Q.25

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)

(C) 6-Benzylamino purine (BAP) and Zeatin

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

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Understanding Synthetic Auxins and Cytokinins in Plant Hormones

Plant hormones regulate growth, development, and responses to stress. Auxins promote cell elongation and rooting, while cytokinins stimulate cell division and delay senescence. Synthetic versions mimic these natural hormones and are widely used in agriculture and tissue culture. A key question in biology exams tests their correct combinations.

This article breaks down MCQ Q.25 on the synthetic auxin and synthetic cytokinin combination, highlighting why one option stands out.

The Correct Answer: Option (B) 2,4-D and 6-Benzylamino Purine (BAP)

The correct combination is (B) 2,4-Dichlorophenoxy acetic acid (2,4-D) and 6-Benzylamino purine (BAP).

• 2,4-D is a synthetic auxin, chemically distinct from natural auxins. It’s a potent herbicide and growth regulator used in weed control and plant tissue culture.

• BAP (6-Benzylaminopurine) is a synthetic cytokinin, artificially derived to promote shoot formation in micropropagation.

This pair is standard in plant biotechnology for balancing root and shoot growth via the auxin-to-cytokinin ratio.

Detailed Explanation of All Options

Let’s evaluate each option to clarify why only (B) fits the synthetic auxin and synthetic cytokinin criteria.

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

• 2,4-D: Synthetic auxin (correct).

• IAA: Natural auxin, the primary endogenous form in plants, not synthetic.

• Why incorrect? IAA isn’t synthetic, so this mixes synthetic and natural auxins.

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

• 2,4-D: Synthetic auxin.

• BAP: Synthetic cytokinin, a benzyl derivative of adenine commonly used in labs.

• Why correct? Both are lab-synthesized, non-naturally occurring hormones. Ideal for synthetic auxin and cytokinin combination in tissue culture.

Option (C): 6-Benzylamino purine (BAP) and Zeatin

• BAP: Synthetic cytokinin (correct).

• Zeatin: Natural cytokinin, first isolated from corn kernels (Zea mays).

• Why incorrect? Zeatin occurs naturally, disqualifying this as a fully synthetic pair.

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

• IAA: Natural auxin.

• Zeatin: Natural cytokinin.

• Why incorrect? Both are endogenous plant hormones, not synthetic.

Why This Matters in Plant Biology and Exams

In plant tissue culture, the 2,4-D and BAP ratio controls morphogenesis—high cytokinin favors shoots, high auxin favors roots. These synthetics offer stability over natural ones like IAA or Zeatin, which degrade faster.

For NEET, CSIR NET, or botany students, mastering synthetic auxin and cytokinin combinations boosts scores on hormone-based MCQs.