Naturally occurring plant auxins: Which of the following listed auxins is naturally produced by plants and widely occurs in vivo?

Introduction / Context:
Auxins regulate cell elongation, apical dominance, vascular differentiation, and root initiation. In tissue culture, both natural and synthetic auxins are used, but it is important to distinguish endogenous from synthetic molecules.

Given Data / Assumptions:

• IAA is the principal endogenous auxin in higher plants.
• NAA and 2,4-D are synthetic auxins commonly used for stability and predictable responses.

Concept / Approach:
Natural occurrence is the key criterion. While NAA and 2,4-D mimic auxin activity, they are not biosynthesized by plants. IAA derives from tryptophan-dependent and independent pathways in planta and is ubiquitous across species.

Step-by-Step Solution:

Verification / Alternative check:
Plant physiology references consistently identify IAA as the main endogenous auxin; NAA and 2,4-D are laboratory-applied analogs.

Why Other Options Are Wrong:

• NAA and 2,4-D: synthetic, not naturally produced.
• All of the above / None of the above: contradict the established classification.

Common Pitfalls:
Assuming frequent use in vitro implies natural occurrence; conflating endogenous hormones with synthetic analogs selected for stability.

Final Answer:
Indole-3-acetic acid (IAA)