- Ethylene is an important plant hormone that regulates several aspects of plant growth and development. Some statements are given below in relation to ethylene signalling pathways:
A. Unbound ethylene receptors work as positive regulators of the response pathway.
B. There are more than two ethylene receptors known to date.
C. The carboxy terminal half of the ethylene receptor, ETR1 (Ethylene response 1), contains a domain homologous to histidine kinase catalytic domain.
D. E1N2 (Ethylene insensitive 2) encodes a transmembrane protein. The ein2 mutation promotes ethylene responses in both seedlings and adult Arabidopsis plants.
Which combination of the above statements is correct?
(1) A and B (2) B and C
(3) C and D (4) D and A
Correct answer: (2) B and C
Explanation:
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(A) Unbound ethylene receptors act as negative regulators of the ethylene response pathway by activating downstream repressors like CTR1 in the absence of ethylene; thus, A is false.
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(B) There are multiple ethylene receptors in plants, more than two have been characterized (e.g., ETR1, ETR2, ERS1, ERS2, EIN4). This is true.
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(C) The carboxy-terminal half of ETR1 contains a domain homologous to histidine kinase catalytic domains, common in two-component signaling systems in plants. This is true.
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(D) EIN2 encodes a membrane protein critical for ethylene signaling, but the ein2 mutation causes ethylene insensitivity, not promotion of ethylene responses. So, D is false.
Option-wise:
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(1) A and B — A false.
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(2) B and C — both true.
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(3) C and D — D false.
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(4) D and A — both false.
In ethylene signaling, multiple receptors including ETR1 function with histidine kinase domains as negative regulators, while EIN2 operates downstream as a key membrane transducer; mutant EIN2 impairs rather than enhances ethylene sensitivity, defining pathway control points.
