Iron Uptake and Transport in Plants

Iron (Fe) is an essential trace element for normal plant life activities. It plays a crucial role in various metabolic pathways, including:

• Chlorophyll synthesis and composition

• Co-factors of many enzymes crucial for photosynthesis and other metabolic processes

• Components of iron containing proteins like cytochromes, Fe-S complexes that are components of electron transport chains.

• Protein synthesis and DNA replication.

Iron Availability

• Although iron is highly abundant in the earth's crust, the amount available for plant absorption is very low.

• In soil, Fe exists mainly in two valence states:

  • Ferrous Iron (Fe²⁺): Can be absorbed and utilized by plants. It exists in dissolved form under acidic and flooded conditions.

  • Ferric Iron (Fe³⁺): Difficult for plants to absorb due to its low solubility and effectiveness. It primarily exists as insoluble ferric oxides under high-pH conditions (pH > 6.5).

• To cope with low availability, plants have evolved two primary adaptive mechanisms for Fe uptake, known as Strategy I and Strategy II.

Iron Uptake Strategies

Plants employ two main strategies for acquiring iron from the rhizosphere, which are traditionally segregated based on plant type.

Strategy I: Reduction-Based Strategy

This mechanism, employed primarily by nongraminaceous plants (e.g., Arabidopsis, soybean, cucumber, dicotyledons), uses an acidification-reduction-transport process.

Step	Process	Key Components
1. Acidification	Plasma membrane-localized H+ -ATPases (e.g., AHA2, HA6) pump protons (H+) into the rhizosphere, lowering the pH. This solubilizes insoluble Fe3+ complexes. Roots also secrete phenolic compounds (coumarins) to mobilize Fe3+.	H+-ATPases (H+-ATPase, AHA2, HA6) and PDR9 (for coumarin efflux)
2. Reduction	Trivalent iron chelating reductase (FRO2) on the root surface converts Fe3+ into the absorbable Fe2+ form. This is considered the limiting step in Mechanism I.	Ferric Chelate Reductase (FRO2)
3. Transport	The ferrous iron transporter (IRT1) then mediates the high-affinity transmembrane transport of Fe2+ into the root cells	Iron-Regulated Transporter (IRT1)

• Complex Formation: The core components, H⁺-ATPase (AHA2), FRO2, and IRT1, can physically interact and form a complex to coordinate and optimize Fe uptake.

• Coumarin's Role: Phenolic compounds, like coumarins, can convert insoluble Fe³⁺ into soluble Fe³⁺ chelates. This Fe³⁺-chelate can then be reduced by FRO2 and taken up by IRT1. Under high-pH, Arabidopsis can also directly take up the Fe³⁺-coumarin complex, a Strategy II-like mechanism.

Iron uptake strategy in non-graminaceous plants (Adapted from Chao and Chao, 2022, New Phytologist)

Strategy II: Chelation-Based Strategy

This mechanism is predominantly used by graminaceous plants (e.g., rice, maize, barley). It involves the synthesis and secretion of strong chelating agents to capture Fe³⁺ for direct transport.

Step	Process	Key Components
1. Synthesis & Secretion	The root system synthesizes and exports low-molecular-weight, non-protein amino acids called phytosiderophores (PSs), such as mugeneic acid (MA), nicotinamide (NA), 2'-deoxymugineic acid (DMA). This is mediated by efflux transporters (e.g., TOM1).	Transporter of Mugineic acid (TOM1).
2. Chelation	PSs have a strong affinity for Fe3+ (with six functional groups) and chelate it from the soil, forming stable, octahedral Fe3+-PS complexes.	Phytosiderophores (PSs).
3. Transport	The stable Fe3+-PS complexes are transported into the root cells by specialized membrane-localized transporters such as YELLOW STRIPE1 (YS1) and YS1-Like (YSL) transporters (e.g., OsYSL15).	YELLOW STRIPE1 or YS1-Like Transporters (YS1/YSL, e.g., OsYSL15).

Iron uptake strategy in graminaceous plants (Adapted from Chao and Chao, 2022, New Phytologist)

Overlap and Crossover

Recent studies indicate that the distinction between Strategy I and Strategy II is not absolute, as plants may utilize both depending on soil conditions.

• Nongraminaceous plants can benefit from the Strategy II system by absorbing Fe³⁺-PS complexes secreted by neighboring graminaceous plants.

