Mobilization of iron by phytosiderophores as affected by other micronutrients

It has been shown previously (Treeby et al., 1989) that phytosiderophores, released by roots of iron deficient grasses (Gramineae), mobilize from calcareous soils not only iron (Fe) but also zinc (Zn), manganese (Mn) and copper (Cu). Mobilization of Fe may therefore be impaired by other micronutrient cations. This has been studied in both, model experiments with Fe hydroxide and with a calcareous soil (15% CaCO₃, pH 8.6) amended with micronutrients as sulfate salts.Mobilization of Fe from Fe hydroxide by phytosiderophores (epi-3-hydroxymugineic acid) was not affected by the addition of CaCl₂, MgSO₄ and MnSO₄, slightly inhibited by ZnSO₄ and strongly inhibited by CuSO₄. In a calcareous soil amended with increasing levels of ZnSO₄, MnSO₄ and CuSO₄, mobilization of Fe by phytosiderophores remained uneffected by Zn and Mn amendments but was progressively impaired by increasing levels of Cu amendment, correlated with corresponding enhancement of Cu mobilization.High concentrations of ZnSO₄ and MnSO₄ and relatively high concentrations of CuSO₄ were required for inhibition of Fe mobilization by phytosiderophores. It is therefore concluded that in most calcareous soils phytosiderophores efficiently mobilize Fe, and that phytosiderophores play an important role in Fe acquisition by grasses grown on calcareous soils.     

Genotypical differences among graminaceous species in release of phytosiderophores and uptake of iron phytosiderophores

Graminaceous species can enhance iron (Fe) acquisition from sparingly soluble inorganic Fe(III) compounds by release of phytosiderophores (PS) which mobilize Fe(III) by chelation. In most graminaceous species Fe deficiency increases the rate of PS release from roots by a factor of 10–20, but in some species, for example sorghum, this increase is much less. The chemical nature of PS can differ between species and even cultivars.The various PS are similarly effective as the microbial siderophore Desferal (ferrioxamine B methane sulfonate) in mobilizing Fe(III) from a calcareous soil. Under the same conditions the synthetic chelator DTPA (diaethylenetriamine pentaacetic acid) is ineffective.The rate of Fe(III)PS uptake by roots of graminaceous species increases by a factor of about 5 under Fe deficiency. In contrast, uptake of Fe from both synthetic and microbial Fe(III) chelates is much lower and not affected by the Fe nutritional status of the plants. This indicates that in graminaceous species under Fe deficiency a specific uptake system for FePS is activated. In contrast, the specific uptake system for FePS is absent in dicots. In a given graminaceous species the uptake rates of the various FePS are similar, but vary between species by a factor of upto 3. In sorghum, despite the low rate of PS release, the rate of FePS uptake is particularly high.The results indicate that release of PS and subsequent uptake of FePS are under different genetic control. The high susceptibility of sorghum to Fe deficiency (lime-chlorosis) is most probably caused by low rates of PS release in the early seedling stage. Therefore in sorghum, and presumably other graminaceous species also, an increase in resistance to lime chlorosis could be best achieved by breeding for cultivars with high rates of PS release. In corresponding screening procedures attention should be paid to the effects of iron nutritional status and daytime on PS release as well as on rapid microbial degradation of PS.     

Mobilization of iron and other micronutrient cations from a calcareous soil by plant-borne,microbial, and synthetic metal chelators

Mobilization of Fe, Zn, Cu, and Mn by various chelators from a calcareous soil was measured using a simple dialysis tube/complexing resin system. Root exudates from Fe-deficient barley increased the concentrations of all four metals in solution by, on average, a factor of 20, and the addition of complexing resin as a sink for heavy metal cations forced steady state solution concentrations to be reached sooner. Root exudates mobilized increasing amounts of the various micronutrients in the following order: Cu<Fe<Zn<Mn. Phytosiderophores isolated from root exudates of Fe-deficient barley mobilized similar amounts of Cu and Zn but somewhat more Fe and considerably more Mn than crude exudate. The synthetic chelators EDDHA and DTPA showed low specificity in micronutrient mobilization, but the microbial siderophore Desferal was relatively more specific, preferentially mobilizing Fe and Mn. The data indicates that phytosiderophores are capable of increasing the amount of complexed cations in solution. Despite their lack of specificity, phytosiderophores were just as effective as Desferal increasing the availability of Fe. Thus, phytosiderophores, as plant-borne chelators, are certainly of significance for the Fe nutrition of cereals grown in calcareous soils.     

