Engineering mannitol accumulation in tobacco To increase the level of mannitol (a compatible solute) in transgenic tobacco plants for stress tolerance, which gene is typically introduced and expressed?

Introduction / Context:
Accumulation of compatible solutes like mannitol can enhance plant tolerance to salinity and drought. Transgenic strategies boost mannitol biosynthesis by introducing a key biosynthetic enzyme into crops such as tobacco.

Given Data / Assumptions:

• Mannitol is synthesized via a mannitol-1-phosphate intermediate in many plants and microbes.
• Engineering focuses on driving flux toward mannitol production, not catabolism.
• The question asks which gene is transferred to raise mannitol content.

Concept / Approach:

Mannitol synthase (often functioning via mannitol-1-phosphate formation/reduction and subsequent dephosphorylation) channels hexose phosphates into mannitol biosynthesis. Overexpressing the synthase increases mannitol pools, improving osmotic adjustment. In contrast, mannitol dehydrogenase generally participates in catabolism (conversion of mannitol back to fructose in some tissues), which would not raise net mannitol levels.

Step-by-Step Solution:

Verification / Alternative check:

Multiple transgenic studies report that expressing synthase-pathway genes elevates mannitol accumulation and confers stress tolerance, validating the strategy.

Why Other Options Are Wrong:

Catabolic mannitol dehydrogenase would reduce mannitol. Mannitol pyrophosphorylase is not a standard step in this pathway. Trehalose-6-phosphate synthase relates to trehalose, a different compatible solute.

Common Pitfalls:

Confusing biosynthetic ‘‘synthase’’ with degradative ‘‘dehydrogenase’’ roles; only the former increases pool size.

Final Answer:

Mannitol synthase (mannitol-1-phosphate pathway enzyme)