The Simultaneous Repression of CCR and CAD, TWO Enzymes of the Lignin Biosynthetic Pathway, Results in Sterility and Dwarfism i

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INTRODUCTION Numerous plant-based secondary metabolites have already been isolated. The functions of most of these biomolecules have also been well-defined. Saponins enhance nutrient absorption. Cocaine is used for protection against foraging insects. Lignins, on the other hand, contribute in the maintenance of the stable structure of secondary plant cellular walls.


Later on, understanding pathways may be used in the laboratory synthesis of these compounds. Moreover, the induction, as well as repression, of their production in plants through anthropogenic intervention to promote optimal plant growth and/or fruit bearing may be conducted. This is important as many secondary metabolites are already used by humans. In the case of lignins, genes CCR and CAD have been identified as the encoders for the last and specific steps for monolignol biosynthesis, the first part of lignin pathway. Because of the complex nature of biochemical production, regulation of a certain pathway may affect other pathways as well. Such effects may be great enough to cause changes in phenotype. In the case of monolignol biosynthesis in tobacco, the silencing of both CCR and CAD genes resulted not only to a reduction of lignin production but to a decrease in plant size as well. The recent study by Thevenin et al. (2011) looked into the effects of silencing monolignol-specific CAD and CCR genes in Arabidopsis thaliana. The plant contains two CCR genes, but only one, CCR 1, is specific for lignification. The mutant lines for this gene, irx4, ccr1s and ccr1g, are dwarves, have a reduced amount of lignin (25-35%), and have a modified set of phenolic metabolites. On the other hand, A. ...
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