caloric basis. There was no evidence of potential chronic toxic exposure based on presumptive intake extrapolated from WHO published toxic intake concentrations for humans or domestic animals. The large range of silica intake from various foods (2.96-83.67 mg/1,000 kcal) may have health implications in dogs prone to silica urolithiasis. Further studies investigating the bioavailability of these ultra-trace minerals and establishing dietary ultra-trace mineral allowance would be ideal; however, based on these findings, consumption of these ultra-trace minerals in over-the-counter dry dog foods appears safe.

Bioavailability of iron from spinach using an in vitro/human Caco-2 cell bioassay model.

Spinach (Spinacia oleracea) cv Whitney was tested for iron bioavailabilty using an in vitro human intestinal cell culture ferritin bioassay technique previously developed. Spinach was cultured in a growth chamber for 33 days, harvested, and freeze-dried. Total iron in the samples was an average of 71 micrograms/g dry weight. Spinach was digested in vitro (pepsin and 0.1 M HCl followed by pancreatin and 0.1 M NaHCO3) with and without the addition of supplemental ascorbic acid. Caco-2 cell cultures were used to determine iron bioavailability from the spinach mixtures. Production of the iron-binding protein ferritin in the Caco-2 cells showed the supplemental ascorbic acid doubled bioavailability of iron from spinach. The data show fresh spinach is a poor source of iron, and emphasize the importance of evaluation of whole meals rather than single food items. The data support the usefulness of the in vitro/Caco-2 cell ferritin bioassay model for prescreening of space flight diets for bioavailable iron.

Genotypic variation of zinc and selenium concentration in grains of Brazilian wheat lines.

Exploration of genetic resources for micronutrient concentrations facilitates the breeding of nutrient-dense crops, which is increasingly seen as an additional, sustainable strategy to combat global micronutrient deficiency. In this work, we evaluated genotypic variation in grain nutrient concentrations of 20 Brazil wheat (Triticum aestivum L.) accessions in response to zinc (Zn) and Zn plus selenium (Se) treatment. Zn and Se concentrations in grains exhibited 2- and 1.5-fold difference, respectively, between these wheat accessions. A variation of up to 3-fold enhancement of grain Zn concentration was observed when additionally Zn was supplied, indicating a wide range capacity of the wheat lines in accumulating Zn in grains. Moreover, grain Zn concentration was further enhanced in some lines following supply of Zn plus Se, showing stimulative effect by Se and the feasibility of simultaneous biofortification of Zn and Se in grains of some wheat lines. In addition, Se supply with Zn improved the accumulation of another important micronutrient, iron (Fe), in grains of half of these wheat lines, suggesting a beneficial role of simultaneous biofortification of Zn with Se. The significant diversity in these wheat accessions offers genetic potential for developing cultivars with better ability to accumulate important micronutrients in grains.

Evaluation of calcium, phosphorus, and selected trace mineral status in commercially available dry foods formulated for dogs.

OBJECTIVE: To evaluate concentrations of calcium, phosphorus, zinc, iron, copper, manganese, and selenium in several commercially available dry dog foods and compare these with current Association of American Feed Control Officials (AAFCO) recommendations for maintenance of healthy dogs. DESIGN: Descriptive study. SAMPLE: 45 over-the-counter dry foods formulated for maintenance of healthy dogs (ie, maintenance foods) and 5 therapeutic dry foods formulated for dogs with hepatic or renal disease. PROCEDURES: Mineral concentrations were measured via inductively coupled plasma mass spectrometry or inductively coupled plasma atomic emission spectroscopy and compared with AAFCO-recommended minimum and maximum values. RESULTS: Most (39/45) maintenance foods were in compliance with AAFCO recommendations for all mineral concentrations evaluated. Calcium concentration was > 7. 1 g/1,000 kcal of metabolizable energy (ME) in 4 of 45 maintenance foods, and phosphorus concentration was > 4.6 g/1,000 kcal ME in 3 of these; 2 maintenance foods contained < 34 mg of zinc/1,000 kcal ME. These values were not within AAFCO-recommended ranges. Calcium-to-phosphorus ratio in foods formulated for dogs with renal disease was above, and copper concentration in foods formulated for dogs with hepatic disease was below, recommended ranges for healthy dogs. CONCLUSIONS AND CLINICAL RELEVANCE: Calcium concentrations exceeded recommended limits in some maintenance foods labeled for all life stages, underscoring the need to feed diets appropriately formulated for specific life stages, particularly for large- and giant-breed puppies. Studies investigating the bioavailability of minerals are necessary before firm recommendations can be made.

