**3. Transcriptional Factors in Floral Volatile Regulation**

Transcription factors (TFs) are sequence-specific DNA-binding proteins that interact with the regulatory regions of the target genes and modulate the transcription initiation rate by RNA polymerases [53]. Although several types of transcription factors have been reported to be involved in the biosynthesis of volatile compounds and secondary metabolites in plants, a limited number of TFs involved in the formation of molecules responsible for floral scent have been identified. Recently, a growing number of research results have reported that several types of TFs, including basic helix-loop-helix (bHLH), basic leucine zipper (bZIP), ethylene response factor (ERF), NAC, MYB, and WRKY family members, are involved in regulation for terpene biosynthesis [53].

TFs play a key role in controlling the expression level of genes involved in various developmental processes and physiological pathways, including plant secondary metabolism [54]. *ODORANT1* (*ODO1*) is the first *MYB* TF identified in flowers. It is a member of the R2R3-MYB TF family, which regulates genes involved in floral scent production through the shikimate and phenylpropanoid pathways [43]. Terpene biosynthesis is regulated by genes encoding required enzymes and their regulators. *Phalaenopsis* TFs regulate the terpenoid pathway; *PbbHLH4* regulates the *geranyl diphosphate synthase* (*GDPS*) gene for the synthesis of monoterpenoids in *P. bellina* [55]. Moreover, a higher expression of five genes encoding TFs (*PbbHLH4*, *PbbHLH6*, *PbbZIP4*, *PbERF1*, and *PbNAC1*) was reported in the scented orchid. Especially, 10-fold higher levels of α-terpineol (a monoterpenoid) were detected in *PbbZIP4*-overexpressing flowers of *P*. *aphrodite* compared to the control [56]. *HY5*, an *bZIP* TF, is known to play a critical role in mastering both the light and circadian signaling pathway. Identification of HY5-interacting motifs on the upstream regulatory fragments of *PbNAC1* implies that the light and circadian clock signals are likely to manage monoterpene biosynthesis in *P. bellina* [56].

Transcriptomic analyses of *C*. *goeringii* flowers led to identification of 1179 genes that were clustered into 64 groups encoding putative TFs with the three largest being bHLH (73 members), ERF (71 members), and C2H2 zinc finger proteins (65 members). *CgbHLH1* and *CgbZIP3* are homologs of *AabHLH1* and *AabZIP1*, respectively. *CgbZIP7* is a homolog of *PbbZIP4*, which regulates monoterpene biosynthesis in *P*. *bellina* (Table 2) [57], while *CgERF2* is a homolog of *CitAP2.10*, which is associated with sesquiterpene (+)-valencene synthesis in sweet orange [34]. *CgNAC5*, a homolog of *AaNAC4*, controls monoterpene synthesis in kiwifruit [35], and *CgWRKY1* and CgWRKY2, which are homologs of GaWRKY1, regulate sesquiterpene (+)-δ-cadinene synthesis in cotton [36]. Cymbidium Sael Bit MYB1 expression is detected at various flower developmental stages and is highest in petals and columns of the fully open flower. *Cymbidium* Sael Bit *MYB1* is regarded as a regulator of phenylpropanoid/benzenoid genes in floral scent profiles. The key component of flower scent in *C. faberi* is MeJA, which is regulated by the crosstalk of many plant hormones. A total of 379 TFs identified as belonging to 37 TF families are differentially expressed between blooming and withered flowers of *C. faberi* [58], and the top 10 groups belong to the *MYB*, *AP2-EREBP*, *bHLH*, *NAC*, *GRAS*, *C2H2*, *C2C2-Dof*, *MADS*, *WRKY*, and

*ABI3VP1* families. Furthermore, an increase in the levels of floral volatiles in tissues resulted from a large increase of various transcription factors in orchids.


**Table 2.** Representative genes responsible for floral scents in orchids.
