Q.49 4′,6 diamidino 2-phenylindole (DAPI) is a fluorescent dye used to stain the nucleus. Which of the
following plant cells, when mature, cannot be stained by DAPI?
(A) Trichomes
(B) Tracheids
(C) Collenchyma
(D) Mesophyll
The correct answer is: (B) Tracheids.
Mature tracheids are dead, highly lignified cells without a nucleus or DNA, so they cannot be stained by the nuclear dye DAPI, which specifically binds to DNA.
What Is DAPI and How Does It Work?
DAPI (4′,6-diamidino-2-phenylindole) is a blue-fluorescent dye that binds strongly to adenine–thymine (A–T) rich regions in double-stranded DNA.
Because DAPI specifically targets DNA in the nucleus, it is widely used as a nuclear counterstain in fluorescence microscopy to visualize nuclei in both fixed and, at higher concentrations, live cells.
For a plant cell to be stained by DAPI, it must contain an intact nucleus with DNA; fully differentiated dead cells without nuclei will not show DAPI fluorescence.
Question Recap
Question:
4′,6-diamidino-2-phenylindole (DAPI) is a fluorescent dye used to stain the nucleus. Which of the following plant cells, when mature, cannot be stained by DAPI?
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(A) Trichomes
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(B) Tracheids
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(C) Collenchyma
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(D) Mesophyll
Correct answer: (B) Tracheids.
Explanation of Each Option
(A) Trichomes
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Trichomes are hair-like outgrowths of the epidermis that may be unicellular or multicellular and, in many cases, remain living and nucleated, especially glandular or non-lignified types.
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Since living trichome cells retain a nucleus with DNA, they can be stained by DAPI, which binds to nuclear DNA and gives bright blue fluorescence under UV excitation.
(B) Tracheids – Correct Answer
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Tracheids are elongated conducting elements of xylem that become dead at maturity, with their protoplast (including the nucleus) completely lost and their walls heavily lignified.
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Because mature tracheids lack nuclei and thus lack DNA, DAPI has no DNA target to bind, so these cells cannot be stained by DAPI when fully mature.
(C) Collenchyma
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Collenchyma cells are living supporting cells, usually located beneath the epidermis in young stems and petioles, characterized by unevenly thickened primary cell walls but retaining a living protoplast and nucleus at maturity.
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As living cells with intact nuclei and DNA, mature collenchyma cells can be stained by DAPI, which will label their nuclei.
(D) Mesophyll
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Mesophyll cells (palisade and spongy parenchyma) in leaves are living, photosynthetic parenchyma cells rich in chloroplasts and possessing clearly defined nuclei.
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Because mesophyll cells remain alive with nuclear DNA, they are readily stained by DAPI in fluorescence microscopy to visualize their nuclei.
Why Only Tracheids Fail to Show DAPI Staining at Maturity
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DAPI is a DNA-specific, nuclear stain, so only cells with nuclei and genomic DNA will fluoresce upon staining.
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Among the given options, mature tracheids are the only cell type that is structurally dead and enucleate, hence they do not retain any DNA and therefore cannot be visualized with DAPI in their mature state.