• Graminaceous plants (e.g., rice) can employ a Strategy I-like mechanism by directly taking up Fe²⁺ using IRT1/2 under low-oxygen conditions.

• Compensatory Uptake: Both plant types seem to share a compensatory mechanism using Natural Resistance-Associated Macrophage Protein (NRAMP) transporters (e.g., NRAMP1, NRAMP5) to facilitate low-affinity Fe²⁺ uptake, especially when IRT1 is disabled.

Iron Transport Mechanisms

The transport of iron in plants is a multi-step process, moving from the soil into the root cell (Uptake), distribution within the cell (intracellular transport), and then systemically throughout the entire plant (long-distance). Fe must be bound to small molecular chelators throughout the plant to prevent its precipitation, which is crucial for efficient movement in both the short and long pathways.

Intracellular Transport and Storage

Once inside the cytoplasm, the free Fe²⁺ must be sequestered to prevent toxicity and delivered to the sites where it is needed.

• Organelle Delivery: Fe²⁺ is transported from the cytoplasm into various organelles (e.g., chloroplasts, mitochondria, vacuoles) by specific transporters. Chloroplasts are a major sink, containing about 80% of the Fe in a leaf, where it is vital for Photosystem I (PS-I).

• Chelation: Nicotianamine (NA) is a critical small molecule in the cytoplasm that chelates Fe. It is essential for buffering free Fe²⁺ in the cytoplasm and for intracellular trafficking.

• Storage: Excess Fe is primarily stored in the vacuole and the ferritin protein complex, serving as an Fe reservoir.

• Transporters: Specific iron transporters are critically regulated for import and export of iron to different organelles to maintain cellular iron homeostasis.

Examples of intracellular iron transporters:

Organelle	Iron import	Iron Export
Plasmamembrane	IRT1	YSL, FPN1
Chloroplast	PIC1	YSL4 and YSL6
Mitochondria	OPT	ATM3
Vacuole	VIT1 and FPN2	NRAMP3 and NRAMP4

Long-Distance (Vascular) Transport

Long-distance transport distributes iron from the roots to the shoot (leaves, growth points) and then redistributes it to new growth tissues and reproductive organs. The main systems for this transport are the xylem and the phloem. The overall pathway is: 

Root apoplasts → Root symplast → Xylem → Shoot → Phloem → Young Tissue/Seeds.

A. Xylem Transport (Root to Shoot)

The xylem is responsible for the unidirectional bulk flow of water and nutrients from the roots to the aerial parts of the plant.

• Fe State and Chelator: The Fe²⁺ taken up by the roots is often oxidized to Fe³⁺ before being loaded into the xylem. The primary chelating agent for Fe³⁺ in the xylem is Citrate (citric acid), forming the soluble Fe³⁺-citrate complex. Malate is also noted as a chelator.

• Xylem Loading (Root Cells → Xylem):

  • FRD3 (Ferric Reductase Defective 3) in Arabidopsis is a major transporter that loads Fe³⁺-citrate complexes into the xylem. FRD3 belongs to the MATE (Multidrug and Toxic Compound Extrusion) family.

  • FRDL1 (FRD-Like 1) is the analogous citrate transporter in rice, performing a similar function in loading Fe into the xylem.

• Systemic Signaling: The amount of Fe loaded into the xylem also carries systemic Fe signals from the roots to the shoots, coordinating the overall Fe homeostasis of the plant.

B. Phloem Transport (Redistribution and Remobilization)

The phloem is crucial for redistributing Fe from older, mature leaves—where Fe content is high—to younger, actively growing tissues (e.g., new leaves, buds, fruits, and seeds) that require it for development.

• Key Chelators for Remobilization:

  • Nicotianamine (NA): The primary chelating agent for phloem-mediated Fe translocation in most plants.

  • Deoxymugineic Acid (DMA): In graminaceous plants, this type of phytosiderophore (PS) is involved in Fe transport and redistribution in the phloem.

• Phloem Transporters: YSL (YELLOW STRIPE1-Like) transporters are essential for loading and unloading the Fe-chelator complexes within the phloem:

  • YSL1 and YSL3 are identified as major Fe-NA transporters mediating movement in Arabidopsis phloem.

  • YSL2 transports the Fe-NA complex in rice phloem, delivering Fe to the developing grain and young leaves.

