Question

The precursor for ethylene synthesis in plants is

• A. Tryptophan
• B. Methionine
• C. Indole
• D. ent-Kaurene

Hint:

To remember hormone precursors: 1. Auxin \(\to\) Tryptophan. 2. Ethylene \(\to\) Methionine (via SAM and ACC). 3. Gibberellins \(\to\) ent-Kaurene (terpene pathway).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Ethylene (\(\text{C}_2\text{H}_4\)) is a simple gaseous plant hormone involved in fruit ripening, leaf abscission, senescence, and triple response in seedlings.
Its biosynthesis occurs via a highly regulated, three-step metabolic pathway known as the Yang cycle.

Step 2: Detailed Explanation:
Let us trace the precursor and pathway of ethylene biosynthesis:
The primary amino acid precursor for ethylene synthesis in all higher plants is the sulfur-containing amino acid Methionine.
The biosynthetic pathway proceeds as follows:
1. Methionine is first converted into S-Adenosylmethionine (SAM) by the enzyme SAM synthetase, requiring ATP.
2. SAM is converted into 1-Aminocyclopropane-1-carboxylic acid (ACC) by the rate-limiting enzyme ACC synthase.
3. Finally, ACC is oxidized by ACC oxidase to yield Ethylene, carbon dioxide, and hydrogen cyanide.
Let us review other options:
- Tryptophan (A) and Indole (C): These are precursors for the biosynthesis of the auxin Indole-3-acetic acid (IAA).
- ent-Kaurene (D): This is an intermediate in the biosynthetic pathway of Gibberellins.
Therefore, Methionine is the correct precursor for ethylene.

Step 3: Final Answer:
The precursor for ethylene synthesis is Methionine, which corresponds to option (B).