23. Among the following which is not involved in plant defense signaling pathway
(1) Gibberellic acid  (2) Ethylene
(3) Salicylic acid      (4) Jasmonic acid

Correct answer: (1) Gibberellic acid.

Explanation:

• Gibberellic acid (GA) is primarily involved in promoting growth processes such as seed germination, stem elongation, and flowering but is not a major hormone in plant defense signaling.

• Salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) are well-established key players in plant defense signaling pathways. SA is linked to systemic acquired resistance and biotrophic pathogen defense, while JA and ET are involved in defense against necrotrophic pathogens and herbivorous insects.

• GA generally promotes growth rather than defense, and its role in immunity is limited or indirect, therefore it is the hormone not involved in classical plant defense signaling.

Option-wise:

• (1) Gibberellic acid – Not involved in the defense signaling pathway; promotes growth.

• (2) Ethylene – Involved in defense signaling, especially in pathogen and stress responses.

• (3) Salicylic acid – Crucial for systemic acquired resistance and defense against biotrophs.

• (4) Jasmonic acid – Key hormone in defense against necrotrophs and herbivore attack.