13/02/2017 · Phenylpropanoid biosynthesis
The AC element sequences uncovered in the lignin biosynthetic genes are similar to the binding site (CCT/AACC) identified through binding site selection for the maize MYB protein P. In addition, the Antirrhinum MYB305 was found to be able to bind to the AC elements and activate the expression of the AC element-containing bean PAL2 promoter-driven reporter gene., Therefore, it was reasoned that transcription factors that bind to the AC elements in lignin biosynthetic genes are also MYB proteins. The first line of genetic evidence on the possible involvement of MYBs in the regulation of lignin biosynthesis came from the study of two MYB proteins, AmMYB308 and AmMYB330, from Antirrhinum (). Overexpression of the Antirrhinum MYB proteins in transgenic tobacco plants caused a reduction in the expression of several lignin biosynthetic genes and a decrease in lignin content, suggesting that the Antirrhinum MYBs are able to regulate the expression of lignin biosynthetic genes and thereby affect lignin biosynthesis. Since then, several MYBs from Arabidopsis and grapes have been shown to alter the expression of phenylpropanoid biosynthetic genes and lignin biosynthesis when overexpressed. However, none of these MYBs have been proven to bind to the AC elements, nor have they been demonstrated to be expressed in lignifying tissues. The latter is especially important because developmental regulators of lignin biosynthetic genes should be expressed in cells undergoing lignification. Thus, it is uncertain whether these MYBs are indeed regulators of lignin biosynthesis or their effects on lignin biosynthesis observed in the overexpressors are indirect. In fact, one of these MYBs, PAP1, has been demonstrated to be a regulator of anthocyanin biosynthesis, and the increased accumulation of lignin caused by PAP1 overexpression is due to the elevation of the common hydroxycinnamoyl CoA esters shared by the biosynthetic pathways of both lignin and anthocyanin.
and metabolites essential for regulation and ..
Capsaicinoid Biosynthetic pathwaysMore than 22 different capsaicinoids are known to be found in pepper fruits which are synthesized and accumulated in the epidermal cells of placenta of the fruits (). The major ones, capsaicin and dihydrocapsaicin, normally occur in the highest concentration. The other capsaicinoids occur in smaller concentration and known as minor capsaicinoids. Three genes were selected and their c DNA clones were used to measure transcript level and found that expression of these genes positively correlated with pungency (). The biosynthetic pathway of capsaicinoids was studied in terms of organic synthesis and biochemistry using a radiotracer technique. It has been proposed that they are synthesized by the condensation of vanillylamine with C9 to C11 isotype branched-chain fatty acids; the former is derived from phenylpropanoid pathway, the latter from valine and leucine (; ; ; ). Two pathways () are involved in the biosynthesis of capsaicinoids (i) metabolism and (ii) phenylpropanoid pathway ().
Several MYBs from pine and eucalyptus have been shown to be strong candidates as regulators of lignin biosynthesis (). The pine PtMYB1 and PtMYB4, and the eucalyptus EgMYB2 bind to the AC elements and their genes are expressed in developing wood that undergoes secondary wall thickening and lignin biosynthesis. When overexpressed in tobacco plants, PtMYB4 and EgMYB2 induced the expression of some of the lignin biosynthetic genes and led to ectopic lignin deposition or increased secondary wall thickening. In addition, the wood-associated pine PtMYB1 and PtMYB8 also caused ectopic lignin deposition and wall thickening when overexpressed in spruce. PtMYB8 is a close homolog of the Arabidopsis MYB61 () whose overexpression could cause ectopic lignin deposition but its exact functions remain to be studied.– It was concluded that these pine and eucalyptus MYBs are involved in regulation of lignin biosynthesis during wood formation. However, PtMYB4 and EgMYB2 are phylogenetically grouped together with the Arabidopsis MYB46 (), which has been shown to be a key regulator of the biosynthesis of all the three major secondary wall components, including cellulose, xylan and lignin. Overexpression of EgMYB2 in Arabidopsis protoplasts was found to be able to activate the expression of the biosynthetic genes of cellulose, xylan and lignin (). Overexpression of PtMYB4 could induce the expression of a cellulose synthase gene and a lignin biosynthetic gene (). These findings indicate that EgMYB2 and perhaps also PtMYB4 are orthologs of MYB46 and they regulate the entire secondary wall biosynthetic program during wood formation.