The CTR/COPT-dependent copper uptake and SPL7-dependent copper deficiency responses are required for basal cadmium tolerance in A. thaliana.

Copper (Cu) homeostasis in plants is maintained by at least two mechanisms: (1) the miRNA-dependent reallocation of intracellular Cu among major Cu-enzymes and important energy-related functions; (2) the regulation of the expression of Cu transporters including members of the CTR/COPT family. These events are controlled by the transcription factor SPL7 in Arabidopsis thaliana. Cadmium (Cd), on the other hand, is a non-essential and a highly toxic metal that interferes with homeostasis of essential elements by competing for cellular binding sites. Whether Cd affects Cu homeostasis in plants is unknown. We found that Cd stimulates Cu accumulation in roots of A. thaliana and increases mRNA expression of three plasma membrane-localized Cu uptake transporters, COPT1, COPT2 and COPT6. Further analysis of Cd sensitivity of single and triple copt1copt2copt6 mutants, and transgenic plants ectopically expressing COPT6 suggested that Cu uptake is an essential component of Cd resistance in A. thaliana. Analysis of the contribution of the SPL7-dependent pathway to Cd-induced expression of COPT1, COPT2 and COPT6 showed that it occurs, in part, through mimicking the SPL7-dependent transcriptional Cu deficiency response. This response also involves components of the Cu reallocation system, miRNA398, FSD1, CSD1 and CSD2. Furthermore, seedlings of the spl7-1 mutant accumulate up to 2-fold less Cu in roots than the wild-type, are hypersensitive to Cd, and are more sensitive to Cd than the triple copt1copt2copt6 mutant. Together these data show that exposure to excess Cd triggers SPL7-dependent Cu deficiency responses that include Cu uptake and reallocation that are required for basal Cd tolerance in A. thaliana.

Genetic and physiological analysis of iron biofortification in maize kernels.

BACKGROUND: Maize is a major cereal crop widely consumed in developing countries, which have a high prevalence of iron (Fe) deficiency anemia. The major cause of Fe deficiency in these countries is inadequate intake of bioavailable Fe, where poverty is a major factor. Therefore, biofortification of maize by increasing Fe concentration and or bioavailability has great potential to alleviate this deficiency. Maize is also a model system for genomic research and thus allows the opportunity for gene discovery. Here we describe an integrated genetic and physiological analysis of Fe nutrition in maize kernels, to identify loci that influence grain Fe concentration and bioavailability. METHODOLOGY: Quantitative trait locus (QTL) analysis was used to dissect grain Fe concentration (FeGC) and Fe bioavailability (FeGB) from the Intermated B73 × Mo17 (IBM) recombinant inbred (RI) population. FeGC was determined by ion coupled argon plasma emission spectroscopy (ICP). FeGB was determined by an in vitro digestion/Caco-2 cell line bioassay. CONCLUSIONS: Three modest QTL for FeGC were detected, in spite of high heritability. This suggests that FeGC is controlled by many small QTL, which may make it a challenging trait to improve by marker assisted breeding. Ten QTL for FeGB were identified and explained 54% of the variance observed in samples from a single year/location. Three of the largest FeGB QTL were isolated in sister derived lines and their effect was observed in three subsequent seasons in New York. Single season evaluations were also made at six other sites around North America, suggesting the enhancement of FeGB was not specific to our farm site. FeGB was not correlated with FeGC or phytic acid, suggesting that novel regulators of Fe nutrition are responsible for the differences observed. Our results indicate that iron biofortification of maize grain is achievable using specialized phenotyping tools and conventional plant breeding techniques.

Dietary inulin supplementation does not promote colonic iron absorption in a porcine model.

