The following statements are made with respect to signal transduction events in phytohormone signalling in plants:
A. Autophosphorylation of a histidine residue in the receiver domain of the response regulator is important for signal transduction through the two-component system.
B. Phosphorylation of a conserved aspartate residue in the transmitter domain of the histidine kinase is important for the two-component system.
C. CYTOKININ RESPONSE 1 (CRE1), a cytokinin receptor, functions as an Arabidopsis histidine-containing phosphotransfer (AHP) factor.
D. In Arabidopsis, pseudo-AHP, called AHP6, acts as an inhibitor of cytokinin signalling.
E. ETR1, an ethylene receptor in Arabidopsis, has histidine kinase activity and a receiver domain.
Which one of the following options is a combination of all correct statements?

Correct option is D

Correct Answer
(d) D and E
Explanation
Statement D is correct because AHP6 is a pseudo-histidine phosphotransfer protein that lacks the conserved histidine and negatively regulates cytokinin signalling. Statement E is correct because ETR1 is an ethylene receptor that functions as a histidine kinase and contains a receiver domain, fitting the two-component signalling framework in plants.
Information booster
• Two-component signalling in plants is adapted from bacterial systems
• Cytokinin signalling involves histidine kinase receptors, AHPs, and response regulators
• Pseudo-AHP proteins fine-tune hormone responses by inhibiting signal flow
• Ethylene receptors structurally resemble bacterial histidine kinases
Additional Knowledge
Statement A is incorrect because response regulators are phosphorylated on a conserved aspartate residue in the receiver domain, not on histidine.
Statement B is incorrect because histidine kinases autophosphorylate on a conserved histidine residue in the transmitter domain, not on aspartate.
Statement C is incorrect because CRE1 is a cytokinin receptor histidine kinase, whereas AHPs are separate soluble phosphotransfer proteins.