The role of potassium in the secretion of mugineic acids family phytosiderophores from iron-deficient barley roots

Mugineic acid family phytosiderophores (MAs) are secreted from iron-deficient barley roots with high equimolar correlation of potassium. To determine the form of MAs when it is secreted, we investigated the effect of anion channel blockers and valinomycin on the secretion of MAs. Among the anion channel blockers, anthracene-9-carboxylic acid and phenylglyoxal drastically reduced the amount of secreted MAs, while 4,4-diisothiocyano-2,2- stilbene disulfonate slightly inhibited the MAs secretion. Trifluoromethyl-3-phenylamino-2-nicotinic acid reduced the secreted amount to the half of non-treated. This result suggested that MAs are secreted in the form of anion through an anion channel. The elimination of potassium gradient between the cytoplasm and the cell exterior by treatment with valinomycin reduced the amount of secreted MAs. Analysis of potassium distribution in root by LV-SEM-XMA indicated that potassium in the cortex cells of iron-deficient roots is released with MAs secretion and the amount of potassium in the cortex cells decreases after secretion. These results suggested that MAs are secreted in the form of a monovalent anion via anion channels using the potassium gradient between the cytoplasm and the cell exterior. This revised version was published online in June 2006 with corrections to the Cover Date.     

Light-mediated release of phytosiderophores in wheat and barley under iron or zinc deficiency

The effect of varied light intensity (50 – 600 mol m-2 s-1) on the rate of phytosiderophore release was studied under zinc (Zn) deficiency using a bread (Triticum aestivum L. cv. Aroona) and a durum wheat cultivar (Triticum durum Desf. cv. Durati) differing in zinc (Zn) efficiency and under iron (Fe) deficiency using a barley cultivar (Hordeum vulgare L. Europe). Plants were grown under controlled environmental conditions in nutrient solution for 15 days (wheat plants) or 11 days (barley plants). Phytosiderophore release was determined by measuring capacity of root exudates to mobilize copper (Cu) from a Cu-loaded resin.With increasing light intensity visual Zn deficiency symptoms such as whitish-brown lesions on leaf blade developed rapidly and severely in wheat, particularly in the durum cultivar Durati. In wheat plants supplied well with Zn, increases in light intensity from 100 to 600 mol m-2 s-1 did not clearly affect the rate of phytosiderophore release. However, under Zn deficiency increases in light intensity markedly enhanced release of phytosiderophores in Zn-deficient Aroona, but not in Zn-inefficient Durati. When Fe-deficient barley cultivar Europe was grown first at 220 mol m-2 s-1 and then exposed to 600 mol m-2 s-1 for 24 and 48 h, the rate of release of phytosiderophores was enhanced about 4-fold and 7-fold, respectively. Transfer of Fe-deficient plants from 600 to 50 mol m-2 s-1 for 48 h reduced the rate of release of phytosiderophores by a factor of 7. The effect of light on phytosiderophore release was similar regardless of whether the rate of phytosiderophore release was expressed per plant or per unit dry weight of roots.The results demonstrate a particular role of light intensity in phytosiderophore release from roots under both Zn and Fe deficiency. It is suggested that in the studies concerning the role of phytosiderophore release in expression of Zn or Fe efficiency among and within cereals, a special attention should be given to the light conditions.     

Changes in trace metal species and other components of the rhizosphere during growth of radish

Changes in the properties of soil solution in the rhizosphere of developing radish plants were investigated. Variations in these properties were expected to affect the distribution and speciation of metals in the soil and soil solution. Applications of essential nutrients were linked to plant transpiration rates and prevented excess addition of nutrient ions, so that subtle changes in soil solution composition would not be obscured. Soil solution pH, the concentration of dissolved organic carbon (DOC) and the concentrations of major and trace elements in solution were found to vary over time. Strict control of fertilizer additions led to the maintenance of a relatively low ionic strength in the soil solution, and under such conditions trace metal solubility appeared to be highy influenced by the concentration of DOC. A chemical speciation analysis was performed which showed that, while dissolved Cd and Zn were largely uncomplexed in unplanted soil, Cd and Zn in the rhizosphere existed mainly as complexed forms. It is hypothesized that this is partly a result of Ca-metal-ligand equilibrium in solution, with higher Ca concentrations in unplanted soil leading to more of the Cd and Zn in solution existing in the uncomplexed state. Changes in the concentrations of uncomplexed Cd and Zn with time gave the best correlations with changes in plant uptake of these metals over time, supporting the hypothesis that plants mainly absorb the free metal ion from soil solution.     

