- Ethylene binding to its receptor does lead to
(1) activation of the receptor
(2) phosphorylation of the receptor
(3) activation of CTR Raf kinase
(4) activation of Ethylene insensitive factor
Correct answer: (3) activation of CTR Raf kinase
Explanation:
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Ethylene is perceived by specific ethyelene receptors located on the endoplasmic reticulum membrane in plants.
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In the absence of ethylene, these receptors actively stimulate the CTR1 (Constitutive Triple Response 1) protein, which is a Raf-like kinase, to repress downstream ethylene signaling.
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Upon ethylene binding to the receptor, the receptor’s kinase activity is inhibited, leading to inactivation of CTR1 kinase activity.
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So, ethylene binding leads to inactivation (not activation) of CTR1, thereby releasing repression on downstream ethylene signaling and gene expression.
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Thus, the statement that ethylene binding leads to activation of CTR1 is incorrect. Instead, ethylene inhibits CTR1 activity.
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Correspondingly, ethylene binding does not activate the receptor or cause its phosphorylation directly; rather, it changes receptor activity to repress CTR1.
Option-wise:
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(1) Activation of receptor — no, binding inhibits receptor kinase activity.
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(2) Phosphorylation of receptor — no direct phosphorylation upon ethylene binding is typical.
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(3) Activation of CTR1 — no, CTR1 is inhibited by ethylene binding, so this is the incorrect response.
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(4) Activation of Ethylene insensitive factor (EIN2/EIN3 related) — ethylene binding eventually leads to activation of downstream factors after CTR1 inhibition.
Ethylene binding inactivates CTR1 Raf kinase associated with the receptor, lifting repression and enabling the downstream ethylene signaling cascade essential for responses like fruit ripening and stress adaptation.
