A quick journey into the diversity of iron uptake strategies in photosynthetic organisms

Iron sensing, signalling and acquisition by microbes and plants under environmental stress: Use of iron-solubilizing bacteria in crop biofortification for sustainable agriculture

Iron is very crucial micronutrient prerequisite for growth of all cellular organisms including plants, microbes, animals and humans. Though iron (Fe) is present in abundance in earth's crust, but most of its forms present in soil are biologically unavailable, thus putting a constraint to utilize it. Plants and microorganisms maintain iron homeostasis to balance the supply of enough Fe for metabolism from their surrounding environments and to avoid excessive toxic levels. Microorganisms and plants employ different strategies for sensing, signaling, transportation and uptake of Fe under different types of stressed environments. Microbial communities present in soil and vicinity of roots contribute in biogeochemical cycling and uptake of different nutrients including Fe resulting into improved soil fertility and plant health. In this review, the regulation of iron uptake and transport under different kinds of biotic and abiotic stresses is described. In addition, the insights have been provided for enhancing bioavailability of Fe in sustainable agriculture practices. The inoculation of different crop plants with iron solubilizing microbes improved bioavailablilty of Fe in soil and increased plant growth and crop yield. Insights were provided about possible role of recent bioengineering techniques to improve Fe availability and uptake by plants. However, well-planned and large-scale field trials are required before recommending particular iron solubilizing microbes as biofertilizers for increasing Fe availability, improving plant development and crop yields in sustainable agriculture.

Developing Iron and Iodine Enrichment in Tomato Fruits to Meet Human Nutritional Needs

Iron (Fe) and iodine (I) are essential microelements required for a healthy life, with Fe playing a vibrant role in oxygen transport, and I is vital for cognitive development and thyroid function. Global Fe and I deficiencies affect a significant portion of the population worldwide, leading to widespread health concerns, especially anemia, impaired cognitive function, and thyroid disorders. This review not only inspects the potential of agronomic biofortification to enrich Fe and I content in tomatoes, but also highlights its bright future for crop nutrition. It discusses the latest developments in agronomic biofortification methods focused on improving the enrichment of Fe and I in tomatoes, emphasizing practical approaches such as seed priming, soil application, and foliar spray. Notably, the review explores the promising impacts of Fe and I biofortification on growth, yield, and improved fruit quality in tomatoes. Moreover, it offers an in-depth investigation of the efficacy of agronomic biofortification in enhancing the nutritional contents of tomatoes by combining the most recent research findings. It highlights the impact of agronomic biofortification in mitigating micronutrient deficiencies worldwide and its capacity to encourage sustainable agriculture and improve community health by enhancing crop nutrition.

Synthesis of 3-hydroxy-4-pyridinone hexadentate chelators, and biophysical evaluation of their affinity towards lipid bilayers

Iron is an essential micronutrient for almost every living organism, namely pathogenic bacteria. In an infection scenario, host-pathogen competitive relationships for the element are present and Fe withholding is a well known response of the host. Also, bacterial resistance is a major concern that can compromise public health and the WHO underlines an urgent need to search for new pharmaceutical ingredients or strategies to fight opportunistic bacteria. Iron metabolism, and in particular, deprivation is a strategy that currently constitutes another option to fight bacterial infection. In this work we report the synthesis of a new hexadentate chelator with enhanced hydrophilicity (MRHT) and the improved synthesis of two other chelators. The affinity towards charged and non-charged phospholipid bilayers was evaluated for three hexadentate chelators: MRHT, CP256 and RH8b using NMR and EPR spectroscopies. The results revealed that these structures, bearing 3,4-HPO units have a high affinity towards the hydrophilic region of the phospholipid bilayer. From the three hexadentate chelators, MRHT stood out, especially for liposomes with a charged surface, suggesting that this molecule could more efficiently compete with natural siderophores, creating an iron gradient near bacteria organisms.

PbbHLH155 enhances iron deficiency tolerance in pear by directly activating PbFRO2 and PbbHLH38

Iron (Fe) deficiency is a general stress for many horticulture crops, causing leaf chlorosis and stunted growth. The basic-helix-loop-helix (bHLH) transcription factor (TF) was reported to function in Fe absorption; however, the regulatory mechanism of bHLH genes on iron absorption remains largely unclear in pear. In this study, we found that PbbHLH155 was significantly induced by Fe deficiency. Overexpression of PbbHLH155 in Arabidopsis thaliana and pear calli significantly increases resistance to Fe deficiency. The PbbHLH155-overexpressed Arabidopsis lines exhibited greener leaf color, higher Fe content, stronger Fe chelate reductase (FCR) and root acidification activity. The PbbHLH155 knockout pear calli showed lower Fe content and weaker FCR activity. Interestingly, PbbHLH155 inhibited the expressions of PbFRO2 and PbbHLH38, which were positive regulators in Fe-deficiency responses (FDR). Furthermore, yeast one-hybrid (Y1H) and Dual-Luciferase Reporter (DLR) assays revealed that PbbHLH155 directly binds to the promoters of PbFRO2 and PbbHLH38, thus activating their expression. Overall, our results showed that PbbHLH155 directly promote the expression of PbFRO2 and PbbHLH38 to activate FCR activity for iron absorption. This study provided valuable information for pear breeding.

