|Vermerris, Wilfred -|
Submitted to: Plant Physiology
Publication Type: Review Article
Publication Acceptance Date: April 25, 2001
Publication Date: August 1, 2001
Technical Abstract: Lignin is a polymeric material composed of phenylpropanoid units derived from three cinnamyl alcohols (monolignols): p-coumaryl, coniferyl and sinapyl alcohols. From a functional point of view, lignins impart strength to cell walls, facilitate water transport, and impede the degradation of wall polysaccharides, thus acting as a major line of defense against pathogens, insects, and other herbivores. The lignification process encompasses the biosynthesis of monolignols, their transport to the cell wall, and polymerization into the final molecule. Bond formation is thought to result from oxidative (radical-mediated) coupling between a monolignol and the growing oligomer/polymer. Currently, there are two models for coupling radicals to produce a functional lignin molecule. One, the random coupling model, centers on the hypothesis that lignin formation proceeds through coupling of individual monolignols to the growing lignin polymer i a near-random fashion. In this view, the amount and type of individual phenolics available at the lignification site and normal chemical coupling properties regulate lignin formation. The second model, the dirigent protein model, is more recent and suggests that lignification must be under strict regulation of specialized proteins that control the formation of individual bonds. The rationale for this new model is the belief that nature would not leave the formation of such an important molecule as lignin "to chance". A critical review of both models is presented based on published evidence. Experimental evidence supports the random coupling theory while adequate experimental support for the dirigent protein model is lacking.