Flavins secreted by roots of iron-deficient Beta vulgaris enable mining of ferric oxide via reductive mechanisms

Hydrolysis of riboflavins in root exudates under iron deficiency and alkaline stress

Riboflavins are secreted under iron deficiency as a part of the iron acquisition Strategy I, mainly when the external pH is acidic. In plants growing under Fe-deficiency and alkaline conditions, riboflavins have been reported to accumulate inside the roots, with very low or negligible secretion. However, the fact that riboflavins may undergo hydrolysis under alkaline conditions has been so far disregarded. In this paper, we report the presence of riboflavin derivatives and products of their alkaline hydrolysis (lumichrome, lumiflavin and carboxymethylflavin) in nutrient solutions of Cucumis sativus plants grown under different iron regimes (soluble Fe-EDDHA in the nutrient solution, total absence of iron in the nutrient solution, or two different doses of FeSO4 supplied as a foliar spray), either cultivated in slightly acidic (pH 6) or alkaline (pH 8.8, 10 mM bicarbonate) nutrient solutions. The results show that root synthesis and exudation of riboflavins is controlled by shoot iron status, and that exuded riboflavins undergo hydrolysis, especially at alkaline pH, with lumichrome being the main product of hydrolysis.

Rhizosphere metabolic cross-talk from plant-soil-microbe tapping into agricultural sustainability: Current advance and perspectives

Rhizosphere interactions from plant-soil-microbiome occur dynamically all the time in the "black microzone" underground, where we can't see intuitively. Rhizosphere metabolites including root exudates and microbial metabolites act as various chemical signalings involving in rhizosphere interactions, and play vital roles on plant growth, development, disease suppression and resistance to stress conditions as well as proper soil health. Although rhizosphere metabolites are a mixture from plant roots and soil microbes, they often are discussed alone. As a rapid appearance of various omics platforms and analytical methods, it offers possibilities and opportunities for exploring rhizosphere interactions in unprecedented breadth and depth. However, our comprehensive understanding about the fine-tuning mechanisms of rhizosphere interactions mediated by these chemical compounds still remain clear. Thus, this review summarizes recent advances systemically including the features of rhizosphere metabolites and their effects on rhizosphere ecosystem, and looks forward to the future research perspectives, which contributes to facilitating better understanding of biochemical communications belowground and helping identify novel rhizosphere metabolites. We also address challenges for promoting the understanding about the roles of rhizosphere metabolites in different environmental stresses.

Iron Nutrition and Its Biochemical Interactions in Plants: Iron Uptake, Biofortification, Bacteria, and Fungi in Focus

Microelements are vital for plant growth and development [...].

Iron transport mechanisms and their evolution focusing on chloroplasts

Iron (Fe) is an essential element for photosynthetic organisms, required for several vital biological functions. Photosynthesis, which takes place in the chloroplasts of higher plants, is the major Fe consumer. Although the components of the root Fe uptake system in dicotyledonous and monocotyledonous plants have been extensively studied, the Fe transport mechanisms of chloroplasts in these two groups of plants have received little attention. This review focuses on the comparative analysis of Fe transport processes in the evolutionary ancestors of chloroplasts (cyanobacteria) with the processes in embryophytes and green algae (Viridiplantae). The aim is to summarize how chloroplasts are integrated into cellular Fe homeostasis and how Fe transporters and Fe transport mechanisms have been modified by evolution.

Heterogeneous nutrient supply modulates root exudation and accumulation of medicinally valuable compounds in Artemisia annua and Hypericum perforatum

Plants have evolved complex mechanisms to adapt to nutrient-deficient environments, including stimulating lateral root proliferation into local soil patches with high nutrient content in response to heterogeneous nutrient distribution. Despite the widespread occurrence of this phenomenon in soil, the effect of heterogeneous nutrient distribution on the accumulation of secondary compounds in plant biomass and their exudation by roots remains largely unknown. This study aims to fill this critical knowledge gap by investigating how deficiency and unequal distributions of nitrogen (N), phosphorus (P), and iron (Fe) affect plant growth and accumulation of the antimalarial drug artemisinin (AN) in leaves and roots of Artemisia annua, as well as AN exudation by roots. Heterogeneous N and P supplies strongly increased root exudation of AN in half of a split-root system exposed to nutrient deficiency. By contrast, exposure to a homogeneous nitrate and phosphate deficiency did not modulate root exudation of AN. This indicates that a combination of local and systemic signals, reflecting low and high nutritional statuses, respectively, were required to enhance AN exudation. This exudation response was independent of the regulation of root hair formation, which was predominantly modulated by the local signal. In contrast to the heterogeneous supply of N and P, heterogeneous Fe supply did not modulate AN root exudation but increased AN accumulation in locally Fe-deficient roots. No modulation of nutrient supply significantly changed the accumulation of AN in A. annua leaves. The impact of a heterogeneous nitrate supply on growth and phytochemical composition was also investigated in Hypericum perforatum plants. Unlike in A. annue, the uneven N supply did not significantly influence the exudation of secondary compounds in the roots of H. perforatum. However, it did enhance the accumulation of several biologically active compounds, such as hypericin, catechin, and rutin isomers, in the leaves of H. perforatum. We propose that the capacity of plants to induce the accumulation and/or differential exudation of secondary compounds under heterogeneous nutrient supply is both species- and compound-specific. The ability to differentially exude AN may contribute to A. annua's adaptation to nutrient disturbances and modulate allelopathic and symbiotic interactions in the rhizosphere.

Cation diffusion facilitator proteins of Beta vulgaris reveal diversity of metal handling in dicotyledons

Manganese (Mn), iron (Fe), and zinc (Zn) are essential for diverse processes in plants, but their availability is often limiting or excessive. Cation diffusion facilitator (CDF) proteins have been implicated in the allocation of those metals in plants, whereby most of our mechanistic understanding has been obtained in Arabidopsis. It is unclear to what extent this can be generalized to other dicots. We characterized all CDFs/metal tolerance proteins of sugar beet (Beta vulgaris spp. vulgaris), which is phylogenetically distant from Arabidopsis. Analysis of subcellular localization, substrate selectivities, and transcriptional regulation upon exposure to metal deficiencies and toxicities revealed unexpected deviations from their Arabidopsis counterparts. Localization and selectivity of some members were modulated by alternative splicing. Notably, unlike in Arabidopsis, Mn- and Zn-sequestrating members were not induced in Fe-deficient roots, pointing to differences in the Fe acquisition machinery. This was supported by low Zn and Mn accumulation under Fe deficiency and a strikingly increased Fe accumulation under Mn and Zn excess, coinciding with an induction of BvIRT1. High Zn load caused a massive upregulation of Zn-BvMTPs. The results suggest that the employment of the CDF toolbox is highly diverse amongst dicots, which questions the general applicability of metal homeostasis models derived from Arabidopsis.