Phenylpropanoid Biosynthesis - ResearchGate
Lignin is a complex phenylpropanoid polymer formed through dehydrogenative polymerization of three monolignols, including p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol. Lignin from gymnosperms is composed of guaiacyl unit polymerized from coniferyl alcohol, whereas that from angiosperms typically consists of both guaiacyl and syringyl units polymerized from coniferyl alcohol and sinapyl alcohol, respectively. Lignin from grasses also contains p-hydroxyphenyl unit polymerized from p-coumaryl alcohol in addition to guaiacyl and syringyl units. The biosynthetic pathway of monolignols starts with the general phenylpropanoid pathway leading to the production of hydroxycinnamoyl CoA esters, which are the common precursors of diverse groups of chemical compounds, such as flavonoids, suberin, coumarins, quinones, phytoalexins, acetosyringone and lignin. For production of monolignols, two successive reductive steps catalyzed by cinnamoyl CoA reductase and cinnamyl alcohol dehydrogenase convert hydroxycinnamoyl CoA esters into the three monolignols. Monolignols are then transported, via an unknown mechanism, into the cell wall, where they are oxidized by oxidases such as peroxidases and laccases for polymerization.
Biosynthesis, function and metabolic engineering of …
Tremendous progress has been made in the last two decades regarding the transcriptional regulation of the biosynthesis of lignin, the second most abundant biopolymer produced by vascular plants. It is established that the AC elements present in the promoters of the lignin biosynthetic genes serve as a common regulatory element driving their expression, and that specific MYB transcription factors bind to the AC elements and thereby regulate the coordinated expression of lignin biosynthetic genes. Recent studies have also provided evidence that the transcriptional regulation of lignin biosynthesis is under the control of the same transcriptional network regulating the biosynthesis of other secondary wall components. Despite these progresses, many outstanding issues regarding the transcriptional regulation of lignin biosynthesis still remain. First, the transcriptional activation of lignin biosynthetic genes is likely mediated through multiple cis-elements and a combinatorial interaction of multiple transcription factors, a scenario similar to the transcriptional regulation of the branch of the phenylpropanoid pathway leading to flavonoid biosynthesis. Earlier studies of the promoter activities of lignin biosynthetic genes have indicated the presence of cis elements besides the AC elements and of multiple proteins binding to the promoter sequences.,,, In addition, some MYBs have been suggested to function as repressors in fine-tuning the expression level of phenylpropanoid biosynthetic genes.– Identification and characterization of all the cis elements and transcription factors involved in regulation of lignin biosynthesis will be essential for gaining a full picture of the complexity of transcriptional control of lignin biosynthesis. Second, little is known about how lignin heterogeneity is regulated at the transcription level. For example, it is well recognized that the lignin composition varies among different cell types, such as vessels and fibers, within the same plant species, which attributes to the differential expression of genes committed to the biosynthesis of sinapyl alcohol. Understanding of the transcriptional regulation of sinapyl alcohol biosynthetic genes will contribute to uncovering the mechanisms underlying the regulation of lignin heterogeneity. Third, lignin biosynthesis is not only developmentally regulated but also induced in response to many environmental stresses, such as wounding, UV light irradiation and pathogen attacks, but little is known about transcriptional activation of stress-induced lignin biosynthesis. Because these environmental stresses typically do not induce secondary wall thickening, it is likely that the transcriptional regulation of stress-induced lignin biosynthesis is different from that of the developmentally activated lignin biosynthesis. Early promoter deletion studies have identified specific regions in the promoters of lignin biosynthetic genes responsible for wounding, UV irradiation and pathogen activations and suggest that the AC elements are implicated in response to environmental stresses., Further investigation on stress-specific cis elements and the corresponding transcription factors will be necessary to understand how stress signals are transduced to activate lignin biosynthesis. Finally, lignin constitutes 15–30% of the biomass of wood and understanding the transcriptional control of lignin biosynthesis during wood formation will have important implications in tree biotechnology. It will be possible to use one or a few transcription factors to down- or up-regulate the entire lignin biosynthetic pathway and thereby alter lignin content in wood based on our needs. Recently, it has been reported that conversion of biomass into electricity instead of to biofuel for automobile propulsion captures more biomass energy. Since lignin has higher energy density than other polysaccharides, it would be desirable to genetically engineer biomass crops with higher lignin content for conversion into electricity. With the availability of the ever-increasing molecular and genomic tools, it is expected that a complete picture of the transcriptional regulation of lignin biosynthesis will soon emerge.