# Plant Volatile Organic Compounds Evolution: Transcriptional Regulation, Epigenetics and Polylploidy
## 3 The Temporal Pattern of Emissions and Its Epigenetic Regulation  
Regulation of the rhythmic release of VOCs under circadian control has been a topic of investigation for the past several years. The release of floral scent at a specific time of the day is thought to be evolutionarily beneficial in order to maximize resource efficiency, attracting only truly effective pollinators and also avoiding the attraction of predators, which reduce herbivory [110; 111]. These temporal patterns are accompanied by oscillations in the expression of genes in the VOC biosynthetic pathways [110; 112]. Temporal patterns of phenylpropanoid/benzenoid emission have been studied in multiple plant genera, such as _Petunia_, _Nicotiana_ and _Antirrhinum_[89; 113; 114; 115]. In addition, other studies have also reported a rhythmic emission of terpen, but little is known about the circadian regulation of volatile fatty acid derivatives [111; 116]. Circadian rhythmic emission of volatiles is regulated by the expression of key genes, which may also determine the rhythmicity of downstream volatile emissions, modifying the expression of genes at the final steps of biosynthetic pathways. Genes implied in VOC biosynthesis can be temporally regulated by transcription factors (TF) coded by clock genes [4; 110], which may act as regulators of the rhythmic emission of volatiles [111]. The main components of the circadian clock have been elucidated in _Arabidopsis thaliana_[117] but they are highly conserved across many plant species [118]. The _CIRCADIAN CLOCK ASSOCIATED 1 (CCA1)_ and _LATE ELONGATED HYPCOCTYL (LHY)_, known as morning components, along with the response regulator CCT-domain protein _TIMING OF CAB EXPRESSISON 1 (TOC1)_ and _ZEITLIPE (ZTL)_, known as evening components, form the main oscillator at the core of the plant circadian clock, based on transcriptional/translational negative feedback loops involving these clock genes (Figure 2A). _CCA1_ and _LHY_ inhibit the expression of _TOC1_; the TOC1 protein also represses _CCA1_/_LHY_; and ZTL ubiquitinates TOC1 for degradation as a response to blue light [119; 120]. The daily oscillation of light and temperature are the main exogenous cues for the plant's circadian clock [121]. In the gray poplar (_Populus \(\times\) causescens_), diurnal variation in terpene emission is associated with oscillations in the expression of genes encoding for isoprene synthases (ISPS), which are controlled by the presence of circadian regulatory elements (CCA1/LHY) in the promoter sequence of _ISPS_[66]. Furthermore, the clock gene _LHY_ in _Petunia \(\times\) hybrida_ and tobacco (_Nicotiana attenuata_) controls the daily expression pattern of many phenylpropanoid/benzenoid genes and TFs (_ODO1_, _EPSPS_, _CM1_, _ADT_, _PAL_, etc.) and hence the production of these floral volatiles [111]. Although the expression of _LHY_ leads to a decrease in the expression of many phenylpropanoid/benzenoid genes and the loss of emission, reducing its constitutive expression has an opposite effect [114; 122]. In addition, _ZTL_ repression in tobacco (and its ortholog in petunia, _Petunia CHANEL (PhCHL)_) leads to a decrease in floral scent emission [122]. Recent studies have demonstrated the epigenetic regulation of circadian clock components [123; 124; 125]. Chromatin reshaping by histone acetylation, methylation and phosphorylation (Figure 2B) may create a flexible loop of clock gene regulation [125]. The CCA1 morning factor binds to the core oscillator evening gene _TOC1_ promoter, inducing deacetylation of the histone H3 by histone deacetylases (HDACs) that causes chromatin compaction and subsequent _TOC1_ repression at dawn [121; 124]. In addition, the REVEILLE 8/LHY-CCA1-LIKE 5 (RVES/LCL5) TF, which also has a morning peak like CCA1 and LHY, promotes hyperacetylation of H3 in the _TOC1_ promoter, activating _TOC1_ expression and having an opposite role in its regulation. This may facilitate _TOC1_ rising phases and provide a fine-tuning mechanism for precisely shaping the rhythmic expression of _TOC1_[126].