Selenium promotes sulfur accumulation and plant growth in wheat (Triticum aestivum).

Selenium (Se) is an essential micronutrient for animals and humans and a target for biofortification in crops. Sulfur (S) is a crucial nutrient for plant growth. To gain better understanding of Se and S nutrition and interaction in plants, the effects of Se dosages and forms on plant growth as well as on S level in seven wheat lines were examined. Low dosages of both selenate and selenite supplements were found to enhance wheat shoot biomass and show no inhibitory effect on grain production. The stimulation on plant growth was correlated with increased APX antioxidant enzyme activity. Se forms were found to exert different effects on S metabolism in wheat plants. Selenate treatment promoted S accumulation, which was not observed with selenite supplement. An over threefold increase of S levels following selenate treatment at low dosages was observed in shoots of all wheat lines. Analysis of the sulfate transporter gene expression revealed an increased transcription of SULTR1;1, SULTR1;3 and SULTR4;1 in roots following 10 µM Na2 SeO4 treatment. Mass spectrometry-based targeted protein quantification confirmed the gene expression results and showed enhanced protein levels. The results suggest that Se treatment mimics S deficiency to activate specific sulfate transporter expression to stimulate S uptake, resulting in the selenate-induced S accumulation. This study supports that plant growth and nutrition benefit from low dosages of Se fertilization and provides information on the basis underlying Se-induced S accumulation in plants.

Higher iron pearl millet (Pennisetum glaucum L.) provides more absorbable iron that is limited by increased polyphenolic content.

BACKGROUND: Our objective was to compare the capacity of iron (Fe) biofortified and standard pearl millet (Pennisetum glaucum L.) to deliver Fe for hemoglobin (Hb)-synthesis. Pearl millet (PM) is common in West-Africa and India, and is well adapted to growing areas characterized by drought, low-soil fertility, and high-temperature. Because of its tolerance to difficult growing conditions, it can be grown in areas where other cereal crops, such as maize, would not survive. It accounts for approximately 50% of the total world-production of millet. Given the widespread use of PM in areas of the world affected by Fe-deficiency, it is important to establish whether biofortified-PM can improve Fe-nutriture. METHODS: Two isolines of PM, a low-Fe-control ("DG-9444", Low-Fe) and biofortified ("ICTP-8203 Fe",High-Fe) in Fe (26 µg and 85 µg-Fe/g, respectively) were used. PM-based diets were formulated to meet the nutrient requirements for the broiler (Gallus-gallus) except for Fe (Fe concentrations were 22.1±0.52 and 78.6±0.51 µg-Fe/g for the Low-Fe and High-Fe diets, respectively). For 6-weeks, Hb, feed-consumption and body-weight were measured (n = 12). RESULTS: Improved Fe-status was observed in the High-Fe group, as suggested by total-Hb-Fe values (15.5±0.8 and 26.7±1.4 mg, Low-Fe and High-Fe respectively, P<0.05). DMT-1, DcytB, and ferroportin mRNA-expression was higher (P<0.05) and liver-ferritin was lower (P>0.05) in the Low-Fe group versus High-Fe group. In-vitro comparisons indicated that the High-Fe PM should provide more absorbable-Fe; however, the cell-ferritin values of the in-vitro bioassay were very low. Such low in-vitro values, and as previously demonstrated, indicate the presence of high-levels of polyphenolic-compounds or/and phytic-acid that inhibit Fe-absorption. LC/MS-analysis yielded 15 unique parent aglycone polyphenolic-compounds elevated in the High-Fe line, corresponding to m/z = 431.09. CONCLUSIONS: The High-Fe diet appeared to deliver more absorbable-Fe as evidenced by the increased Hb and Hb-Fe status. Results suggest that some PM varieties with higher Fe contents also contain elevated polyphenolic concentrations, which inhibit Fe-bioavailability. Our observations are important as these polyphenols-compounds represent potential targets which can perhaps be manipulated during the breeding process to yield improved dietary Fe-bioavailability. Therefore, the polyphenolic and phytate profiles of PM must be carefully evaluated in order to further improve the nutritional benefit of this crop.

Polyphenolic compounds appear to limit the nutritional benefit of biofortified higher iron black bean (Phaseolus vulgaris L.).

