Localization terpenoid pathways

Tissue and Cell-Specific Localization of Terpenoid Pathways in Defense... 

Our recent research suggests organ-, tissue-, and cell-specific localization of constitutive and induced terpenoid defense pathways in conifers. For example, linalool synthase (PaTPS-Lin) seems to be preferentially expressed in needles of Norway spruce and Sitka spruce with little or no expression in sterns. ft is also likely that expression of PaTPS-Lin in spruce needles is not associated with resin ducts but could reside in other cells involved with induced terpenoid emission. In contrast, we can speculate that most other mono-TPS and di-TPS are associated with epithelial cells of constitutive and induced resin ducts. The possible localization of conifer sesqui-TPS is difficult to predict. Furthermore, the exact spatial and temporal patterns of terpenoid pathway gene expression associated with traumatic resin duct development in the cambium zone and outer xylem remain to be studied at the tissue and cell level. In situ hybridization and immuno-localization of TPS will address these open questions. These methods have worked well in identifying cell type specific gene and protein expression of alkaloid formation in opium poppy Papaver somniferum) As the biochemistry of induced terpene defenses and the development of traumatic resin ducts have been well described in spruce, this system is ideal for future studies of tissue- and cell-specific localization of transcripts and proteins associated with oleoresin defense and induced volatile emissions in conifers. In addition, the advent of laser dissection microscopy techniques presents a fascinating means by which to further address RNA and protein analysis in a tissue-and cell-specific manner. These techniques, when applied to the cambium zone, xylem mother cells, and the epithelial cells that surround traumatic resin ducts, and will allow a temporal and spatial analysis of cellular functions occurring in the traumatic resin response. 

The prenyltransferases that catalyse the s)mtheses of GPP, FPP and GGPP may be important regulatory enz)mies in plant terpenoid bios)mthesis since they are situated at the primary branch points of the pathway, directing flux among the various major classes of terpenoids. The level of prenyltransferase activity is, in fact, closely correlated with the rate of terpenoid formation in many experimental systems (Dudley et at, 1986 Hanley et at, 1992 Hugueney et at, 1996) consistent with the regulatory importance of these catalysts. The localization of specific prenyltransferases in particular types of tissue or subcellular compartments may control the flux and direction of terpenoid synthesis at these sites. For example, the GPP synthase in Salvia officinalis is restricted to the secretory cells of the glandular trichomes, which are the sole site of monoterpene bios)mthesis in this species (Croteau and Purkett, 1989). 

This would favour hypothesis II in Fig. 7.15. Even animals share some of the proteins (TDC, ODC, TyrDC, STS and CR). These observations indicate that the proteins very likely evolved in prokaryotes and were transferred into eucaryotes via either protobacteria (Fig. 7.18) or cyanobacteria, the progenitors of mitochondria or chloroplasts, respectively. A number of SM (e.g. many terpenoids, QAs, the piperidine alkaloid coniine) are produced completely or partly in chloroplasts and/or mitochondria (see Chapter 1). The corresponding genes are mostly nuclear today. It is tempting to speculate that these localizations are indirect indicators of a former bacterial origin of the corresponding pathways. The introduction of bacterial genomes into eukaryotes was... 

The universal precursors to terpenoids, the C5-compounds dimethylallyl pyrophosphate (DMAPP) and isopentenyl pyrophosphate (IPP), originate from two pathways in plants (Fig. 1). The mevalonate (MEV) pathway is well described in many eukaryotic organisms. This pathway is present in the cy-tosol/endoplasmic reticulum of plants. More recently, another pathway has been described, the 2C-methyl-D-erythritol-4-phosphate (MEP) pathway, which is found in the plastids of plants (19). The localization of the different pathways and the plastid-directing transit peptides found in hemi-TPS, mono-TPS, and di-TPS, but not in sesqui-TPS, result in the production of terpenoids from at least two different precursors pools. 

This rapid, usually localized, response also includes biochemical changes, in which monoterpene, diterpene, and sesquiterpene concentrations rise. These terpenes are derived from isoprenoids synthesized via the mevalonate or 1-deoxy-D-xylulose-5-phosphate pathways in the cytosol, endoplasmic reticulum, and plastids. A diverse array of terpenoid synthethases yield the parent compounds, and a number of genes have been characterized. Induction can be elicited by applying methyl jasmonates. ... 

Similarly the localization of the biosynthesis of terpenoids is a matter of debate (70-72,74,76). The most likely site for the formation of geraniol is within the plastids, but where the next steps of the pathway leading to secologanin are located is for the most part uncertain. Geraniol 10-hydroxylase is localized in provacuolar membranes, and not in the endo-plasmatic reticulum as are many other cytochrome P-450 enzymes (109). 

Terpenoid indole alkaloid biosynthetic enzymes are associated with at least three different cell types in C. roseus TDC and STR are localized to the epidermis of aerial organs and the apical meristem of roots, D4H and DAT are restricted to the laticifers and idio-blasts of leaves and stems, and GlOH is found in internal parenchyma of aerial organs (St-Pierre et al. 1999 Buriat et al. 2004) thus, vindoline pathway intermediates must be translocated between cell types. Moreover, enzymes involved in terpenoid indole alkaloid biosynthesis in C. roseus are also localized to at least five subcellular compartments TDC, D4H and DAT are in the cytosol, STR and the peroxidase that couples catharanthine to vinblastine are localized to the vacuole indicating transport of tryptamine across the tono-plast, SGD is a soluble enzyme associated with the cytoplasmic face of the endoplasmic reticulum, the P450-dependent monooxygenases are integral endomembrane proteins, and the N-methyltransferase involved in vindoline biosynthesis is localized to thylakoid membranes (De Luca and St-Pierre 2000). 

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