Compatible solutes for abiotic stress engineering: Which compound has been produced in transgenic plants to improve tolerance to salt stress and water deficit?

Introduction / Context:
Salt and drought stress disrupt cellular water balance and ion homeostasis. One successful strategy is to engineer accumulation of compatible solutes (osmoprotectants) that stabilize proteins and membranes while maintaining turgor without interfering with metabolism.

Given Data / Assumptions:

• Plants can be engineered with bacterial genes to synthesize non-native osmolytes.
• The question asks for a well-known engineered compound used for salt/drought tolerance.

Concept / Approach:
Mannitol, produced via mannitol-1-phosphate dehydrogenase (mtlD) or related enzymes, has been expressed in plants to increase osmotic adjustment and reactive oxygen species scavenging capacity. Other compatible solutes (e.g., proline, glycine betaine, trehalose) have also been explored, but mannitol is a classic example in the literature.

Step-by-Step Solution:

Verification / Alternative check:
Transgenic tobacco, wheat, and other species expressing mannitol pathway genes showed improved tolerance to salinity and water deficit in controlled studies.

Why Other Options Are Wrong:

• Sucrose: important carbohydrate but not the hallmark engineered osmoprotectant here.
• Nicotine and octopine: unrelated to osmotic stress protection.
• Putrescine: a polyamine with roles in stress but less the canonical engineered solution in this context.

Common Pitfalls:
Confusing native sugars with engineered compatible solutes; assuming any metabolite increase confers abiotic stress tolerance.

Final Answer:
Mannitol