Aluminum tolerance mechanisms in transgenic plants Which strategy has successfully increased aluminum tolerance by modifying root exudation or chelation?

Introduction / Context:
Aluminum toxicity in acidic soils inhibits root growth. A proven tolerance mechanism in several crops is the exudation of organic acids (citrate/malate) that chelate Al^3+ in the rhizosphere, reducing its bioavailability at the root apex.

Given Data / Assumptions:

• Organic acid transporters (e.g., ALMT, MATE family) mediate efflux of malate or citrate.
• Transgenic overexpression of these transporters increases organic acid exudation.
• Chelated Al-citrate complexes are less toxic to root cells.

Concept / Approach:
Select the option that directly states secretion of an organic acid into soil. While other intracellular sequestration strategies exist, field-validated improvements often rely on enhanced citrate/malate efflux from roots.

Step-by-Step Solution:

Verification / Alternative check:
Transgenic barley, wheat, and Arabidopsis lines overexpressing MATE/ALMT genes show improved root growth on Al-toxic media via increased citrate/malate efflux.

Why Other Options Are Wrong:

• Phytosiderophores primarily chelate Fe, not Al, and are characteristic of grasses for iron uptake.
• Phytochelatins target heavy metals like Cd; evidence for strong Al detox via phytochelatins is limited.
• Binding Al to the cell wall can still impede growth; exudation approach is more validated.
• Lignin induction does not address Al speciation and can hinder growth.

Common Pitfalls:
Confusing iron-acquisition siderophores with Al-chelation strategies; the key for Al is organic acid exudation.

Final Answer:
Secretion of citrate (organic acids) from roots into the rhizosphere