Indole-3-acetaldoxime delays root iron-deficiency responses and modify auxin homeostasis in Medicago truncatula

Iron (Fe) is an essential plant micronutrient, being a major limiting growth factor in calcareous soils. To increase Fe uptake, plants induce lateral roots growth, the expression of a Fe(III)-chelate reductase (FCR), a Fe(II)-transporter and a H⁺-ATPase and the secretion of flavins. Furthermore, auxin hormone family is involved in the Fe-deficiency responses but the action mechanism remains elusive. In this work, we evaluated the effect of the auxin-precursor indole-3-acetaldoxime (IAOx) on hydroponically grown Medicago truncatula plants under different Fe conditions. Upon 4-days of Fe starvation, the pH of the nutrient solution decreased, while both the FCR activity and the presence of flavins increased. Exogenous IAOx increased lateral roots growth contributing to superroot phenotype, decreased chlorosis, and delayed up to 3-days the pH-decrease, the FCR-activity increase, and the presence of flavins, compared to Fe-deficient plants. Gene expression levels were in concordance with the physiological responses. RESULTS: showed that IAOx was immediately transformed to IAN in roots and shoots to maintain auxin homeostasis. IAOx plays an active role in iron homeostasis delaying symptoms and responses in Fe-deficient plants. We may speculate that IAOx or its derivatives remobilize Fe from root cells to alleviate Fe-deficiency. Overall, these results point out that the IAOx-derived phenotype may have advantages to overcome nutritional stresses.

Protein kinase MtCIPK12 modulates iron reduction in Medicago truncatula by regulating riboflavin biosynthesis

Iron (Fe) is an essential micronutrient, and deficiency in available Fe is one of the most important limiting factors for plant growth. In some species including Medicago truncatula, Fe deficiency results in accumulation of riboflavin, a response associated with Fe acquisition. However, how the plant's Fe status is integrated to tune riboflavin biosynthesis and how riboflavin levels affect Fe acquisition and utilization remains largely unexplored. We report that protein kinase CIPK12 regulates ferric reduction by accumulation of riboflavin and its derivatives in roots of M. truncatula via physiological and molecular characterization of its mutants and over-expressing materials. Mutations in CIPK12 enhance Fe accumulation and improve photosynthetic efficiency, whereas overexpression of CIPK12 shows the opposite phenotypes. The Calcineurin B-like proteins CBL3 and CBL8 interact with CIPK12, which negatively regulates the expression of genes encoding key enzymes in the riboflavin biosynthesis pathway. CIPK12 negatively regulates Fe acquisition by suppressing accumulation of riboflavin and its derivatives in roots, which in turn influences ferric reduction activity by riboflavin-dependent electron transport under Fe deficiency. Our findings uncover a new regulatory mechanism by which CIPK12 regulates riboflavin biosynthesis and Fe-deficiency responses in plants.

Entomopathogenic Fungi-Mediated Solubilization and Induction of Fe Related Genes in Melon and Cucumber Plants

Endophytic insect pathogenic fungi have a multifunctional lifestyle; in addition to its well-known function as biocontrol agents, it may also help plants respond to other biotic and abiotic stresses, such as iron (Fe) deficiency. This study explores M. brunneum EAMa 01/58-Su strain attributes for Fe acquisition. Firstly, direct attributes include siderophore exudation (in vitro assay) and Fe content in shoots and in the substrate (in vivo assay) were evaluated for three strains of Beauveria bassiana and Metarhizium bruneum. The M. brunneum EAMa 01/58-Su strain showed a great ability to exudate iron siderophores (58.4% surface siderophores exudation) and provided higher Fe content in both dry matter and substrate compared to the control and was therefore selected for further research to unravel the possible induction of Fe deficiency responses, Ferric Reductase Activity (FRA), and relative expression of Fe acquisition genes by qRT-PCR in melon and cucumber plants.. In addition, root priming by M. brunneum EAMa 01/58-Su strain elicited Fe deficiency responses at transcriptional level. Our results show an early up-regulation (24, 48 or 72 h post inoculation) of the Fe acquisition genes FRO1, FRO2, IRT1, HA1, and FIT as well as the FRA. These results highlight the mechanisms involved in the Fe acquisition as mediated by IPF M. brunneum EAMa 01/58-Su strain.

Apoplast utilisation of nanohaematite initiates parallel suppression of RIBA1 and FRO1&3 in Cucumis sativus

Nanoscale Fe containing particles can penetrate the root apoplast. Nevertheless, cell wall size exclusion questions that for Fe mobilisation, a close contact between the membrane integrating FERRIC REDUCTASE OXIDASE (FRO) enzymes and Fe containing particles is required. Haematite nanoparticle suspension, size of 10-20 nm, characterized by ⁵⁷Fe Mössbauer spectroscopy, TEM, ICP and SAED was subjected to Fe utilisation by the flavin secreting model plant cucumber (Cucumis sativus). Alterations in the structure and distribution of the particles were revealed by ⁵⁷Fe Mössbauer spectroscopy, HRTEM and EDS element mapping. Biological utilisation of Fe resulted in a suppression of Fe deficiency responses (expression of CsFRO 1, 2 & 3 and RIBOFLAVIN A1; CsRIBA1 genes and root ferric chelate reductase activity). Haematite nanoparticles were stacked in the middle lamella of the apoplast. Fe mobilisation is evidenced by the reduction in the particle size. Fe release from nanoparticles does not require a contact with the plasma membrane. Parallel suppression in the CsFRO 1&3 and CsRIBA1 transcript amounts support that flavin biosynthesis is an inclusive Fe deficiency response involved in the reduction-based Fe utilisation of Cucumis sativus roots. CsFRO2 is suggested to play a role in the intracellular Fe homeostasis.

