What strategies are employed by plants for the uptake of iron under conditions of iron stress?

I. Morphological Adaptations

Plants respond to iron deficiency by altering their root system architecture to maximize exploration of the soil volume for iron. These changes include:

• Increased Root Hair Density: Iron-deficient plants often exhibit a significant increase in the number and length of root hairs, expanding the surface area for iron absorption.
• Lateral Root Formation: The development of lateral roots is stimulated under iron stress, further increasing the root surface area.
• Root Elongation: Primary root elongation can be enhanced, allowing the roots to penetrate deeper into the soil where iron availability might be slightly higher.
• Aerenchyma Formation: In some species, aerenchyma (air spaces) formation in roots increases oxygen diffusion to the roots, facilitating iron oxidation and uptake.

II. Physiological Mechanisms – Strategy I Plants (Non-Graminaceous Dicotyledons)

Dicotyledonous plants employ ‘Strategy I’ for iron acquisition, characterized by the following mechanisms:

• Acidification of the Rhizosphere: Plants release protons (H⁺) into the rhizosphere, lowering the pH and increasing iron solubility. This is mediated by proton-ATPases.
• Redox Reactions at the Root Surface: Plants secrete reductases (e.g., ferric reductase) that reduce ferric iron (Fe³⁺) to ferrous iron (Fe²⁺), which is more soluble and readily absorbed.
• Chelation of Iron: Plants synthesize and exude organic acids (e.g., citric acid, malic acid) that chelate Fe²⁺, preventing its precipitation and facilitating its uptake.

III. Physiological Mechanisms – Strategy II Plants (Graminaceous Monocotyledons)

Grasses (Poaceae) utilize ‘Strategy II’, which differs significantly from Strategy I:

• Phytosiderophore Secretion: Grasses release phytosiderophores (PSs), such as deoxymugineic acid (DMA), into the rhizosphere. These are non-proteinogenic amino acids that have a high affinity for Fe³⁺, forming soluble Fe-PS complexes.
• Uptake via Yellow Stripe Protein (YSL): The Fe-PS complexes are then transported back into the root cells via specific transporters, namely the Yellow Stripe proteins (YSLs).
• Reduction of Fe³⁺ to Fe²⁺: While PSs primarily chelate Fe³⁺, reduction to Fe²⁺ can also occur, enhancing uptake.

IV. Symbiotic Interactions

Plants can also enhance iron acquisition through symbiotic relationships:

• Mycorrhizal Associations: Mycorrhizal fungi extend the plant’s root system, increasing the exploration volume for iron. They also release organic acids and siderophores, enhancing iron solubility and uptake.
• Rhizobium Symbiosis: In legumes, rhizobium bacteria fix atmospheric nitrogen, but also contribute to iron acquisition by producing siderophores.
• Iron-Mobilizing Bacteria: Certain bacteria in the rhizosphere can solubilize iron through the production of siderophores or by lowering the pH.

V. Internal Iron Homeostasis

Once iron is absorbed, plants maintain internal iron homeostasis through:

• Iron Storage: Iron is stored in vacuoles as ferritin, preventing toxicity.
• Iron Transport: Iron is transported within the plant via nicotianamine (NA), a non-proteinogenic amino acid.
• Regulation of Iron Uptake: Iron status regulates the expression of genes involved in iron uptake and transport.

Strategy	Plants	Key Mechanisms
Strategy I	Non-Graminaceous Dicotyledons	Rhizosphere acidification, Fe3+ reduction, organic acid chelation
Strategy II	Graminaceous Monocotyledons	Phytosiderophore secretion, YSL transporters, Fe3+ reduction