Floral volatiles: from biosynthesis to function

Abstract

Floral volatiles have attracted humans' attention since antiquity and have since then permeated many aspects of our lives. Indeed, they are heavily used in perfumes, cosmetics, flavourings and medicinal applications. However, their primary function is to mediate ecological interactions between flowers and a diverse array of visitors, including pollinators, florivores and pathogens. As such, they ultimately ensure the plants' reproductive and evolutionary success. To date, over 1700 floral volatile organic compounds (VOCs) have been identified. Interestingly, they are derived from only a few biochemical networks, which include the terpenoid, phenylpropanoid/ benzenoid and fatty acid biosynthetic pathways. These pathways are intricately regulated by endogenous and external factors to enable spatially and temporally controlled emission of floral volatiles, thereby fine-tuning the ecological interactions facilitated by floral volatiles. In this review, we will focus on describing the biosynthetic pathways leading to floral VOCs, the regulation of floral volatile emission, as well as biological functions of emitted volatiles.

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

“…The MVA pathway comprises six enzymatic reactions, carried out through step-by-step condensation of three molecules of acetyl CoA, which then undergo reduction to the MVA pathway followed by two subsequent steps of phosphorylation and decarboxylation, forming IPP as the ultimate product (Lange et al 2000;Tholl 2015). Similarly, the MEP pathway involves seven enzymatic steps and begins with the condensation of pyruvate and glyceraldehydes-3-phosphate (G3P) (Nagegowda 2010;Muhlemann et al 2014). IPP isomerizes to form DMAPP, which serves as a substrate for hemiterpene biosynthesis or combines with one IPP unit to produce geranyl diphosphate (GPP), catalyzed by the GPP synthases (GPS).…”

“…Similarly, iso-, mono-and sesquiterpenes synthases convert DMAPP, GPP and FPP (or Z, Z-FPP) to isoprene, monoterpenes and sesquiterpenes, respectively. Based on their biosynthetic origin, all volatile organic compounds can be divided into numerous classes, while several floral scent volatiles are constituents of the terpenoid, phenylpropanoid/ benzenoid or fatty acid derivative groups of compounds, and each volatile group is recognized by a few compounds that produce a typical scent (Muhlemann et al 2014).…”

“…Terpenoids are structurally diverse and the most abundant group of floral volatiles, encompassing more than 40,000 individual compounds (Buckingham 2004;Muhlemann et al 2014). A majority of these compounds are of plant origin and have several biological functions in higher plants.…”

“…These five-carbon units serve as substrates/precursors for the biosynthesis of terpenoids. Based on their biosynthetic origin, floral volatile organic compounds can be divided into three major classes: terpenoids, benzenoids/phenylpropanoids and derivatives of fatty acids, wherein terpenes constitute 55% of the plant secondary metabolites, alkaloids 27% and phenolics 18% (Muhlemann et al 2014).…”

“…The MEP pathway is mainly responsible for the biosynthesis of mono-and diterpenes, producing approximately 53 and 1% of the total floral terpenoids, respectively. Sesquiterpenes are synthesized from the MVA pathway, contributing approximately 28% of the total floral terpenoids (Muhlemann et al 2014), as shown in Fig. 1.…”