ABSTRACT
All photosynthetic organisms, including higher plants, algae, and cyanobacteria, contain carotenoids, C40 isoprenoids (tetraterpenes), which are essential fat-soluble components for the preservation of animal health. For example, lutein is present in green leafy vegetables and the flowers of marigold (e.g., Tagetes patula) as a dominant pigment, and has been reported to exert a beneficial health effect as prevention against eye disease such as age-related macular degeneration (AMD) (Krinsky et al., 2003). Astaxanthin is biosynthesized in the green alga Haematococcus pluvialis and (marine) bacteria (e.g., the Paracoccus genus) belonging to the Alphaproteobacteria class, and is a commercially important pigment not only in aquaculture but also in nutraceuticals and cosmetics, which exerts strong anti-oxidative and anti-aging effects, e.g., it was shown to improve fine lines/wrinkles and elasticity in the skin of middle-aged women (Misawa, 2011a; Yamashita, 2006; Chapter 28).In this chapter, we present an overview on the structural classification and physiological functions of isoprenoids that originate from higher plants or other organisms, then explore the isoprenoid biosynthetic pathway and pathway engineering for important functional isoprenoids, and finally report our cataloging of novel Tps genes encoding terpene synthases (TPSs), which were isolated from various edible plants. 5.2 Physiological Functions of Isoprenoids
Isoprenoids have been classified according to their carbon number (derived from C5-isoprene units) into monoterpenes (C10), sesquiterpenes (C15), diterpenes (C20), triterpenes (C30), and carotenoids (C40) (Fig. 5.1). Most monoterpenes and sesquiterpenes are low molecular weight, lipophilic, and volatile. Therefore, these isoprenoids are good conveyors of information over long distances (Gershenzon and Dudareva, 2007), and play critical roles in communication between plants and insects, and plants and microorganisms (Lücker et al., 2001).Aphids, the world’s main insect pest, synthesize and release sesquiterpene β-farnesene [(E)-β-farnesene; trans-β-farnesene] (Fig. 5.1) as an alarm pheromone, which causes other aphids in the vicinity to stop feeding and move away when they are attacked
by parasitoids or predators (Beale et al., 2006). Interestingly, β-farnesene released from aphids functions as a so-called kairomone (i.e., a chemical signal that has beneficial effects on the receivers), which attracts the parasitoid wasp Diaeretiella rapae and seven-spot ladybird Coccinella septempunctata (Beale et al., 2006; Abassi et al., 2000). β-Farnesene is also a ubiquitous plant volatile that is often emitted as a mixture with two sesquiterpenes, germacrene D and β-caryophyllene [(-)-β-caryophyllene; (E)-β-caryophyllene] (see Fig. 5.5a for their structures), which are inhibitory to the activities alarming aphids and attracting ladybirds caused by β-farnesene. These findings demonstrate the signal-sensing adaptation of aphids and ladybirds to discriminate between pure β-farnesene and the isoprenoid mixture (Beale et al., 2006; Abassi et al., 2000). Furthermore, the foliar trichomes of the wild potato Solanum berthaultii were shown to release β-farnesene to repel aphids and attract an aphid enemy (Gershenzon and Dudareva, 2007; Gibson and Pickett, 1983). In short, β-farnesene is the alarm pheromone for herbivores, kairomone for carnivores, and is occasionally exploited to repel herbivores and invite carnivores by higher plants to protect themselves. These findings suggest that three organism species have co-evolved around sesquiterpene signals.