  • YSL18 is also involved in Fe transport in phloem tissues and reproductive organs, highlighting the importance of YSL family members in long-distance iron redistribution.

Long distance iron transport in plants (Adapted from Chao and Chao, 2022, New Phytologist)

Effects of Iron Deficiency

Iron deficiency (chlorosis) is widespread, especially in calcareous soils, and impairs normal plant growth and metabolism.

Area of Effect	Symptoms/Consequences
Morphological	Young leaves show vein chlorosis (yellowing between veins while veins remain green), shrunken leaves, and plant dwarfing. Roots may show enlarged and thickened tips with an increase in root hairs.
Chlorophyll & Photosynthesis	Chlorophyll synthesis is impeded, leading to leaf greening and yellowing. Chloroplast ultrastructure is disrupted, the number of cystoid membranes is reduced, and PS-I structure/activity is severely disrupted. The photosynthetic rate decreases due to non-stomatal limitation caused by interruption of the electron transfer chain.
Respiration	Activity of respiration-related enzymes (e.g., cytochrome, CAT, POD, cis-aconitase) is weakened. Electron transfer is hindered, reducing ATP synthesis.
Metabolism	Nitrate reductase activity is reduced. Overall metabolism is activated but stored carbohydrates are depleted. Fe deficiency affects fruit quality and yield (e.g., reduced sugar and vitamin C content in some fruits).

Regulation of iron uptake

Regulation of Iron Transporters under Iron Deficiency in Strategy I plants

The expression of key Fe uptake genes (like FRO2 and IRT1 in Strategy I) is primarily regulated at the transcriptional level in response to Fe deficiency.

• Key Transcriptional Regulator: FIT

• The basic helix-loop-helix (bHLH) transcription factor FIT (FER-LIKE Fe DEFICIENCY-INDUCED TRANSCRIPTION FACTOR) is central to activating Strategy I genes in dicot roots.

• Under Fe deficiency, FIT transcription is induced.

• FIT forms active heterodimers with other bHLH proteins, specifically members of the Ib bHLH clade (bHLH38, bHLH39, bHLH100, and bHLH101).

• This FIT/Ib bHLH heterodimer then binds to the promoters of target genes, activating the transcription of:

  • IRT1 (for Fe²⁺ uptake)

  • FRO2 (for Fe³⁺ reduction)

  • AHA2 (for rhizosphere acidification)

  • NRAMP3/4, PIC1 (for sub-cellular Fe homeostasis)

• The genes for the Ib bHLH proteins themselves are also up-regulated upon Fe deficiency by other bHLH heterodimers (e.g., URI/IVc bHLH).

• Negative feedback control: POPEYE (PYE)

• PYE represses the expression of key positive regulators of Fe uptake, the bHLH Ib genes (bHLH38, bHLH39, bHLH100, and bHLH101), by associating with their promoters. Since these bHLH proteins are necessary partners for FIT to activate IRT1 and FRO2, repressing them effectively shuts down the Fe uptake pathway.

• PYE also negatively regulates its own transcription, which is a common mechanism for feedback control.

• Post-Translational Regulation of IRT1

  • To prevent the accumulation of toxic divalent metal cations (like excess zinc or manganese, which IRT1 can also transport) or excess Fe, IRT1 activity is also controlled after it is synthesized.

  • Under Fe sufficiency (or excess zinc/manganese), IRT1 is rapidly internalized from the plasma membrane and targeted for degradation in the vacuole, often driven by ubiquitination mediated by E3 ubiquitin ligases (e.g., IDF1). This acts as a quality control and a mechanism to shut down uptake when not needed.

• Signaling and Crosstalk

  • Nitric Oxide (NO) and Glutathione (GSH) have been identified as crucial regulators of Fe uptake and homeostasis under Fe deficient conditions.

  • Crosstalk with other micronutrients (e.g., zinc) and hormones (e.g., ethylene) further fine-tunes the Fe deficiency response.

Transcriptional regulation of iron deficiency response in Arabidopsis (Adapted from Gao et. al., 2019, Frontiers in Plant Science)

Transcriptional control in strategy II plants

• Two iron deficiency factors, IDEF1 and IDEF2, bind to the iron deficiency element (IDE), present in the upstream of iron uptake associated genes like IRO2, IRT1, and NAS to activate their expression. 

• Accumulated IRO2 also binds to different downstream iron-related genes like NAS, FDH etc.