Plasmid tools — In Agrobacterium-mediated plant transformation workflows, which statement is NOT true about helper plasmids?
Correct Answer: These can't replicate in Agrobacterium
Introduction / Context:Helper plasmids are central to both co-integrative and binary vector systems used with Agrobacterium. Distinguishing their real properties from misconceptions helps avoid design errors in cloning and conjugation steps.
Given Data / Assumptions:
- Helper plasmids typically carry transfer (tra) functions and sometimes vir genes.
- They replicate in Agrobacterium and can mediate mobilization of non-self-transmissible vectors.
- Intermediate vectors may be conjugation-deficient and need a helper.
Concept / Approach:By definition, helper plasmids are compatible replicons in Agrobacterium that provide necessary functions for conjugation and/or virulence in trans. Therefore, the claim that they cannot replicate in Agrobacterium is incorrect.
Step-by-Step Solution:
Identify required traits: replication in Agrobacterium and provision of transfer functions.Intermediate vectors lacking tra genes require helper plasmids to mobilize into Agrobacterium.Hence, statements about replication and conjugation assistance are true; the statement that they cannot replicate is false.Verification / Alternative check:Widely used helpers like pRK2013 function in Agrobacterium, enabling triparental mating and plasmid mobilization.
Why Other Options Are Wrong:
- Replicate in Agrobacterium: correct.
- Mediate conjugation: correct.
- Provide vir in trans (in some systems): correct.
- “All of the above”: cannot be correct because one choice is false.
Common Pitfalls:Confusing helper plasmids with E. coli-only cloning plasmids that lack replication origins compatible with Agrobacterium.
Final Answer:These can't replicate in Agrobacterium