Lecture Notes on Iron Uptake Mechanisms in Plants

Nutrient Uptake and Transport: Iron as an Example

The lecture focuses on the mechanisms behind nutrient uptake and transport, using iron as a specific example.
Iron has extensive literature compared to other elements, especially micronutrients.
Iron is a complex topic with two principal forms: Iron three plus (Fe^{3+}) and Iron two plus (Fe^{2+}).
Fe^{3+} + electron \rightarrow Fe^{2+} (Reduction).

Reduction: Acceptance of electrons (reduces positive charge).
Oxidation: Removal of electrons (increases positive charge).
In aerated soils, iron is mostly present as Fe^{3+}, which plants find difficult to utilize due to its high charge and low solubility.
Plants evolved mechanisms to cope with low iron availability over millions of years.

Plants developed two main strategies to acquire iron:
- Strategy One: Applicable to almost all plants except grasses (dicots and non-graminaceous monocots).
- Strategy Two: Utilized by grasses (graminaceous plants).
Gramineae (old term) \rightarrow Poaceae (new term); graminaceous is still used.

Increased Iron three plus (Fe^{3+}) Reductase Activity: Up-regulation of the capacity to reduce Fe^{3+} to Fe^{2+}.
Hydrogen Efflux Increase: Acidification of the external environment occurs by increasing the activity of H plus ATPase, an enzyme that pumps hydrogen out of the cytoplasm. Acidic conditions favor the conversion of Fe^{3+} to Fe^{2+}, hence increasing iron availability.
Citrate and Malate Upregulation: Internal chelators like malate and citrate bind iron in the cytoplasm, aiding its distribution and transport to other plant parts.
Release of Chelators: Exudation of phenolics to increase the solubility of iron-containing compounds in the external environment.
Increased Iron Uptake: The combination of these mechanisms leads to enhanced iron uptake.
Localization: These responses are most active 1-2 cm behind the root tip.

Iron Reductase: Enzyme in the plasma membrane that reduces Fe^{3+} to Fe^{2+} (external part does the reduction).
- Constitutive: Always expressed.
- Inducible: Induced under iron deficiency.
Iron Channels: Specific channels for Fe^{2+} transport across the plasma membrane.
Increased Efflux of Hydrogen Ions: Via H plus ATPase to acidify the external environment and create an electrochemical gradient.

Cytoplasmic судьба: Fe^{2+} might be oxidized back to Fe^{3+} due to the relatively high pH of the cytoplasm (pH 7.1-7.4, conducive to oxidation).
Malate and Citrate Binding: Chelators like malate and citrate bind to iron for transport to other plant parts.
Photosynthesis Requirement: Photosynthesis is essential to provide food from leaves to roots, enabling the production of chelators and energy for H plus ATPase activity.

Cucumber (a non-grass plant using Strategy One) shows significant chlorosis under iron deficiency.
Rhizosphere Acidification: pH drops from 6 to almost 5, indicating a substantial increase in hydrogen concentration.
Increased Reducing Capacity: Activity of iron reductase increases significantly under iron deficiency.
Uptake Rate Increase: Plants deficient in iron exhibit a much higher iron uptake rate when iron is resupplied.

Root epidermal cells show changes under iron deficiency:
- More mitochondria for energy production.
- More Golgi apparatus components for biosynthesis of maleate, citrate, and phenolics.

Mainly used by grasses.
Involves phytosiderophores: molecules that plants use to carry iron (phyto = plant, sidero = iron, phore = carry).

Non-proteinogenic amino acids (amino acids not used to build proteins).
Examples: Neugenic acid and Aveenoic acids.
Different grass species have different types and amounts of phytosiderophores.
Effectiveness varies among species (wheat is more effective than sorghum in iron uptake).

Production of Phytosiderophores: Methionine (a sulfur-containing amino acid) is a precursor. Nicotinamide is also important as it is involved in the transport of micronutrients.
Release into the External Environment: Phytosiderophores are released via a transporter (TORM - Transporter of Muginetic Acid).
Iron Acquisition from Soil Particles: Phytosiderophores have a high affinity for iron and chelate Fe^{3+} from soil particles.
Transport to the Root Surface: The phytosiderophore-Fe^{3+} complex is transported to the root surface via transpiration stream.
Uptake into the Cytoplasm: The Yellow Stripe transporter transports the phytosiderophore-Fe^{3+} complex into the cytoplasm.
Internal Processes: Iron is dissociated from the phytosiderophore complex, and the phytosiderophore is recycled. Malate and citrate bind to iron for transport to other plant parts.

Discovered in maize (May 1).
Named due to yellow striping in maize leaves under iron deficiency (lack of chlorophyll synthesis between veins).
Maize mutants lacking this transporter struggle with iron uptake.
Strategy Two is generally more effective than Strategy One in soils with low iron availability.

Phytosiderophores also have affinity for other metals, such as zinc, copper, and manganese.
Affinity: Iron > Zinc > Copper (low) > Manganese (very low).
The role of phytosiderophores in zinc uptake is debated.
Yellow Stripe transporter might be involved in the uptake of zinc-bound phytosiderophores.

Grasses do have H plus ATPase and perform acidification of the external environment, though not necessarily in response to iron availability due to the efficiency of Strategy Two.

Similar to Strategy One, phytosiderophore exudation is highest at the root tip.
Root tip exploration: Root tips explore new volumes of soil, making it ecologically advantageous to concentrate responses there.

Phytosiderophore release is also temporally controlled.
Exudation peaks a few hours after sunrise (8:00 AM in controlled experiments) due to increased photosynthesis and energy availability.
Plants economize by releasing phytosiderophores during the most effective part of the day.

Knowledge of iron uptake mechanisms is more advanced than that of other micronutrients (e.g., zinc, boron).
Concentrations of micronutrients required by plants are very low (micromolar range).
Uptake Kinetics: Saturation uptake kinetics.
Apoplastic Accumulation: Cations are attracted to negatively charged cell walls in the apoplast.
Plasma Membrane Transport: Requires energy expenditure for electrochemical gradients (H plus ATPase).
Cation Channels: Specific and non-specific cation channels are involved (e.g., zinc can sneak through calcium channels).
Boron: May be transported via diffusion as boric acid, though transporters are also involved.