64 Which one of the following plant hormones use the two-component histidine kinase receptor system for signal transduction?
(1) Auxin                 (2) Gibberellin
(3) Cytokinin          (4) Abscisic acid

Correct answer: (3) Cytokinin

Explanation:

• Cytokinins are plant hormones that use a two-component histidine kinase receptor system for signal transduction. This system consists of a membrane-bound histidine kinase receptor that perceives the cytokinin signal and initiates a phosphorelay cascade involving histidine phosphotransfer proteins and response regulators, ultimately modulating gene expression.

• Auxin, gibberellin, and abscisic acid do not use this two-component system for their signaling pathways.

• The two-component system is an evolutionary conserved mechanism widely involved in cytokinin perception and signaling in plants.

Option-wise:

• (1) Auxin — no two-component system.

• (2) Gibberellin — no two-component system.

• (3) Cytokinin — correct; uses two-component histidine kinase receptor system.

• (4) Abscisic acid — no two-component system.

Cytokinins signal through a sophisticated two-component histidine kinase receptor system, involving phosphorylation cascades that regulate diverse physiological processes, making this pathway a key feature of cytokinin hormone action in plant development.

This makes cytokinin the correct answer as the plant hormone using the two-component histidine kinase receptor system.