Iron Availability and Homeostasis in Plants: A Review of Responses, Adaptive Mechanisms, and Signaling

Iron is an essential element for all living organisms, playing a major role in plant biochemistry as a redox catalyst based on iron redox properties. Iron is the fourth most abundant element of the Earth's crust, but its uptake by plants is complex because it is often in insoluble forms that are not easily accessible for plants to use. The physical and chemical speciation of iron, as well as rhizosphere activity, are key factors controlling the bioavailability of Fe. Iron can be under reduced (Fe²⁺) or oxidized (Fe³⁺) ionic forms, adsorbed onto mineral surfaces, forming complexes with organic molecules, precipitated to form poorly crystalline hydroxides to highly crystalline iron oxides, or included in crystalline Fe-rich mineral phases. Plants must thus adapt to a complex and changing iron environment, and their response is finely regulated by multiple signaling pathways initiated by a diversity of stimulus perceptions. Higher plants possess two separate strategies to uptake iron from rhizosphere soil: the chelation strategy and the reduction strategy in grass and non-grass plants, respectively. Molecular actors involved in iron uptake and mobilization through the plant have been characterized for both strategies. All these processes that contribute to iron homeostasis in plants are highly regulated in response to iron availability by downstream signaling responses, some of which are characteristic signaling signatures of iron dynamics, while others are shared with other environmental stimuli. Recent research has thus revealed key transcription factors, cis-acting elements, post-translational regulators, and other molecular mechanisms controlling these genes or their encoded proteins in response to iron availability. In addition, the most recent research is increasingly highlighting the crosstalk between iron homeostasis and nutrient response regulation. These regulatory processes help to avoid plant iron concentrations building up to potential cell functioning disruptions that could adversely affect plant fitness. Indeed, when iron is in excess in the plant, it can lead to the production and accumulation of dangerous reactive oxygen species and free radicals (H₂O₂, HO^•, O₂^•-, HO^•₂) that can cause considerable damages to most cellular components. To cope with iron oxidative stress, plants have developed defense systems involving the complementary action of antioxidant enzymes and molecular antioxidants, safe iron-storage mechanisms, and appropriate morphological adaptations.

The active Fe chelator proline-2'-deoxymugineic acid enhances peanut yield by improving soil Fe availability and plant Fe status

Iron (Fe) deficiency restricts crop yields in calcareous soil. Thus, a novel Fe chelator, proline-2'-deoxymugineic acid (PDMA), based on the natural phytosiderophore 2'-deoxymugineic acid (DMA), was developed to solve the Fe deficiency problem. However, the effects and mechanisms of PDMA relevant to the Fe nutrition and yield of dicots grown under field conditions require further exploration. In this study, pot and field experiments with calcareous soil were conducted to investigate the effects of PDMA on the Fe nutrition and yield of peanuts. The results demonstrated that PDMA could dissolve insoluble Fe in the rhizosphere and up-regulate the expression of the yellow stripe-like family gene AhYSL1 to improve the Fe nutrition of peanut plants. Moreover, the chlorosis and growth inhibition caused by Fe deficiency were significantly diminished. Notably, under field conditions, the peanut yield and kernel micronutrient contents were promoted by PDMA application. Our results indicate that PDMA promotes the dissolution of insoluble Fe and a rich supply of Fe in the rhizosphere, increasing yields through integrated improvements in soil-plant Fe nutrition at the molecular and ecological levels. In conclusion, the efficacy of PDMA for improving the Fe nutrition and yield of peanut indicates its outstanding potential for agricultural applications.

Root exudates contribute to belowground ecosystem hotspots: A review

Root exudates are an essential carrier for material cycling, energy exchange, and information transfer between the belowground parts of plants and the soil. We synthesize current properties and regulators of root exudates and their role in the belowground ecosystem as substances cycle and signal regulation. We discussed the composition and amount of root exudates and their production mechanism, indicating that plant species, growth stage, environmental factors, and microorganisms are primary influence factors. The specific mechanisms by which root secretions mobilize the soil nutrients were summarized. First, plants improve the nutrient status of the soil by releasing organic acids for acidification and chelation. Then, root exudates accelerated the SOC turnover due to their dual impacts, forming and destabilizing aggregates and MASOC. Eventually, root exudates mediate the plant-plant interaction and plant-microbe interaction. Additionally, a summary of the current collection methods of root exudates is presented.

Iron uptake, signaling, and sensing in plants

Iron (Fe) is an essential micronutrient that affects the growth and development of plants because it participates as a cofactor in numerous physiological and biochemical reactions. As a transition metal, Fe is redox active. Fe often exists in soil in the form of insoluble ferric hydroxides that are not bioavailable to plants. Plants have developed sophisticated mechanisms to ensure an adequate supply of Fe in a fluctuating environment. Plants can sense Fe status and modulate the transcription of Fe uptake-associated genes, finally controlling Fe uptake from soil to root. There is a critical need to understand the molecular mechanisms by which plants maintain Fe homeostasis in response to Fe fluctuations. This review focuses on recent advances in elucidating the functions of Fe signaling components. Taking Arabidopsis thaliana and Oryza sativa as examples, this review begins by discussing the Fe acquisition systems that control Fe uptake from soil, the major components that regulate Fe uptake systems, and the perception of Fe status. Future explorations of Fe signal transduction will pave the way for understanding the regulatory mechanisms that underlie the maintenance of plant Fe homeostasis.

Strategies and Bottlenecks in Hexaploid Wheat to Mobilize Soil Iron to Grains

Our knowledge of iron (Fe) uptake and mobilization in plants is mainly based on Arabidopsis and rice. Although multiple players of Fe homeostasis have been elucidated, there is a significant gap in our understanding of crop species, such as wheat. It is, therefore, imperative not only to understand the different hurdles for Fe enrichment in tissues but also to address specifically the knowns/unknowns involved in the plausible mechanism of Fe sensing, signaling, transport, and subsequent storage in plants. In the present review, a unique perspective has been described in light of recent knowledge generated in wheat, an economically important crop. The strategies to boost efficient Fe uptake, transcriptional regulation, and long-distance mobilization in grains have been discussed, emphasizing recent biotechnological routes to load Fe in grains. This article also highlights the new elements of physiological and molecular genetics that underpin the mechanistic insight for the identified Fe-related genes and discusses the bottlenecks in unloading the Fe in grains. The information presented here will provide much-needed resources and directions to overcome challenges and design efficient strategies to enhance the Fe density in wheat grains.

Plant iron nutrition: the long road from soil to seeds

Iron (Fe) is an essential plant micronutrient since many cellular processes including photosynthesis, respiration, and the scavenging of reactive oxygen species depend on adequate Fe levels; however, non-complexed Fe ions can be dangerous for cells, as they can act as pro-oxidants. Hence, plants possess a complex homeostatic control system for safely taking up Fe from the soil and transporting it to its various cellular destinations, and for its subcellular compartmentalization. At the end of the plant's life cycle, maturing seeds are loaded with the required amount of Fe needed for germination and early seedling establishment. In this review, we discuss recent findings on how the microbiota in the rhizosphere influence and interact with the strategies adopted by plants to take up iron from the soil. We also focus on the process of seed-loading with Fe, and for crop species we also consider its associated metabolism in wild relatives. These two aspects of plant Fe nutrition may provide promising avenues for a better comprehension of the long pathway of Fe from soil to seeds.

