## 2 Phytotoxic Effects of Volatile Terpenes from the Lamiaceae Family
### Phytotoxic Effects of Monoterpenes from the Genus Nepeta  
The genus _Nepeta_, (subfamily Nepetoideae, tribe Mentheae), commonly known as catmint, is one of the largest genera within the Lamiaceae family and comprises about 300 species [75; 76]. It includes perennial herbaceous plants that are mainly distributed in Central and Southern Europe, the Middle East, Central and South Asia, and parts of Africa [77; 78] (Plants of the World online 2024--[[https://powo.science.kew.org](https://powo.science.kew.org)]([https://powo.science.kew.org](https://powo.science.kew.org)), accessed on 4 January 2025). _Nepeta_ species are widely recognized in traditional medicine for their multiple therapeutic properties [78; 79], but also as an effective natural insect repellent against mosquitoes, cockroaches, flies, and termites and as a powerful attractant for cats [80; 81]. Field studies have identified inhibition zones around certain _Nepeta_ species, where competing vegetation is notably absent. This phenomenon strongly suggests that _Nepeta_ species possess allebotathic properties [82]. Studies on _Nepeta_ species have identified monoterpenoids, sesquiterpenoids, triterpenoids, flavonoids, and phenolic compounds as major specialized metabolites that could play a role in allebotathic interactions [80]. The majority of the biological and pharmacological activities in _Nepeta_ species are attributed to their iridoid compounds [58]. Iridoids, a group of cyclopentane pyran monoterpenoids, are predominantly found as glycosides, with glucose often linked to their C-1 or C-11 positions. Of the over 3000 known iridoids, only about 80 are aglycones, a notable proportion of which are exclusively found in the genus _Nepeta_. Nepetalactone, a volatile iridoid aglycone, is considered the most important bioactive compound in the EOs of iridoid-rich _Nepeta_ species and occurs in eight stereoisomeric forms, including four diastereoisomers and the corresponding enantiomers [76]. Although these NL isomers differ only in the orientation of a single chemical bond, they exhibit different biological activities [83]. With a few exceptions, only the 75 diastereoisomers have been identified in nature. The yield and the chemical composition of EOs vary in different _Nepeta_ species depending on genetic and environmental factors [84]. The biosynthesis of NL in _Nepeta_ begins with GPP, a precursor derived from the MEP pathway, as is the case for all monoterpenes. GPP is converted into geraniol by the enzyme geraniol synthase (GES) [85]. Geraniol is then hydroxylated by the cytochrome P450 enzyme geraniol 8-hydroxylase (GSH) [86] and further oxidized by hydroxygeraniol oxidase (HGO) [87] to form 8-oxogeraniol. The homologs of GES, G8H, and HGO in _Nepeta_ are highly similar to those found in other iridoid-producing plants [86]. Subsequently, the enzyme iridoid synthase (ISY) catalyzes the 1,4-reduction of 8-oxogeranial, producing 8-oxocitronellyl enol, which spontaneously cyclizes to predominantly form the _cis,trans_-stereoisomer of NL [88]. The discovery of stereochemical isomers of NL in _Nepeta_ (_cis,trans-, trans,cis-_, and _cis,cis-_) revealed that while the ISY product 8-oxo-65-citronellyl enol can non-enzymatically cyclize into the _cis,trans_-NL stereoisomer, cyclization in the plant is facilitated by specific enzymes that regulate the formation of distinct NL stereoisomers [89]. Three stereoselective cyclase enzymes were identified in the trichomes of _Nepeta cataria_ and _N. mussiniiiii_[86; 89]. These enzymes, which resemble short-chain dehydrogenase-like (SLN-like) enzymes, including NAD-dependent repetalactol-related short-chain dehydrogenase/reductases (NEPSs) NEPS3 and NEPS4, catalyze the cyclization of 8-oxocitronellyl enol into the _cis,cis-_ and _trans,cis_-NL isomers. Coding sequences for NrNEPS1, identified in the _N. tranjensis_ transcriptome [90], and NcNEPS5 from _N. cataria_[86] were also characterized.