Iron Deficiency Response Research Articles

Playing with the switches

Iron availability in soils is often limiting for plants, especially in soils with basic pH. Iron deficiency is responsible for decreased yield and nutritional values of the crops. To cope with iron deficiency, plants activate a syndrome of physiological, developmental and metabolic responses that stimulate iron acquisition. In non-grasses, these responses include the development of ectopic root hairs or of transfer cells specialized in mineral absorption, the upregulation of the ferric chelate reductase that produces FeII from FeIII chelates, and the up-regulation of FeII uptake in roots. The ferric chelate reductase FRO2 and the Fe-uptake transporter IRT1 have been recently characterized at the molecular level in Arabidopsis, tomato and pea. Manipulation of a common regulatory gene could be an efficient way to engineer iron efficiency in crops. Several mutants, including the pea mutants brz and dgl and the Arabidopsis mutant frd3/man1, display a constitutive iron-deficiency response under iron-replete growth conditions. By contrast, the tomato fer mutant fails to activate iron-deficiency responses under iron-starvation conditions. It was hypothesized that this mutant might be defective in a master switch responsible for the activation of iron-deficiency responses in roots.

FRD3, a member of the multidrug and toxin efflux family, controls iron deficiency responses in Arabidopsis.

We present the cloning and characterization of an Arabidopsis gene, FRD3, involved in iron homeostasis. Plants carrying any of the three alleles of frd3 constitutively express three strategy I iron deficiency responses and misexpress a number of iron deficiency-regulated genes. Mutant plants also accumulate approximately twofold excess iron, fourfold excess manganese, and twofold excess zinc in their shoots. frd3-3 was first identified as man1. The FRD3 gene is expressed at detectable levels in roots but not in shoots and is predicted to encode a membrane protein belonging to the multidrug and toxin efflux family. Other members of this family have been implicated in a variety of processes and are likely to transport small organic molecules. The phenotypes of frd3 mutant plants, which are consistent with a defect in either iron deficiency signaling or iron distribution, indicate that FRD3 is an important component of iron homeostasis in Arabidopsis.

Functional iron deficiency, infection and systemic inflammatory response syndrome in critical illness.

To investigate the prevalence and clinical relevance of functional iron deficiency in the critically ill, we performed a prospective observational study in a university hospital general intensive care unit. We collected patient demographics, severity of illness data, haematological and biochemical variables in 51 consecutive admissions. We recorded episodes of culture-positive infection. Functional iron deficiency (FID), measured by red cell hypochromasia on flow cytometry, was present in 35% of patients at admission to intensive care. FID patients were of similar age, diagnosis, APACHE score, sequential organ failure assessment (SOFA) score, haemoglobin, serum B12, folate and ferritin to patients without FID. However, patients with FID had a prolonged intensive care stay compared with non-FID patients (P<0.001) and increased time to hospital discharge (P=0.09). Duration of intensive care stay correlated with severity of FID (r=0.33, P<0.02). Systemic inflammatory response syndrome (SIRS) was present for longer in those with FID (P<0.02). Overall mortality did not differ between groups. No difference was seen in the incidence of positive cultures between those with FID (9/18 patients) and those without FID (15/33 patients). FID was independently associated only with abnormal white blood cell count (WBC < 4 or > 11 x 10(9) x l(-1)) at admission to ICU, P=0.007, but not with positive cultures. There is a high prevalence of FID in intensive care, associated with an increased duration of stay and duration of SIRS. We have been unable to demonstrate a link with infection, either as a predisposing factor or as an acute response.

Towards map‐based cloning of two genes involved in iron acquisition and metabolism in tomato

Iron is an important micronutrient element of plants. For mobilization and uptake of iron from soil, plants use different active mechanisms. In the last two decades, many researches have focused on the physiological and biochemical basis of iron uptake and metabolism in plants. Little is known about genes involved in the uptake process from soil and their regulation in vivo. To study molecular biology of iron uptake and metabolism in strategy I plants, the mutants chloronerva (chln) and T3238fer (fer) described in tomato (Lycopersicon esculentum Mill.) are very informative mutants. The fer gene is epstatic over the chln gene in double mutants and controls the whole process of iron deficiency response reaction in strategy I plants while the chln gene is responsible for synthesis of the non‐protein amino acid, nicotianamine, which is a key substance in iron metabolism of all higher plants. We try to isolate the both genes by map‐based cloning approach. In this paper we describe our strategy and an actual state of progress for the isolation of the fer and chln genes in tomato. The isolation of the two genes would represent an important first step for understanding the molecular mechanisms controlling strategy I iron‐uptake responses.

