Biosynthesis of Plant Volatiles: Nature's Diversity and Ingenuity

Abstract

Plant volatiles (PVs) are lipophilic molecules with high vapor pressure that serve various ecological roles. The synthesis of PVs involves the removal of hydrophilic moieties and oxidation/ hydroxylation, reduction, methylation, and acylation reactions. Some PV biosynthetic enzymes produce multiple products from a single substrate or act on multiple substrates. Genes for PV biosynthesis evolve by duplication of genes that direct other aspects of plant metabolism; these duplicated genes then diverge from each other over time. Changes in the preferred substrate or resultant product of PV enzymes may occur through minimal changes of critical residues. Convergent evolution is often responsible for the ability of distally related species to synthesize the same volatile.Plant volatiles (PV) are typically lipophilic liquids with high vapor pressures. Nonconjugated PVs can cross membranes freely and evaporate into the atmosphere when there are no barriers to diffusion. The number of identified volatile chemicals synthesized by various plants exceeds 1000 and is likely to grow as more plants are examined with new methods for detecting and analyzing quantities of volatiles that are often minute (1-3).PVs serve multiple functions in both floral and vegetative organs, and these roles are not always related to their volatility (1). Most PVs are restricted to specific lineages and are involved in species-specific ecological interactions, leading to their designation as specialized, or secondary, metabolites (4). To humans, pollinator-attracting floral scents have been a source of olfactory pleasure since antiquity, and we also use a large number of aromatic plants as flavorings, preservatives, and herbal remedies (5). Flower and herb aromas may contain many individual chemicals, sometimes with very little overlap in the PV profiles of even closely related species (6). It is unlikely that the observed differences are due entirely to differential gene expression. That most PVs are restricted to a few lineages, both ancient and derived, argues for frequent changes in enzymatic profiles through evolution. Considerable diversity and fast rates of change have also been observed in nonvolatile specialized metabolites in plants, raising similar questions regarding the underlying evolutionary mechanisms (7).Improvements in analytical techniques and molecular and biochemical methods have made PVs one of the best-studied groups of plant secondary metabolites. Here we describe

Synergistic effects of floral phytochemicals against a bumble bee parasite

“…In addition to the documented presence of these specific compounds, the biochemical pathways that produce eugenol and thymol give rise to many structurally similar compounds that may have similar individual and interactive effects. Eugenol is produced via the shikimate pathway, and as a phenylpropene, it belongs to the second most diverse class of plant volatiles (Pichersky, Noel, & Dudareva, 2006). Thymol is produced from isoprenoid precursors via the methylerythritol phosphate (MEP) pathway from substrates involved in primary metabolism (Pichersky et al., 2006), meaning that precursors of thymol and related compounds are found in all plant species.…”

Biosynthesis of plant‐derived flavor compounds

“…Many of the terpenoids produced are non-volatile and are involved in important plant processes such as membrane structure (sterols), photosynthesis (chlorophyll side chains, carotenoids), redox chemistry (quinones) and growth regulation (gibberellins, abscisic acid, brassinosteroids) . The volatile terpenoids -hemiterpenoids (C 5 ), monoterpenoids (C 10 ), sesquiterpenoids (C 15 ) and some diterpenoids (C 20 ) -are involved in interactions between plants and insect herbivores or pollinators and are also implicated in general defense or stress responses Pichersky and Gershenzon, 2002;Pichersky et al, 2006). Terpenoids, mainly the C 10 and C 15 members of this family, were found to affect the flavor profiles of most fruits and the scent of flowers at varying levels ( Figure 5).…”

“…Benzenoid and phenylpropanoid volatile compounds, primarily derived from phenylalanine, contribute to the aromas and scents of many plant species and play important roles in plant communication with the environment Knudsen and Gershenzon, 2006;Pichersky et al, 2006). Several enzymes that catalyze the final steps in the biosynthesis of these compounds have been isolated and characterized.…”

“…Many terpene synthases have been isolated and characterized from various plant species (Bohlmann et al, 1998;Tholl, 2006). While many terpene volatiles are direct products of terpene synthases, many others are formed through transformation of the initial products by oxidation, dehydrogenation, acylation and other reactions (Croteau and Karp, 1991;Croteau et al, 2000;Dudareva et al, 2004;Pichersky et al, 2006). For example, ())-(1R,2S,5R)-menthol, the principal monoterpene of commercial peppermint essential oil and the component responsible for the familiar cooling sensation of peppermint and its products, is formed by eight enzymatic steps involving monoterpene synthases, isomerases and reductases (Ringer et al, 2005;Turner and Croteau, 2004).…”

Volatile terpenoids: multiple functions, biosynthesis, modulation and manipulation by genetic engineering

“…Floral volatiles are lipophilic liquids in nature, having high vapor pressure and low molecular weight at ambient temperatures. These properties allow them to freely pass through the cellular membranes for release into the adjacent environment (Pichersky et al 2006). Among all terpenoids, mono-and sesquiterpenes are the most commonly studied classes because of their extensive distribution in the plant kingdom and their essential roles in both human society and plants.…”