The most important auxin in higher plants is IAA (option 1).

Option Breakdown

Auxins are key plant growth regulators promoting cell elongation, root initiation, and apical dominance, primarily synthesized from tryptophan.

• (1) IAA (Indole-3-Acetic Acid): The primary natural auxin in higher plants, most abundant and potent, driving majority of growth responses like phototropism and fruit development.

• (2) 4-Cl-IAA (4-Chloro-IAA): Minor naturally occurring auxin found in specific plants like peas, less common and not the dominant form across higher plants.​

• (3) IBA (Indole-3-Butyric Acid): Naturally present in low levels, often used synthetically for rooting, but not the main endogenous auxin.​

• (4) 2,4-D (2,4-Dichlorophenoxyacetic Acid): Synthetic herbicide mimicking auxin, disrupts growth at high doses; not naturally occurring in plants.​

The most important auxin in higher plants, IAA (Indole-3-acetic acid), regulates essential processes like cell elongation, root formation, and tropisms, making it vital for competitive exams such as GATE Life Sciences. Synthesized mainly from tryptophan in shoot tips and roots, IAA maintains polar transport gradients critical for plant architecture.

Auxin Types and Functions

Higher plants feature several auxins, but IAA dominates as the most important auxin in higher plants due to its abundance and bioactivity.

• IAA: Core natural auxin; peaks in young tissues, promotes apical dominance and vascular differentiation.

• 4-Cl-IAA: Rare chlorinated variant, active in some species but minor overall.​

• IBA: Auxiliary natural auxin, converts to IAA intracellularly; low native levels.​

• 2,4-D: Artificial, used in weed control; excess mimics auxin overload.​

Biosynthesis pathways (IPA, IAM) converge on IAA, with stability controlled by conjugation and degradation.​

Key Roles in Plant Growth

As the most important auxin in higher plants, IAA drives:

• Phototropism and gravitropism via asymmetric distribution.

• Embryo patterning and lateral root initiation.

• Fruit parthenocarpy and stress responses.

Exam questions often test IAA’s primacy over synthetic analogs.​

Exam Tips for Auxin MCQs

For “The most important auxin present in higher plant is:”, select IAA—recall its natural prevalence vs. others’ niche/synthetic roles. Link to NCERT plant growth chapters for GATE prep.