Root exudates impact plant performance under abiotic stress

Plant root exudates serve pivotal roles in supporting plant development and interactions with the physicochemical and biological factors in the rhizosphere. Under stress conditions, root exudation is involved in enhancing plant resource-use efficiency and facilitating the crosstalk between plant and soil microbes to ameliorate stress. Although there are a large number of root exudates that remain to be characterized, recent technological advancements have allowed for the function of many exudate compounds to be elucidated. In this review, we discuss current knowledge about the key root exudates that modulate plant resource-use efficiency under various abiotic stresses including drought, aluminum toxicity, phosphorus, nitrogen, and iron deficiency. The role that key root exudates play in shaping microbial communities in the rhizosphere under stress conditions is also an important consideration addressed in this review.

Reactions of two xeric-congeneric species of Centaurea (Asteraceae) to soils with different pH values and iron availability

Centaurea scabiosa L. and C. stoebe Tausch are known to co-exist naturally in two extremely different types of open dry habitats in the temperate zone, alkaline xerothermic grasslands and acidic dry grasslands. However, knowledge about their preferences to edaphic conditions, including soil acidity (pH), and iron (Fe) availability is scarce. Therefore, experimental comparison of soil requirements (acidic Podzol vs alkaline Rendzina) of these species was carried out. The study was designed as a pot experiment and conducted under field conditions. Fe availability was increased by application of Fe-HBED. Reactions of plants to edaphic conditions were determined using growth measurements, leaf morphometric measurements, chlorosis scoring, chlorophyll content and chlorophyll a fluorescence (OJIP) quantification as well as determination of element content (Ca, Mg, Fe, Mn, Zn and Cu). Growth and leaf morphometrical traits of the studied congeneric species were affected similarly by the soil type and differently by the chelate treatment. Increased availability of Fe in Rendzina contrasted the species, as treatment with 25 µmol Fe-HBED kg⁻¹ soil promoted growth only in C. stoebe. Both species turned out to be resistant to Fe-dependent chlorosis which was also reflected in only minor changes in chlorophyll a fluorescence parameters. Both species showed relatively low nutritional demands. Surprisingly, Fe-HBED did not stimulate Fe acquisition in the studied species, nor its translocation along the root:shoot axis. Furthermore, contrary to expectations, C. scabiosa took up less Fe from the acidic than alkaline soil. C. scabiosa not only absorbed more Ca and Zn but also translocated greater amounts of these elements to shoots than C. stoebe. Both species acquired more Mg on Podzol than on Rendzina which suggests adaptation allowing avoidance of aluminum (Al) toxicity on acidic soils. Overall, it seems that C. scabiosa prefers alkaline soils, whilst C. stoebe prefers acidic ones.

Ethylene and Nitric Oxide Involvement in the Regulation of Fe and P Deficiency Responses in Dicotyledonous Plants

Iron (Fe) and phosphorus (P) are two essential elements for plant growth. Both elements are abundant in soils but with poor availability for plants, which favor their acquisition by developing morphological and physiological responses in their roots. Although the regulation of the genes related to these responses is not totally known, ethylene (ET) and nitric oxide (NO) have been involved in the activation of both Fe-related and P-related genes. The common involvement of ET and NO suggests that they must act in conjunction with other specific signals, more closely related to each deficiency. Among the specific signals involved in the regulation of Fe- or P-related genes have been proposed Fe-peptides (or Fe ion itself) and microRNAs, like miR399 (P), moving through the phloem. These Fe- or P-related phloem signals could interact with ET/NO and confer specificity to the responses to each deficiency, avoiding the induction of the specific responses when ET/NO increase due to other nutrient deficiencies or stresses. Besides the specificity conferred by these signals, ET itself could confer specificity to the responses to Fe- or P-deficiency by acting through different signaling pathways in each case. Given the above considerations, there are preliminary results suggesting that ET could regulate different nutrient responses by acting both in conjunction with other signals and through different signaling pathways. Because of the close relationship among these two elements, a better knowledge of the physiological and molecular basis of their interaction is necessary to improve their nutrition and to avoid the problems associated with their misuse. As examples of this interaction, it is known that Fe chlorosis can be induced, under certain circumstances, by a P over- fertilization. On the other hand, Fe oxides can have a role in the immobilization of P in soils. Qualitative and quantitative assessment of the dynamic of known Fe- and P-related genes expression, selected ad hoc and involved in each of these deficiencies, would allow us to get a profound knowledge of the processes that regulate the responses to both deficiencies. The better knowledge of the regulation by ET of the responses to these deficiencies is necessary to properly understand the interactions between Fe and P. This will allow the obtention of more efficient varieties in the absorption of P and Fe, and the use of more rational management techniques for P and Fe fertilization. This will contribute to minimize the environmental impacts caused by the use of P and Fe fertilizers (Fe chelates) in agriculture and to adjust the costs for farmers, due to the high prices and/or scarcity of Fe and P fertilizers. This review aims to summarize the latest advances in the knowledge about Fe and P deficiency responses, analyzing the similarities and differences among them and considering the interactions among their main regulators, including some hormones (ethylene) and signaling substances (NO and GSNO) as well as other P- and Fe-related signals.

Iron deficiency-inducible peptide-coding genes OsIMA1 and OsIMA2 positively regulate a major pathway of iron uptake and translocation in rice

Under low iron (Fe) availability, plants transcriptionally induce various genes responsible for Fe uptake and translocation to obtain adequate amounts of Fe. Although transcription factors and ubiquitin ligases involved in these Fe deficiency responses have been identified, the mechanisms coordinating these pathways have not been clarified in rice. Recently identified Fe-deficiency-inducible IRON MAN (IMA)/FE UPTAKE-INDUCING PEPTIDE (FEP) positively regulates many Fe-deficiency-inducible genes for Fe uptake in Arabidopsis. Here, we report that the expression of two IMA/FEP genes in rice, OsIMA1 and OsIMA2, is strongly induced under Fe deficiency, positively regulated by the transcription factors IDEF1, OsbHLH058, and OsbHLH059, as well as OsIMA1 and OsIMA2 themselves, and negatively regulated by HRZ ubiquitin ligases. Overexpression of OsIMA1 or OsIMA2 in rice conferred tolerance to Fe deficiency and accumulation of Fe in leaves and seeds. These OsIMA-overexpressing rice exhibited enhanced expression of all of the known Fe-deficiency-inducible genes involved in Fe uptake and translocation, except for OsYSL2, a Fe-nicotianamine transporter gene, in roots but not in leaves. Knockdown of OsIMA1 or OsIMA2 caused minor effects, including repression of some Fe uptake- and translocation-related genes in OsIMA1 knockdown roots. These results indicate that OsIMA1 and OsIMA2 play key roles in enhancing the major pathway of the Fe deficiency response in rice.

