Root Fe Uptake Research Articles

Transcriptome analysis of iron over-accumulating Arabidopsis genotypes uncover putative novel regulators of systemic and retrograde signaling.

On account of its competence to accept and donate electrons, iron (Fe) is an essential element across all forms of life, including plants. Maintaining Fe homeostasis requires precise orchestration of its uptake, trafficking, and translocation in order to meet the demand for Fe sinks such as plastids. Plants harboring defects in the systemic Fe transporter OPT3 (OLIGOPEPTIDE TRANSPORTER 3) display constitutive Fe deficiency responses and accumulate toxic levels of Fe in their leaves. Similarly, ectopic expression of IRONMAN (IMA) genes, encoding a family of phloem-localized signaling peptides, triggers the uptake and accumulation of Fe by inhibiting the putative Fe sensor BRUTUS. This study aims at elucidating the mechanisms operating between OPT3-mediated systemic Fe transport, activation of IMA genes in the phloem, and activation of Fe uptake in the root epidermis. Transcriptional profiling of opt3-2 mutant and IMA1/IMA3 overexpressing (IMA Ox) lines uncovered a small subset of genes that were consistently differentially expressed across all three genotypes and Fe-deficient control plants, constituting potential novel regulators of cellular Fe homeostasis. In particular, expression of the the F-box protein At1g73120 was robustly induced in all genotypes, suggesting a putative function in the posttranslational regulation of cellular Fe homeostasis. As further constituents of this module, two plastid-encoded loci that putatively produce transfer ribonucleic acid (tRNA)-derived small ribonucleic acids are possibly involved in retrograde control of root Fe uptake.

Differences in Fe toxicity response index and associated growth characteristics among rice genotypes

ABSTRACT Acid sulfate soils in tropical coastal rice areas contain high Fe levels, and excess ferrous (Fe2+) ions are released during the early growth stage in the wet season. In such environments, Fe toxicity can cause significant yield losses. The objectives of this study were to evaluate the seedling growth of diverse rice germplasm (from japonica, aus and indica groups) under high-Fe conditions in hydroponic culture and to elucidate the growth characteristics of Fe toxicity tolerant genotypes. We calculated an Fe toxicity response index (FTRI) as a selection criterion that reflects both the response of shoot biomass to high Fe and the amount of Fe-induced leaf bronzing damage: higher FTRI values indicate greater tolerance to high Fe levels. We first compared Fe levels to determine the appropriate conditions for genotype selection, and then screened 2- to 3-week-old seedlings grown for 2 more weeks in hydroponic solution containing 500 mg Fe2+ L−1. Sixteen genotypes were screened, and for five of them, the differences in FTRI corresponded with previously reported yield performance in high-Fe fields. We identified Taichung65 (temperate japonica) as a promising donor, as it showed little bronzing damage and high relative (stress/control) biomass under high-Fe conditions, resulting in a high FTRI. Plant nutrient analyses of Taichung65 suggested that maintenance of low Fe concentration and low total Fe content in shoots despite high Fe concentration in the rhizosphere, through prevention of either root Fe uptake or Fe translocation from the root to the shoot, should be an important mechanism of Fe toxicity tolerance.

Effect of calcium and trace metals (Cd, Cu, Mn, Ni, Zn) on root iron uptake in relation to chemical properties of the root-excreted ligands.

Interferences of major cations (Ca2+, Mg2+) and trace metals (TM, i.e. Cd2+, Cu2+, Mn2+, Ni2+ and Zn2+) in root Fe uptakewere evaluated. Root Fe uptake was modelled including the reactions of the root exuded ligand with the soil major and trace cations. Fe uptake was simulated with different ligands representing various affinities for the cations, the latter varying in concentration. The stability constant of Fe complexes (KFeL) does not influence Fe uptake, contrarily to the ligand parameters for Fe-hydroxide dissolution. Fe uptake decreases when KCaL or Ca2+ in solution increases. Presence of TM has nearly no influence on Fe uptake when the TM complexes have low stability constants (KML), as in the case of oxalate and citrate complexes. When ligands have high KML, like EDTA, DFO-B or mugineic acid (MA), TM reduces Fe uptake by 51-55%, and much more in the case of TM contamination. Exudation of Fe ligands with low KML has no negative effect on TM uptake, which can increase if the dissociation rate is high, as for Cu complexes. Ligands with high KML (EDTA, DFO-B, MA) greatly reduce TM uptake, only if their hydrated cations can be absorbed. Calcium does not significantly reduce Fe uptake when Ca-complexes have KCaL < 104. Consequently, ligands like oxalate or MA should be efficient in most soils. TM should perturbate Fe uptake mediated by ligands with high KML such as MA, but not oxalate. Plants exuding phytosiderophores should also absorb TM complexes to avoid micronutrient deficiencies.

