102. For metabolic engineering in plants having improved tolerance to osmotic stress, mannitol is overproduced
because it:
(1) lowers the water potential in the plant cell
(2) increases the water potential in the plant cell
(3) lowers membrane potential in the plant cell
(4) increases membrane potential in the plant cell
🌿 Mannitol and Osmotic Stress Tolerance in Plants
Mannitol, a sugar alcohol (polyol), plays a significant role in enhancing plant resistance to osmotic stress such as drought, salinity, or cold conditions. Metabolic engineering of plants to overproduce mannitol has been a successful strategy to improve water retention and stress resilience.
✅ Correct Answer:
(1) Mannitol lowers the water potential in the plant cell
🌱 What Is Water Potential and Why It Matters?
Water potential (Ψ) is a measure of the potential energy of water in a system. Water moves from areas of high water potential to low water potential.
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Under osmotic stress (like drought or high salt), the environment outside the plant cell has lower water potential, causing the plant to lose water.
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To combat this, cells must lower their internal water potential to maintain or attract water flow inward and preserve turgor pressure (cell stiffness vital for growth).
💧 How Does Mannitol Help?
Mannitol acts as a compatible solute or osmoprotectant. It is non-toxic at high concentrations and does not interfere with cellular functions.
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When mannitol accumulates in the cytoplasm, it increases the osmolarity of the cell.
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This leads to a decrease in water potential, allowing the cell to retain or absorb more water from the surroundings.
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Thus, the plant maintains hydration, cell structure, and photosynthetic activity, even under stress conditions.
🧬 Mannitol and Metabolic Engineering
Plants do not naturally produce mannitol in large amounts. To confer this trait:
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Scientists introduce genes like mannitol-1-phosphate dehydrogenase (mtlD) from bacteria into plant genomes.
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This leads to endogenous production of mannitol, enhancing the plant’s osmotic stress tolerance.
Engineered crops like tobacco, tomato, and Arabidopsis expressing mtlD gene show:
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Improved drought resistance
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Enhanced growth under salt stress
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Better survival rates during cold spells
🌾 Applications in Agriculture
Mannitol-overproducing plants can be used to:
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Increase crop productivity in arid regions
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Sustain yields under climate change-induced stress
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Reduce dependency on irrigation or chemical osmoprotectants
🧪 Summary Table
| Feature | Role of Mannitol |
|---|---|
| Type | Sugar alcohol (polyol) |
| Function in Stress | Osmoprotectant |
| Effect on Water Potential | Lowers it |
| Benefit to Plant | Retains water, enhances survival |
| Genetic Source for Engineering | Bacterial mtlD gene |
📌 Final Takeaway
Overproduction of mannitol helps plants tolerate osmotic stress by lowering their internal water potential, allowing them to retain more water and survive under harsh environmental conditions. This strategy is a powerful tool in the hands of agricultural biotechnology to develop stress-resilient crops.
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