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.

Fig. 1

Maize TPS4 and TPS5 catalyze the formation of the same complement of sesquiterpene products, albeit in distinctly different proportions. (A) Abbreviated TPS4 and TPS5 mechanisms accounting for the major products of each enzyme [29% (S)-β-bisabolene, 9% (E)-β-farnesene, 24% 7-epi-sequithujene, and 6% sesquithujene for TPS4; and 27% (S)-β-bisabolene, 13% (E)-β-farnesene, 2% 7-epi-sesquithujene, and 28% sesquithujene for TPS5]. For clarity, only the major products are shown or those products whose levels are considerably different for TPS4 and TPS5. OPP signifies a pyrophosphate. Gray arrows depict pathways equally shared among TPS4 and TPS5. Red and green arrows depict TPS5- and TPS4-preferred pathways, respectively [adapted from (12)]. (B) Active site models of TPS4 and TPS5 based on the crystal structure of tobacco 5-epi-aristolochene synthase (10), shown with the FPP nonhydrolyzable analog farnesyl hydroxyphosphonate (FHP). The four key residues responsible for functional divergence—Thr⁴⁰⁷, Ala⁴⁰⁹, Thr⁴¹⁰, and Ile⁴¹¹ in TPS4, and Ser⁴⁰⁷, Gly⁴⁰⁹, Ala⁴¹⁰, and Asn⁴¹¹ in TPS5— are shown as color-coded sticks, with the underlying secondary structure of the three-dimensional models colored lavender.

Fig. 2

Structural relatedness of some methyltransferases and their substrates. POMT, OOMT1, and OOMT2 are involved in synthesizing the rose floral volatile 1,3,5-trimethoxybenzene. Basil EOMT and C. breweri IEMT methylate eugenol to produce methyleugenol. Basil CVOMT methylates chavicol to produce methylchavicol. Eugenol, chavicol, methylchavicol, and methyleugenol are all volatiles with distinct aromas. COMT is an enzyme in the lignin biosynthetic pathway, and IOMT methylates the nonvolatile isoflavone daidzein to the nonvolatile isoformonentin. All these enzymes use S-adenosyl-l-methionine as the methyl donor [adapted from (–34)]. The branches in the schematic phylogenetic tree are not drawn to scale.

Fig. 3

Localization of storage and synthesis of PVs. (A) Conical cells on the surface of snapdragon petals synthesize and emit terpenes and benzenoids. (B) Glands of Cistus creticus, a shrub native to Crete, are rich in volatile and nonvolatile terpenes.

Fig. 4

Summary of the cellular processes involved in the synthesis of PVs. Most modification reactions occur in the cytosol, but some may take place in other subcellular compartments, including plastids, mitochondria, peroxisomes, and the endoplasmic reticulum.