Mediation of Zinc and Iron Accumulation in Maize by ZmIRT2, a Novel Iron-Regulated Transporter.

Iron (Fe) is an essential micronutrient for plant growth. Iron-regulated transporters (IRTs) play important roles in Fe2+ uptake and transport in strategy I plants. Maize (Zea mays) belongs to a strategy II plant, in which mugineic acid (MA)-Fe3+ uptake is mainly carried out by Yellow Stripe 1 (YS1). However, ZmIRT1 was previously identified by our laboratory. In this study, we isolated a novel gene from maize (ZmIRT2), which is highly homologous to OsIRT2 and ZmIRT1. ZmIRT2 was expressed in roots and anther and was induced by Fe and zinc (Zn) deficiencies. ZmIRT2-GFP fusion protein localized to the plasma membrane and endoplasmic reticulum. ZmIRT2 reversed growth defects involving Zn and Fe uptake in mutant yeast. ZmIRT2 overexpression in maize led to elevated Zn and Fe levels in roots, shoots and seeds of transgenic plants. Transcript levels of ZmIRT1 were elevated in roots, while levels of YS1 were reduced in shoots of ZmIRT2 transgenic plants. Our results imply that ZmIRT2 may function solely with ZmIRT1 to mediate Fe uptake in roots. ZmIRT1, ZmIRT2 and ZmYS1 may function in a cooperative manner to maintain Zn and Fe homeostasis in ZmIRT2 overexpressing plants. Furthermore, ZmIRT2 could be used in fortification efforts to elevate Zn and Fe levels in crop plants.

Root-to-shoot iron partitioning in Arabidopsis requires IRON-REGULATED TRANSPORTER1 (IRT1) protein but not its iron(II) transport function.

IRON-REGULATED TRANSPORTER1 (IRT1) is the root high-affinity ferrous iron (Fe) uptake system and indispensable for the completion of the life cycle of Arabidopsis thaliana without vigorous Fe supplementation. Here we provide evidence supporting a second role of IRT1 in root-to-shoot partitioning of Fe. We show that irt1 mutants overaccumulate Fe in roots, most prominently in the cortex of the differentiation zone in irt1-2, compared to the wild type. Shoots of irt1-2 are severely Fe-deficient according to Fe content and marker transcripts, as expected. We generated irt1-2 lines producing IRT1 mutant variants carrying single amino-acid substitutions of key residues in transmembrane helices IV and V, Ser206 and His232, which are required for transport activity in yeast. Root short-term 55 Fe uptake rates were uninformative concerning IRT1-mediated transport. Overall irt1-like concentrations of the secondary substrate Mn suggested that the transgenic Arabidopsis lines also remain incapable of IRT1-mediated root Fe uptake. Yet, IRT1S206A partially complements rosette dwarfing and leaf chlorosis of irt1-2, as well as root-to-shoot Fe partitioning and gene expression defects of irt1-2, all of which are fully complemented by wild-type IRT1. Taken together, these results suggest a regulatory function for IRT1 in root-to-shoot Fe partitioning that does not require Fe transport activity of IRT1. Among the genes of which transcript levels are partially dependent on IRT1, we identify MYB DOMAIN PROTEIN10, MYB DOMAIN PROTEIN72 and NICOTIANAMINE SYNTHASE4 as candidates for effecting IRT1-dependent Fe mobilization in roots. Understanding the biological functions of IRT1 will help to improve Fe nutrition and the nutritional quality of agricultural crops.

