Low Zn Conditions Research Articles

Arbuscular mycorrhizal fungi increase grain zinc concentration and modify the expression of root ZIP transporter genes in a modern barley (Hordeum vulgare) cultivar

The positive effects of arbuscular mycorrhizal fungi (AMF) on the zinc (Zn) nutrition of a number of cereal species has been demonstrated, but for Hordeum vulgare (barley), this has been scarcely investigated. Zn is taken up by ZIP transporters in the roots, and several barley ZIP transporter genes are up-regulated under Zn deficient conditions.We grew a modern cultivar of barley (cv. Compass) at five different soil Zn concentrations ranging from no addition through to a toxic concentration. The plants were either inoculated with the AMF Rhizophagus irregularis, or mock-inoculated. At harvest, measurements of biomass, tissue Zn concentration, and expression of ZIP transporter genes were taken.Inoculation of barley with AMF resulted in improved grain and straw Zn concentrations, especially at low soil Zn concentrations, but did not increase the biomass of the plants. Of the five HvZIP genes tested that are up-regulated under low Zn conditions, one gene (HvZIP13) was significantly up-regulated by mycorrhizal colonisation at the lowest Zn treatment. Two other ZIP genes were down-regulated in mycorrhizal plants under low soil Zn.Inoculation with AMF has an effect on ZIP transporter genes in the roots of barley plants. Furthermore, AMF may be more useful for improving quality of barley grain in terms of Zn concentrations, rather than improving yield.

Zinc ions have a potential to attenuate both Ni ion uptake and Ni ion-induced inflammation

Nickel ions (Ni2+) are eluted from various metallic materials, such as medical devices implanted in human tissues. Previous studies have shown that Ni2+ enters inflammatory cells inducing inflammation. However, the regulation of Ni2+ uptake in cells has not yet been reported in detail. In the present study, we investigated the effects of various divalent cations on Ni2+ uptake and Ni2+-induced interleukin (IL)-8 production in the human monocytic cell line, THP-1. We demonstrated that ZnCl2, MnCl2, and CoCl2 inhibited the Ni2+ uptake, while CuCl2, FeCl2, MgCl2, and divalent metal transporter (DMT)-1 inhibitor, Chlorazol Black, did not. Furthermore, ZnCl2 inhibited Ni2+-induced IL-8 production, correlating with the inhibition of Ni2+ uptake. These results suggested that Ni2+ uptake occurred through Zn2+, Mn2+, and Co2+-sensitive transporters and that the inhibition of Ni2+ uptake resulted in the inhibition of IL-8 production. Furthermore, using an Ni wire-implanted mouse model, we found that Ni wire-induced expression of mouse macrophage inflammatory protein-2 (MIP-2) and cyclooxygenase-2 (COX-2) mRNA in the skin tissue surrounding the wire were enhanced by low Zn conditions. These results suggested that the physiological concentration of Zn2+ modulates Ni2+ uptake by inflammatory cells, and a Zn deficient state might increase sensitivity to Ni.

Increased expression of six ZIP family genes by zinc (Zn) deficiency is associated with enhanced uptake and root-to-shoot translocation of Zn in barley (Hordeum vulgare).

Low zinc (Zn) in soils reduces yield and grain Zn content. Regulation of ZRT/IRT-like protein (ZIP) family genes is a major mechanism in plant adaptation to low and fluctuating Zn in soil. Although several Zn deficiency-inducible ZIP genes are identified in cereals, there has been no systematic study on the association of Zn deficiency-induced uptake and root-to-shoot translocation with expression of ZIP family genes. We measured Zn deficiency-induced uptake and root-to-shoot translocation of Zn in barley (Hordeum vulgare) plants by resupplying 0.5 μM Zn, and quantified the transcripts of thirteen HvZIP genes. Subcellular localization and tissue-specific expression were also determined for Zn deficiency-inducible HvZIP genes. Zn deficiency enhanced the capacity of uptake and root-to-shoot translocation of Zn, and sustained the enhanced capacity for 6 d after Zn resupply. Six HvZIP genes were highly induced in roots of Zn-deficient plants, and their proteins were localized in the plasma membrane. Tissue-specific expression in roots supports their roles in uptake and root-to-shoot translocation of Zn under low Zn conditions. Our results provide a comprehensive view on the physiological roles of ZIP genes in plant adaptation to low and fluctuating Zn in soil, and pave the way for development of new strategies to improve Zn-deficiency tolerance and biofortification in cereals.

EFFECT OF ZINC AVAILABILITY ON GROWTH, MORPHOLOGY, AND NUTRIENT INCORPORATION IN A COASTAL AND AN OCEANIC DIATOM(1).

