Q. 54 Which of the following stains is used to visualize callose under the microscope?
(A) Alcian blue
(B) Aniline blue
(C) Toluidine blue
(D) Thymol blue
The correct answer is (B) Aniline blue. This special fluorescent dye is routinely used in plant histology to visualize callose deposits under the microscope by binding to β-1,3-glucans in the cell wall.
Question and correct answer
In many plant physiology and microscopy MCQs, students are asked:
“Which of the following stains is used to visualize callose under the microscope?”
Options:
(A) Alcian blue
(B) Aniline blue
(C) Toluidine blue
(D) Thymol blue
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Correct answer: (B) Aniline blue
Aniline blue is a fluorochrome that specifically complexes with β‑1,3‑glucans such as callose and makes them fluoresce when excited with UV or violet light, allowing clear visualization of callose deposits in plant tissues.
Why aniline blue is correct
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Aniline blue binds to callose, which is a β‑1,3‑glucan polymer deposited in plant cell walls, especially at plasmodesmata, sieve plates, and during pathogen response.
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When viewed under epifluorescence microscopy with UV excitation, aniline blue–callose complexes emit bright fluorescence, making callose structures easy to detect and quantify.
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Standard protocols for imaging callose in Arabidopsis leaves, pollen tubes, and plasmodesmata all recommend aniline blue staining as the primary method.
Because of this high affinity for β‑1,3‑glucans and strong fluorescence signal, aniline blue is considered the preferred stain for visualizing callose under the microscope in teaching and research laboratories.
Explanation of all options
Option (A) Alcian blue
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Alcian blue is a basic copper phthalocyanine dye mainly used to stain acidic mucopolysaccharides and glycosaminoglycans (mucins) in animal tissues, such as in histopathology of gastrointestinal and respiratory tracts.
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Its primary application is in detecting acid mucins (e.g., in goblet cells), not plant callose, and it does not have the same specificity or fluorescence properties for β‑1,3‑glucans as aniline blue.
Therefore, Alcian blue is not the correct stain for visualizing callose in plant cells.
Option (B) Aniline blue
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Aniline blue is widely used in plant biology to detect callose because it forms a fluorescent complex with β‑1,3‑glucans, which are the main component of callose.
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It is routinely applied to observe callose deposition in pollen tubes, plasmodesmata, and pathogen-induced cell wall thickenings using UV excitation in fluorescence microscopy.
Hence, aniline blue is the correct answer and the key stain used to visualize callose under the microscope.
Option (C) Toluidine blue
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Toluidine blue is a basic thiazine dye used as a general metachromatic stain for plant and animal tissues, frequently applied to show lignin, nucleic acids, and acidic tissue components at the light microscope level.
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Although very useful for general histology and differential staining of cell wall components, toluidine blue does not specifically target callose and is not the standard choice for selective visualization of β‑1,3‑glucans.
Therefore, toluidine blue is not the preferred stain for callose visualization.
Option (D) Thymol blue
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Thymol blue is an acid–base indicator commonly used in chemistry laboratories to detect pH changes, with characteristic color transitions in specific pH ranges.
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It is not a histological stain and has no particular affinity or fluorescence for callose or other specific plant cell wall polysaccharides, so it is irrelevant for microscopy-based callose detection.
Thus, thymol blue is not used to visualize callose under the microscope.
SEO-focused summary for students
For exam preparation and search optimization, remember the keyphrase “stain used to visualize callose under the microscope” and link it mentally with aniline blue. This dye binds to callose (β‑1,3‑glucan) and fluoresces under UV light, making it the standard and correct answer in MCQs on plant histology and plant–pathogen interactions.