Iron deficient Medicago scutellata grown in nutrient solution at high pH accumulates and secretes large amounts of flavins

Flavin synthesis and secretion is an integral part of the toolbox of root-borne Fe facilitators used by Strategy I species upon Fe deficiency. The Fe-deficiency responses of the wild legume Medicago scutellata grown in nutrient solution have been studied at two different pH values (5.5 and 7.5). Parameters studied include leaf chlorophyll, nutrient solution pH, concentrations and contents of micronutrients, flavin accumulation in roots, flavin export to the medium, and root ferric chelate reductase and acidification activities. Results show that M. scutellata behaves upon Fe deficiency as a Strategy I species, with a marked capacity for synthesizing flavins (riboflavin and three hydroxylated riboflavin derivatives), which becomes more intense at high pH. Results also show that this species is capable of exporting a large amount of flavins to the external medium, both at pH 5.5 and 7.5. This is the first report of a species having a major flavin secretion at pH 7.5, in contrast with the very low flavin secretion found in other flavin-producing species such as Beta vulgaris and M. truncatula. These results provide further support to the hypothesis that flavin secretion is relevant for Fe acquisition at high pH, and open the possibility to improve the Fe-efficiency responses in legumes of agronomic interest.

Importance of the Rhizosphere Microbiota in Iron Biofortification of Plants

Increasing the iron content of plant products and iron assimilability represents a major issue for human nutrition and health. This is also a major challenge because iron is not readily available for plants in most cultivated soils despite its abundance in the Earth's crust. Iron biofortification is defined as the enhancement of the iron content in edible parts of plants. This biofortification aims to reach the objectives defined by world organizations for human nutrition and health while being environment friendly. A series of options has been proposed to enhance plant iron uptake and fight against hidden hunger, but they all show limitations. The present review addresses the potential of soil microorganisms to promote plant iron nutrition. Increasing knowledge on the plant microbiota and plant-microbe interactions related to the iron dynamics has highlighted a considerable contribution of microorganisms to plant iron uptake and homeostasis. The present overview of the state of the art sheds light on plant iron uptake and homeostasis, and on the contribution of plant-microorganism (plant-microbe and plant-plant-microbe) interactions to plant nutritition. It highlights the effects of microorganisms on the plant iron status and on the co-occurring mechanisms, and shows how this knowledge may be valued through genetic and agronomic approaches. We propose a change of paradigm based on a more holistic approach gathering plant and microbial traits mediating iron uptake. Then, we present the possible applications in plant breeding, based on plant traits mediating plant-microbe interactions involved in plant iron uptake and physiology.

Oxidation of Fe(II) by Flavins under Anoxic Conditions

Flavin-mediated electron transfer is an important pathway for Fe(III) reduction by dissimilatory iron-reducing bacteria. Although the mechanisms and kinetics of Fe(III) reduction by reduced flavins have been widely studied, the reaction between Fe(II) and oxidized flavins is rarely investigated. Results of this study show that under anoxic conditions, Fe(II) can be oxidized by the oxidized forms of riboflavin (RBF) and flavin mononucleotide (FMN) at pH 7-9. For instance, at pH 9, 73% of 17.8 μM Fe(II) was oxidized by 10 μM RBF within 20 min. Both the rate and extent of oxidation increased with increasing concentrations of oxidized flavins and increasing solution pH. Thermodynamic calculations and kinetic analyses implied that the oxidation of Fe(II) proceeded predominantly via the autodecomposition of Fe²⁺-RBF^- and Fe²⁺-FMN^- complexes, along with minor contributions from direct oxidation of Fe(II) by flavins and flavin radicals. Our findings suggest that the reoxidation of Fe(II) by oxidized flavins may be a rate-controlling factor in microbial Fe(III) reduction via flavin-mediated electron transfer.

Metal ligands in micronutrient acquisition and homeostasis

Acquisition and homeostasis of micronutrients such as iron (Fe) and zinc (Zn) pose specific challenges. Poor solubility and high reactivity require controlled synthesis and supply of ligands to complex these metals extracellularly and intracellularly. Cytosolic labile pools represent only a minute fraction of the total cellular content. Several low-molecular-weight ligands are known in plants, including sulfur ligands (cysteine and peptides), nitrogen/oxygen ligands (S-adenosyl-l-methionine-derived molecules and histidine), and oxygen ligands (phenolics and organic acids). Some ligands are secreted into the extracellular space and influence the phytoavailability of metal ions. A second principal function is the intracellular buffering of micronutrients as well as the facilitation of long-distance transport in xylem and phloem. Furthermore, low-molecular-weight ligands are involved in the storage of metals, predominantly in vacuoles. A detailed molecular understanding is hampered by technical limitations, in particular the difficulty to detect and quantify cellular metal-ligand complexes. More, but still too little, is known about ligand synthesis and the transport across membranes, either with or without a complexed metal. Metal ligands have an immediate impact on human well-being. Engineering metal ligand synthesis and distribution in crops has tremendous potential to improve the nutritional quality of food and to tackle major human health risks.

Iron acquisition strategies in land plants: not so different after all

Due to its ability to accept and donate electrons, iron (Fe) is an indispensable component of electron transport chains and a cofactor in many vital enzymes. Except for waterlogged conditions, under which the lack of oxygen prevents oxidation and precipitation of iron as Fe³⁺ hydroxides, the availability of iron in soils is generally far below the plant's demand for optimal growth. Plants have evolved two phylogenetically separated and elaborately regulated strategies to mobilize iron from the soil, featuring mechanisms which are thought to be mutually exclusive. However, recent studies uncovered several shared components of the two strategies, questioning the validity of the concept of mutual exclusivity. Here, we use publicly available data obtained from the model species rice (Oryza sativa) to unveil similarities and incongruities between co-expression networks derived from transcriptomic profiling of iron-deficient rice and Arabidopsis plants. This approach revealed striking similarities in the topographies of the resulting co-expression networks with relatively minor deviations in the molecular attributes of the comprised genes, which nonetheless lead to different physiological functions. The analysis also discovered several novel players that are possibly involved in the regulation plant adaptation to iron deficiency.

