Plants uptake iron through a combination of mechanisms, primarily focusing on solubilizing iron in the soil and then transporting it into the root cells.
Iron uptake is a complex process because iron is often present in the soil in insoluble forms (like iron oxides). Plants have evolved strategies to overcome this limitation. The primary method involves mobilizing iron in the rhizosphere (the soil immediately surrounding the roots) and then transporting it across the root cell membranes.
There are two main strategies plants employ:
1. Strategy I: Reduction Strategy (Typical of dicots and non-grass monocots)
- Rhizosphere Acidification: Plants release protons (H+) into the soil, lowering the pH and increasing the solubility of Fe3+ (ferric iron).
- Reductant Release: Plants secrete reductants that reduce Fe3+ to Fe2+ (ferrous iron), which is more soluble and easier to absorb.
- Fe2+ Transport: A specific iron transporter protein, often IRT1 (Iron-Regulated Transporter 1), located in the root cell membrane, transports Fe2+ into the cell.
2. Strategy II: Chelation Strategy (Typical of grasses)
- Phytosiderophore Release: Grasses synthesize and release phytosiderophores (iron-chelating compounds) into the rhizosphere. These compounds bind to Fe3+, forming a soluble Fe3+-phytosiderophore complex.
- Complex Uptake: The Fe3+-phytosiderophore complex is then taken up by the plant via a specific transporter protein that recognizes and binds the entire complex.
Additional Factors Influencing Iron Uptake:
- Diffusion: Iron can move to the root surface by diffusion from areas of high concentration to areas of lower concentration as the plant takes it up.
- Root Interception: As roots grow, they physically intercept iron compounds present in the soil.
- Mycorrhizal Associations: Certain fungi (mycorrhizae) form symbiotic relationships with plant roots, enhancing nutrient uptake, including iron.
In summary, plants use either a reduction strategy (releasing acids and reductants) or a chelation strategy (releasing phytosiderophores) to solubilize iron in the soil, followed by specific transporter proteins to absorb the iron into their root cells. Diffusion and root interception also play minor roles.
