Tracy Punshon

Localization of Iron in Arabidopsis Seed Requires the Vacuolar Membrane Transporter VIT1

Iron deficiency is a major human nutritional problem wherever plant-based diets are common. Using synchrotron x-ray fluorescence microtomography to directly visualize iron in Arabidopsis seeds, we show that iron is localized primarily to the provascular strands of the embryo. This localization is completely abolished when the vacuolar iron uptake transporter VIT1 is disrupted. Vacuolar iron storage is also critical for seedling development because vit1-1 seedlings grow poorly when iron is limiting. We have uncovered a fundamental aspect of seed biology that will ultimately aid the development of nutrient-rich seed, benefiting both human health and agricultural productivity.

Rice consumption contributes to arsenic exposure in US women

Emerging data indicate that rice consumption may lead to potentially harmful arsenic exposure. However, few human data are available, and virtually none exist for vulnerable periods such as pregnancy. Here we document a positive association between rice consumption and urinary arsenic excretion, a biomarker of recent arsenic exposure, in 229 pregnant women. At a 6-mo prenatal visit, we collected a urine sample and 3-d dietary record for water, fish/seafood, and rice. We also tested womens home tap water for arsenic, which we combined with tap water consumption to estimate arsenic exposure through water. Women who reported rice intake (n = 73) consumed a median of 28.3 g/d, which is ∼0.5 cup of cooked rice each day. In general linear models adjusted for age and urinary dilution, both rice consumption (g, dry mass/d) and arsenic exposure through water (μg/d) were significantly associated with natural log-transformed total urinary arsenic (, , both P < 0.0001), as well as inorganic arsenic, monomethylarsonic acid, and dimethylarsinic acid (each P < 0.005). Based on total arsenic, consumption of 0.56 cup/d of cooked rice was comparable to drinking 1 L/d of 10 μg As/L water, the current US maximum contaminant limit. US rice consumption varies, averaging ∼0.5 cup/d, with Asian Americans consuming an average of >2 cups/d. Rice arsenic content and speciation also vary, with some strains predominated by dimethylarsinic acid, particularly those grown in the United States. Our findings along with others indicate that rice consumption should be considered when designing arsenic reduction strategies in the United States.

Arsenic, Organic Foods, and Brown Rice Syrup

Background: Rice can be a major source of inorganic arsenic (Asi) for many sub-populations. Rice products are also used as ingredients in prepared foods, some of which may not be obviously rice based. Organic brown rice syrup (OBRS) is used as a sweetener in organic food products as an alternative to high-fructose corn syrup. We hypothesized that OBRS introduces As into these products. Objective: We determined the concentration and speciation of As in commercially available brown rice syrups and in products containing OBRS, including toddler formula, cereal/energy bars, and high-energy foods used by endurance athletes. Methods: We used inductively coupled plasma mass spectrometry (ICP-MS) and ion chromatography coupled to ICP-MS to determine total As (Astotal) concentrations and As speciation in products purchased via the Internet or in stores in the Hanover, New Hampshire, area. Discussion: We found that OBRS can contain high concentrations of Asi and dimethyl-arsenate (DMA). An “organic” toddler milk formula containing OBRS as the primary ingredient had Astotal concentrations up to six times the U.S. Environmental Protection Agency safe drinking water limit. Cereal bars and high-energy foods containing OBRS also had higher As concentrations than equivalent products that did not contain OBRS. Asi was the main As species in most food products tested in this study. Conclusions: There are currently no U.S. regulations applicable to As in food, but our findings suggest that the OBRS products we evaluated may introduce significant concentrations of Asi into an individual’s diet. Thus, we conclude that there is an urgent need for regulatory limits on As in food.

Rice consumption and urinary arsenic concentrations in U.S. children.

Background: In adult populations, emerging evidence indicates that humans are exposed to arsenic by ingestion of contaminated foods such as rice, grains, and juice; yet little is known about arsenic exposure among children. Objectives: Our goal was to determine whether rice consumption contributes to arsenic exposure in U.S. children. Methods: We used data from the nationally representative National Health and Nutrition Examination Survey (NHANES) to examine the relationship between rice consumption (measured in 0.25 cups of cooked rice per day) over a 24-hr period and subsequent urinary arsenic concentration among the 2,323 children (6–17 years of age) who participated in NHANES from 2003 to 2008. We examined total urinary arsenic (excluding arsenobetaine and arsenocholine) and dimethylarsinic acid (DMA) concentrations overall and by age group: 6–11 years and 12–17 years. Results: The median [interquartile range (IQR)] total urinary arsenic concentration among children who reported consuming rice was 8.9 μg/L (IQR: 5.3–15.6) compared with 5.5 μg/L (IQR: 3.1–8.4) among those who did not consume rice. After adjusting for potentially confounding factors, and restricting the study to participants who did not consume seafood in the preceding 24 hr, total urinary arsenic concentration increased 14.2% (95% confidence interval: 11.3, 17.1%) with each 0.25 cup increase in cooked rice consumption. Conclusions: Our study suggests that rice consumption is a potential source of arsenic exposure in U.S. children.

