Identify the non-auxin — which of the following is not classified as an auxin used in plant tissue culture?

Introduction / Context:
Auxins and cytokinins are the cornerstone hormones of plant tissue culture. Being able to distinguish which compounds belong to which class is essential for designing media that drive callus, root, or shoot formation.

Given Data / Assumptions:

• Common auxins: IAA, NAA, IBA, 2,4-D.
• Common cytokinins: zeatin, BAP/BA, kinetin, TDZ.
• We must select the compound that is not an auxin.

Concept / Approach:

IAA, NAA, IBA, and 2,4-D are all widely used auxins that promote cell elongation, callusing, and rooting. Zeatin, however, is a naturally occurring cytokinin that promotes cell division and shoot morphogenesis, not an auxin.

Step-by-Step Solution:

Verification / Alternative check:

Standard references and MS-based protocols classify zeatin as a cytokinin; it is never grouped among auxins.

Why Other Options Are Wrong:

IAA, NAA, IBA, and 2,4-D are bona fide auxins with clear auxinic effects on tissues.

Common Pitfalls:

Confusing “plant growth regulator” with a specific hormone class; remember zeatin is cytokinin.

Final Answer:

Zeatin