Agrobacterium-induced tumor phenotype in plant cells The tumorigenic phenotype that can be maintained indefinitely in tissue culture arises from the expression of genes located on which DNA segment?

Introduction / Context:
Crown gall disease and related tumor phenotypes in plants are caused by Agrobacterium transferring a defined DNA region, the T-DNA, into the host genome. Once integrated, genes on the T-DNA are expressed, reprogramming hormone metabolism and causing autonomous growth even in vitro.

Given Data / Assumptions:

• T-DNA integrates stably into plant nuclear chromosomes.
• Oncogenes alter auxin/cytokinin balance and opine synthesis.
• Transformed tissue retains tumorous growth without the bacterium.

Concept / Approach:
Distinguish the unique, transferred DNA (T-DNA) from other nucleic acid terms. cDNA is a laboratory construct; rDNA often refers to ribosomal DNA repeats; mRNA is a transient transcript not the heritable cause of the phenotype.

Step-by-Step Solution:

Verification / Alternative check:
Removal of bacteria does not reverse the tumor phenotype; molecular assays detect integrated T-DNA in cultured tumor cells.

Why Other Options Are Wrong:

• cDNA is synthesized from mRNA in vitro for cloning; not causal in planta.
• rDNA encodes ribosomal RNAs; unrelated to tumor induction.
• mRNA is not a genomic locus; it is a gene expression product.

Common Pitfalls:
Assuming vir genes are transferred; they are required in the bacterium but do not integrate with T-DNA into the plant genome.

Final Answer:
T-DNA