Increase in root branching enhanced ferric-chelate reductase activity under iron stress in potato (Solanum tuberosum)

In response to Fe-deficiency, various dicots increase their root branching to improve ferric-chelate reductase activity. It still remains unclear, whether the response caused by Fe-deficiency ultimately improves the plant's ability to withstand Fe-deficiency. In this experiment conducted at ICAR-Central Potato Research Institute, Regional Station, Shillong during 2020, we demonstrated a substantial increase in the growth of the lateral root of potato genotype (CP 3443), when grown in the iron-stress, in relation to control plants, and the total lateral root number is well linked to ferric-chelate reductase (FCR) activity. These findings showed that FCR is involved in root Fe uptake in potato (Solanum tuberosum L.) and they suggest a role in Fe distribution throughout the plant. In view of these findings, the Fe-deficiency induced increases in the lateral roots suggested that these play a significant role in Fe-deficiency tolerance in potato, which can serve as useful trait for the identification of chlorosis tolerance and/or nutrient-deficiency stress.

Inhibition of nitric oxide production under alkaline conditions regulates iron homeostasis in rice.

Rice is one of the most susceptible plants to iron (Fe) deficiency under neutral and alkaline conditions. Alkaline stress induces H2 O2 production and increases the deposition of Fe on the root surface, which causes leaf chlorosis and Fe deficiency in rice. Gene chip and qRT-PCR analysis indicated that the expression of the nitrate reductase (NR) genes were downregulated by alkaline treatment, which resulted in significantly decreased nitrate activity and nitric oxide (NO) production in the epidermis and stele, where H2 O2 accumulated. In contrast, treatment with sodium nitroprusside (SNP), a NO donor, strongly alleviated alkaline-induced Fe deficiency by limiting Fe plaque formation. Increasing the NO signal significantly reduced the accumulation of H2 O2 and the lignin barrier but enhanced phenolic acid secretion in the root epidermis and stele under alkaline conditions. The secreted phenolic acid effectively mobilized the apoplast Fe and increased Fe uptake in roots, thereby alleviating the Fe-deficiency response and downregulating the expressions of Fe-uptake genes under alkaline conditions. In conclusion, alkaline stress inhibits NR activity and NO production in the roots of rice, which play vital roles in the mobilization of the apoplast Fe by regulation of H2 O2 and phenolic acid concentrations.

Divergence in response of japonica and hybrid rice to titanium dioxide nanoparticles

The present study aimed to compare the responses of two rice cultivars to titanium dioxide nanoparticles (nTiO2), in plant performance and root-associated soil microbes, and provide a reference for the selection of nanoparticle-responsive varieties. The field experiment was conducted to study the effects of nTiO2 (applied at 200 mg/kg) on typical Chinese rice cultivars, Wuyunjing23 (WYJ23) and Yongyou2640 (YY2640), for the whole life cycle. We evaluated agronomic and physiological parameters including chlorophyll content and antioxidant enzyme activities at the heading stage, and biomass, Ti, and nutrient elements uptake after harvest. Soil bacterial communities were also measured through 16S rRNA gene sequencing. Exposure to nTiO2 led to neither Ti accumulation nor changed biomass in either cultivar, but it increased root Fe uptake and reduced grain Cu content. Specifically, nTiO2 exposure decreased the chlorophyll content but increased the peroxidase activity in WYJ23; such altered physiology might affect root exudates and further modify bacterial communities. Although nTiO2 did not induce visible phytotoxicity in YY2640, it affected the abundance of bacteria genera associated with N turnover and plant-C-metabolizing (e.g., Geobacter and Gaiella), resulting in changed C and N cycling in the rhizosphere. This study revealed divergent responses of WYJ23 and YY2640 to nTiO2, both in plant physiology and rhizosphere system. nTiO2 induced greater physiological response but affected the rhizosphere little in WYJ23. By contrast, nTiO2 changed soil microbes and enzyme activities without affecting the physiology of YY2640, suggesting its superior adaptivity to nTiO2.

Putative cis-Regulatory Elements Predict Iron Deficiency Responses in Arabidopsis Roots.

