Directing morphogenesis — which growth regulator chiefly stimulates embryo/shoot development in vitro?

Introduction / Context:
In organogenesis and somatic embryogenesis, plant growth regulator balance determines developmental outcomes. Identifying the hormone class most associated with shoot/embryo promotion guides medium design for regeneration protocols.

Given Data / Assumptions:

• Auxin–cytokinin balance is the classical framework (Skoog–Miller concept).
• We focus on stimulating embryo or shoot formation.
• Common cytokinins include BA/BAP, kinetin, zeatin, TDZ.

Concept / Approach:

Relatively higher cytokinin to auxin ratios favor shoot organogenesis and often support somatic embryo development or maturation in combination with other cues. Auxins are essential for callus induction and rooting; gibberellins promote elongation; brassinosteroids are modulators; abscisic acid is used later to mature somatic embryos.

Step-by-Step Solution:

Verification / Alternative check:

Most shoot regeneration protocols employ BAP or kinetin; somatic embryo protocols often adjust cytokinin levels or use TDZ.

Why Other Options Are Wrong:

Auxins tend to promote roots or callus; gibberellins elongate; brassinosteroids and ABA are not the principal inducers of shoot/embryo initiation.

Common Pitfalls:

Overusing auxin during shoot induction can suppress bud formation or cause vitrification.

Final Answer:

Cytokinins (e.g., BAP, kinetin, zeatin) to drive shoot/embryo development