Phenylpropanoid in plant cell walls

As regards the second topic, namely that of phenylpropanoid reactions within plant cell walls, a more comprehensive discussion is possible and also timely, due to the recent increase in interest in this area. For the purpose of this review, the phenylpropanoids present in plant cell walls are first classified according to structural complexity (monomers, dimers, polymers, etc.), following which their main reactions are discussed. 

There are at least five types of phenylpropanoid related reactions which appear to occur in plant cell walls. Two are UV-mediated photochemical reactions, and hence may be restricted only to the first few layers of cells under the plant surface due to poor penetrability of the light (3). The other reactions appear to be enzymatically mediated, and result in the formation of dimers or polymers from the corresponding monomeric units. 

Lignification so called is the polymerization process in plant cell walls that takes phenolic monomers, produces radicals, and couples them with other monomer radicals (only during initiation reactions), or more typically cross-couples them with the growing lignin oligomer, to build up a phenylpropanoid polymer [44-47]. 

Vascular plant cell walls contain a wide variety of phenylpropanoids, such as monomers, dimers and polymers. Of these, the polymers (i.e., lignins and suberins) are the most abundant. According to our current knowledge, all cell-wall phenylpropanoids are derived from monomers synthesized in the cytoplasm. Following their excretion into the plant cell wall, these monomers can then be either photochemically or biochemically modified within the cell wall. 

Lignins are a group of macromolecular heteropolymers (600-1000 kDa) found in the cell wall of vascular plants that are made up of phenylpropanoid units. Lignin has proven to be a useful chemical biomarker for vascular-plant inputs to estuarine/coastal margin sediments. Cellulose, hemicellulose, and lignin generally make up > 75% of the biomass of woody plant materials. 

Lignins a group of macromolecular heteropolymers (600-1000 kDa) found in the cell wall of vascular plants that are made up of phenylpropanoid units. 

The analysis of mutants of the phenylpropanoid pathway in Arabidopsis, as outlined in this review, has led to numerous revisions of the pathway over the past decade. The presently accepted pathway clarifies some of the contradictory data of the past, but also poses new questions for which we do not yet have answers. For example, a growing body of evidence suggests that neither ferulic acid nor sinapic acid are intermediates in phenylpropanoid biosynthesis. This is problematic in that many plant cell walls contain esterified ferulic acid, " and sinapic acid esters are major soluble secondary metabolites in Arabidopsis leaves and seeds. If the most current model of the pathway is correct, how are these molecules synthesized ... 

Heldt HW and Heldt F. 2005. Phenylpropanoids comprise a multitude of plant secondary metabolites and cell wall components. In Plant Biochemistry, 3rd ed. San Diego, CA Elsevier Academic Press, pp. 435 154. 

The wall material of plant cells is one of their distinguishing characteristics. As a result, lignin, cellulose, and other wall constituents have been studied in many plant tissue cultures. Phenylpropanoids. for example, have been shown lo be precursors of lignin formation in while pine. Set/noiii. lilac, rose, carrot, and geranium tissue cultures. Moreover, the biosynthesis of lignin has been shown to be alTeeted by kinetin. boron, and major elements, such as calcium. 

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