铜稳态对铁代谢平衡的影响

铜是人体必需的微量元素,参与体内多种蛋白和酶的组成,机体内存在严格的铜稳态调控机制。作为血浆中最主要的多铜亚铁氧化酶——铜蓝蛋白,与另外两种同源亚铁氧化酶——膜铁转运辅助蛋白和zyklopen,共同参与体内铁的转运,维持铁代谢的平衡。将对调节铜和铁平衡的重要意义以及铜和铁在机体代谢过程中的相互作用、发展动态进行讨论。     

An underground tale: contribution of microbial activity to plant iron acquisition via ecological processes

Background

Iron (Fe) deficiency in crops is a worldwide agricultural problem. Plants have evolved several strategies to enhance Fe acquisition, but increasing evidence has shown that the intrinsic plant-based strategies alone are insufficient to avoid Fe deficiency in Fe-limited soils. Soil micro-organisms also play a critical role in plant Fe acquisition; however, the mechanisms behind their promotion of Fe acquisition remain largely unknown.

Scope

This review focuses on the possible mechanisms underlying the promotion of plant Fe acquisition by soil micro-organisms.

Conclusions

Fe-deficiency-induced root exudates alter the microbial community in the rhizosphere by modifying the physicochemical properties of soil, and/or by their antimicrobial and/or growth-promoting effects. The altered microbial community may in turn benefit plant Fe acquisition via production of siderophores and protons, both of which improve Fe bioavailability in soil, and via hormone generation that triggers the enhancement of Fe uptake capacity in plants. In addition, symbiotic interactions between micro-organisms and host plants could also enhance plant Fe acquisition, possibly including: rhizobium nodulation enhancing plant Fe uptake capacity and mycorrhizal fungal infection enhancing root length and the nutrient acquisition area of the root system, as well as increasing the production of Fe³⁺ chelators and protons.     

The role of siderophores in iron acquisition by photosynthetic marine microorganisms

The photosynthetic picocyanobacteria and eukaryotic microorganisms that inhabit the open ocean must be able to supply iron for their photosynthetic and respiratory needs from the subnanomolar concentrations available in seawater. Neither group appears to produce siderophores, although some coastal cyanobacteria do. This is interpreted as an adaptation to the dilute oceanic environment rather than a phylogenetic constraint, since there are cases in which related taxa from different environments have the capacity to produce siderophores. Most photosynthetic marine microorganisms are presumably, however, capable of accessing iron from strong chelates since the majority of dissolved iron in seawater is complexed by organic ligands, including siderophores. Rather than direct internalization of siderophores and other iron chelates, marine organisms primarily appear to use uptake pathways that involve a reduction step to free bound iron, closely coupled with transport into the cell.     

Serum and hair levels of zinc and other elements in dogs with visceral leishmaniasis

The aim of this study was to determine the zinc, iron, copper, calcium, phosphorus, and magnesium levels in blood serum and zinc and copper levels in hair of dogs with canine visceral leishmaniasis. The serum zinc and iron levels were found to be significantly lower in diseased dogs than those of healthy controls. Serum copper levels were significantly higher, whereas no significant differences were observed for calcium, phosphorus, and magnesium. There were no significant differences in the zinc and copper levels in hair. Our results show that the serum zinc, iron, and copper levels are altered in canine leishmaniasis.     

The influence of gallium and other metal ions on the uptake of non-transferrin-bound iron by rat hepatocytes

Background. – Under conditions of iron overload non-transferrin-bound iron (NTBI) occurs in the circulation and is mainly cleared by the liver. Beside iron, gallium and aluminum enhance accumulation of NTBI. We try to characterize the mechanism and metal-mediated regulation of NTBI uptake using cultivated primary rat hepatocytes.     

Effects of iron,copper, zinc,calcium, and magnesium on human lymphocytes in culture

The effects of simultaneous changes of calcium, magnesium, iron, copper, and zinc concentrations were evaluated in normal human T and B lymphocytes, cultured in cation-depleted media. Optimal concentrations for thymidine incorporation (TI) in both cell populations were Fe and Zn 15 μM and Cu 5 μM; for t cells Ca 2 mM and Mg 4 mM; for B cells Ca 4 mM and Mg 6 mM. TI decreased with increasing molarity of cations and the decrease was particularly apparent with Cu. Minimal amounts of Ca and Mg (0.5 mM) were necessary for growth, even in presence of optimal concentrations of Fe, Cu, and Zn. Fe and Cu showed synergistic stimulatory effects at low concentrations and synergistic inhibitory effects at high concentrations. Antagonism between Fe and Zn, Cu and Zn, and Ca and Zn was also demonstrated. CD4/CD8 increased with PHA stimulation in presence of Zn, and decreased with ConA stimulation in presence of Zn or Fe. The results demonstrate: (1) the relationship and interdependence of Fe, Cu, and Zn concentrations in modulating the growth of normal lymphocytes; (2) the stimulatory effects of Fe on B cells and Zn on CD8 positive cells; (3) the inhibitory effect of Cu at concentrations lower than those of Fe and Zn; (4) the requirement of Ca and Mg in certain concentration and ratio for the action of the other cations; and (5) the Ca and Mg requirement for the growth of B cells higher than T cells.     

