Mechanisms and regulation of reduction‐based iron uptake in plants

The effects of copper, manganese and zinc on plant growth and elemental accumulation in the manganese-hyperaccumulator Phytolacca americana

Iron (Fe) and copper (Cu) homeostasis are tightly linked across biology. In previous work, Fe deficiency interacted with Cu-regulated genes and stimulated Cu accumulation. The C940-fe (fefe) Fe-uptake mutant of melon (Cucumis melo) was characterized, and the fefe mutant was used to test whether Cu deficiency could stimulate Fe uptake. Wild-type and fefe mutant transcriptomes were determined by RNA-seq under Fe and Cu deficiency. FeFe-regulated genes included core Fe uptake, metal homeostasis, and transcription factor genes. Numerous genes were regulated by both Fe and Cu. The fefe mutant was rescued by high Fe or by Cu deficiency, which stimulated ferric-chelate reductase activity, FRO2 expression, and Fe accumulation. Accumulation of Fe in Cu-deficient plants was independent of the normal Fe-uptake system. One of the four FRO genes in the melon and cucumber (Cucumis sativus) genomes was Fe-regulated, and one was Cu-regulated. Simultaneous Fe and Cu deficiency synergistically up-regulated Fe-uptake gene expression. Overlap in Fe and Cu deficiency transcriptomes highlights the importance of Fe-Cu crosstalk in metal homeostasis. The fefe gene is not orthologous to FIT, and thus identification of this gene will provide clues to help understand regulation of Fe uptake in plants.

Mining iron: Iron uptake and 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.

Hydroxyiminodisuccinic acid (HIDS): A novel biodegradable chelating ligand for the increase of iron bioavailability and arsenic phytoextraction

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

Significance of the concentration of chelating ligands on Fe3+-solubility, bioavailability, and uptake in rice plant

The objective of this study was to investigate whether a specific siderophore‐mediated iron uptake (possibly preceded by a reduction step) occurs in corn roots at a specific recognition site. The activity of ferrichrome analogs and ferrioxamine B, as iron carriers into corn roots, was studied. Stereo‐specific recognition of Fe‐ferrichrome analogs was observed during Fe uptake by corn roots. All ferrichrome analogs forming Fe(III) complexes of the same absolute configuration as does natural ferrichrome (cis), enhance Fe uptake, while their enantiomers lack biological activity. Gallium (III) failed to be taken up under analogous conditions, indicating that synthetic ferrichrome analogs mediate the Fe uptake via a specific reduction step. Within certain limits, binding energy of the chelator did not play a role in enhancing iron uptake. It is suggested that the iron uptake mechanism in corn roots involves binding to a specific chiral receptor in conjunction with a reductive process.

The mechanism of drug resistance to gallium nitrate is not known. Since gallium can be incorporated into ferritin, an iron storage protein that protects cells from iron toxicity, we investigated whether ferritin expression was altered in gallium-resistant (R) CCRF-CEM cells. We found that the ferritin content of R cells was decreased, while heavy chain ferritin mRNA levels and iron regulatory protein-1 (IRP-1) RNA binding activity were increased. IRP-1 protein levels were similar in gallium-sensitive (S) and R cells, indicating that R cells contain a greater proportion of IRP-1 in a high affinity mRNA binding state. 59Fe uptake and transferrin receptor expression were decreased in R cells. In both S and R cells, gallium inhibited cellular 59Fe uptake, increased ferritin mRNA and protein, and decreased IRP-1 binding activity. Gallium uptake by R cells was markedly diminished; however, the sensitivity of R cells to gallium could be restored by increasing their uptake of gallium with excess transferrin. Our results suggest that R cells have developed resistance to gallium by down-regulating their uptake of gallium. In parallel, iron uptake by R cells is also decreased, leading to changes in iron homeostasis. Furthermore, since gallium has divergent effects on iron uptake and ferritin synthesis, its action may also include a direct effect on ferritin mRNA induction and IRP-1 activity.

Abstract. Iron (Fe) uptake by the microbial community and the contribution of three different size fractions was determined during spring phytoplankton blooms in the naturally Fe-fertilized area off the Kerguelen Islands (KEOPS2). Total Fe uptake in surface waters was on average 34 ± 6 pmol Fe L-1 d-1, and microplankton (> 25 μm size fraction; 40–69%) and pico-nanoplankton (0.8–25 μm size fraction; 29–59%) were the main contributors. The contribution of heterotrophic bacteria (0.2–0.8 μm size fraction) to total Fe uptake was low at all stations (1–2%). Iron uptake rates normalized to carbon biomass were highest for pico-nanoplankton above the Kerguelen Plateau and for microplankton in the downstream plume. We also investigated the potential competition between heterotrophic bacteria and phytoplankton for the access to Fe. Bacterial Fe uptake rates normalized to carbon biomass were highest in incubations with bacteria alone, and dropped in incubations containing other components of the microbial community. Interestingly, the decrease in bacterial Fe uptake rate (up to 26-fold) was most pronounced in incubations containing pico-nanoplankton and bacteria, while the bacterial Fe uptake was only reduced by 2- to 8-fold in incubations containing the whole community (bacteria + pico-nanoplankton + microplankton). In Fe-fertilized waters, the bacterial Fe uptake rates normalized to carbon biomass were positively correlated with primary production. Taken together, these results suggest that heterotrophic bacteria are outcompeted by small-sized phytoplankton cells for the access to Fe during the spring bloom development, most likely due to the limitation by organic matter. We conclude that the Fe and carbon cycles are tightly coupled and driven by a complex interplay of competition and synergy between different members of the microbial community.

