# DBT JRF 2020 — Question 2

Exam: DBT JRF 2020 · Section: B · Topic: Plant Biology · Difficulty: Hard

## Question

Which of the following plant hormones employs a phosphorelay system to regulate gene expression?

## Options

- **A.** auxin
- **B.** cytokinin
- **C.** brassinosteroid
- **D.** ethylene

## Answer

Option B

## Explanation

Cytokinin signalling employs a multi-step phosphorelay (analogous to two-component systems in bacteria): (1) Cytokinin binds to histidine kinase receptors (AHK1, AHK2, AHK3, AHK4/CRE1) at the plasma membrane or ER, triggering autophosphorylation of a conserved His residue in the kinase domain. (2) The phosphoryl group is transferred to an Asp in the receiver domain of the same receptor (His→Asp). (3) Arabidopsis Histidine Phosphotransfer proteins (AHPs) shuttle the phosphoryl group from the receptor to the nucleus (Asp→His). (4) AHPs phosphorylate ARR (Arabidopsis Response Regulator) type-B transcription factors (Asp) in the nucleus, which then activate cytokinin-responsive genes. This multi-step relay allows signal amplification, integration, and fine-tuned regulation.

Source: https://mypyqbuddy.com/questions/which-of-the-following-plant-hormones-employs-a-6a05a8a02adafec47029321a