The tiny giant of the sea, Ostreococcus's unique adaptations

Ostreococcus spp. are unicellular organisms with one of the simplest cellular organizations. The sequencing of the genomes of different Ostreococcus species has reinforced this status since Ostreococcus tauri has one most compact nuclear genomes among eukaryotic organisms. Despite this, it has retained a number of genes, setting it apart from other organisms with similar small genomes. Ostreococcus spp. feature a substantial number of selenocysteine-containing proteins, which, due to their higher catalytic activity compared to their selenium-lacking counterparts, may require a reduced quantity of proteins. Notably, O. tauri encodes several ammonium transporter genes, that may provide it with a competitive edge for acquiring nitrogen (N). This characteristic makes it an intriguing model for studying the efficient use of N in eukaryotes. Under conditions of low N availability, O. tauri utilizes N from abundant proteins or amino acids, such as L-arginine, similar to higher plants. However, the presence of a nitric oxide synthase (L-arg substrate) sheds light on a new metabolic pathway for L-arg in algae. The metabolic adaptations of O. tauri to day and night cycles offer valuable insights into carbon and iron metabolic configuration. O. tauri has evolved novel strategies to optimize iron uptake, lacking the classic components of the iron absorption mechanism. Overall, the cellular and genetic characteristics of Ostreococcus contribute to its evolutionary success, making it an excellent model for studying the physiological and genetic aspects of how green algae have adapted to the marine environment. Furthermore, given its potential for lipid accumulation and its marine habitat, it may represent a promising avenue for third-generation biofuels.

Molecular insights into the mutualism that induces iron deficiency tolerance in sorghum inoculated with Trichoderma harzianum

Iron (Fe) deficiency is a common mineral stress in plants, including sorghum. Although the soil fungus Trichoderma harzianum has been shown to mitigate Fe deficiency in some circumstances, neither the range nor mechanism(s) of this process are well understood. In this study, high pH-induced Fe deficiency in sorghum cultivated in pots with natural field soil exhibited a significant decrease in biomass, photosynthetic rate, transpiration rate, stomatal conductance, water use efficiency, and Fe-uptake in both the root and shoot. However, the establishment of T. harzianum colonization in roots of Fe-deprived sorghum showed significant improvements in morpho-physiological traits, Fe levels, and redox status. Molecular detection of the fungal ThAOX1 (L-aminoacid oxidase) gene showed the highest colonization of T. harzianum in the root tips of Fe-deficient sorghum, a location thus targeted for further analysis. Expression studies by RNA-seq and qPCR in sorghum root tips revealed a significant upregulation of several genes associated with Fe uptake (SbTOM2), auxin synthesis (SbSAURX15), nicotianamine synthase 3 (SbNAS3), and a phytosiderophore transporter (SbYS1). Also induced was the siderophore synthesis gene (ThSIT1) in T. harzianum, a result supported by biochemical evidence for elevated siderophore and IAA (indole acetic acid) levels in roots. Given the high affinity of fungal siderophore to chelate insoluble Fe3+ ions, it is likely that elevated siderophore released by T. harzianum led to Fe(III)-siderophore complexes in the rhizosphere that were then transported into roots by the induced SbYS1 (yellow-stripe 1) transporter. In addition, the observed induction of several plant peroxidase genes and ABA (abscisic acid) under Fe deficiency after inoculation with T. harzianum may have helped induce tolerance to Fe-deficiency-induced oxidative stress and adaptive responses. This is the first mechanistic explanation for T. harzianum's role in helping alleviate Fe deficiency in sorghum and suggests that biofertilizers using T. harzianum will improve Fe availability to crops in high pH environments.

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.

