In plant–microbe interactions (for example, Agrobacterium tumefaciens infecting dicots), what are “R factors” with respect to DNA transfer into plant cells?
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APlant-encoded resistance (R) proteins that recognize pathogen effectors
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BBacterial ribosomal proteins that bind host DNA
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CBacterial factors essential for transferring DNA into plant cells
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DLow–molecular weight molecules called opines produced by tumors
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EA class of eukaryotic transcription factors that activate Ti plasmids
Answer
Correct Answer: Bacterial factors essential for transferring DNA into plant cells
Explanation
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