The effect of iron source on formation and persistence of root iron pools in cereals

Abstract

Five chelated iron sources have been applied to barley and maize to investigate the effect of differing chemical form on the formation and persistence of root apoplastic Fe pools. Short-term Fe exposure (barley) experiments indicated that the charged state of the Fe complex was the most important factor regulating the initial formation and magnitude of the apoplastic pool. Longer term experiments (maize), incorporating a period of Fe deprivation, produced more complex results. Differences in plant growth during the experiment produced changes in the magnitude of the root Fe pool; these interacted with the chemical form of the applied Fe to regulate the release, utilisation and hence the ultimate size of the apoplastic pool produced by each Fe source. It is concluded that such experiments are poor indicators of the potential performance of novel chelated Fe sources.     

Chronological changes in tissue copper, zinc and iron in the toxic milk mouse and effects of copper loading

The toxic milk (tx) mouse is a rodent model for Wilson disease, an inherited disorder of copper overload. Here we assessed the effect of copper accumulation in the tx mouse on zinc and iron metabolism. Copper, zinc and iron concentrations were determined in the liver, kidney, spleen and brain of control and copper-loaded animals by atomic absorption spectroscopy. Copper concentration increased dramatically in the liver, and was also significantly higher in the spleen, kidney and brain of control tx mice in the first few months of life compared with normal DL mice. Hepatic zinc was increased with age in the tx mouse, but zinc concentrations in the other organs were normal. Liver and kidney iron concentrations were significantly lower at birth in tx mice, but increased quickly to be comparable with control mice by 2 months of age. Iron concentration in the spleen was significantly higher in tx mice, but was lower in 5 day old tx pups. Copper-loading studies showed that normal DL mice ingesting 300 mg/l copper in their diet for 3 months maintained normal liver, kidney and brain copper, zinc and iron levels. Copper-loading of tx mice did not increase the already high liver copper concentrations, but spleen and brain copper concentrations were increased. Despite a significant elevation of copper in the brain of the copper-loaded tx mice no behavioural changes were observed. The livers of copper-loaded tx mice had a lower zinc concentration than control tx mice, whilst the kidney had double the concentration of iron suggesting that there was increased erythrocyte hemolysis in the copper-loaded mutants.     

Changes in iron, calcium, magnesium, copper, and zinc levels in different tissues of riboflavin-deficient rats

To clarify the changes of mineral levels in different tissues of riboflavin-deficient rats, Wistar rats were separated into three groups. One group was fed a diet ad libitum that was deficient in riboflavin. The other two were fed either the complete diet that was weight-matched to the riboflavin-deficient group or fed a complete diet ad libitum. In riboflavin-deficient rats, the hemoglobin concentration and riboflavin contents of blood, liver, and kidney were significantly decreased, compared with weight-matched and ad libitum-fed controls. The mineral concentrations of tissues are summarized as follows: The iron (Fe) concentration in the heart, liver, and spleen was decreased in the riboflavin-deficient group compared with the other groups. Calcium (Ca) and magnesium (Mg) concentrations in tibia were decreased in the riboflavin-deficient group compared with the other two groups. Copper (Cu) concentration was increased in the heart and liver when the riboflavin-deficient group was compared with the other groups. Zinc (Zn) concentration was increased in tibia when the riboflavin-deficient group was compared with the other groups.     

