25. Which of the following is(are) auxins?
(A) 1-Naphthaleneacetic acid (NAA)
(B) Indole-3-butyric acid (IBA)
(C) 2,4-Dichlorophenoxyacetic acid (2,4-D)
(D) Indole-3-acetic acid (IAA)
Auxins in Plants Explained: IAA, IBA, NAA, and 2,4-D
Introduction
Auxins are the first discovered class of plant growth regulators and play a central role in regulating plant growth, development, and responses to environmental stimuli. These hormones control a wide range of physiological processes including cell elongation, apical dominance, vascular differentiation, root initiation, tropic movements, fruit development, and tissue differentiation. Because of their diverse biological functions, auxins are among the most extensively studied phytohormones in plant physiology and biotechnology.
Auxins may occur naturally within plants or may be synthesized artificially for agricultural and horticultural applications. The major naturally occurring auxin is Indole-3-acetic acid (IAA), whereas compounds such as Indole-3-butyric acid (IBA), 1-Naphthaleneacetic acid (NAA), and 2,4-Dichlorophenoxyacetic acid (2,4-D) exhibit strong auxin activity and are widely used in plant tissue culture, rooting of cuttings, callus induction, and selective weed control.
Correct Answer
Correct Answers: (A), (B), (C), and (D)
Detailed Explanation
All four compounds listed in the question belong to the auxin group of plant growth regulators. They differ mainly in their origin and practical applications, but each possesses auxin activity and regulates plant growth.
Indole-3-acetic acid (IAA) is the principal naturally occurring auxin found in higher plants. It is synthesized mainly in young leaves, shoot apical meristems, and developing seeds, where it regulates cell elongation, apical dominance, vascular differentiation, phototropism, gravitropism, and fruit development.
Indole-3-butyric acid (IBA) is another naturally occurring auxin-like compound that is particularly effective in stimulating adventitious root formation. Because of its stability and high rooting efficiency, IBA is widely used in horticulture and plant tissue culture for vegetative propagation.
1-Naphthaleneacetic acid (NAA) is a synthetic auxin that promotes root initiation, prevents premature fruit drop, induces parthenocarpy, and is extensively used in commercial agriculture and plant propagation.
2,4-Dichlorophenoxyacetic acid (2,4-D) is also a synthetic auxin. At low concentrations it promotes callus formation and cell division in plant tissue culture, whereas at higher concentrations it functions as a selective herbicide that effectively controls broad-leaved weeds without severely affecting grasses.
Since every compound listed exhibits auxin activity, all four options are correct.
Explanation of Each Option
Option (A): 1-Naphthaleneacetic Acid (NAA)
This statement is correct. NAA is a synthetic auxin widely used to stimulate rooting, prevent premature fruit abscission, and improve fruit development.
Option (B): Indole-3-butyric Acid (IBA)
This statement is correct. IBA is an auxin commonly used for adventitious root induction in stem cuttings and plant tissue culture.
Option (C): 2,4-Dichlorophenoxyacetic Acid (2,4-D)
This statement is correct. 2,4-D is a synthetic auxin extensively used in plant biotechnology for callus induction and as a selective herbicide.
Option (D): Indole-3-acetic Acid (IAA)
This statement is correct. IAA is the major naturally occurring auxin responsible for regulating normal plant growth and development.
Why All Four Options are Correct
Each of the four compounds possesses auxin activity. IAA and IBA occur naturally in plants, whereas NAA and 2,4-D are synthetic auxins developed for agricultural, horticultural, and biotechnological applications.
Classification of the Given Auxins
| Compound | Type | Classification |
|---|---|---|
| Indole-3-acetic acid (IAA) | Natural | Auxin |
| Indole-3-butyric acid (IBA) | Natural | Auxin |
| 1-Naphthaleneacetic acid (NAA) | Synthetic | Auxin |
| 2,4-Dichlorophenoxyacetic acid (2,4-D) | Synthetic | Auxin |
Major Functions of Auxins
| Function | Role |
|---|---|
| Cell Elongation | Promotes shoot cell expansion |
| Apical Dominance | Suppresses lateral bud growth |
| Phototropism | Causes bending toward light |
| Gravitropism | Regulates directional growth in response to gravity |
| Root Initiation | Stimulates adventitious root formation |
| Fruit Development | Promotes fruit growth and parthenocarpy |
| Vascular Differentiation | Enhances xylem and phloem formation |
Applications of Different Auxins
| Auxin | Major Application |
|---|---|
| IAA | Natural regulation of plant growth |
| IBA | Rooting hormone for stem cuttings |
| NAA | Fruit thinning, rooting, prevention of fruit drop |
| 2,4-D | Callus induction and selective weed control |
Natural vs Synthetic Auxins
| Feature | Natural Auxins | Synthetic Auxins |
|---|---|---|
| Examples | IAA, IBA | NAA, 2,4-D |
| Occurrence | Produced naturally by plants | Artificially synthesized |
| Common Uses | Growth regulation | Agriculture, tissue culture, herbicides |
Biological Significance
Auxins coordinate numerous developmental processes throughout the plant life cycle. Natural auxins regulate growth under normal physiological conditions, while synthetic auxins have become indispensable tools in agriculture, horticulture, and biotechnology. Their applications include vegetative propagation, tissue culture, production of seedless fruits, prevention of premature fruit drop, and selective control of weeds, making auxins one of the most economically important classes of plant hormones.
Final Answer
Correct Answers: (A), (B), (C), and (D)
1-Naphthaleneacetic acid (NAA), Indole-3-butyric acid (IBA), 2,4-Dichlorophenoxyacetic acid (2,4-D), and Indole-3-acetic acid (IAA) are all auxins. Among them, IAA and IBA are naturally occurring auxins, while NAA and 2,4-D are synthetic auxins widely used in agriculture, horticulture, and plant biotechnology.
