# Plant Volatile Organic Compounds Evolution: Transcriptional Regulation, Epigenetics and Polylploidy
## 2 Biosynthesis and Epigenetic Regulation of VOCs
### Regulation of Transcription Factors Affects Phenylpropanol/Benzenoid Biosynthesis  
Phenylpropanoids and benzenoids constitute the second largest class of plant VOCs [5] and are derived from the aromatic amino acid phenylalanine (Phe), which is synthesized via two alternative branched pathways that connect central carbon metabolism to Phe--the shikimate and the arogenate pathways [69; 70]. This VOC class can be subdivided into three different subclasses depending on the structure of their carbon skeleton--phenylpropanoids (with a C\({}_{6}\)-C\({}_{3}\) backbone), benzenoids (C\({}_{6}\)-C\({}_{1}\)) and phenylpropanol-related compounds (C\({}_{6}\)-C\({}_{2}\)) [42] (Figure 1). The first committed step in the biosynthesis of the majority of phenylpropanol and benzenoid compounds is catalyzed by the well-known and widely distributed enzyme L-phenylalanine ammonia-lyase (PAL), which catalyzes the deamination of Phe to produce trans-cinamic acid (CA) [70]. Benzenoid formation from CA can occur via a \(\beta\)-oxidative pathway, a non- \(\beta\)-oxidative pathway or a combination of both, which are well characterized [71; 72]. Recently, the BAHD superfamily of acetyltransferases [73] and the SABATH family of methyltransferases [74] have been found to contribute to the final biosynthetic steps of volatile benzenoids. The formation of volatile phenylpropanoids shares initial biosynthetic steps with lignin biosynthesis up to phenylpropenol (monolignol precursor), which is then converted into coniferyl alcohol [75; 76]. A coniferyl alcohol acetyltransferase from the BAHD superfamily forms coniferyl acetate [75], which is the precursor of the common phenylpropanoids eugenol and isoequenol [76; 77]. Furthermore, volatile phenylpropanoid-related compounds originate directly from Phe, not via CA [78; 79]. Only genes and enzymes responsible for the biosynthesis of phenylacetaldehyde and 2-phenylethanol have been isolated and characterized. Phenylacetaldehyde production by phenylacetaldehyde synthesis depends on biosynthetic pathways that differ between organisms [78; 79; 80; 81; 82]. Further conversion of phenylacetaldehyde to 2-phenylethanol is catalyzed by a phenylacetaldehyde reductase, as shown in roses [83; 84]. Transcriptional regulation of phenylpropanoid/benzenoid biosynthesis (Figure 1), and hence scent emission, is controlled by TFs [85]. ODORANT1 (ODO1) and EMISSION OF BENZENOIDS II (EBOII) are R2R3-type MYB TFs responsible of the broad control of phenylpropanoids/benzenoids in _Petunia_\(\times\)_hybrida_ by regulation of the shikimate pathway, as well as the entry points into both the Phe and phenylpropanoid branchways [86]. Suppression of _ODO1_ and _EBOII_ expression in _P. \(\times\)_hybrida_ significantly decreased the transcript level of many phenylpropanoid/benzenoid genes (including _DAHPS_, _EPSPS_, _PAL_, _CM_ and _SAMS_) and therefore decreased the emission of most floral volatiles [86; 87]. EBOII also activates the _ODO1_ promoter, favoring the emission of volatile phenylpropanoids/benzenoids [88]. EMISSION OF BENZENOIDS I (EBOI) is a flower-specific R2R3-type TF, closely related to _EBOII_ and upregulated by the EBOII TF, that directly activates _ODO1_ and numerous scent-related genes [86]. In contrast to ODO1, EBOII and EBOI, the MYB4 TF is a specific repressor of the cinnate-4-hydroxylase from the phenylpropanoid pathway, consequently controlling the flux towards phenylpropanoid volatile compounds in petunias [89]. Recently, R2R3-MYB TFs were found to regulate flavonoid biosynthesis through targeting flavonol synthase (FLS) genes in _Fresia_\(\times\)_hybrida_[90]. _FLS_ genes are responsible for the spatio-temporal biosynthesis of floral flavonoids and are regulated by the well-known SG7 MYB protein family [85; 91], but MYB21 was also found to regulate _FLS_[90]. The MYB21 TF has been considered a terpenoid biosynthesis regulator, so there might exist a connection between terpenoids and flavonoid biosynthesis, but it is still unclear.