Q.64. Selection markers and the corresponding genes used in plant genetic engineering are given
below
Selection Marker
P. Kanamycin
Q. Hygromycin
R. Bialaphos
S. Mannose
Gene
i. ℎ𝑝𝑡𝐼𝑉
ii. bar
iii. pmi
iv. nptII
Choose the CORRECT combination
(A) P-ii, Q-i, R-iv, S-iii
(B) P-iv, Q-ii, R-i, S-iii
(C) P-iv, Q-i, R-ii, S-iii
(D) P-iii, Q-iv, R-ii, S-i

The correct answer is option (C): P-iv, Q-i, R-ii, S-iii. This matching reflects the standard selectable marker–selection agent pairs used in plant genetic engineering.

Correct matching (answer)

The standard, widely accepted combinations in plant genetic engineering are:

• Kanamycin → nptII (neomycin phosphotransferase II)

• Hygromycin → hpt (often written as hpt or hph; here given as hptIV)

• Bialaphos (phosphinothricin / glufosinate) → bar

• Mannose → pmi (phosphomannose isomerase)

So the correct combination is:
P-iv (Kanamycin–nptII), Q-i (Hygromycin–hptIV), R-ii (Bialaphos–bar), S-iii (Mannose–pmi), which corresponds to option (C).

Why these marker–gene pairs are correct

Kanamycin – nptII (P-iv)

• The nptII gene encodes neomycin phosphotransferase II, which inactivates aminoglycoside antibiotics like kanamycin by phosphorylation.

• Transgenic plant cells expressing nptII survive on kanamycin-containing medium, while non-transformed cells die, making this a classic antibiotic selection system.

Hygromycin – hptIV (Q-i)

• Hygromycin B is another commonly used antibiotic in plant transformation, and resistance is conferred by hygromycin phosphotransferase genes (hpt/hph family, here noted as hptIV).

• Cells expressing hpt can grow on hygromycin-containing media, enabling efficient selection of transformed tissues.

Bialaphos – bar (R-ii)

• Bialaphos (and its active component phosphinothricin, as in glufosinate herbicides) is detoxified by the product of the bar gene, phosphinothricin acetyltransferase (PAT).

• bar-expressing plants tolerate phosphinothricin-based herbicides, so bar is widely used as a selectable marker for herbicide-tolerant transgenic plants.

Mannose – pmi (S-iii)

• The pmi gene encodes phosphomannose isomerase, which converts mannose-6-phosphate to fructose-6-phosphate, allowing transformed cells to use mannose as a carbon source.

• Non-transgenic cells cannot efficiently metabolize mannose and are growth-inhibited, so pmi provides a positive, non-antibiotic selection system using mannose.

Explanation of all options

Option (A): P-ii, Q-i, R-iv, S-iii – Incorrect

• P-ii pairs Kanamycin with bar, but bar is a herbicide-resistance gene specific for phosphinothricin/bialaphos, not aminoglycoside antibiotics such as kanamycin.

• R-iv pairs Bialaphos with nptII, which is wrong because nptII targets kanamycin-like antibiotics, not herbicides; bar is the correct gene for bialaphos.

• Q-i (Hygromycin–hptIV) and S-iii (Mannose–pmi) are individually correct, but because P and R are wrong, the overall option is incorrect.

Option (B): P-iv, Q-ii, R-i, S-iii – Incorrect

• P-iv (Kanamycin–nptII) is correct.

• Q-ii pairs Hygromycin with bar, but bar does not confer hygromycin resistance; hygromycin resistance requires an hpt-type gene.

• R-i pairs Bialaphos with hptIV, which is wrong because bialaphos/glufosinate selection uses bar, not hygromycin-resistance genes.

• S-iii (Mannose–pmi) is correct, but two mismatches (Q and R) make the option overall incorrect.

Option (C): P-iv, Q-i, R-ii, S-iii – Correct

• Matches each selection agent with its standard, experimentally validated marker gene: nptII with kanamycin, hpt with hygromycin, bar with bialaphos, and pmi with mannose.

• This pattern reflects the typical use of antibiotic, herbicide and metabolic selection systems in plant genetic engineering.

Option (D): P-iii, Q-iv, R-ii, S-i – Incorrect

• P-iii pairs Kanamycin with pmi, but pmi is a metabolic marker for mannose selection, not for kanamycin resistance.

• Q-iv pairs Hygromycin with nptII, but nptII targets kanamycin/neomycin-like antibiotics, not hygromycin B; hygromycin selection requires hpt.

• R-ii (Bialaphos–bar) is correct, but S-i (Mannose–hptIV) is wrong because hpt confers hygromycin resistance, not mannose metabolism.

How to remember these pairs for exams

• Antibiotics:

  • Kanamycin → nptII (think “neo/kan” → neomycin phosphotransferase).

  • Hygromycin → hpt (hygromycin phosphotransferase).

• Herbicide:

  • Bialaphos / phosphinothricin / glufosinate → bar (herbicide-tolerant crops).

• Metabolic marker:

  • Mannose → pmi (phosphomannose isomerase, mannose metabolism).