Hormonal control in callus culture—Which regulator promotes shoots vs roots? In a plant callus culture, how do altered levels of cytokinins versus auxins typically influence organogenesis from undifferentiated tissue?

Introduction / Context:
Organogenesis from callus depends largely on the ratio of two plant growth regulators: cytokinins and auxins. Mastering this balance is fundamental for predictable shoot and root induction in vitro.

Given Data / Assumptions:

• Callus is competent to respond to hormone cues.
• Media components and culture conditions are otherwise suitable.
• Species differences exist, but the general trend holds widely.

Concept / Approach:
Classic Skoog and Miller findings established that a higher cytokinin/auxin ratio favors shoot primordia, whereas a higher auxin/cytokinin ratio favors root primordia. This hormonal antagonism is used throughout micropropagation protocols.

Step-by-Step Solution:

Verification / Alternative check:
Histology shows meristem formation under cytokinin-rich conditions and root initials under auxin-rich conditions; this is standard across manuals and research articles.

Why Other Options Are Wrong:

• (b) Reverses the well-known relationship.
• (c) and (d)/(e) ignore the synergistic requirement and balance between both hormones in most systems.

Common Pitfalls:
Using absolute concentrations instead of considering the ratio; species-specific fine-tuning is often necessary.

Final Answer:
Increasing cytokinin induces shoot formation; increasing auxin promotes root formation