Plant iron deficiency (-Fe) activates a complex regulatory network that coordinates root Fe uptake and distribution to sink tissues. In Arabidopsis (Arabidopsis thaliana), FER-LIKE FE DEFICIENCY-INDUCED TRANSCRIPTION FACTOR (FIT), a basic helix-loop-helix (bHLH) transcription factor (TF), regulates root Fe acquisition genes. Many other -Fe-induced genes are FIT independent, and instead regulated by other bHLH TFs and by yet unknown TFs. The cis-regulatory code, that is, the cis-regulatory elements (CREs) and their combinations that regulate plant -Fe-responses, remains largely elusive. Using Arabidopsis root transcriptome data and coexpression clustering, we identified over 100 putative CREs (pCREs) that predicted -Fe-induced gene expression in computational models. To assess pCRE properties and possible functions, we used large-scale in vitro TF binding data, positional bias, and evolutionary conservation. As one example, our approach uncovered pCREs resembling IDE1 (iron deficiency-responsive element 1), a known grass -Fe response CRE. Arabidopsis IDE1-likes were associated with FIT-dependent gene expression, more specifically with biosynthesis of Fe-chelating compounds. Thus, IDE1 seems to be conserved in grass and nongrass species. Our pCREs matched among others in vitro binding sites of B3, NAC, bZIP, and TCP TFs, which might be regulators of -Fe responses. Altogether, our findings provide a comprehensive source of cis-regulatory information for -Fe-responsive genes that advance our mechanistic understanding and inform future efforts in engineering plants with more efficient Fe uptake or transport systems.

The PAP/SAL1 retrograde signaling pathway is involved in iron homeostasis.

There is a link between PAP/SAL retrograde pathway, ethylene signaling and Fe metabolism in Arabidopsis. Nuclear gene expression is regulated by a diversity of retrograde signals that travel from organelles to the nucleus in a lineal or classical model. One such signal molecule is 3'-phosphoadenisine-5'-phosphate (PAP) and it's in vivo levels are regulated by SAL1/FRY1, a phosphatase enzyme located in chloroplast and mitochondria. This metabolite inhibits the action of a group of exorribonucleases which participate in post-transcriptional gene expression regulation. Transcriptome analysis of Arabidopsis thaliana mutant plants in PAP-SAL1 pathway revealed that the ferritin genes AtFER1, AtFER3, and AtFER4 are up-regulated. In this work we studied Fe metabolism in three different mutants of the PAP/SAL1 retrograde pathway. Mutant plants showed increased Fe accumulation in roots, shoots and seeds when grown in Fe-sufficient condition, and a constitutive activation of the Strategy I Fe uptake genes. As a consequence, they grew more vigorously than wild type plants in Fe-deficient medium. However, when mutant plants grown in Fe-deficient conditions were sprayed with Fe in their leaves, they were unable to deactivate root Fe uptake. Ethylene synthesis inhibition revert the constitutive Fe uptake phenotype. We propose that there is a link between PAP/SAL pathway, ethylene signaling and Fe metabolism.

IRON MAN is a ubiquitous family of peptides that control iron transport in plants.

Iron (Fe) is an essential mineral nutrient that severely affects the growth, yield and nutritional quality of plants if not supplied in sufficient quantities. Here, we report that a short C-terminal amino-acid sequence consensus motif (IRON MAN; IMA) conserved across numerous, highly diverse peptides in angiosperms is essential for Fe uptake in plants. Overexpression of the IMA sequence in Arabidopsis induced Fe uptake genes in roots, causing accumulation of Fe and manganese in all plant parts including seeds. Silencing of all eight IMA genes harboured in the Arabidopsis genome abolished Fe uptake and caused severe chlorosis; increasing the Fe supply or expressing IMA1 restored the wild-type phenotype. IMA1 is predominantly expressed in the phloem, preferentially in leaves, and reciprocal grafting showed that IMA1 peptides in shoots positively regulate Fe uptake in roots. IMA homologues are highly responsive to the Fe status and functional when heterologously expressed across species. IMA constitutes a novel family of peptides that are critical for the acquisition and cellular homeostasis of Fe across land plants.

