In plant tissue culture, what is meant by a “nurse” or conditioned medium, and how is it typically obtained?

Introduction / Context:
Conditioned (or “nurse”) medium is a practical tool to improve growth or regeneration of difficult explants by providing secreted factors from actively growing cells. It can enhance viability, reduce lag phases, and improve morphogenesis in sensitive cultures.

Given Data / Assumptions:

• Fast-growing cell suspensions release vitamins, peptides, phytohormone-like compounds, and other metabolites into the medium.
• This “conditioned” medium can be transferred to new cultures that benefit from these factors.

Concept / Approach:
The concept mirrors feeder-layer strategies in animal cell culture. Instead of co-culture, the supernatant medium itself is used, carrying diffusible growth-promoting molecules that are difficult to define or synthesize individually.

Step-by-Step Solution:

Verification / Alternative check:
Numerous regeneration protocols report improved outcomes when a fraction of conditioned medium is included, especially for recalcitrant species or stages.

Why Other Options Are Wrong:

• Generic “growth factor-rich” descriptions lack the defining feature (derivation from an active culture).
• Antibiotic-containing medium is unrelated to conditioning.

Common Pitfalls:
Using over-aged or nutrient-exhausted medium; failing to sterile-filter before use; ignoring batch variability.

Final Answer:
The liquid medium removed from a vigorously growing suspension culture and used to support other cultures