Role of shoot in regulation of iron deficiency responses in cucumber and bean plants

Decapitation of the shoot apex or application to the stem of 2‐chloro‐9‐hydroxyfluorenecarboxylic acid‐(9)‐methylester (CFM), a synthetic inhibitor of polar IAA transport, prevented the increase of Fe3+ reductase activity in the roots of Fe deficient bean (Phaseolus vulgaris L., cv. American Supportor) plants. In Fe deficient cucumber (Cucumis sativus L., cv. Xintaimici) plants neither of these treatments inhibited the Fe efficiency response. In a split‐root experiment with cucumber plants, the root half growing in a medium without Fe did not show any Fe efficiency response in spite of very low Fe concentration in the roots. By contrast, in the other half of the roots growing in a medium with Fe supply, the Fe3+ reductase activity increased at an early stage of the treatment, although this was obviously a less marked response as compared with roots of Fe deficient plants. Fe concentrations in different parts of the shoot in split‐root cucumber plants were the same as that in control plants supplied with Fe. The results suggest that the shoot plays an important role in regulation of Fe3+ reductase activity in roots of Fe deficient plants, but the signal compound may be different in cucumber and bean plants.

Role of hormones in the induction of iron deficiency responses in Arabidopsis roots.

In "strategy I" plants, several alterations in root physiology and morphology are induced by Fe deficiency, although the mechanisms by which low Fe levels are translated into reactions aimed at alleviating Fe shortage are largely unknown. To prove whether changes in hormone concentration or sensitivity are involved in the adaptation to suboptimal Fe availability, we tested 45 mutants of Arabidopsis defective in hormone metabolism and/or root hair formation for their ability to increase Fe(III) chelate reductase activity and to initiate the formation and enlargement of root hairs. Activity staining for ferric chelate reductase revealed that all mutants were responsive to Fe deficiency, suggesting that hormones are not necessary for the induction. Treatment of wild-type plants with the ethylene precursor 1-aminocyclopropane-1-carboxylic acid caused the development of root hairs in locations normally occupied by non-hair cells, but did not stimulate ferric reductase activity. Ectopic root hairs were also formed in -Fe roots, suggesting a role for ethylene in the morphological responses to Fe deficiency. Ultrastructural analysis of rhizodermal cells indicated that neither Fe deficiency nor 1-aminocyclopropane-1-carboxylic acid treatment caused transfer-cell-like alterations in Arabidopsis roots. Our data indicate that the morphological and physiological components of the Fe stress syndrome are regulated separately.

Water-extractable humic substances enhance iron deficiency responses by Fe-deficient cucumber plants

The ability of Fe-deficient cucumber plants to use iron complexed to a water-extractable humic substances fraction (WEHS), was investigated. Seven-day-old Fe-deficient plants were transferred to a nutrient solution supplemented daily for 5 days with 0.2 μM Fe as Fe-WEHS (5 μg org. C mL-1), Fe-EDTA, Fe-citrate or FeCl3. These treatments all allowed re-greening of the leaf tissue, and partial recovery of dry matter accumulation, chlorophyll and iron contents. However, the recovery was faster in plants supplied with Fe-WEHS and was already evident 48 h after Fe supply. The addition of 0.2 μM Fe to the nutrient solution caused also a partial recovery of the dry matter and iron accumulation in roots of Fe-deficient cucumber plants, particularly in those supplied with Fe-WEHS. The addition of WEHS alone (5 μg org. C mL-1, 0.04 μM Fe) to the nutrient solution slightly but significantly increased iron and chlorophyll contents in leaves of Fe-deficient plants; in these plants, dry matter accumulation in leaves and roots was comparable or even higher than that measured in plants treated with Fe-citrate or FeCl3. After addition of the different iron sources for 5 days to Fe-deficient roots, morphological modifications (proliferation of lateral roots, increase in the diameter of the sub-apical zones and amplified root-hair formation) and physiological responses (enhanced Fe(III)-chelate reductase and acidification of the nutrient solution) induced by Fe deficiency, were still evident, particularly in plants treated with the humic molecules. The presence of WEHS caused also a further acidification of the nutrient medium by Fe-deficient plants. The Fe-WEHS complex (1 μM Fe) could be reduced by intact cucumber roots, at rates of reduction higher than those measured for Fe-EDTA at equimolar iron concentration. Plasma membrane vesicles, purified by two-phase partition from root microsomes of Fe-deficient plants, were also able to reduce Fe-WEHS. Results show that Fe-deficient cucumber plants can use iron complexed to water soluble humic substances, at least in part via reduction of complexed Fe(III) by the plasma membrane Fe(III)-chelate reductase of root cells. In addition, the stimulating effect of humic substances on H+ release might be of relevance for the overall response of the plants to iron shortage.

