# Trepenes and Trepenoids in Plants: Interactions with Environment and Insects
## 1 Plant Trepenes in Abiotic Stress
### Stress Response and Terpenoid Biosynthetic Genes  
The perception and response of plants to abiotic stresses is a complex process involving multiple genes, proteins, enzymes, and metabolites in a highly regulated network [10; 11] (Figure 1). The central component of the salt overly sensitive (SOS) pathway SnRK2 subfamily participates in the terpene phytohormone abscisic acid (ABA) signaling pathway that leads to the activation of transcription factors (TFs) and of the mitogen-activated protein kinase (MAPK) cascade [11; 12; 13]. Besides membrane-based stress perception, physical stimuli such as thermal stress can be sensed at subcellular locations where disruption of the structure-function relationship of biomolecules occurs. Given the advancement of next-generation sequencing and its much-reduced cost per base, whole-genome sequencing has become more feasible. The unprecedented chemical diversity of terpenoids is commensurate to the genetic diversity of TPSs found in plant genomes (Figure 2) [14; 15; 16]. TPS genes were annotated in different numbers, from 29 potentially functional TPS genes in tomato to 69 putatively functional TPS genes in grapevine [17; 18]. Enzyme analysis has shown that the emergence of species-specific clades of TPS genes was brought about by neofunctionalization and gene duplication [19]. TPS genes are classified into seven distinct subfamilies/clades, i.e., _TPSa_, _TPSb_, _TPSc_, _TPSd_, _TPSe_/_, _TPSg_, and _TPSh_[20; 21]. These subfamilies are mainly discriminated based on the presence and modifications of their N- and C-terminal motifs R(R)X\({}_{8}\)W and D(D)X(X)(D)D, respectively [9]. Phylogenetic analysis of TPS protein families based on PFAM PF01397 and PF03936 illustrates that there is a preferential representation of particular TPS subfamilies (_TPSa_, _TPSb_, _TPSc_, _TPSe_/_, and _TPSg_, Figure 2). Thus far, no genome has been found to host all seven subfamilies of TPS. These TPSs have also undergone lineage-specific expansion wherein each species on average has only 1-2 subfamilies of TPSwith probably at least 100 members. Apart from classical plant TPS, microbial TPS-like (MTPSL) exists in non-vascular plants including bryophytes, merchantiophytes, anthropotophytes, and algae [15]. A recent study has attempted to include \(DXS\) genes in TPS gene families, suggesting that duplication of DXS and TPS gene families may lead to the separation of biological features in terpenoid production, and overexpression of these genes may result in the production of major component of terpen [22]. In many plant genomes, \(CYP450\) and \(TPS\) gene pairs are juxtaposed right next to each other (Figure 3) [23; 24]. Integrated analysis of gene co-expression identified two \(CYP450\) genes, namely, \(CYP88D6\) and \(CYP72A154\), involved in the triterpen saponin glycyrrhizin pathway [25]. In _Arabidopsis_, \(CYP6C1\) acts as linalool metabolizing oxygenase and was found to be co-expressed with TPS10 and TPS14 [26]. In addition to the diverse \(TPS\) genes, oxidation provided by enzymes of the CYP450 superfamily gives rise to a variety of terpenoid products. Figure 1: Plant abiotic stress perception and response, concerning different cell compartments/organelles, and the terpenoid biosynthesis pathway. Stress perception is sensed anywhere in the cell where there is a perturbation of biomolecules and metabolic reactions in various cellular compartments/organelles. Ca2+ signaling is initiated through \(\mathrm{[C^{2+}]_{374}}\) changes due to the influx of Ca2+ or its release from intracellular calcium stores. Changes in \(\mathrm{[C^{2+}]_{10}}\) are sensed by various calcium-binding proteins (e.g., calmodulin, CBL), and signals are relayed to the nucleus through the SOS pathway. Ca2+ also activates other pathways such as ABA signaling. Reactive oxygen species (ROS) production in the apoplast is initiated in response to Ca2+ and ABA signaling, and, together, Ca2+/ROS signals are automatically propagated to initiate an SAA or distal response.