Prebiotics may enhance iron bioavailability by increasing iron absorption in the colon. Anemic pigs fitted with cecal cannulas were fed a low-iron diet with or without 4% inulin. Over 7 days, pigs were administered 1 mg of (54)Fe in the morning feed followed by cannula infusion of 0.5 mg of (58)Fe to measure total and colonic iron absorption, respectively. Whole blood was drawn prior to the initial dosing and 14 days thereafter for hemoglobin concentration and stable isotope ratio analyses. The prebiotic role of inulin was confirmed by increases in lactobacilli and bifidobacteria with reductions in clostridia using terminal restriction fragment length polymorphism (TRFLP). Total iron absorption was 23.2 +/- 2.7 and 20.7 +/- 3.5% (mean +/- SEM; p > 0.05), while colonic iron absorption was 0.4 +/- 0.1 and 1.0 +/- 0.2% (mean +/- SEM; p > 0.05) in inulin-fed and control pigs, respectively. These results show that the colon does not make a significant contribution to total iron absorption in iron-deficient pigs and that inulin does not affect iron absorption in the colon.

Drosophila ABC transporter, DmHMT-1, confers tolerance to cadmium. DmHMT-1 and its yeast homolog, SpHMT-1, are not essential for vacuolar phytochelatin sequestration.

Half-molecule ATP-binding cassette transporters of the HMT-1 (heavy metal tolerance factor 1) subfamily are required for Cd2+ tolerance in Schizosaccharomyces pombe, Caenorhabditis elegans, and Chlamydomonas reinhardtii. Based on studies of S. pombe, it has been proposed that SpHMT-1 transports heavy metal.phytochelatin (PC) complexes into the vacuolysosomal compartment. PCs are glutathione derivatives synthesized by PC synthases (PCS) in plants, fungi, and C. elegans in response to heavy metals. Our previous studies in C. elegans, however, suggested that HMT-1 and PCS-1 do not necessarily act in concert in metal detoxification. To further explore this inconsistency, we have gone on to test whether DmHMT-1, an HMT-1 from a new source, Drosophila, whose genome lacks PCS homologs, functions in heavy metal detoxification. In so doing, we show that heterologously expressed DmHMT-1 suppresses the Cd2+ hypersensitivity of S. pombe hmt-1 mutants and localizes to the vacuolar membrane but does not transport Cd.PC complexes. Crucially, similar analyses of S. pombe hmt-1 mutants extend this finding to show that SpHMT-1 itself either does not transport Cd.PC complexes or is not the principal Cd.PC/apoPC transporter. Consistent with this discovery and with our previous suggestion that HMT-1 and PCS-1 do not operate in a simple linear metal detoxification pathway, we demonstrate that, unlike PCS-deficient cells, which are hypersensitive to several heavy metals, SpHMT-1-deficient cells are hypersensitive to Cd2+, but not to Hg2+ or As3+. These findings significantly change our current understanding of the function of HMT-1 proteins and invoke a PC-independent role for these transporters in Cd2+ detoxification.

Extrinsic labeling method may not accurately measure Fe absorption from cooked pinto beans (Phaseolus vulgaris): comparison of extrinsic and intrinsic labeling of beans.

Isotopic labeling of food has been widely used for the measurement of Fe absorption in determining requirements and evaluating the factors involved in Fe bioavailability. An extrinsic labeling technique will not accurately predict the total Fe absorption from foods unless complete isotopic exchange takes place between an extrinsically added isotope label and the intrinsic Fe of the food. We examined isotopic exchange in the case of both white beans and colored beans (Phaseolus vulgaris) with an in vitro digestion model. There are significant differences in (58)Fe/(56)Fe ratios between the sample digest supernatant and the pellet of extrinsically labeled pinto bean. The white bean digest shows significantly better equilibration of the extrinsic (58)Fe with the intrinsic (56)Fe. In contrast to the extrinsically labeled samples, both white and red beans labeled intrinsically with (58)Fe demonstrated consistent ratios of (58)Fe/(56)Fe in the bean meal, digest, supernatant, and pellet. It is possible that the polyphenolics in the bean seed coat may bind Fe and thus interfere with extrinsic labeling of the bean meals. These observations raise questions on the accuracy of studies that used extrinsic tags to measure Fe absorption from beans. Intrinsic labeling appears necessary to accurately measure Fe bioavailability from beans.