BACKGROUND: Our objective was to determine if a biofortified variety of black bean can provide more bioavailable-iron (Fe) than a standard variety. Two lines of black beans (Phaseolus-vulgaris L.), a standard (DOR500; 59µg Fe/g) and biofortified (MIB465; 88µg Fe/g) were used. The DOR500 is a common commercial variety, and the MIB465 is a line developed for higher-Fe content. Given the high prevalence of Fe-deficiency anemia worldwide, it is important to determine if Fe-biofortified black beans can provide more absorbable-Fe. METHODS: Black bean based diets were formulated to meet the nutrient requirements for the broiler (Gallus-gallus) except for Fe (dietary Fe-concentrations were 39.4±0.2 and 52.9±0.9 mg/kg diet, standard vs. biofortified, respectively). Birds (n=14) were fed the diets for 6-weeks. Hemoglobin-(Hb), liver-ferritin and Fe-related transporter/enzyme gene-expression were measured. Hemoglobin-maintenance-efficiency and total-body-Hb-Fe values were used to estimate Fe-bioavailability. RESULTS: Hemoglobin-maintenance-efficiency values were higher (P<0.05) in the group consuming the standard-Fe beans on days 14, 21 and 28; indicating a compensatory response to lower dietary-Fe. Final total-Hb-Fe body content was higher in the biofortified vs. the standard group (26.6±0.9 and 24.4±0.8 mg, respectively; P<0.05). There were no differences in liver-ferritin or in expression of DMT-1, Dcyt-B, and ferroportin. In-vitro Fe-bioavailability assessment indicated very low Fe-bioavailability from both diets and between the two bean varieties (P>0.05). Such extremely-low in-vitro Fe-bioavailability measurement is indicative of the presence of high levels of polyphenolic-compounds that may inhibit Fe-absorption. High levels of these compounds would be expected in the black bean seed-coats. CONCLUSIONS: The parameters of Fe-status measured in this study indicate that only a minor increase in absorbable-Fe was achieved with the higher-Fe beans. The results also raise the possibility that breeding for increased Fe-concentration elevated the levels of polyphenolic-compounds that can reduce bean Fe-bioavailability, although the higher levels of polyphenolics in the higher-Fe beans may simply be coincidental or an environmental effect. Regardless, Fe-biofortified beans remain a promising vehicle for increasing intakes of bioavailable-Fe in human populations that consume high levels of these beans as a dietary staple, and the bean polyphenol profile must be further evaluated and modified if possible in order to improve the nutritional quality of higher-Fe beans.

Polyphenolic Profiles of Yellow Bean Seed Coats and Their Relationship with Iron Bioavailability.

Previous work with Caco-2 cell cultures has shown that individual polyphenols can either promote or inhibit iron uptake. This investigation was designed to characterize the relationship between iron bioavailability and seed coat polyphenol composition in a panel of 14 yellow beans representing five market classes with the potential for fast cooking time and high iron content. The study included two white and two red mottled bean lines, which represent high and low iron bioavailability capacity in dry beans, respectively. Polyphenols were measured quantitatively by high-performance liquid chromatography-mass spectrometry (HPLC-MS)/UV and iron bioavailability of seed coat extracts was measured in Caco-2 assays. Thirteen of the yellow bean seed types contained high concentrations (up to 35.3 ± 2.7 µmol/g) of kaempferol 3-glucoside (k 3-g), a known promoter of iron uptake. A general association between the ratio of promoting to inhibiting polyphenols (P/I) and iron uptake was observed. The presence of iron uptake inhibiting condensed tannins proportionately countered the promotional effects of kaempferol compounds. Unidentified factors present in seed coats other than polyphenols also appeared to affect iron uptake.

An In Vivo (Gallus gallus) Feeding Trial Demonstrating the Enhanced Iron Bioavailability Properties of the Fast Cooking Manteca Yellow Bean (Phaseolus vulgaris L.).

