Octopine Ti-plasmid T-DNA organization — role of the left segment (TL) In octopine-type Ti-plasmids, what is the primary function encoded by the left T-DNA segment (TL)?

Introduction / Context:
Octopine-type Ti plasmids often contain two T-DNA regions, termed TL (left) and TR (right). Their genetic content determines tumor induction and opine synthesis in infected plant tissues.

Given Data / Assumptions:

• TL and TR carry different functional gene sets.
• Auxin and cytokinin biosynthesis genes drive tumor formation.
• Opine synthesis genes determine the opine type produced by transformed cells.

Concept / Approach:

In octopine plasmids, TL typically harbors genes required for phytohormone overproduction leading to uncontrolled cell division (tumor formation). TR frequently encodes the opine synthesis enzymes (e.g., octopine synthase). Conjugation genes are not part of T-DNA; they reside elsewhere on the plasmid. Agropine biosynthesis pertains to agropine-type plasmids, not octopine types.

Step-by-Step Solution:

Verification / Alternative check:

Molecular maps of octopine Ti plasmids assign auxin/cytokinin genes to TL and octopine synthase to TR, corroborating the selection.

Why Other Options Are Wrong:

Agropine biosynthesis is associated with a different Ti plasmid type. Conjugative transfer functions are not within T-DNA. ‘‘All of these’’ cannot be true because these functions are distributed differently.

Common Pitfalls:

Confusing opine class names and assuming all Ti plasmids share identical T-DNA organization.

Final Answer:

Tumor (crown gall) formation via auxin/cytokinin synthesis genes