# Bacterial Semichemicals and Transkingdom Interactions with Insets and Plants
## 1 Introduction
### Bacteria and Insect-Plant Communication  
As exemplified above with the _Chrysolina herbacea--Mentha aquatica_ model, bacteria play a pivotal role in plant-insect interaction systems. The vetiver grass (_Vetiveria zizanoides_ ([PERSON], syn. _Chrysopogon zizanoides_ [PERSON][PERSON]) provide us with another intriguing example. Vetiver is the only grass cultivated worldwide for to its multiple properties. The vetiver grass is very suitable for soil erosion management [72], sustaining agricultural productivity [73], and phytoremediation [74; 75]. From the long roots of vetiver, an essential oil is extracted, which is used to formulate many perfumes. In addition, vetiver is extremely resistant to (or sometimes repellent to) many insect pests, although it may be often infested by pest of other crops, such as stem borers, white grubs, cicadas, and termites, which use vetiver as a refuge [76; 77; 78]. A study focused on _Chilo partellus_, a lepidopterous stem borer of grasses that may infest the vetiver and is a serious pest to maize, sorghum, rice, and other crops, showed that the vetiver grass is highly preferred for oviposition with respect to the aforementioned crops. However, larval survival on vetiver was extremely low, raising the possibility that this plant can be used as a plant trap around crops on which stem borers are a problem [79; 80]. There is evidence suggesting that many insects use vetiver grass as refuge, and that the insect biodiversity of vetiver is actually very high including not only potential insect pests, but also a large number of general predators and parasitoids of insects [81]. The ability of the vetiver grass to attract (and manage life cycle) or repel numerous species of insects relies on the high plant potential for biosynthesis of secondary metabolites. The biosynthesis of these compounds is particularly intense in the vetiver roots that are traditionally used to repel cloth moths, head lice, and bed bugs. The vetiver oil that is extracted from the vetiver roots is a complex mixture of hundreds of sesquiterpene alcohols and hydrocarbon compounds. Several of which possess insect repellent properties. \(\alpha\)-vetivone, \(\beta\)-vetivone, khusimone, zizanal, epizzanal, and (C)-(1S, 10R)-1,10-dimethylbicyclo (4,4,0)-dec-6-en- 3-one) were reported to be repellent to different insects. In particular, zizanal and epizzanal have topical irritant activity on cockroaches and flies [82]. Nootkatone, zizanol, and bicyclovetivenol are strong repellents and toxicants to Formosan subterranean termites _Coptoremes formosanus_ (Isoptera, Rhinitermitiidae) [83]. Indeed, the efficacy of vetiver oil and nootkatone as soil barriers against these termites was proven [84]. Intriguingly, there is evidence that root-associated bacteria may be involved in the biosynthesis of several constituents of the vetiver oil. These bacteria, living in the essential oil-producing cells and in the lysigen lacunae in close association with the essential oil, metabolize biosynthetic precursors that are synthetized by the plant [85; 86]. This is supported by evidence that axenic vetiver produced only trace amounts of oil in vitro, with a strikingly different composition compared to the oils from in vivo vetiver plants. Most root-isolated bacteria were grown using oil sesquiterpenes as a carbon source and were biotrasformed into a large number of compounds typically found in commercial vitier oils [85] (Table S1). Some of these compounds have repellent activity to insects, such as bicyclovetivenol, which was synthesized in vitro by vetiver root-associated _Pseudomonas_ sp. VET-3 and _Pseudomonas_ sp. VET-5 when fed on plant precursor \(\beta\)-caryophyllene, one of the few compounds produced by axenic vitiver [86]. This finding is consistent with a leading role of vetiver root-associated bacteria in overseeing the complex interaction between the plant and its hosted insects. Other volatile terpenoid produced by vetiver root-associated bacteria are listed in Table S1.