Alleviation of seeding chlorosis by plant growth regulators in drip-irrigated rice

ABSTRACTDrip irrigation can produce high rice yields with significant water savings; therefore, it extends rapidly in water-scarce northern China. However, drip-irrigated rice seedlings often exhibit Fe chlorosis. The objective of this field experiment was thus to determine the ability of plant growth regulators to alleviate chlorosis in drip-irrigated rice seedlings. The study compared three plant growth regulators (1-naphthylacetic acid, NAA; sodium nitrophenolate, CSN; and diethyl aminoethyl hexanoate, DA-6) applied in two ways (seed-soaking and drip-application). The results showed that CSN increased root oxidation activity by 37% in the seed-soaking treatment and by 45% in the soil-application treatment. Seed soaking with NAA, CSN, and DA-6 increased the active Fe content in leaves by 8.8%, 17.5%, and 11.4%, respectively, compared with untreated seedlings. Iron absorption and SPAD values were both greater in the soil-application plots than in the seed-soaking plots. Among the plant growth regulators, CSN resulted in the highest yield (2.2% greater than untreated rice in the seed-soaking treatment and 12.8% greater than untreated rice in the soil-application treatment). In conclusion, CSN significantly improved root Fe uptake at the seedling stage and reduced chlorosis in drip-irrigated rice. Therefore, CSN drip application can be recommended for alleviating rice chlorosis in practical use.

Sulfur regulates iron uptake and iron use efficiency in bread and durum wheat

Present study elucidates the role and physiological basis of sulfur (S) mediated iron (Fe) regulation, uptake and use efficiency under deficient availability of soil Fe (4–6 ppm) at different level of applied S [0 (S0), 30 (S30) and 60 (S60) kg ha−1] in bread and durum wheat. Sulfur supply at S30 and S60 greatly improved the Fe and S uptake of the wheat cultivars over S0. Bread wheat recorded a greater agronomic SUE than the durum wheat but it declined with S application in a dose dependent manner in both the wheat cultivars. Further, the increase in grain yield and grain Fe under S application than without it in both the cultivars may also be related to phytosiderophore mediated higher root Fe uptake, and root to shoot and to grain translocation of Fe and subsequent induction of Fe deficiency tolerance response, more so in bread wheat than the durum wheat. It is suggested that the effect of S on Fe uptake under Fe deficient condition may be mediated via an increase in S supply regulated biosynthesis of phytosiderophore and their consequent release in rhizosphere. Results indicate two way interaction between Fe and S, at the rhizosphere and at the plant level.

Cytochrome b5 Reductase 1 Triggers Serial Reactions that Lead to Iron Uptake in Plants

Rhizosphere acidification is essential for iron (Fe) uptake into plant roots. Plasma membrane (PM) H+-ATPases play key roles in rhizosphere acidification. However, it is not fully understood how PM H+-ATPase activity is regulated to enhance root Fe uptake under Fe-deficient conditions. Here, we present evidence that cytochrome b5 reductase 1 (CBR1) increases the levels of unsaturated fatty acids, which stimulate PM H+-ATPase activity and thus lead to rhizosphere acidification. CBR1-overexpressing (CBR1-OX) Arabidopsis thaliana plants had higher levels of unsaturated fatty acids (18:2 and 18:3), higher PM H+-ATPase activity, and lower rhizosphere pH than wild-type plants. By contrast, cbr1 loss-of-function mutant plants showed lower levels of unsaturated fatty acids and lower PM H+-ATPase activity but higher rhizosphere pH. Reduced PM H+-ATPase activity in cbr1 could be restored in vitro by addition of unsaturated fatty acids. Transcript levels of CBR1, fatty acids desaturase2 (FAD2), and fatty acids desaturase3 (FAD3) were increased under Fe-deficient conditions. We propose that CBR1 has a crucial role in increasing the levels of unsaturated fatty acids, which activate the PM H+-ATPase and thus reduce rhizosphere pH. This reaction cascade ultimately promotes root Fe uptake.

Root iron uptake efficiency of Ulmus laevis and U. minor and their distribution in soils of the Iberian Peninsula.