Using synchrotron X-ray fluorescence microprobes in the study of metal homeostasis in plants.

BACKGROUND AND AIMS This Botanical Briefing reviews the application of synchrotron X-ray fluorescence (SXRF) microprobes to the plant sciences; how the technique has expanded our knowledge of metal(loid) homeostasis, and how it can be used in the future. SCOPE The use of SXRF microspectroscopy and microtomography in research on metal homeostasis in plants is reviewed. The potential use of SXRF as part of the ionomics toolbox, where it is able to provide fundamental information on the way that plants control metal homeostasis, is recommended. CONCLUSIONS SXRF is one of the few techniques capable of providing spatially resolved in-vivo metal abundance data on a sub-micrometre scale, without the need for chemical fixation, coating, drying or even sectioning of samples. This gives researchers the ability to uncover mechanisms of plant metal homeostasis that can potentially be obscured by the artefacts of sample preparation. Further, new generation synchrotrons with smaller beam sizes and more sensitive detection systems will allow for the imaging of metal distribution within single living plant cells. Even greater advances in our understanding of metal homeostasis in plants can be gained by overcoming some of the practical boundaries that exist in the use of SXRF analysis.

Phloem transport of arsenic species from flag leaf to grain during grain filling

• Strategies to reduce arsenic (As) in rice grain, below concentrations that represent a serious human health concern, require that the mechanisms of As accumulation within grain be established. Therefore, retranslocation of As species from flag leaves into filling rice grain was investigated. • Arsenic species were delivered through cut flag leaves during grain fill. Spatial unloading within grains was investigated using synchrotron X-ray fluorescence (SXRF) microtomography. Additionally, the effect of germanic acid (a silicic acid analog) on grain As accumulation in arsenite-treated panicles was examined. • Dimethylarsinic acid (DMA) and monomethylarsonic acid (MMA) were extremely efficiently retranslocated from flag leaves to rice grain; arsenate was poorly retranslocated, and was rapidly reduced to arsenite within flag leaves; arsenite displayed no retranslocation. Within grains, DMA rapidly dispersed while MMA and inorganic As remained close to the entry point. Germanic acid addition did not affect grain As in arsenite-treated panicles. Three-dimensional SXRF microtomography gave further information on arsenite localization in the ovular vascular trace (OVT) of rice grains. • These results demonstrate that inorganic As is poorly remobilized, while organic species are readily remobilized, from leaves to grain. Stem translocation of inorganic As may not rely solely on silicic acid transporters.

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.

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

This work identifies a physiological substrate and a physiological function of the Arabidopsis oligopeptide transporter, OPT3, in iron (Fe) homeostasis, provides a mechanistic explanation of the role of OPT3 in systemic Fe signaling, and uncovers an aspect of crosstalk between Fe homeostasis and cadmium partitioning. 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.

Heavy Metal Resistance and Accumulation Characteristics in Willows

ABSTRACT The resistance of Salix to Cu, Cd, Ni, and Zn was investigated in hydroponic culture, with phytoextraction potential evaluated for Cu. Root elongation (indicative of resistance level) was significantly affected, with considerable variation between and within individual clones. Resistance appeared to be clone- or hybrid-specific, rather than species-specific. S. caprea clones (and hybrids) were among the most resistant, but a secondary S. caprea clone from a different provenance was much less tolerant. S. viminalis and S. triandra clones were the most sensitive. Highest resistance was found in response to Cd, while Cu and Ni were extremely toxic. A resistant S. caprea ecotype originating from a metalliferous mine spoil was identified using this technique. Copper concentration reached a maximum of 2000, 400, and 82 μg g-1 (d.wt) in roots, wood, and foliage, respectively, after 1 month in hydroponic culture. The level of variation in the response of Salix to metals may cause difficulties in phytorem...

Arsenic concentration and speciation in infant formulas and first foods

Arsenic (As) exposure to humans is pervasive, and, increasingly, studies are revealing adverse health effects at ever lower doses. Drinking water is the main route of exposure for many individuals; however, food can be a significant source of As to individuals, especially if their diet is rice-based. Infants are particularly susceptible to dietary exposure, since many first foods contain rice and infants have a low body mass. Here we report on As concentration and speciation in infant formulas and first foods. Speciation is essential for food analysis because of the much greater toxicity of inorganic As species and the possibility that As in food (unlike water) may be present in either inorganic or organic forms. Infant milk formulas were low in total As (2.2–12.6 ng g–1, n = 15). Non-dairy formulas were significantly higher in As than dairy-based formulas. Arsenic in formula was almost exclusively inorganic. Arsenic concentration in purees (n = 41) and stage 2/3 foods (n = 18) ranged from 0.3 to 22 ng g–1. Rice-fortified foods had significantly higher total As concentrations than non-rice-based foods. Again, As speciation was predominantly inorganic with lower concentrations of dimethylarsenic acid (DMA) also present. These data confirm that infants are exposed to As via diet, and suggest that careful attention to diet choices may limit this exposure.