Iron is a critical micronutrient for plants, acting as a catalyst in key growth processes, including photosynthesis, respiration, and enzyme activation. However, iron availability is often limited due to soil oxidation and the neutral to alkaline pH of aerobic soils. This study investigates the evolutionary and functional aspects of the FRO2 gene, which is essential for reducing Fe3⁺ to the more soluble Fe2⁺ form, thereby enhancing iron uptake in plants. A genome-wide analysis in Oryza sativa, alongside comparisons with Arabidopsis thaliana, Solanum lycopersicum, Sorghum bicolor, and Zea mays, revealed significant FRO2 gene diversity across these species, organized into four main phylogenetic clusters. We identified 58 FRO2 genes with conserved domains, supporting their functionality across species. Expression analysis of cis-regulatory elements, including the TATA-box and CAAT-box, pointed to role of FRO2 in abiotic stress responses and hormonal regulation. Ka/Ks analysis indicated purifying selection, underscoring the conserved role of gene. The findings highlight the evolutionary adaptation of FRO2 genes, suggesting that their functionality in Oryza sativa can be extrapolated to other species in Poaceae, Solanaceae, and Brassicaceae. These insights offer a foundation for biofortification strategies aimed at improving iron content and stress resilience in crops, advancing crop breeding efforts under iron-limiting conditions.

Using Fe chemistry to predict Fe uptake rates for natural plankton assemblages from the Southern Ocean

The mechanisms of iron (Fe) and transferrin (Tf) uptake by the human melanoma cell line, SK-MEL-28, have been investigated using chelators and metabolic probes. These data provide evidence for two saturable processes of Fe uptake from Tf, namely, specific receptor-mediated endocytosis and a second nonspecific, non-receptor-mediated mechanisms which saturated with respect to Fe uptake at a Tf concentration of approximately 0.3 mg/ml. In contrast to Fe uptake, Tf uptake increased linearly up to at least 1 mg/ml. Furthermore, under the culture conditions used, the second nonspecific, non-receptor-mediated mechanism was the most important process in terms of quantitative Fe uptake. Two concentrations of Tf-125I-59Fe (0.01 and 0.1 mg/ml) were used in order to characterise the specific and nonspecific Fe uptake pathways. Membrane permeable chelators were equally effective at both Tf concentrations, whereas membrane impermeable chelators were significantly (P < 0.001) more effective at reducing the internalisation of Fe at the higher Tf concentration, consistent with a mechanism of Fe uptake which occurred at a site in contact with the extracellular medium. The oxidoreductase inhibitor, amiloride, only slightly inhibited Fe uptake at the higher Tf concentration, suggesting that the second nonspecific process was not mediated by a diferric Tf reductase. Three lysosomotrophic agents and the endocytosis inhibitor, phenylglyoxal, markedly reduced Fe uptake at both Tf concentrations, and it is concluded that a saturable process consistent with receptor-mediated endocytosis of Tf occurred at the lower Tf concentration, while the predominant mechanism of Fe uptake at high Tf concentrations was a second saturable process consistent with adsorptive pinocytosis.

Phytoremediation using Vetiver grass (VG) has been used in the past few decades all over the world, but the effectiveness of its uptake mechanism in water has less been discussed especially in mixed metal conditions. Hence, the focus of this research was to determine and evaluate the heavy metal removal effectiveness of two heavy metals, copper (Cu) and iron (Fe), based on vetiver grass uptake and toxicity. Three treatments of Fe and Cu mixture were chosen, and individual concentrations act as a control. The vetiver grass at a height of 10 cm and a root length of 25 cm for each treatment was used and harvested on days 0,1, 3, 6 and 7. The roots were randomly cut, oven-dried and used the digestion method to extract the Fe and Cu in the root. From the result, vetiver grass has better Fe and Cu uptake ability in the mixture treatment compared to the individual treatment. Fe uptake is increasing up to 25.3 % in the presence of Cu (mixture 1). Meanwhile, for Cu, mixture 3 is an ideal ratio to increase the Cu uptake by 14.5%. It shows that both Fe and Cu facilitated each other’s uptake in real wastewater conditions. There were also observed minimal toxicity signs such as necrosis throughout the experiment. The appearance of vetiver grass has changed in this experiment starting day 3 and remains yellowish. In conclusion, wastewater that contains a high concentration of Fe than Cu can promise the improvement of both Fe and Cu uptake by vetiver grass. This study benefits researchers that apply phytoremediation to remove heavy metals in wastewater by providing a guideline for Fe and Cu removal by vetiver grass. In future, the study will focus on the mechanism and interaction of Fe and Cu uptake by root

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.