The lignin-deficient mutant of Arabidopsis, a plant equivalent of a boneless chicken ranch?
by Leslie Sieburth
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Lignin is a polymer composed of aromatic subunits that is found in the cell walls of some plant cells (especially xylem tracheary elements). Acquisition of lignin is considered to have been a critical event for plant colonization of land. Based mostly on its position of deposition and chemical properties, lignin is believed to confer both strength and waterproofing to cell walls.
To more carefully analyze lignin functions, we have isolated two (allelic) mutants that are devoid of lignin.
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Phloroglucinol staining allows easy detection of lignin by producing a bright red color in wild type plants (right). In the lignin mutants, no color reaction was observed (right). Size bars = 2 mm
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Using both phloroglucinol and Maule reactions, no lignin was detectable in the lignin mutants. Because lignin plays such a fundamental role in plant development, we analyzed the phenotype of these mutants.
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Seedling Phenotype of Lignin-Deficient Mutant. Wild type (L. er) at 11 days (top), mutant at 11 days (bottom left) and at 20 days (bottom right).
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Lignin-deficient mutants germinate as a normally-sized seedling (not shown), but by 11 days are visibly smaller than the wild type, have epinastic cotyledons, and very small leaves. By 20 days, aerial portions of the mutant has not gotten much larger, but the root has proliferated greatly, and has initiated many (abnormal) roots.
To determine what the effect of lignin loss was on cell structure, we prepared and examined plastic sections of hypocotyls.
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Micrographs (TEM) of wild type and mutant hypocotyls. At the top left are portions of two tracheary elements (TEs). At the right is a lower magnification image of the vein itself. The lower two images show sections from mutant hypocotyl. The arrow points to a partially differentiated TE (autolysis of the cell has not yet occurred).
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TEs of wild type plants have large heavily fortified walls. They are circular in cross-section. In contrast, the mutant has crumpled TEs. We could easily identify the partially differentiated TEs in the mutant we could observe their thick cell walls. However, it appeared that as soon as the cell contents lyse, the cell walls implode.
We have identified the lesion in this mutant it has a stop codon at amino acid 46 of the C4H gene. This gene encodes the second biosynthetic step of the phenylpropanoid biosynthetic pathway. These studies confirm that lignin does play essential roles in providing strength to plant cell walls.
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