Arsenic Oxidation by Flavin-Derived Reactive Species under Oxic and Anoxic Conditions: Oxidant Formation and pH Dependence

Flavins are ubiquitous redox-active compounds capable of producing reactive oxygen (O₂^•-, ^•OH, and H₂O₂) and flavin radical species in natural environments, yet their roles in the redox transformations of environmental contaminants, such as arsenic (As), remain to be investigated. Here, we show that reduced flavins can be a source of effective oxidants for As(III) under both oxic and anoxic conditions. For instance, in the presence of 15 μM reduced riboflavin (RBFH₂), 22% of 30 μM As(III) is oxidized in aerated solution at pH 7.0. The co-oxidation of As(III) with RBFH₂ is pH-dependent, with a faster reaction rate under mildly acidic relative to alkaline conditions. Quencher tests with 2-propanol (for ^•OH) and catalase (for H₂O₂) indicate that As(III) oxidation under oxic conditions is likely controlled by flavin-derived ^•OH at pH 5.2 and 7.0, and by H₂O₂ at pH 9.0. Kinetic modeling further implies that flavin-derived reactive oxygen species are mainly responsible for As(III) oxidation under oxic conditions, whereas oxidation of As(III) under anoxic conditions at pH 9.0 is attributed to riboflavin radicals (RBFH^•) generated from co-existing oxidized and reduced riboflavin. The demonstrated ability of flavins to catalyze As(III) oxidation has potential implications for As redox cycling in the environment.

'Candidatus Phytoplasma solani' interferes with the distribution and uptake of iron in tomato

Background

‘Candidatus Phytoplasma solani’ is endemic in Europe and infects a wide range of weeds and cultivated plants. Phytoplasmas are prokaryotic plant pathogens that colonize the sieve elements of their host plant, causing severe alterations in phloem function and impairment of assimilate translocation. Typical symptoms of infected plants include yellowing of leaves or shoots, leaf curling, and general stunting, but the molecular mechanisms underlying most of the reported changes remain largely enigmatic. To infer a possible involvement of Fe in the host-phytoplasma interaction, we investigated the effects of ‘Candidatus Phytoplasma solani’ infection on tomato plants (Solanum lycopersicum cv. Micro-Tom) grown under different Fe regimes.

Results

Both phytoplasma infection and Fe starvation led to the development of chlorotic leaves and altered thylakoid organization. In infected plants, Fe accumulated in phloem tissue, altering the local distribution of Fe. In infected plants, Fe starvation had additive effects on chlorophyll content and leaf chlorosis, suggesting that the two conditions affected the phenotypic readout via separate routes. To gain insights into the transcriptional response to phytoplasma infection, or Fe deficiency, transcriptome profiling was performed on midrib-enriched leaves. RNA-seq analysis revealed that both stress conditions altered the expression of a large (> 800) subset of common genes involved in photosynthetic light reactions, porphyrin / chlorophyll metabolism, and in flowering control. In Fe-deficient plants, phytoplasma infection perturbed the Fe deficiency response in roots, possibly by interference with the synthesis or transport of a promotive signal transmitted from the leaves to the roots.

Conclusions

‘Candidatus Phytoplasma solani’ infection changes the Fe distribution in tomato leaves, affects the photosynthetic machinery and perturbs the orchestration of root-mediated transport processes by compromising shoot-to-root communication.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-6062-x) contains supplementary material, which is available to authorized users.

How Plants Handle Trivalent (+3) Elements

Plant development and fitness largely depend on the adequate availability of mineral elements in the soil. Most essential nutrients are available and can be membrane transported either as mono or divalent cations or as mono- or divalent anions. Trivalent cations are highly toxic to membranes, and plants have evolved different mechanisms to handle +3 elements in a safe way. The essential functional role of a few metal ions, with the possibility to gain a trivalent state, mainly resides in the ion's redox activity; examples are iron (Fe) and manganese. Among the required nutrients, the only element with +3 as a unique oxidation state is the non-metal, boron. However, plants also can take up non-essential trivalent elements that occur in biologically relevant concentrations in soils. Examples are, among others, aluminum (Al), chromium (Cr), arsenic (As), and antimony (Sb). Plants have evolved different mechanisms to take up and tolerate these potentially toxic elements. This review considers recent studies describing the transporters, and specific and unspecific channels in different cell compartments and tissues, thereby providing a global vision of trivalent element homeostasis in plants.

Soil carbonate drives local adaptation in Arabidopsis thaliana

High soil carbonate limits crop performance especially in semiarid or arid climates. To understand how plants adapt to such soils, we explored natural variation in tolerance to soil carbonate in small local populations (demes) of Arabidopsis thaliana growing on soils differing in carbonate content. Reciprocal field-based transplants on soils with elevated carbonate (+C) and without carbonate (-C) over several years revealed that demes native to (+C) soils showed higher fitness than those native to (-C) soils when both were grown together on carbonate-rich soil. This supports the role of soil carbonate as a driving factor for local adaptation. Analyses of contrasting demes revealed key mechanisms associated with these fitness differences. Under controlled conditions, plants from the tolerant deme A1(+C) native to (+C) soil were more resistant to both elevated carbonate and iron deficiency than plants from the sensitive T6(-C) deme native to (-C) soil. Resistance of A1(+C) to elevated carbonate was associated with higher root extrusion of both protons and coumarin-type phenolics. Tolerant A1(+C) also had better Ca-exclusion than sensitive T6(-C) . We conclude that Arabidopsis demes are locally adapted in their native habitat to soils with moderately elevated carbonate. This adaptation is associated with both enhanced iron acquisition and calcium exclusion.

From Leguminosae/Gramineae Intercropping Systems to See Benefits of Intercropping on Iron Nutrition

To achieve sustainable development with a growing population while sustaining natural resources, a sustainable intensification of agriculture is necessary. Intercropping is useful for low-input/resource-limited agricultural systems. Iron (Fe) deficiency is a worldwide agricultural problem owing to the low solubility and bioavailability of Fe in alkaline and calcareous soils. Here, we summarize the effects of intercropping systems on Fe nutrition. Several cases showed that intercropping with graminaceous plants could be used to correct Fe nutrition of Leguminosae such as peanut and soybean or fruits such as Psidium guajava L., Citrus, grape and pear in calcareous soils. Intercropping systems have strong positive effects on the physicochemical and biochemical characteristics of soil and the microbial community due to interspecific differences and interactions in the rhizosphere. Rhizosphere interactions can increase the bioavailability of Fe with the help of phytosiderophores. Enriched microorganisms may also facilitate the Fe nutrition of crops. A peanut/maize intercropping system could help us understand the dynamics in rhizosphere and molecular mechanism. However, the role of microbiome in regulating Fe acquisition of root and the mechanisms underlying these phenomena in other intercropping system except peanut/maize need further work, which will help better utilize intercropping to increase the efficiency of Fe foraging.

