66. Ethylene signaling pathway is important for a fruit ripening. Which one of the following responses is routinely used to identity ethylene signaling pathway components?
(1) Cotyledon expansion response
(2) Lateral root formation response
(3) Triple response
(4) Flowering time response

Correct answer: (3) Triple response

Explanation:

• The triple response is a classical physiological response used to identify components of the ethylene signaling pathway, especially in young seedlings of Arabidopsis and other plants.

• This response consists of inhibition of hypocotyl and root elongation, radial swelling of the hypocotyl, and an exaggerated apical hook formation.

• It is routinely used as a bioassay to screen for mutants defective in ethylene perception or signaling.

• Other options like cotyledon expansion response, lateral root formation, and flowering time response are not standard assays for ethylene signaling.

Option-wise:

• (1) Cotyledon expansion response — not associated with ethylene pathway screening.

• (2) Lateral root formation response — not commonly used for ethylene signaling identification.

• (3) Triple response — correct; classic test for ethylene signaling components.

• (4) Flowering time response — influenced by multiple hormones, not specific for ethylene signaling component identification.

The triple response assay is essential for studying ethylene signaling, revealing alterations in growth morphology in seedlings and providing insights into hormone perception and downstream pathways critical for fruit ripening and plant development.