The AUX1-AFB1-CNGC14 module establishes a longitudinal root surface pH profile

Plant roots navigate in the soil environment following the gravity vector. Cell divisions in the meristem and rapid cell growth in the elongation zone propel the root tips through the soil. Actively elongating cells acidify their apoplast to enable cell wall extension by the activity of plasma membrane AHA H+-ATPases. The phytohormone auxin, central regulator of gravitropic response and root development, inhibits root cell growth, likely by rising the pH of the apoplast. However, the role of auxin in the regulation of the apoplastic pH gradient along the root tip is unclear. Here, we show, by using an improved method for visualization and quantification of root surface pH, that the Arabidopsis thaliana root surface pH shows distinct acidic and alkaline zones, which are not primarily determined by the activity of AHA H+-ATPases. Instead, the distinct domain of alkaline pH in the root transition zone is controlled by a rapid auxin response module, consisting of the AUX1 auxin influx carrier, the AFB1 auxin co-receptor, and the CNCG14 calcium channel. We demonstrate that the rapid auxin response pathway is required for an efficient navigation of the root tip.

Ferrous iron uptake via IRT1/ZIP evolved at least twice in green plants

Iron (Fe) is essential for virtually all organisms, being irreplaceable because of its electrochemical properties that enable many biochemical processes, including photosynthesis. Besides its abundance, Fe is generally found in the poorly soluble form of ferric iron (Fe³⁺ ), while most plants uptake the soluble form Fe²⁺ . The model angiosperm Arabidopsis thaliana, for example, captures Fe through a mechanism that lowers rhizosphere pH through proton pumping that increases Fe³⁺ solubility, which is then reduced by a membrane-bound reductase and transported into the cell by the zinc-regulated, iron-regulated transporter-like protein (ZIP) family protein AtIRT1. ZIP proteins are transmembrane transporters of divalent metals such as Fe²⁺ , Zn²⁺ , Mn²⁺ , and Cd²⁺ . In this work, we investigated the evolution of functional homologs of IRON-REGULATED TRANSPORTER 1/ZIP in the supergroup Archaeplastida (Viridiplantae + Rhodophyta + Glaucophyta) using 51 genomes of diverse lineages. Our analyses suggest that Fe is acquired through deeply divergent ZIP proteins in land plants and chlorophyte green algae, indicating that Fe²⁺ uptake by ZIP proteins evolved independently at least twice throughout green plant evolution. Our results indicate that the archetypical IRON-REGULATED TRANSPORTER (IRT) proteins from angiosperms likely emerged before the origin of land plants during early streptophyte algae terrestrialization, a process that required the evolution of Fe acquisition in terrestrial subaerial settings.

MCO1 and MCO3, two putative ascorbate oxidases with ferroxidase activity, new candidates for the regulation of apoplastic iron excess in Arabidopsis

Iron (Fe) is an essential metal ion that plays a major role as a cofactor in many biological processes. The balance between the Fe2+ and Fe3+ forms is central for cellular Fe homeostasis because it regulates its transport, utilization, and storage. Contrary to Fe3+ reduction that is crucial for Fe uptake by roots in deficiency conditions, ferroxidation has been much less studied. In this work, we have focused on the molecular characterization of two members of the MultiCopper Oxidase family (MCO1 and MCO3) that share high identity with the Saccharomyces cerevisiae ferroxidase Fet3. The heterologous expression of MCO1 and MCO3 restored the growth of the yeast fet3fet4 mutant, impaired in high and low affinity Fe uptake and otherwise unable to grow in Fe deficient media, suggesting that MCO1 and MCO3 were functional ferroxidases. The ferroxidase enzymatic activity of MCO3 was further confirmed by the measurement of Fe2+-dependent oxygen consumption, because ferroxidases use oxygen as electron acceptor to generate water molecules. In planta, the expression of MCO1 and MCO3 was induced by increasing Fe concentrations in the medium. Promoter-GUS reporter lines showed that MCO1 and MCO3 were mostly expressed in shoots and histochemical analyses further showed that both promoters were highly active in mesophyll cells. Transient expression of MCO1-RFP and MCO3-RFP in tobacco leaves revealed that both proteins were localized in the apoplast. Moreover, cell plasmolysis experiments showed that MCO1 remained closely associated to the plasma membrane whereas MCO3 filled the entire apoplast compartment. Although the four knock out mutant lines isolated (mco1-1, mco1-2, mco3-1, and mco3-2) did not display any macroscopic phenotype, histochemical staining of Fe with the Perls/DAB procedure revealed that mesophyll cells of all four mutants overaccumulated Fe inside the cells in Fe-rich structures in the chloroplasts, compared with wild-type. These results suggested that the regulation of Fe transport in mesophyll cells had been disturbed in the mutants, in both standard condition and Fe excess. Taken together, our findings strongly suggest that MCO1 and MCO3 participate in the control of Fe transport in the mesophyll cells, most likely by displacing the Fe2+/Fe3+ balance toward Fe3+ in the apoplast and therefore limiting the accumulation of Fe2+, which is more mobile and prone to be transported across the plasma membrane.