In plant–microbe interactions (for example, Agrobacterium tumefaciens infecting dicots), what are “R factors” with respect to DNA transfer into plant cells?

Introduction / Context:
Plant–microbe interactions often hinge on DNA movement from bacteria into plant nuclei. In the classic Agrobacterium tumefaciens system, bacterial factors encoded on the tumor-inducing (Ti) plasmid orchestrate the transfer of a defined DNA segment (T-DNA) into plant cells, where it integrates and is expressed. The question tests understanding of which “R factors” are functionally tied to this transfer step.

Given Data / Assumptions:

• Focus is on bacterial–plant DNA transfer (not antibiotic resistance).
• Agrobacterium uses plasmid-encoded proteins to process and deliver T-DNA.
• “R factors” here denote bacterial determinants required for transfer.

Concept / Approach:
Agrobacterium carries vir (virulence) gene clusters on the Ti plasmid. These proteins generate a single-stranded T-strand, coat it, and export the nucleoprotein through a Type IV secretion system into the plant cell. Inside the plant, additional bacterial and host factors guide nuclear import and integration. Thus, the bacterial factors are essential for DNA transfer; opines are metabolites synthesized by transformed plant cells, not the transfer machinery.

Step-by-Step Solution:
Identify the functional role under test: DNA transfer from bacterium to plant.Map role to players: Ti-plasmid–encoded vir proteins (bacterial factors) perform this task.Exclude plant R proteins (host immunity) and opines (metabolites), which do not mediate DNA transfer.

Verification / Alternative check:
Binary vector systems retain vir genes in a helper plasmid; when present, T-DNA transfer proceeds. Without vir factors, transfer fails.

Why Other Options Are Wrong:

• Plant R proteins: host surveillance; they do not transfer DNA.
• Bacterial ribosomal proteins: translation, not DNA export.
• Opines: products of transformed tissues, not transfer components.
• Eukaryotic transcription factors: not required to activate Ti plasmids for transfer.

Common Pitfalls:
Confusing “R factors” with plant resistance genes or with opines.

Final Answer:
Bacterial factors essential for transferring DNA into plant cells