The role of trace metals in photosynthetic electron transport in O2-evolving organisms

Iron is the quantitatively most important trace metal involved in thylakoid reactions of all oxygenic organisms since linear (= non-cyclic) electron flow from H₂O to NADP⁺ involves PS II (2–3 Fe), cytochrome b₆-f (5 Fe), PS I (12 Fe), and ferredoxin (2 Fe); (replaceable by metal-free flavodoxin in certain cyanobacteria and algae under iron deficiency). Cytochrome c₆ (1 Fe) is the only redox catalyst linking the cytochrome b₆-f complex to PS I in most algae; in many cyanobacteria and Chlorophyta cytochrome c₆ and the copper-containing plastocyanin are alternatives, with the availability of iron and copper regulating their relative expression, while higher plants only have plastocyanin. Iron, copper and zinc occur in enzymes that remove active oxygen species and that are in part bound to the thylakoid membrane. These enzymes are ascorbate peroxidase (Fe) and iron-(cyanobacteria, and most al gae) and copper-zinc- (some algae; higher plants) superoxide dismutase. Iron-containing NAD(P)H-PQ oxidoreductase in thylakoids of cyanobacteria and many eukaryotes may be involved in cyclic electron transport around PS I and in chlororespiration. Manganese is second to iron in its quantitative role in the thylakoids, with four Mn (and 1 Ca) per PS II involved in O₂ evolution. The roles of the transition metals in redox catalysts can in broad terms be related to their redox chemistry and to their availability to organisms at the time when the pathways evolved. The quantitative roles of these trace metals varies genotypically (e.g. the greater need for iron in thylakoid reactions of cyanobacteria and rhodophytes than in other O₂-evolvers as a result of their lower PS II:PS I ratio) and phenotypically (e.g. as a result of variations in PS II:PS I ratio with the spectral quality of incident radiation).     

Alterations in plasma essential trace elements selenium, manganese, zinc, copper, and iron concentrations and the possible role of these elements on oxidative status in patients with childhood asthma

The aim of the present study is to evaluate the status of plasma essential trace element selenium (Se), manganese (Mn), copper (Cu), zinc (Zn), and iron (Fe) concentrations and the effect of these elements on oxidative status in patients with childhood asthma. Plasma Se, Mn, Cu, and Zn concentrations were determined by atomic absorption spectrophotometry (AAS) and Fe concentrations, malondialdehyde (MDA), and total antioxidant capacity (TAC) were determined by the colorimetric method. The plasma MDA/TAC ratio was calculated as an index of oxidative status. Plasma albumin levels were measured to determine nutritional status. Plasma Fe concentrations, MDA levels and the MDA/TAC ratio were significantly higher (p<0.001, p<0.001, and p<0.01, respectively) and Se and Mn concentrations and TAC were lower (p<0.01, p<0.05, and p<0.01, respectively) in patients when compared to the healthy subjects. Plasma Zn, Cu, and albumin levels were not found to be significantly different in patients and controls (p>0.05). There were positive relationships between plasma MDA and Fe (r=0.545, p<0.001) and TAC and Se (r=0.485, p<0.021), and a negative correlation between TAC and MDA values (r= −0.337, p<0.031) in patients with childhood asthma. However, there was no correlation between these trace elements and albumin content in patient groups. These observations suggest that increased Fe and decreased Se concentrations in patients with childhood asthma may be responsible for the oxidant/antioxidant imbalance.     

Genotypic variation of rice in phosphorus acquisition from iron phosphate: Contributions of root morphology and phosphorus uptake kinetics

To elucidate the contributions of rice root morphology and phosphorus uptake kinetics to P uptake by rice from iron phosphate, a sand culture experiment with either sufficient P supply (control treatment, 10 mg P/l as NaH₂PO₄) or Fe-P as the only source of P (40 mg P/pot as FePO₄ × 4H₂O) and a solution culture experiment supplied with either sufficient P (10 mg P/l) or deficient P (0.5 mg P/l) were conducted. Eight rice cultivars, which differed in P uptake from Fe-P, were investigated. Plant P uptake, root morphology, and P uptake kinetics were determined. There were significant (P < 0.05) genotypic variations in both plant dry weight and P uptake per plant among eight rice (Oryza sativa L.) cultivars when supplied with Fe-P as the P source. The Fe-P treatment significantly (P < 0.05) decreased plant dry weight, P uptake per plant, and P concentration in plant dry matter of all cultivars in comparison with the control plants. In Fe-P treated plants, significant (P < 0.05) genotypic variation was shown in root morphology, including root length, surface area, volume, and number of lateral roots. The P uptake per plant from Fe-P by rice was significantly (P < 0.05) correlated with root surface area and root volume as well as with the number of lateral roots, suggesting that the ability of rice to absorb P from Fe-P was closely related to root morphology. Low P supply in solution significantly increased the I _max (P < 0.05), but significantly decreased the K _M (P < 0.05) for P absorption by all rice cultivars. We supposed that kinetic characteristics of root P uptake could not account for the ability of rice to absorb P from Fe-P. Published in Russian in Fiziologiya Rastenii, 2007, Vol. 54, No. 2, pp. 260–266. The text was submitted by the authors in English.