# Trepenes and Trepenoids in Plants: Interactions with Environment and Insects
## 1 Plant Trepenes in Abiotic Stress  
### Diversity of Plant Trepenes  
Plant secondary metabolites involved in environmental adaptation and stress tolerance can be broadly classified into phenolics, flavonoids, alkaloids, and terpenoids. Among these classes of compounds, terpenoids are the most diverse, with significant properties in the context of chemical ecology [1]. These compounds are synthesized from the five-carbon precursor units isopentenyl pyrophosphate (IPP) and its functional isomer dimethylallyl pyrophosphate (DMAPP). Subsequent condensation of IPP and DMAPP through the action of isopenyl diphosphate synthase (IDS--a type of prenyltransferase (PT)) produces acyclic and achiral isopenyl diphosphate/pyrophosphate (ID, C\({}_{5\text{n}}\)) intermediates including geranyl pyrophosphate (GPP, C\({}_{10}\)), farnesyl pyrophosphate (FPP, C\({}_{15}\)), and geranylgeranyl pyrophosphate (GGPP, C\({}_{20}\)), which are considered universal terpenoid precursors [2]. Trepene synthases (TPSs) act on one or more of these universal precursors (including DMAPP) to produce a combinatorial diversity of terpenes. To clear out any ambiguity, terpenes are simple hydrocarbons based on combinations of DMAPP and ID, while terpenoids (which are also known as isoprenoids) are terpenes with an oxygen moiety and additional structural rearrangements.  
Nonetheless, these two terms are used interchangeably. Terpenoids are conveniently classified on the basis of the number of carbon atoms they possess: hemiterpenoids (C\({}_{5}\)), monoterpenoids (C\({}_{10}\)), homoterpenoids (C\({}_{11,16}\)), sesquiterpenoids (C\({}_{15}\)), diterpenoids (C\({}_{20}\)), seasterpenoids (C\({}_{25}\)), triterpenoids (C\({}_{30}\)), tetrterpenoids (C\({}_{40}\)), and polyterpenoids (C\({}_{>40}\), higher-order terpenoids).  
A small fraction (1%) of low-molecular-weight (300 Da) compounds known as plant volatiles are implicated in plant-environment interactions and, to some extent, in the abiotic stress response [3; 4]. Terpenoids that are emitted to communicate with the environment are referred to as volatile terpen (VTs) and include hemi-, mono-, homo-, sesqui-, and some diterpenoids. The storage and volatility of these terpenoids are largely influenced by their chemical properties (i.e., vapor pressure and hydrophobicity), while their emission rates and patterns depend on abiotic and biotic factors including temperature, humidity, seasonality, irradiance, and interactions with other plants and organisms [5]. The storage of terpenoids concerns plant tissues with specialized structures such as secretory cavities, resin canals, latex canals, glandular trichomes [6]. A study [5] has emphasized that some VTs, referred to as 'hidden terpenoids', are chemically modified by glycosylation for storage and upon exposure to stress, may undergo hydrolysis releasing VTs to the atmosphere on demand. There is a consensus agreement that VT production and emission are tightly associated with stress alleviation, allelopathy, and plant-herbivore-carnivore (tritrophic or multitrophic) interactions.  
The chemical diversity of terpenoids is driven by the stereo-specific carbocation cyclization, rearrangement, and elimination reactions that transform a few universal ID precursors into core scaffolds of numerous structurally distinct terpenoids. This indicates that a single TPS can give rise to multiple products due to the stochasticity of bond rearrangement following the generation of the unusual carbocation intermediates. Remarkably, the existence of multi-substrate TPS has also been demonstrated to be dependent on the physiological and development status of plants. This further suggests that substrate preference and terpene product profiles may vary in response to fluctuations in the environment [7]. Functional decoration of these core scaffolds by cytochrome P450 monooxygenases (P450) augments the already-existing chemical diversity through oxygenation and further structural rearrangements, resulting in an estimated 80,000 distinct compounds [8; 9].