We investigated the effect of Zn availability on growth rate (μ), cell morphology, and elemental stoichiometry and incorporation rate in two marine diatoms. For the coastal diatom Skeletonema costatum (Grev.) Cleve, the half-saturation constant (KS ) for growth was 4.1 pM Zn(2+) , and growth ceased at ≤ 2.6 pM Zn(2+) , whereas for the oceanic diatom Thalassiosira oceanica Hasle, KS was 0.5 pM Zn(2+) , and μ remained at ∼40%μmax even at 0.3 pM Zn(2+) . Under Zn-limiting (Zn-L) conditions, S. costatum decreased cell size significantly, leading to an 80% increase in surface area to volume ratio (SA/V) at Zn(2+) of 3.5 pM compared to Zn-replete (Zn-R) conditions (at Zn(2+) of 13.2 pM), whereas T. oceanica's morphology did not change appreciably. Cell quotas of C, N, P, Si, and chl a significantly decreased under Zn limitation in S. costatum (at Zn(2+) of 3.5 pM), whereas Zn limitation in T. oceanica (at Zn(2+) of 0.3 pM) had little effect on quotas. Elemental stoichiometry was ∼85C:10N:9Si:1P and 81C:9N:5Si:1P for S. costatum, and 66C:5N:2Si:1P and 52C:6N:2Si:1P for T. oceanica, under Zn-R and Zn-L conditions, respectively. Incorporation rates of all elements were significantly reduced under Zn limitation for both diatoms, but particularly for Si in S. costatum, and for C in T. oceanica, despite its apparent tolerance of low Zn conditions. With [Zn(2+) ] in some parts of the ocean being of the same order (∼0.2 to 2 pM) as our low Zn conditions for T. oceanica, our results support the hypothesis that in situ growth and C acquisition may be limited by Zn in some oceanic species.

Metal accumulation in tobacco expressing Arabidopsis halleri metal hyperaccumulation gene depends on external supply

Engineering enhanced transport of zinc to the aerial parts of plants is a major goal in bio-fortification. In Arabidopsis halleri, high constitutive expression of the AhHMA4 gene encoding a metal pump of the P1B-ATPase family is necessary for both Zn hyperaccumulation and the full extent of Zn and Cd hypertolerance that are characteristic of this species. In this study, an AhHMA4 cDNA was introduced into N. tabacum var. Xanthi for expression under the control of its endogenous A. halleri promoter known to confer high and cell-type specific expression levels in both A. halleri and the non-hyperaccumulator A. thaliana. The transgene was expressed at similar levels in both roots and shoots upon long-term exposure to low Zn, control, and increased Zn concentrations. A down-regulation of AhHMA4 transcript levels was detected with 10 μM Zn resupply to tobacco plants cultivated in low Zn concentrations. In general, a transcriptional regulation of AhHMA4 in tobacco contrasted with the constitutively high expression previously observed in A. halleri. Differences in root/shoot partitioning of Zn and Cd between transgenic lines and the wild type were strongly dependent on metal concentrations in the hydroponic medium. Under low Zn conditions, an increased Zn accumulation in the upper leaves in the AhHMA4-expressing lines was detected. Moreover, transgenic plants exposed to cadmium accumulated less metal than the wild type. Both modifications of zinc and cadmium accumulation are noteworthy outcomes from the biofortification perspective and healthy food production. Expression of AhHMA4 may be useful in crops grown on soils poor in Zn.

Screening Common Bean Genotypes for Tolerance to Low Zinc Availability Using a Chelate-Buffered Hydroponics System

A chelate-buffered hydroponics system was assessed for its ability to induce zinc (Zn) deficiency in common bean and for its usefulness as a tool to select genotypes tolerant of Zn deficiency stress. Twenty-two common bean genotypes were evaluated for tolerance to Zn deficient conditions using the buffered hydroponics system. Relative yield (comparing growth under low Zn to adequate Zn conditions) and Zn and phosphorus (P) accumulation in plants were measured. Significant Zn deficiency stress was induced in all of the genotypes with relative yields ranging from 11.5 to 52.6%. Six genotypes were identified as being tolerant to low Zn. These six genotypes (E 101, AND 684, LSA 102, SUG 55, PVA 773, and CENTRO) all had relative yields >29%, were able to accumulate >60 µg Zn per plant from solution and were better able to regulate P uptake to avoid excessive P accumulation than the other genotypes. In a second experiment, a subset of seven genotypes grown in the chelate-buffered hydroponics systems were compared directly to plants grown in field soil. The two systems correlated well for total dry weight (r = 0.57, P < 0.001), shoot Zn content (µg g−1) (r = 0.58, P < 0.001), and total Zn content (µg g−1) (r = 0.64, P < 0.001) when plants were grown under low Zn conditions.

Genotypic variation in common bean in response to zinc deficiency in calcareous soil

