A plants volatile organic compound (VOC) composition is the hand that swats unwanted visitors, welcomes visitation from much needed pollinators, and the substantial determinant in a plants identity and overall evolutionary fitness within the plant community. Understanding the mechanics of volatile formation is essential in interpreting their importance in plant life, and in plant-animal interactions. A huge allocation of energy is assumed to be given to the production of volatiles, and is therefore deemed costly to the plants, yet this allocation has seldom been quantified (Grison-Pige et al 2001). Volatile release rates that could be found for flowers (mostly Ophrys flowers) ranged from 0.001-0.008 micrograms/hour and this represented between 0.001%-1% of total energy committed to reproduction (Grison-Pige et al 2001). It is the integration of visual cues; beautiful elaborate petals, rich colors, and enchanting aromas (olfactory cues) that give each individual plant, and each species a unique set of assets to woo the otherwise non-specific group of pollinators known as generalist pollinators. It is this same association that drives the increased evolutionary fitness of plants containing both visual and olfactory cues as well as bountiful nectar rewards. In this essay I will outline. The purpose of this essay is to outline the biosynthetic pathways important in producing plant VOC’s, focusing mainly on those producing terpenes, and outlining the applications of volatile research in determining its relationship to sexual evolution within gender dimorphic plant population of Interior British Columbia. Production Center of Volatile Organic Compounds:
VOC’s are produced within the organelles of the plant cell (Pichersky 2006) (Figure 1). The plastid produces such precursors to the volatiles as methylketones, aldehydes, sesquiterpenes, diterpenes, carotenoids, phenylpropenoids and benzenoids (Pichersky 2006). These pre-cursors can then be oxidized, reduced, hydroxylated, methylated, acetylated, or degraded to produce the final VOC’s (Pichersky 2006). These volatiles are stored within membranes, extracellular space in glands, within oil bodies, or conjugated and stored within a vacuole (Pichersky 2006). The production of volatile precursors that are synthesized in the Shikimate pathway produce trans-cinnamic acid; this acid is involved in the production of benzenoids by peroxisome within the cytosol (Pichersky 2006). Other VOC production occurs within the endoplasmic reticulum as well as within mitochondria of plant cells (Starcks et al 1997).
Figure 1: Volatile synthesis within the cell and sub-cellular compartments as represented in Picherksy et al, 2006.
The largest class of secondary metabolites is called the terpenes. Terpenes are composed of n number of isoprene (C5H8) subunits (Raven 2005). Terpenes also encompass the largest number of plant volatile organic compounds which are the focus of this paper (Borg-Karlson 1990; Raven 2005; Pichersky 2006). All terpenes are synthesized in the isoprenoid pathways occurring within the plastid of a plant cell. Isomers, isopentenyl pyrophosphate and dimethylallyl pyrophosphate are generated via phosphate pathways (these are five carbon activated isoprenes) (Pichersky 2006). One molecule of each of these isomers undergoes a condensation reaction regulated by a plastidic prenyltransferase (PPT) which results in the synthesis of one geranyl pyrophosphate (Pichersky 2006). Another reaction occurring in conjunction with the first involves a second type of PPT which again generates a several step condensation reaction involving one dimethylallyl pyrophosphate, with three isopentenyl pyrophosphates (Pichersky 2006). This reaction yields geranylgeranyl pyrosphosphate. All of these products in conjunction with enzymes called terpene synthases yield the precursors to the final VOC’s found within the plastid (Bohlmann 1998). Terpene synthase reactions are...
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