Iron deficiency responses in roots of kiwi

Many dicotyledonous species respond to iron (Fe) deficiency by morphological and physiological changes at root level, which are usually defined as Strategy I. Particularly, these latter modifications include a higher acidification of the external medium and the induction of a high root Fe reductase activity. The aim of this work was to investigate the response of kiwi (Actinidia deliciosa cv. Hayward) plants, which often exhibit Fe chlorosis in the field, to Fe deficiency. Actinidia kept for two weeks in nutrient solution without Fe showed visual deficiency symptoms (leaf chlorosis). Moreover, upon prolonged micronutrient shortage shoot, and to a lesser extent, root dry weight accumulation was greatly impaired. Roots of Fe‐deficient Actinidia showed an increased capacity of net proton extrusion and higher ferric ethylenediaminetetraacetate [Fe(III)EDTA] reductase activity as compared to plants grown in the presence of 10 μM Fe(III)EDTA. Localization of the increased acidification and reductase capacity by means of agar‐technique revealed that these activities are both present in the sub‐apical region of the roots. Re‐supply of Fe after two weeks partially reversed the tendency of the roots to acidify the nutrient solution and to reduce Fe(III)EDTA.

Genetic evidence that induction of root Fe(III) chelate reductase activity is necessary for iron uptake under iron deficiency.

Reduction of Fe(III) to Fe(II) by Fe(III) chelate reductase is thought to be an obligatory step in iron uptake as well as the primary factor in making iron available for absorption by all plants except grasses. Fe(III) chelate reductase has also been suggested to play a more general role in the regulation of cation absorption. In order to experimentally address the importance of Fe(III) chelate reductase activity in the mineral nutrition of plants, three Arabidopsis thaliana mutans (frd1-1, frd1-2 and frd1-3), that do not show induction of Fe(III) chelate reductase activity under iron-deficient growth conditions, have been isolated and characterized. These mutants are still capable of acidifying the rhizosphere under iron-deficiency and accumulate more Zn and Mn in their shoots relative to wild-type plants regardless of iron status. frd1 mutants do not translocate radiolabeled iron to the shoots when roots are presented with a tightly chelated form of Fe(III). These results: (1) confirm that iron must be reduced before it can be transported, (2) show that Fe(III) reduction can be uncoupled from proton release, the other major iron-deficiency response, and (3) demonstrate that Fe(III) chelate reductase activity per se is not necessarily responsible for accumulation of cations previously observed in pea and tomato mutants with constitutively high levels of Fe(III) chelate reductase activity.

Iron deficiency and immune responses.

Iron deficiency is often associated with the impairment of the immune system, particularly the functioning of T-cell lymphocytes. Studies were therefore carried out in mice to investigate the involvement of iron in T-cell functions. Iron deficiency in mice was induced nutritionally by feeding diets differing in iron content for 4 weeks to produce normal, moderately low and severely low haematological indices. The proliferation of lymphocytes stimulated by Con A from these groups of mice was significantly affected by the degree of iron deficiency. Iron depletion also resulted in decreased production of interleukin 2 (IL-2) in the proliferating cells. The implications of these findings in the manifestation of iron deficiency anaemia are discussed.

Nicotianamine ‐ a common constituent of strategies I and II of iron acquisition by plants: A review

Abstract Nicotianamine (NA) is present in all multicellular plants hitherto investigated. The only known exception is the semiletal, recessive tomato mutant chloronerva which develops normally only after exogenous supply of NA. NA is an intermediate in the biosynthesis of phytosiderophores from L‐methionine and hence a component of strategy II of iron acquisition. It accumulates in iron deficient barley root tips. NA is a chelator for Fe(II) and other divalent transition metals. In tomato (strategy I), but probably also in strategy II plants, NA is essential for the proper function of Fe(II) dependent processes, such as the regulation of iron deficiency response mechanisms, iron enzymes, and phloem loading/unloading of iron.

Iron deficiency response in relation to iron uptake in two cultivars and a local strain ofMentha arvensisL.

The tolerance to iron-deficiency stress and the iron uptake were studied in two Japanese mint(Mentha arvensis L.) cultivars MAS1 and MS77 and their local strain MA2. A considerable reduction of pH of the nutrient medium in MAS1 and MS77 treatments associated with different degree of chlorosis was found. A rapid recovery from chlorosis was found only in MS77 and to some degree in MAS1, but not in the MA2. The results indicated that iron uptake and translocation were inversely related to iron stress tolerance.

