Lignin is the durable biopolymer that gives carrots their fiber and crunch. Acting as the glue that holds the plant cell wall together, lignin imparts tremendous mechanical strength to the plant. Present in all land plants except mosses, lignin performs three important functions: it allows plants to grow upright as they compete for sunlight, it facilitates the upward movement of water and minerals through the plant's vascular tissue, and it protects plants from pathogens and foraging animals. Lignin also sequesters atmospheric carbon and thereby plays an important role in the carbon cycle. Approximately 30% of non-fossil organic carbon is stored in lignin, and, after cellulose, lignin is the most abundant biological polymer on Earth. Lignin consists of three phenylpropanoid subunits, G (guaiacyl), S (syringyl), and H (p-hydroxyphenyl). The precursors of these subunits are generated inside the cell and transported to the cell wall, where they are oxidized by enzymes and then join together to form lignin's highly complex and heterogeneous three-dimensional structure. Biologists have long wondered how this process of lignification is regulated. Two families of enzymes, the peroxidases and laccases, occur in plant cell walls and have been proposed to catalyze the oxidation of lignin precursors. Whereas the involvement of peroxidases in lignification has been confirmed, that of laccases had not. Now, a team of researchers at the Institut Jean Pierre Bourgin INRA, France, provide compelling evidence that laccases do indeed contribute to lignification in the model plant Arabidopsis (a member of the mustard and cabbage family). The work was published online in The Plant Cell on March 29, 2011. Seventeen laccase genes are present in Arabidopsis.
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