This article discusses the significance of lignin, particularly S-lignin, in plant cell walls and its role in providing structural support. It details a staining method to visualize S-lignin in plant tissues, highlighting the differences in intensity between various cell types.
Lignin is an important structural component in the plant cell wall. It is a polymer composed primarily of syringyl alcohols, or S-lignin units, and guaiacyl alcohols, or, G-lignin units, and a minor population of para-coumaryl alcohols called the H-lignin units.
S-lignin’s content differs in various secondary plant tissues. It is mainly found in the xylary vessels and fibers and interfascicular fibers, where it provides structural support.
To detect the presence of S-lignin, take agarose-embedded dicot stem cross-sections in a tube. Treat the stem sections sequentially with a potassium permanganate staining solution, followed by diluted hydrochloric acid, and, finally, with ammonium hydroxide - a weak base.
During this treatment, the S-Lignin moiety present in the cell wall undergoes a series of changes to finally generate a product that appears red.
The intensity of red coloration is lower in the xylem cells owing to the presence of less S-lignin compared to the interfascicular fibers, which appear a deeper red. Transfer the sections onto a slide, and visualize them under a light microscope.
The variation in red intensity helps to understand the relative ratio of S-lignin in lignified plant tissues.
For Maule staining, transfer the stem sections to a microcentrifuge tube. Add 1 milliliter of the 0.5% potassium permanganate solution to the tube containing the sections. Pipette the 0.5% potassium permanganate solution up and down gently, preferably, without disturbing the sections.
After a 2-minute incubation and repeat pipetting, let the microcentrifuge tube stand until all the sections settle down. Using a 1-milliliter pipette, draw out 700 microliters of 0.5% potassium permanganate solution. Add 700 microliters of distilled water to rinse out the potassium permanganate. Repeat three to four times or until the water solution stays clear.
After discarding the water, quickly add 1 milliliter of 3% hydrochloric acid until the deep brown color is discharged from the sections. This may happen within 3 to 5 minutes or may require two washes of 5 minutes each. Pipette out all of the 3% hydrochloric acid solution and immediately add 1 milliliter of concentrated ammonium hydroxide solution before imaging under bright-field lighting.
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