The common dry bean (Phaseolus vulgaris L.) is a globally produced pulse crop and an important source of micronutrients for millions of people across Latin America and Africa. Many of the preferred black and red seed types in these regions have seed coat polyphenols that inhibit the absorption of iron. Yellow beans are distinct from other market classes because they accumulate the antioxidant kaempferol 3-glucoside in their seed coats. Due to their fast cooking tendencies, yellow beans are often marketed at premium prices in the same geographical regions where dietary iron deficiency is a major health concern. Hence, this study compared the iron bioavailability of three faster cooking yellow beans with contrasting seed coat colors from Africa (Manteca, Amarillo, and Njano) to slower cooking white and red kidney commercial varieties. Iron status and iron bioavailability was assessed by the capacity of a bean based diet to generate and maintain total body hemoglobin iron (Hb-Fe) during a 6 week in vivo (Gallus gallus) feeding trial. Over the course of the experiment, animals fed yellow bean diets had significantly (p ≤ 0.05) higher Hb-Fe than animals fed the white or red kidney bean diet. This study shows that the Manteca yellow bean possess a rare combination of biochemical traits that result in faster cooking times and improved iron bioavailability. The Manteca yellow bean is worthy of germplasm enhancement to address iron deficiency in regions where beans are consumed as a dietary staple.

Investigation of Nicotianamine and 2' Deoxymugineic Acid as Enhancers of Iron Bioavailability in Caco-2 Cells.

Nicotianamine (NA) is a low-molecular weight metal chelator in plants with high affinity for ferrous iron (Fe2+) and other divalent metal cations. In graminaceous plant species, NA serves as the biosynthetic precursor to 2' deoxymugineic acid (DMA), a root-secreted mugineic acid family phytosiderophore that chelates ferric iron (Fe3+) in the rhizosphere for subsequent uptake by the plant. Previous studies have flagged NA and/or DMA as enhancers of Fe bioavailability in cereal grain although the extent of this promotion has not been quantified. In this study, we utilized the Caco-2 cell system to compare NA and DMA to two known enhancers of Fe bioavailability-epicatechin (Epi) and ascorbic acid (AsA)-and found that both NA and DMA are stronger enhancers of Fe bioavailability than Epi, and NA is a stronger enhancer of Fe bioavailability than AsA. Furthermore, NA reversed Fe uptake inhibition by Myricetin (Myr) more than Epi, highlighting NA as an important target for biofortification strategies aimed at improving Fe bioavailability in staple plant foods.

Characterization of Polyphenol Effects on Inhibition and Promotion of Iron Uptake by Caco-2 Cells.

Polyphenolic compounds present in the seed coat of common bean (Phaseolus vulgaris L.) are known to act collectively as inhibitors of iron bioavailability. Recent research identified specific polyphenols as being potent Fe uptake inhibitors. That research also identified other polyphenols as being promoters of Fe uptake. The present study extends that work using a Caco-2 cell model to characterize the effects of 43 additional polyphenols on Fe uptake. In addition, this study indicates that the inhibitory compounds have a more potent effect that outweighs the ability of promoting compounds to increase Fe uptake. For example, a ratio of 100:0 epicatechin (a promoter)/myricetin (an inhibitor) produced 78.5 ± 6.7 ng ferritin/mg protein, 90:10 yielded 27.4 ± 3.0, 50:50 yielded 3.42 ± 0.54, and 0:100 yielded 2.26 ± 0.25 ng ferritin/mg protein. A simulation of the relative concentrations of eight major polyphenols (four inhibitors, four promoters) present in a sample of black bean seed coats demonstrated that most of the inhibitory compounds would need to be removed to reduce the negative effect on Fe uptake. In vivo studies are now warranted to confirm the above in vitro effects. Such work would be significant as other bean color classes exist that are likely to have polyphenolic profiles that are more favorable to Fe bioavailability.

Zinc and selenium accumulation and their effect on iron bioavailability in common bean seeds.

Common beans (Phaseolus vulgaris) are the most important legume crops. They represent a major source of micronutrients and a target for essential trace mineral enhancement (i.e. biofortification). To investigate mineral accumulation during seed maturation and to examine whether it is possible to biofortify seeds with multi-micronutrients without affecting mineral bioavailability, three common bean cultivars were treated independently with zinc (Zn) and selenium (Se), the two critical micronutrients that can be effectively enhanced via fertilization. The seed mineral concentrations during seed maturation and the seed Fe bioavailability were analyzed. Common bean seeds were found to respond positively to Zn and Se treatments in accumulating these micronutrients. While the seed pods showed a decrease in Zn and Se along with Fe content during pod development, the seeds maintained relatively constant mineral concentrations during seed maturation. Selenium treatment had minimal effect on the seed accumulation of phytic acid and polyphenols, the compounds affecting Fe bioavailability. Zinc treatment reduced phytic acid level, but did not dramatically affect the concentrations of total polyphenols. Iron bioavailability was found not to be greatly affected in seeds biofortified with Se and Zn. In contrast, the inhibitory polyphenol compounds in the black bean profoundly reduced Fe bioavailability. These results provide valuable information for Se and Zn enhancement in common bean seeds and suggest the possibility to biofortify with these essential nutrients without greatly affecting mineral bioavailability to increase the food quality of common bean seeds.