The calcifuge and calcicole character of wild plants has been related to nutrient availability shortages, including iron (Fe)-deficiency. Surprisingly, just a few studies examined the relation between root Fe uptake and plant distribution in different soil types. We assessed the root Fe acquisition efficiency of two Ulmus species with calcareous (Ulmus minor) and siliceous (U. laevis) soil distribution patterns in the Iberian Peninsula. Seedlings of both elm species were grown hydroponically with different Fe concentrations during 6 weeks. Plant physiological responses to Fe-limiting conditions were evaluated as were the ferric reductase activity and proton (H+) extrusion capacity of the roots. Iron deprived elm seedlings of both species were stunted and suffered severe Fe-chlorosis symptoms. After Fe re-supply leaf chlorophyll concentrations rose according to species-dependent patterns. While U. minor leaves and seedlings re-greened evenly, U. laevis did so along the nerves of new growing leaves. U. minor had a higher root ferric reductase activity and H+-extrusion capability than U. laevis and maintained a better nutrient balance when grown under Fe-limiting conditions. The two elm species were found to have different Fe acquisition efficiencies which may be related to their natural distribution in calcareous and siliceous soils of the Iberian Peninsula.

Arabidopsis CYP82C4 expression is dependent on Fe availability and circadian rhythm, and correlates with genes involved in the early Fe deficiency response

Under conditions of reduced iron availability, most frequent in calcareous soils, plants induce the “Fe Deficiency Response” to improve root Fe uptake. The transcription factor FIT is essential for such a response in strategy I plants, such as Arabidopsis thaliana. From microarray analysis of Arabidopsis roots, it is known that three different cytochrome P450 genes, CYP82C4, CYP82C3 and CYP71B5 are up-regulated under Fe deficiency through a FIT-dependent pathway. We show that, out of these three P450 genes, only CYP82C4 strongly correlates with genes involved in metal uptake/transport. The CYP82C4 promoter, unlike those of CYP82C3 and CYP71B5, contains several IDE1-like sequences ( iron deficiency-responsive element) as well as an RY element. While confirming that the CYP82C4 transcript accumulates in Fe-deficient Arabidopsis seedlings, with circadian fluctuations in a light-dependent way, we also demonstrate that such accumulation is suppressed under Fe excess. Full suppression of CYP82C4 expression, as observed in the atc82c4-1 KO mutant, is associated with longer roots at the seedling stage. We propose that CYP82C4 is involved in the early Fe deficiency response, possibly through an IDE1-like mediated pathway.

Kinetic properties of a micronutrient transporter from Pisum sativum indicate a primary function in Fe uptake from the soil.

Fe uptake in dicotyledonous plants is mediated by a root plasma membrane-bound ferric reductase that reduces extracellular Fe(III)-chelates, releasing Fe(2+) ions, which are then absorbed via a metal ion transporter. We previously showed that Fe deficiency induces an increased capacity to absorb Fe and other micronutrient and heavy metals such as Zn(2+) and Cd(2+) into pea ( Pisum sativum L.) roots [Cohen et al. (1998) Plant Physiol 116:1063-1072). To investigate the molecular basis for this phenomenon, an Fe-regulated transporter that is a homologue of the Arabidopsis IRT1 micronutrient transporter was isolated from pea seedlings. This cDNA clone, designated RIT1 for root iron transporter, encodes a 348 amino acid polypeptide with eight putative membrane-spanning domains that is induced under Fe deficiency and can functionally complement yeast mutants defective in high- and low-affinity Fe transport. Chelate buffer techniques were used to control Fe(2+) in the uptake solution at nanomolar activities representative of those found in the rhizosphere, and radiotracer methodologies were employed to show that RIT1 is a very high-affinity (59)Fe(2+) uptake system ( K(m) =54-93 nM). Additionally, radiotracer ((65)Zn, (109)Cd) flux techniques were used to show that RIT can also mediate a lower affinity Zn and Cd influx ( K(m) of 4 and 100 microM, for Zn(2+) and Cd(2+), respectively). These findings suggest that, in typical agricultural soils, RIT1 functions primarily as a high-affinity Fe(2+) transporter that mediates root Fe acquisition. This is consistent with recent findings with Arabidopsis IRT1 knockout mutants that strongly suggest that this transporter plays a key role in root Fe uptake and nutrition. However, the ability of RIT1 to facilitate Zn and Cd uptake when these metals are present at elevated concentrations suggests that RIT1 may be one pathway for the entry of toxic metals into the food chain. Furthermore, the finding that plant Fe deficiency status may promote heavy metal uptake via increased expression of this transporter could have implications both for human nutrition and also for phytoremediation, the use of terrestrial plants to sequester toxic metals from contaminated soil.