The Nicotiana tabacum ABC transporter NtPDR3 secretes O-methylated coumarins in response to iron deficiency

Although iron is present in large amounts in the soil, its poor solubility means that plants have to use various strategies to facilitate its uptake. In this study, we show that expression of NtPDR3/NtABCG3, a Nicotiana tabacum plasma-membrane ABC transporter in the pleiotropic drug resistance (PDR) subfamily, is strongly induced in the root epidermis under iron deficiency conditions. Prevention of NtPDR3 expression resulted in N. tabacum plants that were less tolerant to iron-deficient conditions, displaying stronger chlorosis and slower growth than those of the wild-type when not supplied with iron. Metabolic profiling of roots and root exudates revealed that, upon iron deficiency, secretion of catechol-bearing O-methylated coumarins such as fraxetin, hydroxyfraxetin, and methoxyfraxetin to the rhizosphere was compromised in NtPDR3-silenced plants. However, exudation of flavins such as riboflavin was not markedly affected by NtPDR3-silencing. Expression of NtPDR3 in N. tabacum Bright Yellow-2 (BY-2) cells resulted in altered intra- and extracellular coumarin pools, supporting coumarin transport by this transporter. The results demonstrate that N. tabacum secretes both coumarins and flavins in response to iron deficiency and that NtPDR3 plays an essential role in the plant response to iron deficiency by mediating secretion of O-methylated coumarins to the rhizosphere.

Biosynthesis of redox-active metabolites in response to iron deficiency in plants

Iron is an essential but poorly bioavailable nutrient because of its low solubility, especially in alkaline soils. Here, we describe the discovery of a previously undescribed redox-active catecholic metabolite, termed sideretin, which derives from the coumarin fraxetin and is the primary molecule exuded by Arabidopsis thaliana roots in response to iron deficiency. We identified two enzymes that complete the biosynthetic pathway of fraxetin and sideretin. Chemical characterization of fraxetin and sideretin, and biological assays with pathway mutants, suggest that these coumarins are critical for iron nutrition in A. thaliana. Further, we show that sideretin production also occurs in eudicot species only distantly related to A. thaliana. Untargeted metabolomics of the root exudates of various eudicots revealed production of structurally diverse redox-active molecules in response to iron deficiency. Our results indicate that secretion of small-molecule reductants by roots may be a widespread and previously underappreciated component of reduction-based iron uptake.

The multiple facets of root iron reduction

The biological significance of iron (Fe) is based on its propensity to oscillate between the ferric and ferrous forms, a transition that also affects its phyto-availability in soils. With the exception of grasses, Fe3+ is unavailable to plants. Most angiosperms employ a reduction-based Fe uptake mechanism, which relies on enzymatic reduction of ferric iron as an obligatory, rate-limiting step prior to uptake. This system functions optimally in acidic soils. Calcicole plants are, however, exposed to environments that are alkaline and/or have suboptimal availability of phosphorous, conditions under which the enzymatic reduction mechanism ceases to work effectively. We propose that auxiliary, non-enzymatic Fe reduction can be of critical importance for conferring fitness to plants thriving in alkaline soils with low bioavailability of Fe and/or phosphorus.

Role of root exudates in metal acquisition and tolerance

Plants acquire mineral nutrients mostly through the rhizosphere; they secrete a large number of metabolites into the rhizosphere to regulate nutrient availability and to detoxify undesirable metal pollutants in soils. The secreted metabolites are inorganic ions, gaseous molecules, and mainly carbon-based compounds. This review focuses on the mechanisms and regulation of low-molecular-weight organic-compound exudation in terms of metal acquisition. We summarize findings on riboflavin/phenolic-facilitated and phytosiderophore-facilitated iron acquisition and discuss recent studies of the functions and secretion mechanisms of low-molecular-weight organic acids in heavy-metal detoxification.

Iron homeostasis in plants - a brief overview

Iron plays a crucial role in biochemistry and is an essential micronutrient for plants and humans alike. Although plentiful in the Earth's crust it is not usually found in a form readily accessible for plants to use. They must therefore sense and interact with their environment, and have evolved two different molecular strategies to take up iron in the root. Once inside, iron is complexed with chelators and distributed to sink tissues where it is used predominantly in the production of enzyme cofactors or components of electron transport chains. The processes of iron uptake, distribution and metabolism are overseen by tight regulatory mechanisms, at the transcriptional and post-transcriptional level, to avoid iron concentrations building to toxic excess. Iron is also loaded into seeds, where it is stored in vacuoles or in ferritin. This is important for human nutrition as seeds form the edible parts of many crop species. As such, increasing iron in seeds and other tissues is a major goal for biofortification efforts by both traditional breeding and biotechnological approaches.

Mobilization of Iron by Plant-Borne Coumarins

Iron is one of the most abundant elements in soils, but its low phytoavailability at high pH restricts plant communities on alkaline soils to taxa that have evolved efficient strategies to increase iron solubility. Recent evidence provides support for a previously underestimated role of root-secreted coumarins in mobilizing iron through reduction and chelation as part of an orchestrated strategy evolved to improve the acquisition of iron from recalcitrant pools. Understanding the mechanisms that tune the production of iron-mobilizing coumarins and their intricate interplay with other biosynthesis pathways could yield clues for deciphering the molecular basis of 'iron efficiency' - the ability of plants to thrive on soils with limited iron availability - and may open avenues for generating iron-fortified crops.

Small molecules below-ground: the role of specialized metabolites in the rhizosphere

Soil communities are diverse taxonomically and functionally. This ecosystem experiences highly complex networks of interactions, but may also present functionally independent entities. Plant roots, a metabolically active hotspot in the soil, take an essential part in below-ground interactions. While plants are known to release an extremely high portion of the fixated carbon to the soil, less information is known about the composition and role of C-containing compounds in the rhizosphere, in particular those involved in chemical communication. Specialized metabolites (or secondary metabolites) produced by plants and their associated microbes have a critical role in various biological activities that modulate the behavior of neighboring organisms. Thus, elucidating the chemical composition and function of specialized metabolites in the rhizosphere is a key element in understanding interactions in this below-ground environment. Here, we review key classes of specialized metabolites that occur as mostly non-volatile compounds in root exudates or are emitted as volatile organic compounds (VOCs). The role of these metabolites in below-ground interactions and response to nutrient deficiency, as well as their tissue and cell type-specific biosynthesis and release are discussed in detail.