Identification of Black Bean (Phaseolus vulgaris L.) Polyphenols That Inhibit and Promote Iron Uptake by Caco-2 Cells.

In nutritional studies, polyphenolic compounds are considered to be inhibitors of Fe bioavailability. Because they are presumed to act in a similar manner, total polyphenols are commonly measured via the Folin-Ciocalteu colorimetric assay. This study measured the content of polyphenolic compounds in white and black beans and examined the effect of individual polyphenols on iron uptake by Caco-2 cells. Analysis of seed coat extracts by LC-MS revealed the presence of a range of polyphenols in black bean, but no detectable polyphenols in white bean. Extracts from black bean seed coats strongly inhibited iron uptake. Examination of the eight most abundant black bean seed coat, non-anthocyanin polyphenols via Caco-2 cell assays showed that four (catechin, 3,4-dihydroxybenzoic acid, kaempferol, and kaempferol 3-glucoside) clearly promoted iron uptake and four (myricetin, myricetin 3-glucoside, quercetin, and quercetin 3-glucoside) inhibited iron uptake. The four inhibitors were present in 3-fold higher total concentration than the promoters (143 ± 7.2 vs 43.6 ± 4.4 µM), consistent with the net inhibitory effect observed for black bean seed coats. The ability of some polyphenols to promote iron uptake and the identification of specific polyphenols that inhibit Fe uptake suggest a potential for breeding bean lines with improved iron nutritional qualities.

Studies of Cream Seeded Carioca Beans (Phaseolus vulgaris L.) from a Rwandan Efficacy Trial: In Vitro and In Vivo Screening Tools Reflect Human Studies and Predict Beneficial Results from Iron Biofortified Beans.

Iron (Fe) deficiency is a highly prevalent micronutrient insufficiency predominantly caused by a lack of bioavailable Fe from the diet. The consumption of beans as a major food crop in some populations suffering from Fe deficiency is relatively high. Therefore, our objective was to determine whether a biofortified variety of cream seeded carioca bean (Phaseolus vulgaris L.) could provide more bioavailable-Fe than a standard variety using in-vivo (broiler chicken, Gallus gallus) and in-vitro (Caco-2 cell) models. Studies were conducted under conditions designed to mimic the actual human feeding protocol. Two carioca-beans, a standard (G4825; 58 µg Fe/g) and a biofortified (SMC; 106 µg Fe/g), were utilized. Diets were formulated to meet the nutrient requirements of Gallus gallus except for Fe (33.7 and 48.7 µg Fe/g, standard and biofortified diets, respectively). In-vitro observations indicated that more bioavailable-Fe was present in the biofortified beans and diet (P<0.05). In-vivo, improvements in Fe-status were observed in the biofortified bean treatment, as indicated by the increased total-body-Hemoglobin-Fe, and hepatic Fe-concentration (P<0.05). Also, DMT-1 mRNA-expression was increased in the standard bean treatment (P<0.05), indicating an upregulation of absorption to compensate for less bioavailable-Fe. These results demonstrate that the biofortified beans provided more bioavailable Fe; however, the in vitro results revealed that ferritin formation values were relatively low. Such observations are indicative of the presence of high levels of polyphenols and phytate that inhibit Fe absorption. Indeed, we identified higher levels of phytate and quercetin 3-glucoside in the Fe biofortified bean variety. Our results indicate that the biofortified bean line was able to moderately improve Fe-status, and that concurrent increase in the concentration of phytate and polyphenols in beans may limit the benefit of increased Fe-concentration. Therefore, specific targeting of such compounds during the breeding process may yield improved dietary Fe-bioavailability. Our findings are in agreement with the human efficacy trial that demonstrated that the biofortified carioca beans improved the Fe-status of Rwandan women. We suggest the utilization of these in vitro and in vivo screening tools to guide studies aimed to develop and evaluate biofortified staple food crops. This approach has the potential to more effectively utilize research funds and provides a means to monitor the nutritional quality of the Fe-biofortified crops once released to farmers.

