# Bacterial Semichemicals and Transkingdom Interactions with Insets and Plants
## 1 Introduction
### Bacteria and Insect-Plant Communication
Plant-insect mutualism, based on pollination performed by Hymenoptera, is established thanks to the microbiota of nectar. [PERSON] and colleagues [87] demonstrated that yeasts (_Metschnikowia reukaufii_ and _Aureobasidium pullulans_) and bacteria (_Neokomagataea_ sp. and _Asaia astilbes_) produce volatiles compound in nectar. All microbial strains produced n-hexanol, while only bacteria produced the characteristic compound 2,5-dimethylfuran, while alcohols, esters, and ketones were more abundant among fungal metabolites [87]. In plant Hymenoptera systems, bacterial symbionts are also involved in seed preservation. The first insect-produced herbicide myrmicacin, which was isolated from the ants, stopped seed germination and preserved the seeds from structural damages [88]. Myrmicacin analogs play similar roles. These compounds can also stop pollen germination and mitotic division in plants and fungal cells and may inhibit fungal growth on seed surfaces [89]. Some Lactobacillus, such as _Lactobacillus plantarum_, produce myrmicacin-related hydroxy acids [90]. All these functions can be counteracted by the production of antibiotics by Actinomycetes in the \"fungal garden\", a specialized structure managed by ants that cultivate fungi as a source of food [91]. Another interesting point is the ability of nonpathogenic bacteria to mediate physiological response and signaling pathways in plants. A review by [PERSON] shows that phytobiome, which is formed by microbiome and macrobiome (insect), manages phenotypic plasticity in plants, inducing changes in VOCs emitted by the plant and influencing the relationship between herbivores and plants [92]. Some nonpathogenic bacteria cause transcriptomic and metabolic changes in plants. For example, root-colonizing _Pseudomonas fluorescens_ SS10 induces resistance against another pathogenic _Pseudomonas_ species (_P. syringae_ pv _tomato_, _Pst_), but also enhances resistance against the insect pest _Spodoptera exigua_ (Lepidoptera, Nocutiidae). This induction of resistance depends on a signaling pathway based on salicylic acid, while other signaling pathway systems are based on phytohormones as jasmonic acid and ethylene. Using _Arabidopsis thaliana_, [PERSON] and coworkers identified approximately 50 metabolites related to the salicylic acid signaling pathway and demonstrated that camalexin and glucosinolate were principal compounds related to resistance against _Pst_, using gene-disruption approach to silence signaling [93]. Another study showed that the root colonization by _P. fluorescens_ in _A. thaliana_ influences not only the defense plant response to herbivores, but also the relationship between herbivores and their parasitoid. As shown in experiments with _A. thaliana_, the VOCs profile emitted by plants infested by aphid _Myzus persicae_ (Hemiptera, Aphididae) is less attractive for the parasitoid _Diaeretiella rapae_ (Hymenoptera, Braconidae) if the roots of the plant are colonized by root bacteria [94]. This phenomenon is lost when the jasmonic acid production is silenced in the plant [94]. The jasmonic acid signaling pathway is related also to the mediation of plant defenses in bitrophic systems. Caterpillars of _Pieris brassicae_ (Lepidoptera, Pieridae) develop faster and bigger when the plants are simultaneously infected by aphid _Brevicoryne brassicae_ (Hemiptera, Aphididae). This phenomenon can occur because the infestation of aphids reduces the levels of jasmonic acids by ten-fold as a consequence of low transcriptions of the _lox_ and _myc_ genes, coding for enzyme-related to jasmonic acid biosynthesis [95]. The facilitation also affects the way herbivores influence parassitoids. As reported in the example described previously, the development of the parasitic wasps _Cotesia glomerata_ (Hymenoptera, Braconidae) is faster if the parasitized caterpillars are fed on by plant-infested aphids, probably because a better fitness of herbivoreouse hosts leads to a better fitness of parasitoid [95].