Analysis of Iron-Deficiency Response Systems on the Plant Root, Using the Hairy Root

Analysis of Iron-Deficiency Response Systems on the Plant Root, Using the Hairy Root

Effect of iron deficiency on electron transport processes of plasmalemma-enriched vesicles isolated from cotton roots

To investigate the accelerated transplasmalemma redox activity induced by iron starvation, cotton seedlings ( Gossypium hirsutum L., cv Acala San Jose 2) are grown in nutrient solutions with or without Fe 3+-ethylenediamine tetraacetic acid (FeEDTA). Vesicles enriched in plasmalemma are then isolated from the roots using a sucrose step gradient or by two-phase partitioning. Iron starvation stimulates by approximately two-fold the transport of electrons from NADH to FeEDTA, ferricyanide or cytochrome c when both donor and acceptor are added at the same face of the membrane. Under these conditions, NADH:duroquinone oxidoreductase activity is unaffected. It is concluded that not all of the redox systems at the plasmalemma are stimulated by iron starvation. Regardless of iron nutrition, Triton X-100 stimulates NADH oxidation of the vesicles when ferricyanide and duroquinone are the electron acceptors, but the detergent inhibits oxidation in the presence of cytochrome c. Transplasmalemma redox activity, which resembles more closely that occurring in vivo, is also detected in the vesicle system by measuring the pH gradient or the membrane potential generated as electrons are transported from ascorbate trapped within the vesicle to exogenous ferricyanide. The pH gradient observed while the membrane is not polarized, is steeper in vesicles from iron-starved roots; however, when the membrane potential is measured, Fe starvation results in its becoming less positive. These results are interpreted as being due to a stimulation by iron starvation of both transplasmalemma electron transport and of proton efflux. Based on the above observations and conclusions, we also suggest that plasmalemma-enriched vesicles provide a satisfactory model system to study aspects of the iron deficiency response.

Physiological Disorders of the Nicotianamine-auxothroph Tomato Mutant chloronerva at Different Levels of Iron Nutrition II. Iron Deficiency Response and Heavy Metal Metabolism

The tomato mutant chloronerva exhibits typical symptoms of iron deficiency. Iron deficiencyinduced proton secretion occurs up to 10 µM FeEDTA in the nutrient medium (wild-type only at iron supply lower than 1 µM). Thickened root tips and root hair zones are formed up to 20 µM Fe (wild-type root hairs up to 5 µM). The ferric reductase activity of the rhizodermis remains higher than in the wildtype up to 100 µM Fe supply. In the concentration range of 5 to 10 µM Fe supply the mutant accumulates the 2- to 3-fold amount of iron in the shoot compared with the wild-type. The increased uptake of certain heavy metals (e. g. Cu, Mn, Zn) by the wild-type under iron deficiency is extended to a higher iron supply range in the mutant. All these deviations from the wild-type behaviour are overcome by the supply with nicotianamine (NA) to the leaves. The role of NA as an intracellular transporter for Fe(II) in the regulation of iron metabolism and iron deficiency response mechanisms is discussed.

Association of soil properties and soil and plant iron to iron deficiency response in rice (Oryza Sativa L.)

Abstract Problems are invariably encountered when attempts are made to explain the variability in Bray percent yields or plant response in terms of soil or plant iron (Fe). To resolve this inconsistency, the present investigation was initiated to identify a combination of soil extractable Fe, soil properties and form of plant Fe that may be used as a measure of Fe deficiency. The study involved 16 diverse soils, using upland rice (Oryza sativa L.) as the test crop and Fe‐EDDHA [ferric ethylenediamine di (o‐hydroxyl‐phenyl acetic acid)] as source of Fe. The results showed that Bray percent yields were neither related to DTPA (diethylenetriamine pentaacetic acid) or EDTA (ethylenediamine tetraacetic acid) extractable Fe nor with total plant Fe. Even the inclusion of pH, lime, organic carbon and clay data in the regression equations was of no value. However, Bray percent yields were significantly and positively (r = 0.57* ) associated with ferrous Fe (Fe2+) in 40‐day‐old rice plants. The explanation concerni...

Serum Ferritin in an Elderly Population

Serum ferritin was assayed by an immunoradiometric method in 82 people above 60 years of age. For comparison purposes, the same assay was performed in 71 younger normal adults. The serum ferritin distribution in the elderly had a larger variance than in the younger adults and in addition, there was a clear shift to higher values in the elderly. The latter was more notable in females than in males but there was still a statistically lower mean in elderly females than in elderly males. Ten out of 55 elderly subjects with evidence of iron deficiency (response to oral iron therapy) had a normal or high serum ferritin which suggests that variables unrelated to iron status may operate in determining serum ferritin levels in the elderly. The shift to higher values appears to occur upon reaching grossly 70 years of age. Whether the shift is a physiologically normal event is at present an open question.