# Plant Volatile Organic Compounds Evolution: Transcriptional Regulation, Epigenetics and Polylploidy
## 1 Plant Volatile Organic Compounds in the Epigenetic Era  
Plants synthesize an amazing diversity of secondary metabolites, which have been selected throughout their evolutionary history as a response to specific needs [1; 2]. Secondary metabolites are distinguished from primary metabolites (such as nucleic acids, amino acids, carbohydrates, etc.) in that they are specialized metabolites which have a wide range of functions, mediating interaction with both biotic and abiotic environments [3; 4]. Volatile organic compounds (VOCs) are one of the most important secondary metabolites produced by plants. These lipophilic compounds have a low molecular weight and high vapor pressures at ambient temperature. More than 1700 VOCs have been identified in different species from both angiosperms and gymnosperms, including a total of 90 families and 38 orders [5]. They are fundamentally released from flowers, but also fruits, leaves, stems and even roots [6]. VOCs define the chemical landscape of numerous ecosystems by taking part in intra- and interspecific interactions [7; 8; 9], being highly context dependent, and functioning in direct and indirect ways from the landscape to the intraloral scale [10]. Plant VOCs have effects on plant-pollinator, plant-herbivore, plant-plant and other interactions and, consequently, on fitness [11]. Among all these functions, the most important and understood one is the attraction of pollinators, ensuring the plant's reproductive success [12; 9]. Floral scent promotes plant-pollinator specialization, as well as outcrossing and reproductive isolation through floral constancy [10; 13]. Thus, such a sexual signal is subject to high selective pressure, being fundamental for plants' evolution and adaptation to the environment [11]. Nevertheless, VOCs are also crucial in plant defense against herbivores and protection from pathogens [8; 14]. VOCs can act as repellents, being constitutively emitted [10; 15] or showing an increased emission during herbivory attack [16]. In addition, specific VOCs can perform as indirect defenses, which can drastically reduce the number of herbivores, especially insects and larvae, by attracting parasitoids and predators [7]. Furthermore, bacteria-specific VOCs are emitted by plants with a diagnostic purpose, triggering defense signaling pathways and acting as direct inhibitors of bacterial growth, therefore making plants more resistant to pathogen invasion [17]. Although pollinator attraction and plant defense may be the most important functions of VOCs, they play many other roles in the interaction of plants with their environment. VOCs are essential in plan-to-plant signaling, allowing other plants to respond to herbivores and activate defenses before being attacked [8; 18] They also work as allelochemicals and neighbor detection signals, taking part in competition between plants and also having an effect on the availability of environmental factors such as light, nutrients and water [19]. Moreover, plant VOCs are also involved in other less studied interactions, like symbiotic relationships with microorganisms and fungi [6] or attraction of seed dispersers [20]. In addition, abiotic stress factors, such as temperature and high light intensity, also stimulate VOC biosynthesis and emission [21; 22]. Water, salt and oxidative stresses may increase VOC emissions, but their effects are not consistent throughout the literature [23]. VOC emission, especially that of terpen, has been proven to mitigate these stresses, allowing plants to recover rapidly from high temperature exposure [24] or alleviating oxidative stress [25] and consequently increasing plant fitness. To fulfill all these functions, plant VOCs are biosynthesized through entangled pathways which need complex regulation. VOCs are differentially--both temporally and spatially--synthesized throughout the development of the plant, due to differences in gene expression in distinct cell types [26].