18. Which of the following is the CORRECT combination of a synthetic auxin and a synthetic cytokinin?

(A) 2,4-Dichlorophenoxy acetic acid (2,4-D) and Indole-3-acetic acid (IAA)

(B) 2,4-Dichlorophenoxy acetic acid (2,4-D) and 6-Benzylamino purine (BAP)

(D) Indole-3-acetic acid (IAA) and Zeatin

Synthetic Auxin and Synthetic Cytokinin Explained | 2,4-D and BAP

Introduction

Plant growth and development are regulated by a group of naturally occurring chemical messengers known as plant growth regulators (PGRs) or phytohormones. Among these, auxins and cytokinins play a central role in controlling cell division, cell elongation, differentiation, organ formation, apical dominance, vascular differentiation, and tissue regeneration. In addition to naturally occurring hormones, scientists have developed several synthetic compounds that mimic the biological activity of auxins and cytokinins. These synthetic plant growth regulators are widely used in agriculture, horticulture, and plant tissue culture because of their stability, efficiency, and predictable physiological effects.

One of the most commonly used synthetic auxins is 2,4-Dichlorophenoxy acetic acid (2,4-D), while one of the most widely used synthetic cytokinins is 6-Benzylaminopurine (BAP). These compounds are extensively employed in plant biotechnology for callus induction, organogenesis, micropropagation, and crop improvement.

Correct Answer

Correct Option: (B) 2,4-Dichlorophenoxy acetic acid (2,4-D) and 6-Benzylamino purine (BAP)

Detailed Explanation

2,4-Dichlorophenoxy acetic acid (2,4-D) is a synthetic auxin that mimics the physiological activity of the natural auxin Indole-3-acetic acid (IAA). It promotes cell elongation, stimulates cell division in tissue culture, induces callus formation, and is widely used as a selective herbicide against broadleaf weeds.

6-Benzylaminopurine (BAP), also known as benzyl adenine (BA), is a synthetic cytokinin. It promotes cell division, shoot initiation, axillary bud proliferation, delay of leaf senescence, and regeneration of complete plants during tissue culture. BAP is one of the most frequently used cytokinins in plant micropropagation because of its high effectiveness and stability.

In contrast, Indole-3-acetic acid (IAA) is the principal natural auxin produced by plants, whereas Zeatin is the major natural cytokinin first isolated from maize (Zea mays). Since the question specifically asks for a combination of a synthetic auxin and a synthetic cytokinin, only Option (B) satisfies both conditions.

Explanation of Each Option

Option (A): 2,4-D and Indole-3-acetic Acid (IAA)

This option is incorrect. Although 2,4-D is a synthetic auxin, IAA is a naturally occurring auxin rather than a synthetic cytokinin.

Option (B): 2,4-D and 6-Benzylaminopurine (BAP)

This option is correct. 2,4-D is a synthetic auxin, while BAP is a synthetic cytokinin. This combination is extensively used in plant tissue culture for regulating callus formation and organ development.

Option (C): 6-Benzylaminopurine (BAP) and Zeatin

This option is incorrect. BAP is a synthetic cytokinin, but Zeatin is a natural cytokinin. There is no auxin in this combination.

Option (D): Indole-3-acetic Acid (IAA) and Zeatin

This option is incorrect. Both IAA and Zeatin are naturally occurring plant hormones. IAA is a natural auxin and Zeatin is a natural cytokinin.

Why Option (B) is Correct

Option (B) correctly combines 2,4-D, a synthetic auxin, with BAP, a synthetic cytokinin. These two compounds are routinely used together in plant tissue culture because varying their concentration ratio determines whether cultured cells form roots, shoots, or undifferentiated callus.

Why the Other Options are Incorrect

Why Option (A) is Incorrect

IAA is a natural auxin rather than a synthetic cytokinin.

Why Option (C) is Incorrect

This combination contains two cytokinins, one synthetic (BAP) and one natural (Zeatin), but no auxin.

Why Option (D) is Incorrect

Both hormones are naturally occurring and therefore do not satisfy the requirement of identifying synthetic plant growth regulators.

Comparison of All Options

Option	Hormone Combination	Correct or Incorrect
A	Synthetic Auxin + Natural Auxin	Incorrect
B	Synthetic Auxin + Synthetic Cytokinin	Correct
C	Synthetic Cytokinin + Natural Cytokinin	Incorrect
D	Natural Auxin + Natural Cytokinin	Incorrect

Natural and Synthetic Plant Hormones

Hormone Class	Natural Example	Synthetic Example
Auxin	Indole-3-acetic acid (IAA)	2,4-D, NAA
Cytokinin	Zeatin	6-Benzylaminopurine (BAP), Kinetin

Major Functions of Auxins and Cytokinins

Hormone	Major Functions
Auxins	Cell elongation, apical dominance, root initiation, vascular differentiation
Cytokinins	Cell division, shoot initiation, delay of senescence, bud proliferation

Applications in Plant Tissue Culture

Growth Regulator	Major Application
2,4-D	Callus induction and somatic embryogenesis
BAP	Shoot multiplication and regeneration
IAA	Root development
Zeatin	Natural cytokinin for shoot development

Auxin-Cytokinin Ratio in Tissue Culture

Auxin : Cytokinin Ratio	Developmental Response
High Auxin, Low Cytokinin	Root formation
Low Auxin, High Cytokinin	Shoot formation
Balanced Ratio	Callus formation

Biological Significance

Synthetic auxins and cytokinins have revolutionized plant biotechnology by enabling rapid propagation of disease-free plants, conservation of endangered species, production of genetically transformed plants, and large-scale commercial micropropagation. The coordinated action of auxins and cytokinins regulates cell division and differentiation, making them indispensable tools in tissue culture laboratories and modern agriculture.

Final Answer