ReviewBiogenesis, molecular regulation and function of plant isoprenoids
Introduction
It has been estimated that 15–25% of plant genes are dedicated to plant secondary metabolism pathways [1]. The latter comprise three major class of products including isoprenoids, alkaloids and flavonoids. When considering the presence of these products among different taxa, it is apparent that the isoprenoid pathway is the oldest acquisition. In supporting this idea, one could note that plant dihydroflavonol reductase, the first specific enzyme of anthocyanin biosynthesis has extensive homology with isoprenoid dehydrogenases [2]. Isoprenoids are compounds built up of simple or multiple C5 units and are divided into different subgroups synthesized in different plant cell compartments including plastids, mitochondria and the endoplasmic reticulum–cytosol (Fig. 1). Their synthesis is often tightly linked to the function, differentiation state and organization of the compartment with which they are associated.
Here, we review our current understanding of plant isoprenoid biogenesis integrating biochemical, molecular and functional data. The metabolic engineering will not be covered, as this was comprehensively addressed [3]. For information concerning plant protein prenylation, readers are referred to previous reviews [4], [5].
Section snippets
Plastid and cytosolic pathways
It is now well established that in cyanobacteria and plants, isopentenyl diphosphate (IPP) is synthesized in the plastids through 1-deoxy-d-xylulose-5-phosphate (DXP) pathway and in the cytosol through the mevalonic acid (MVA) pathway according to the pathways outlined in Fig. 2 (for a review, see [6], [7], [8], [9], [10]). It is worth noting that alga belonging to the chlorophyta have apparently lost the mevalonic pathway and use exclusively the DXP pathway [11]. Similarly, the malaria
General aspects
The prenyltransferase reaction involves the condensation of an acceptor isoprenoid or non-isoprenoid molecule to an allylic diphosphate. Three types of reactions have been described [73]. The first and most studied reaction involves a head-to-tail or 1′-4 condensation of the 5-carbon IPP, to DMAPP or to an elongating allylic diphosphate chain. Depending on their specificities these prenyltransferases yield linear trans- or cis-prenyl diphosphates. The second class of prenyltransferases reaction
cis-Polyprenyltransferases
In eucaryotes, cis-prenyltransferases are involved in the synthesis of C55–C100 dolichols, which serve as sugar carriers in protein glycosylation [118], [119]. A dehydrodolichyl diphosphate synthase has been characterized from Arabidopsis [120]. The Arabidopsis gene was mainly expressed in roots [120]. In general, these prenyl transferases are activated by lipids and detergents. The peptide sequence of these enzymes do not have the most conservative FARM and the SARM that are characteristic of
Isoprene and methylbutenol
Several plants belonging to mosses, ferns, gymnosperms and angiosperms emit isoprene and methylbutenol (for a review, see [130]; Fig. 4). The reactions are catalyzed by isoprene synthase (IS) and methylbutenol synthase (MBS), two plastid enzymes that convert DMAPP to isoprene and methylbutenol [130]. In contrast to several plastid biosynthetic enzymes, IS and MBS possess a relatively high KmDMAPP. The gene encoding IS has been characterized from poplar species [131], [132], [133]. IS display
Background
Sterols are essential components of fungal, animal and plant membranes. They regulate membrane fluidity and permeability and interact with lipids and proteins within the membrane. These structural and membrane roles of sterols in different organisms or models, including the formation of “rafts”, have been reviewed elsewhere in a number of well documented papers [203], [204], [205], [206], [207], [208], [209], including some dedicated more specifically to plant sterols [210]. The sterols and the
Ubiquinones
It can be inferred from previous studies that IPP is imported from the cytosol and used for ubiquinone biosynthesis [353] according to a pathway not yet definitively established in plants, but probably very similar to that operating in Gram-negative bacteria and yeasts [354] (Fig. 17). Feeding experiments show that the side-chain of ubiquinone is synthesized from mevalonate-derived IPP [355]. Although mitochondrial FPPS [356] and GGPPS [85] isoforms have been characterized, specific
Diterpene biogenesis
The plastid is the main if not the unique site for the synthesis of GGPP that constitutes the backbone for the synthesis of diverse diterpenes. However, it must be mentioned that in the case of irregular diterpenes such as anisotomenes, two GPPs are used instead of one GGPP [79] (Fig. 4). The initial steps of GGPP transformation are catalyzed by diterpene cyclases and is usually followed by further modification in the ER–cytosol. The gibberellin pathway serves to illustrate this point (Fig. 18
Conclusion
The regulation of the biosynthesis of isoprenoids in plant cells is poorly understood. In general, the amount of induced accumulation of isoprenoids in plant tissues is low. In fact we do not master several factors. We do not know how the metabolite fluxes between the primary and the secondary metabolisms are orchestrated. Practically, for all studied pathways, the concentration of the substrates and products of each step are presently unknown to evaluate which reactions are in equilibrium and
Acknowledgements
We are indebted to the Europe Union Research Program QLK3-2000-00809 for research grants.
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