Transport interactions between cadmium and zinc in roots of bread and durum wheat seedlings.

Field studies have shown that the addition of Zn to Cd-containing soils can help reduce accumulation of Cd in crop plants. To understand the mechanisms involved, this study used 109Cd and 65Zn to examine the transport interactions of Zn and Cd at the root cell plasma membrane of bread wheat (Triticum aestivum L.) and durum wheat (Triticum turgidum L. var. durum). Results showed that Cd2+ uptake was inhibited by Zn2+ and Zn2+ uptake was inhibited by Cd2+. Concentration-dependent uptake of both Cd2+ and Zn2+ consisted of a combination of linear binding by cell walls and saturable, Michaelis-Menten influx across the plasma membrane. Saturable influx data from experiments with and without 10 micro m concentrations of the corresponding inhibiting ion were converted to double reciprocal plots. The results revealed a competitive interaction between Cd2+ and Zn2+, confirming that Cd2+ and Zn2+ share a common transport system at the root cell plasma membrane in both bread and durum wheat. The study suggests that breeding or agronomic strategies that aim to decrease Cd uptake or increase Zn uptake must take into account the potential accompanying change in transport of the competing ion.

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.

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.

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.

Characterization of cadmium uptake, translocation and storage in near-isogenic lines of durum wheat that differ in grain cadmium concentration.

Here we examined several physiological properties of two near-isogenic lines of durum wheat (Triticum turgidum var. durum) that differ in grain cadmium accumulation, to identify the function of a gene locus that confers differential grain Cd concentrations. Time- and concentration-dependent uptake and translocation studies using 109Cd were conducted on nutrient solution-grown seedlings. Root extracts were analysed by inductively coupled plasma emission spectrometry, gel filtration and capillary electrophoresis to determine the interaction between Cd and phytochelatins (PCs) in storage of Cd in roots. The two isolines did not differ in time- or concentration-dependent root Cd uptake, but the low grain-Cd-accumulating isoline showed decreased movement of Cd from roots to shoots. All buffer-soluble Cd extracted from roots of both isolines was in the form of a low-molecular-weight PC-containing complex. The data suggest that PC synthesis is not a limiting factor in the differential storage of Cd in roots, and that movement of Cd through the root and into the transpiration stream may be the cause of differential Cd partitioning in the two isolines.

Zinc effects on cadmium accumulation and partitioning in near-isogenic lines of durum wheat that differ in grain cadmium concentration.

Here, we examined the effectiveness of two approaches for reducing cadmium (Cd) accumulation in durum wheat (Triticum turgidum L. var durum) grain: the application of supplemental zinc (Zn), and the use of cultivars exhibiting reduced grain Cd concentrations. Two durum wheat near-isogenic lines (NIL) that differ in grain Cd accumulation were grown to maturity in solution culture containing a chelating agent to buffer the free activities of Zn and Cd at levels approximating those of field conditions. The low Cd accumulating (L-Cd) isoline had Cd concentrations, in grains and shoot parts, which were 60-70% lower than those of the high Cd accumulating (H-Cd) isoline. Increasing the Zn activities in the nutrient solution from deficient to sufficient levels reduced the concentration of Cd in grains and vegetative shoot parts of both isolines. The results suggest that supplemental Zn reduces Cd tissue concentrations by inhibiting Cd uptake into roots. Cd partitioning patterns between roots and shoots and between spike components suggest that the physiological basis for the low Cd trait is related to the compartmentation or symplasmic translocation of Cd.

Zinc efficiency is correlated with enhanced expression and activity of zinc-requiring enzymes in wheat.

