In plant genetic engineering for herbicide tolerance, resistance to sulfonylurea compounds in transgenic tobacco has most reliably been achieved by transferring which enzyme gene target of these herbicides?

Introduction / Context:
Sulfonylurea herbicides act by inhibiting the branched-chain amino acid biosynthesis pathway in plants. Understanding the precise enzyme target allows scientists to engineer transgenic crops that are selectively resistant while weeds remain sensitive. This question probes knowledge of herbicide targets and corresponding resistance genes used in plant biotechnology.

Given Data / Assumptions:

• Sulfonylureas inhibit a single, well-defined enzyme in plants.
• Transgenic resistance generally comes from either overexpression or mutation of the target enzyme.
• The crop mentioned is tobacco, but the mechanism is general across species.

Concept / Approach:
The primary cellular target of sulfonylureas (and the related imidazolinones) is acetolactate synthase (ALS), also called acetohydroxyacid synthase (AHAS). Point mutations (e.g., in the catalytic subunit) reduce herbicide binding, conferring robust resistance. By contrast, EPSPS is the target of glyphosate, and GS is the target of glufosinate; transferring or altering those genes would not address sulfonylurea action.

Step-by-Step Solution:

Verification / Alternative check:
Published cases across multiple crops (tobacco, canola, rice) show ALS point mutations (e.g., Pro197, Trp574, Ser653 positions depending on species numbering) that produce sulfonylurea resistance without strong growth penalties.

Why Other Options Are Wrong:

• EPSPS: target of glyphosate, not sulfonylureas.
• GS: target of glufosinate (phosphinothricin), not sulfonylureas.
• “Any of the above”: incorrect because herbicide targets are specific.
• ACCase: target of aryloxyphenoxypropionates and cyclohexanediones in grasses, not sulfonylureas.

Common Pitfalls:
Confusing popular herbicides (glyphosate, glufosinate) with sulfonylureas; assuming resistance can be conferred by any metabolic enzyme gene.

Final Answer:
Gene for ALS (acetolactate synthase; also called AHAS)