Plant tissue culture – definition focus: Callus cultures are amorphous, undifferentiated cell aggregates arising from the unorganized growth of explants on which type of culture medium and conditions?

Introduction / Context:
In plant biotechnology, a “callus” refers to an unorganized mass of cells that proliferates from plant explants under culture. Identifying the precise conditions that promote callus formation is foundational for downstream applications such as organogenesis, somatic embryogenesis, protoplast culture, and genetic transformation.

Given Data / Assumptions:

• Callus is undifferentiated plant tissue induced under controlled culture conditions.
• Explants are placed on nutrient media containing mineral salts, vitamins, a carbon source, and plant growth regulators (typically auxin and/or cytokinin).
• Plant tissue culture must be performed aseptically to avoid microbial contamination.

Concept / Approach:
While both liquid and solid media are used in plant tissue culture, classical callus induction is carried out on a solidified (e.g., agar or gellan gum) nutrient medium under sterile conditions. Solid surfaces help stabilize the explant and provide a uniform microenvironment where auxin–cytokinin balance promotes dedifferentiation and unorganized cell proliferation. Asepsis is non-negotiable: microbial contaminants outgrow plant cells and secrete inhibitory metabolites.

Step-by-Step Solution:
Recognize that “callus culture” implies undifferentiated growth from plant explants.Recall that solid media (with agar) are standard for initial callus induction.Note that all plant tissue culture must be aseptic to succeed.Therefore, select “aseptic solid nutrient medium.”

Verification / Alternative check:
Protocols for callus induction (e.g., from leaf discs or stem segments) universally specify sterile explant preparation and plating onto solid media supplemented with appropriate auxin/cytokinin ratios, confirming the chosen condition.

Why Other Options Are Wrong:

• Liquid medium: used for suspension cultures, not ideal for initial callus initiation from intact explants.
• Solid nutrient medium (without “aseptic”): incomplete; sterility is essential.
• Solid or liquid medium: too vague; ignores the standard for callus induction and the requirement of asepsis.
• Hormone-free tap water: lacks nutrients, regulators, and sterility.

Common Pitfalls:
Assuming any medium will work; ignoring hormones; overlooking the absolute necessity of sterile technique.

Final Answer:
Aseptic solid nutrient medium.