Alkaline stress and iron deficiency regulate iron uptake and riboflavin synthesis gene expression differently in root and leaf tissue: implications for iron deficiency chlorosis

Iron (Fe) is an essential mineral that has low solubility in alkaline soils, where its deficiency results in chlorosis. Whether low Fe supply and alkaline pH stress are equivalent is unclear, as they have not been treated as separate variables in molecular physiological studies. Additionally, molecular responses to these stresses have not been studied in leaf and root tissues simultaneously. We tested how plants with the Strategy I Fe uptake system respond to Fe deficiency at mildly acidic and alkaline pH by measuring root ferric chelate reductase (FCR) activity and expression of selected Fe uptake genes and riboflavin synthesis genes. Alkaline pH increased cucumber (Cucumis sativus L.) root FCR activity at full Fe supply, but alkaline stress abolished FCR response to low Fe supply. Alkaline pH or low Fe supply resulted in increased expression of Fe uptake genes, but riboflavin synthesis genes responded to Fe deficiency but not alkalinity. Iron deficiency increased expression of some common genes in roots and leaves, but alkaline stress blocked up-regulation of these genes in Fe-deficient leaves. In roots of the melon (Cucumis melo L.) fefe mutant, in which Fe uptake responses are blocked upstream of Fe uptake genes, alkaline stress or Fe deficiency up-regulation of certain Fe uptake and riboflavin synthesis genes was inhibited, indicating a central role for the FeFe protein. These results suggest a model implicating shoot-to-root signaling of Fe status to induce Fe uptake gene expression in roots.

Transition Metal Transport in Plants and Associated Endosymbionts: Arbuscular Mycorrhizal Fungi and Rhizobia

Transition metals such as iron, copper, zinc, or molybdenum are essential nutrients for plants. These elements are involved in almost every biological process, including photosynthesis, tolerance to biotic and abiotic stress, or symbiotic nitrogen fixation. However, plants often grow in soils with limiting metallic oligonutrient bioavailability. Consequently, to ensure the proper metal levels, plants have developed a complex metal uptake and distribution system, that not only involves the plant itself, but also its associated microorganisms. These microorganisms can simply increase metal solubility in soils and making them more accessible to the host plant, as well as induce the plant metal deficiency response, or directly deliver transition elements to cortical cells. Other, instead of providing metals, can act as metal sinks, such as endosymbiotic rhizobia in legume nodules that requires relatively large amounts to carry out nitrogen fixation. In this review, we propose to do an overview of metal transport mechanisms in the plant-microbe system, emphasizing the role of arbuscular mycorrhizal fungi and endosymbiotic rhizobia.

Revisiting the iron pools in cucumber roots: identification and localization

Fe deficiency responses in Strategy I causes a shift from the formation of partially removable hydrous ferric oxide on the root surface to the accumulation of Fe-citrate in the xylem. Iron may accumulate in various chemical forms during its uptake and assimilation in roots. The permanent and transient Fe microenvironments formed during these processes in cucumber which takes up Fe in a reduction based process (Strategy I) have been investigated. The identification of Fe microenvironments was carried out with (57)Fe Mössbauer spectroscopy and immunoblotting, whereas reductive washing and high-resolution microscopy was applied for the localization. In plants supplied with (57)Fe(III)-citrate, a transient presence of Fe-carboxylates in removable forms and the accumulation of partly removable, amorphous hydrous ferric oxide/hydroxyde have been identified in the apoplast and on the root surface, respectively. The latter may at least partly be the consequence of bacterial activity at the root surface. Ferritin accumulation did not occur at optimal Fe supply. Under Fe deficiency, highly soluble ferrous hexaaqua complex is transiently formed along with the accumulation of Fe-carboxylates, likely Fe-citrate. As (57)Fe-citrate is non-removable from the root samples of Fe deficient plants, the major site of accumulation is suggested to be the root xylem. Reductive washing results in another ferrous microenvironment remaining in the root apoplast, the Fe(II)-bipyridyl complex, which accounts for ~30 % of the total Fe content of the root samples treated for 10 min and rinsed with CaSO4 solution. When (57)Fe(III)-EDTA or (57)Fe(III)-EDDHA was applied as Fe-source higher soluble ferrous Fe accumulation was accompanied by a lower total Fe content, confirming that chelates are more efficient in maintaining soluble Fe in the medium while less stable natural complexes as Fe-citrate may perform better in Fe accumulation.

Accumulation and Secretion of Coumarinolignans and other Coumarins in Arabidopsis thaliana Roots in Response to Iron Deficiency at High pH

Root secretion of coumarin-phenolic type compounds has been recently shown to be related to Arabidopsis thaliana tolerance to Fe deficiency at high pH. Previous studies revealed the identity of a few simple coumarins occurring in roots and exudates of Fe-deficient A. thaliana plants, and left open the possible existence of other unknown phenolics. We used HPLC-UV/VIS/ESI-MS(TOF), HPLC/ESI-MS(ion trap) and HPLC/ESI-MS(Q-TOF) to characterize (identify and quantify) phenolic-type compounds accumulated in roots or secreted into the nutrient solution of A. thaliana plants in response to Fe deficiency. Plants grown with or without Fe and using nutrient solutions buffered at pH 5.5 or 7.5 enabled to identify an array of phenolics. These include several coumarinolignans not previously reported in A. thaliana (cleomiscosins A, B, C, and D and the 5'-hydroxycleomiscosins A and/or B), as well as some coumarin precursors (ferulic acid and coniferyl and sinapyl aldehydes), and previously reported cathecol (fraxetin) and non-cathecol coumarins (scopoletin, isofraxidin and fraxinol), some of them in hexoside forms not previously characterized. The production and secretion of phenolics were more intense when the plant accessibility to Fe was diminished and the plant Fe status deteriorated, as it occurs when plants are grown in the absence of Fe at pH 7.5. Aglycones and hexosides of the four coumarins were abundant in roots, whereas only the aglycone forms could be quantified in the nutrient solution. A comprehensive quantification of coumarins, first carried out in this study, revealed that the catechol coumarin fraxetin was predominant in exudates (but not in roots) of Fe-deficient A. thaliana plants grown at pH 7.5. Also, fraxetin was able to mobilize efficiently Fe from a Fe(III)-oxide at pH 5.5 and pH 7.5. On the other hand, non-catechol coumarins were much less efficient in mobilizing Fe and were present in much lower concentrations, making unlikely that they could play a role in Fe mobilization. The structural features of the array of coumarin type-compounds produced suggest some can mobilize Fe from the soil and others can be more efficient as allelochemicals.