# DBT JRF 2022 — Question 90

Exam: DBT JRF 2022 · Section: B · Topic: Plant Biotechnology › Golden Rice & Selectable Markers · Difficulty: Medium

## Question

Given below are two statements one is labelled as Assertion A and the other is labelled as Reason R: Assertion A: Phosphomannose isomerase (PMI) from E. coli was used as selection marker to develop 'golden rice 2'. Reason R: When mannose is added in the selection media, endogenous hexokinase converts mannose into mannose-6-phosphate which blocks glycolysis, ATP production, and represses transcription of photosynthetic genes, resulting in slower growth of non-transformed plants. PMI, on the other hand, converts mannose-6-phosphate to fructose-6-phosphate. In the light of the above statements, choose the correct answer from the options given below:

## Options

- **A.** Both A and R are true and R is the correct explanation of A
- **B.** Both A and R are true but R is not the correct explanation of A
- **C.** A is true but R is false
- **D.** A is false but R is true

## Answer

Option A

## Explanation

PMI selectable marker in Golden Rice 2 transformation: Assertion: PMI (phosphomannose isomerase) was used as a selectable marker in Golden Rice 2. Reason: PMI converts mannose-6-phosphate (M6P) to fructose-6-phosphate (F6P), rescuing transformed cells on mannose-containing medium. Mechanism of PMI selection: • Mannose (present in growth medium) is taken up and phosphorylated to mannose-6-phosphate by hexokinase in plant cells. • M6P cannot enter glycolysis further (no plant phosphomannose isomerase) → M6P ACCUMULATES → blocks glycolysis → non-transformed cells starve and die. • Transformed cells expressing PMI convert M6P → F6P → can enter glycolysis → SURVIVE and GROW. • This is a POSITIVE selectable marker system (growth advantage, not antibiotic resistance). Advantages: No antibiotic resistance genes → better regulatory acceptance for food crops. Both Assertion A and Reason R are true, and R correctly explains A. Answer: Both A and R are true; R is the correct explanation of A.

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