During plant regeneration from callus, which growth regulator is primarily added to initiate and promote shoot formation (shoot organogenesis)?

Introduction / Context:
Reprogramming callus to regenerate shoots requires a hormonal signal that favors shoot meristem initiation over root formation. In classic Skoog and Miller paradigms, the auxin:cytokinin ratio determines organogenic direction.

Given Data / Assumptions:

• Callus is already established.
• The goal is shoot initiation and proliferation.
• Hormone balance can be adjusted by adding cytokinins.

Concept / Approach:
High cytokinin relative to auxin promotes shoot organogenesis by stimulating cell division in shoot-forming regions and activating shoot meristem identity networks. Conversely, high auxin favors rooting or undifferentiated callus proliferation depending on context.

Step-by-Step Solution:

Verification / Alternative check:
Protocols for many species (e.g., BAP or kinetin supplementation) demonstrate robust shoot formation when cytokinin dominates the hormonal regime.

Why Other Options Are Wrong:

• Auxins: typically favor roots or callus.
• Gibberellins: elongation and growth, not de novo shoot meristem initiation.
• Brassinosteroids and ABA: play roles in growth and stress responses but are not the primary drivers of shoot organogenesis from callus.

Common Pitfalls:
Using excessive auxin during the shoot induction phase; failing to reduce auxin after callus induction.

Final Answer:
Cytokinins