Greenhouse experiments have been carried out to study the genotypic variation among 35 bean (Phaseolus vulgaris L.) genotypes with regards to tolerance to zinc (Zn) deficiency (Zn efficiency). Plants were grown for 45 days in Zn deficient soil supplemented with 0 or 5 μg Zn g−1 soil) and analyzed for Zn efficiency, plant Zn concentration and content, and the distribution of Zn between old and young parts of the shoot. Zn efficiency (ZE) was defined as the ratio of dry matter production at low and high Zn supply and was calculated for the whole shoot as well as for young and old parts of the shoot. There were marked differences in ZE among the bean genotypes. Genotypes G4449 and G11360 were about 2-fold and 10-fold more Zn-efficient than G11229 and G3871 in whole shoot and young-part based ZE, respectively. Interestingly, the older portions of the shoot for most genotypes had higher dry matter production under Zn deficiency than under sufficient Zn supply, suggesting that there was a significant inhibition of new shoot growth and transport of photosynthates from source to sink organs under low-Zn conditions. Zinc concentrations of both old and young portions of the shoot did not correlate with ZE, but shoot Zn content was found to be significantly correlated with ZE. Furthermore, Zn-efficient genotypes distributed more Zn into young parts of the shoot under Zn-deficient conditions than did the inefficient lines. Variation in seed Zn content did not significantly influence the determination of ZE. We concluded that there is a substantial variation in Zn efficiency in the bean genome, and ZE based on analysis of the young shoot tissues represents a suitable screening technique for the evaluation of ZE in low-Zn soils.

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.

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.

High- and low-affinity zinc transport systems and their possible role in zinc efficiency in bread wheat.

There is considerable variability among wheat (Triticum aestivum L.) cultivars in their ability to grow and yield well in soils that contain very low levels of available Zn. The physiological basis for this tolerance, termed Zn efficiency, is unknown. We investigated the possible role of Zn(2+) influx across the root cell plasma membrane in conferring Zn efficiency by measuring short-term (65)Zn(2+) uptake in two contrasting wheat cultivars, Zn-efficient cv Dagdas and Zn-inefficient cv BDME-10. Plants were grown hydroponically under sufficient and deficient Zn levels, and uptake of (65)Zn(2+) was measured over a wide range of Zn activities (0.1 nM-80 microM). Under low-Zn conditions, cv BDME-10 displayed more severe Zn deficiency symptoms than cv Dagdas. Uptake experiments revealed the presence of two separate Zn transport systems mediating high- and low-affinity Zn influx. The low-affinity system showed apparent K(m) values similar to those previously reported for wheat (2-5 microM). Using chelate buffered solutions to quantify Zn(2+) influx in the nanomolar activity range, we uncovered the existence of a second, high-affinity Zn transport system with apparent K(m) values in the range of 0.6 to 2 nM. Because it functions in the range of the low available Zn levels found in most soils, this novel high-affinity uptake system is likely to be the predominant Zn(2+) uptake system. Zn(2+) uptake was similar for cv Dagdas and cv BDME-10 over both the high- and low-affinity Zn(2+) activity ranges, indicating that root Zn(2+) influx does not play a significant role in Zn efficiency.

Uptake and distribution of65Zn and54Mn in wheat grownat sufficient and deficient levels of Zn and Mn

The uptake and distribution of 65 Zn and 54 Mn by wheat (Triticum aestivum cv. Aroona) was investigated. Plants were grown in a chelate-buffered nutrient solution with either sufficient Zn and Mn, low Zn or low Mn. A single representative seminal root from 14-d-old and 42-d-old plants was dual-labelled with 65 Zn and 54 Mn. The 14-d-old plants were harvested every 10 min from 10-140 min of labelling, whilst the 42-d-old plants were harvested after 2 h of labelling. At harvest, each plant was separated into leaves, main stem, unexposed roots, and tillers. In addition, the crown was separated from the stem in the 14-d-old plants. In the control plants labelled at 14 d, 65 Zn was first detected and accumulated in the crown of the roots after 40-60 min. Labelled Zn was then detected in the stem, followed by the leaves. The oldest and youngest leaves received less 65 Zn than the second and third oldest leaves. The plants grown under low Zn conditions accumulated more 65 Zn in their older leaves and transferred 65 Zn to the unexposed roots. Distribution of 54 Mn was similar in the controls to that of 65 Zn, except the older leaves received no 54 Mn. At the second harvest, a similar distribution pattern of 65 Zn and 54 Mn was observed with regard to leaf age. Large amounts of 65 Zn and 54 Mn were detected within the unexposed roots of all treatments. It is suggested that the distribution of root-supplied Zn and Mn may be determined by micronutrient status and its relationship with leaf transpiration rates.

Differential Response of Maize Inbreds to Zn1

AbstractMaize (Zea mays L.) inbreds differ in their ability to take up and use Zn and these differences could be important for development and adaptation of plants to low Zn conditions. In pursuing the need for such information growth measurements and mineral element analysis were made on the inbreds A635 and H84 which showed differential Zn deficiency when grown with varied Zn in nutrient solutions. H84 was more “Zn‐efficient” than A645. H84 produced more total dry matter and more dry matter per unit Zn, developed fewer Zn deficiency symptoms, had more Zn in the tops compared to roots, and had fewer imbalances of other mineral elements when both were grown at low levels of Zn. At higher Zn levels, A635 produced as much as or more dry matter and had similar mineral element balances as H84. H84 also grew better than A635 when the two inbreds were grown together in the same low Zn solution. A635 showed greater changes in P, Ca, Mg, Mn, and Fe than H84 as Zn varied in solution. Differential responses between these two inbreds for Zn appeared to be caused by differences in the inbreds for translocation, requirements, and utilization of Zn, and for accumulation of imbalanced amounts of mineral elements which interact with Zn.