SummaryThe nature of isoprenoids synthesized in plants is primarily determined by the speci®city of prenyltransferases. Several of these enzymes have been characterized at the molecular level. The compartmentation and molecular regulation of geranyl diphosphate (GPP), the carbon skeleton that is the backbone of myriad monoterpene constituents involved in plant defence, allelopathic interactions and pollination, is poorly understood. We describe here the cloning and functional expression of a GPP synthase (GPPS) from Arabidopsis thaliana. Immunohistological analyses of diverse non-secretory and secretory plant tissues reveal that GPPS and its congeners, monoterpene synthase, deoxy-xylulose phosphate synthase and geranylgeranyl diphosphate synthase, are equally compartmentalized and distributed in non-green plastids as well in chloroplasts of photosynthetic cells. This argues that monoterpene synthesis is not solely restricted to specialized secretory structures but can also occur in photosynthetic parenchyma. These data provide new information as to how monoterpene biosynthesis is compartmentalized and induced de novo in response to biotic and abiotic stress in diverse plants.
The accumulation of three major carotenoid derivatives-crocetin glycosides, picrocrocin, and safranal-is in large part responsible for the color, bitter taste, and aroma of saffron, which is obtained from the dried styles of Crocus. We have identified and functionally characterized the Crocus zeaxanthin 7,8(7 ,8 )-cleavage dioxygenase gene ( CsZCD ), which codes for a chromoplast enzyme that initiates the biogenesis of these derivatives. The Crocus carotenoid 9,10(9 ,10 )-cleavage dioxygenase gene ( CsCCD ) also has been cloned, and the comparison of substrate specificities between these two enzymes has shown that the CsCCD enzyme acts on a broader range of precursors. CsZCD expression is restricted to the style branch tissues and is enhanced under dehydration stress, whereas CsCCD is expressed constitutively in flower and leaf tissues irrespective of dehydration stress. Electron microscopy revealed that the accumulation of saffron metabolites is accompanied by the differentiation of amyloplasts and chromoplasts and by interactions between chromoplasts and the vacuole. Our data suggest that a stepwise sequence exists that involves the oxidative cleavage of zeaxanthin in chromoplasts followed by the sequestration of modified water-soluble derivatives into the central vacuole.
Isopentenyl diphosphate (IPP), which is produced from mevalonic acid or other nonmevalonic substrates, is the universal precursor of isoprenoids in nature. Despite the presence of several isoprenoid compounds in plastids, enzymes of the mevalonate pathway leading to IPP formation have never been isolated or identified to our knowledge. We now describe the characterization of two pepper (Capsicum annuum L.) cDNAs, CapTKT1 and CapTKT2, that encode transketolases having distinct and dedicated specificities. CapTKT1 is primarily involved in plastidial pentose phosphate and glycolytic cycle integration, whereas CapTKT2 initiates the synthesis of isoprenoids in plastids via the nonmevalonic acid pathway. From pyruvate and glyceraldehyde-3-phosphate, CapTKT2 catalyzes the formation of 1-deoxy-xylulose-5-phosphate, the IPP precursor. CapTKT1 is almost constitutively expressed during the chloroplast-to-chromoplast transition, whereas CapTKT2 is overexpressed during this period, probably to furnish the IPP necessary for increased carotenoid biosynthesis. Because deoxy-xylulose phosphate is shared by the plastid pathways of isoprenoid, thiamine (vitamin B 1 ), and pyridoxine (vitamin B 6 ) biosynthesis, our results may explain why albino phenotypes usually occur in thiaminedeficient plants.Because of the combined activities of mevalonatesynthesizing and -activating enzymes, IPP is known as the universal isoprenoid building block. How IPP is synthesized and channeled into plastid isoprenoids to support the production of carotenoids, chlorophylls, prenylquinones, and diterpenes is largely unknown. Two hypotheses have been proposed. One is that plastids operate autonomously, synthesizing plastid isoprenoids directly from carbon dioxide or from plastid glycolytic intermediates such as pyruvate (Goodwin, 1971;Moore and Shephard, 1978; Heintze et al., 1990 Heintze et al., , 1994McCaskill and Croteau, 1995). However, the mechanism of carbon flow via a pyruvate intermediate is unknown for plants. A second hypothesis is that IPP is transported from the cytosol (Kleinig, 1989), which is based on the finding that hydroxymethylglutaryl CoA reductase and mevalonate-activating enzymes are absent in plastids (Gray, 1987). This view is reinforced by the fact that mevilonin, a specific inhibitor of 3-hydroxy-3-methylglutaryl-CoA reductase, drastically inhibits cytosolic sterol biosynthesis at moderate concentrations but does not affect isoprenoid synthesis in plastids (Bach and Lichtenthaler, 1983). This led to consideration of an alternative IPP-generation system. In fact, such a pathway is known for prokaryotes, in which IPP is formed via deoxy-xylulose phosphate rather than by mevalonate (Rohmer et al., 1993) in a transketolation reaction between pyruvate and glyceraldehyde-3-phosphate . In vivo precursor labeling indicates that a similar pathway operates for the synthesis of ginkgolides (Schwarz, 1994) and plastid isoprenoids (Schwender et al., 1996; Arigoni et al., 1997;Lichtenthaler et al., 1997aLichtenthaler et al., , 1997b.In t...
Like seed plants, liverworts synthesize and accumulate a myriad of isoprenoid compounds. Using antibodies raised against several isoprenoid biosynthetic enzymes, we investigated their intracellular compartmentation by in situ immunolocalization from Marchantia polymorpha. The enzymes examined were deoxy-xylulose phosphate synthase, geranyl diphosphate synthase, farnesyl diphosphate synthase, geranylgeranyl diphosphate synthase, monoterpene synthase, geranylgeranyl diphosphate reductase, phytoene synthase, and phytoene desaturase. Our results show that liverwort oil bodies, which are organelles bound by a single unit membrane, possess isoprenoid biosynthetic enzymes similar to those found in plastids and the cytosol. We postulate that oil bodies play a dynamic role in cell metabolism in addition to their role as sites of essential oil accumulation and sequestration. The occurrence of such enzymes in different cellular compartments might be due to multiple targeting of gene products to various organelles.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.