Flux of protons released by wild type and ferritin over-expressor tobacco plants : effect of phosphorus and iron nutrition

Tobacco ( Nicotiana tabacum) plants over-expressing the iron storage protein ferritin, either in the cytoplasm or in the plastids, were grown under various P and Fe conditions. The crossed effects of both the genotypes and the environmental conditions on iron and chlorophyll concentrations in leaves, ferric reductase (EC 1.6.99.13) and plasmalemma H +-ATPase (EC 3.6.3.6) activities in roots, and fluxes of H + released by roots were determined. The increase in leaf Fe concentration observed in plants over-expressing ferritin was accompanied by an increase in root ferric reductase and H +-ATPase activities. Iron deficient conditions induced a decrease in Fe and chlorophyll concentrations in leaves, an increase in ferric reductase and H +-ATPase activities in roots and an increase in H +-flux released by roots in all genotypes. Phosphorus abundant conditions induced also an increase in ferric reductase and H +-ATPase activities in roots in all genotypes, and an increase in H +-flux released by roots in the genotype over-expressing ferritin in the cytoplasm. These results suggest that P could regulate the root Fe uptake system, at the gene expression level, as already reported for Zn in barley. Moreover, they show that H +-flux was not always consistent with H +-ATPase activity, revealing that the global measurement of root plasmalemma H +-ATPase activity has some limits compared to the localized determination of the H +-flux. These limits could be due to the fact that part of the H +-flux is not directly linked to H +-ATPase activity in roots.

Characterization of FRO1, a pea ferric-chelate reductase involved in root iron acquisition.

To acquire iron, many plant species reduce soil Fe(III) to Fe(II) by Fe(III)-chelate reductases embedded in the plasma membrane of root epidermal cells. The reduced product is then taken up by Fe(II) transporter proteins. These activities are induced under Fe deficiency. We describe here the FRO1 gene from pea (Pisum sativum), which encodes an Fe(III)-chelate reductase. Consistent with this proposed role, FRO1 shows similarity to other oxidoreductase proteins, and expression of FRO1 in yeast conferred increased Fe(III)-chelate reductase activity. Furthermore, FRO1 mRNA levels in plants correlated with Fe(III)-chelate reductase activity. Sites of FRO1 expression in roots, leaves, and nodules were determined. FRO1 mRNA was detected throughout the root, but was most abundant in the outer epidermal cells. Expression was detected in mesophyll cells in leaves. In root nodules, mRNA was detected in the infection zone and nitrogen-fixing region. These results indicate that FRO1 acts in root Fe uptake and they suggest a role in Fe distribution throughout the plant. Characterization of FRO1 has also provided new insights into the regulation of Fe uptake. FRO1 expression and reductase activity was detected only in Fe-deficient roots of Sparkle, whereas both were constitutive in brz and dgl, two mutants with incorrectly regulated Fe accumulation. In contrast, FRO1 expression was responsive to Fe status in shoots of all three plant lines. These results indicate differential regulation of FRO1 in roots and shoots, and improper FRO1 regulation in response to a shoot-derived signal of iron status in the roots of the brz and dgl mutants.

Mechanism of iron uptake by plants

Abstract. Green plants require a continuous supply of Fe as they grow, because Fe does not not move from the older to the newer leaves. Soils do not lack Fe per se, but it may not be available to plants grown in alkaline soils. Plants are classed ‘Fe‐efficient’ if they respond to Fe‐deficiency stress by inducing biochemical reactions that make Fe available in a useful form, and ‘Fe‐inefficienT’ if they do not. Iron uptake induced in response to Fe stress involves release of hydrogen ions and reductants by the root. The lowered pH and presence of reductant at the root zone, along with reduction of Fe3+ to Fe2+ at the root surface, enables Fe2+ to be taken up primarily through the young lateral roots. Ferrous iron is present throughout the protozylem and may or may not have entered the root by a carrier. The root‐absorbed Fe2+ is oxidized to Fe3+ at the junction of the protoxylem and the metaxylem, chelated by citrate, and then transported in the metaxylem to the plant top. In the plant, the chemical reactions injuced by Fe‐deficiency stress may affect nitrate reductase activity, use of Fe from Fe3+ phosphate and chelating agents, and tolerance to heavy metals. An efficient mechanism for Fe uptake in roots appears to be important for the efficient use of Fe in plant tops.