Zinc (Zn) is an essential micronutrient for plants. The ability of plants to maintain significant yields under low Zn is termed Zn efficiency (ZE) and its genetic and mechanistic basis is still not well understood. Previously, we showed that root Zn uptake did not play a role in ZE. In the current study, Zn-efficient and -inefficient wheat (Triticum aestivum) genotypes were grown for 13 d in chelate buffer nutrient solutions at low (0.1 pM), sufficient (150 pM), and high (1 microM) Zn(2+) activities and analyzed for root-to-shoot translocation of Zn, subcellular leaf Zn distribution, and activity and expression of the Zn-requiring enzymes in leaves. No correlation between ZE and Zn translocation to the shoot was found. Furthermore, total and water-soluble concentrations of leaf Zn were not associated with ZE, and no differences in subcellular Zn compartmentation were found between Zn-efficient and -inefficient genotypes. However, the expression and activity of the Zn-requiring enzymes copper (Cu)/Zn superoxide dismutase (SOD) and carbonic anhydrase did correlate with differences in ZE. Northern analysis suggested that Cu/ZnSOD gene expression was up-regulated in the Zn-efficient genotype, Kirgiz, but not in inefficient BDME. Under Zn deficiency stress, the very Zn-efficient genotype Kirgiz and moderately Zn-efficient Dagdas exhibited an increased activity of Cu/ZnSOD and carbonic anhydrase when compared with Zn-inefficient BDME. These results suggest that Zn-efficient genotypes may be able to maintain the functioning of Zn-requiring enzymes under low Zn conditions; thus, biochemical Zn utilization may be an important component of ZE in wheat.

Measurement of thiol-containing amino acids and phytochelatin (PC2) via capillary electrophoresis with laser-induced fluorescence detection.

An analytical method for determining thiols and phytochelatins using high-performance capillary electrophoresis coupled with laser-induced fluorescence detection is presented. The technique utilizes the labeling of thiols with the fluorescent reagent 5-bromomethylfluorescein (5-BMF), which is excited by a 488 nm argon ion laser and fluoresces at 515 nm. The paper describes the determination of the optimal conditions for reaction of 5-BMF with thiols as well as the parameters for electrophoresis runs that produce optimal electropherogram peaks. The technique is shown to be very sensitive for cysteine, cysteinyl-glycine, gamma-glutamyl-cysteine, glutathione and (gamma-glutamylcysteinyl)2-glycine (PC2). Concentrations as low as 25 nmol/L and amounts as low as 1 fmol were detected for glutathione. Sensitivity for detection of PC2 was somewhat lower. The method was shown to be simple, rapid and accurate and should facilitate measurement of thiol-containing amino acids, peptides and phytochelatin (PC2) in small volumes of extracts obtained from biological tissue.

The role of shoot-localized processes in the mechanism of Zn efficiency in common bean.

Zn efficiency (ZE) is the ability of plants to maintain high yield under Zn-deficiency stress in the soil. Two bean ( Phaseolus vulgaris L.) genotypes that differed in ZE, Voyager (Zn-efficient) and Avanti (Zn-inefficient), were used for this investigation. Plants were grown under controlled-environment conditions in chelate-buffered nutrient solution where Zn(2+) activities were controlled at low (0.1 pM) or sufficient (150 pM) levels. To investigate the relative contribution of the root versus the shoot to ZE, observations of Zn-deficiency symptoms in reciprocal grafts of the two genotypes were made. After growth under low-Zn conditions, plants of nongrafted Avanti, self-grafted Avanti and reciprocal grafts that had the Avanti shoot scion exhibited Zn-deficiency symptoms. However nongrafted and self-grafted Voyager, as well as reciprocal grafts with the Voyager shoot scion, were healthy with no visible Zn-deficiency symptoms under the same growth conditions. More detailed investigations into putative shoot-localized ZE mechanisms involved determinations of leaf biomass production and Zn accumulation, measurements of subcellular Zn compartmentation, activities of two Zn-requiring enzymes, carbonic anhydrase and Cu/Zn-dependent superoxide dismutase (Co/ZnSOD), as well as the non-Zn-requiring enzyme nitrate reductase. There were no differences in shoot tissue Zn concentrations between the Zn-inefficient and Zn-efficient genotypes grown under the low-Zn conditions where differences in ZE were exhibited. Shoot Zn compartmentation was investigated using radiotracer ((65)Zn) efflux analysis and suggested that the Zn-efficient genotype maintains higher cytoplasmic Zn concentrations and less Zn in the leaf-cell vacuole, compared to leaves from the Zn-inefficient genotype under Zn deficiency. Analysis of Zn-requiring enzymes in bean leaves revealed that the Zn-efficient genotype maintains significantly higher levels of carbonic anhydrase and Cu/ZnSOD activity under Zn deficiency. While these data are not sufficient to allow us to determine the specific mechanisms underlying ZE, they certainly point to the shoot as a key site where ZE mechanisms are functioning, and could involve processes associated with Zn compartmentation and biochemical Zn utilization.