## 1 Introduction
The phototrophic non-sulfur \(\alpha\)-proteobacteria of the genus _Rhodobacter_ feature some unique physiological properties, making them interesting microbial hosts for heterologous terpene production: (i) the cell membrane is commonly considered to be a critical determinant in terpenoid production since it can function as a storage compartment for the involved enzymes and metabolites [42; 43]. In this context, _Rhodobacter_ seems to be particularly suited for terpene production since the bacterium can form an extended intra-cytoplasmic membrane system (ICM), thereby providing a naturally enlarged reservoir for membrane-bound enzymes and terpenes [44; 45]. (ii) As these phototrophic bacteria produce the carotenoids spheroidene and spheroidenone using the DXP pathway [46; 47], they further offer a robust and effective isoprenoid metabolism that can be engineered for efficient terpenoid production. (iii) _Rhodobacter_ species enable photo(hetero)tropic growth in low-cost minimal media at relatively high growth rates, allowing the utilization of sunlight as an energy source for sustainable cultivation and production processes. Recent studies could demonstrate that engineering the isoprenoid precursor biosynthesis can lead to a strong increase of sesqui- and triterpen formation in _R. capsulatus_[39; 48; 49] and _R. sphearoides_[38; 40; 50; 51; 52]. In particular, the co-expression of a terpene synthase with the FPP synthase IspA, and/or enzymes constituting the heterologous MVA pathway, resulted in enhanced production of the corresponding plant terpenoids. A major problem in agricultural crop production is the large number of plant-damaging animals such as insects, mites, and nematodes or pathogens including viruses, bacteria, and fungi, some of which lead to high economic losses of around 60% globally [53]. One of the most widely distributed and destructive pathogens of plants causing white mold disease in more than 400 host plants all over the world is the fungus _Sclerotinia scleroorum_ ([PERSON] [54]. Another devastating example of fungal diseases is the plant vascular wilt caused by the _Fusarium_ species [55]. Other fungal pathogens such as _Phona lingam_[56; 57], _Alternaria brassicicola_[58], and _Rhizoctonia solani_[59; 60] also cause major yield reduction in important crops. To control these pathogens and due to the rapidly growing world population and the resulting increase of food consumption, farmers are using synthetic and biological substances as fertilizers, pesticides, or growth regulators side by side with the cultivation of resistant or tolerant plant varieties [61; 62; 63; 64]. Each of these methods has its limitations, but so far, the use of pesticides is the most convenient and commonly used method. Nevertheless, these can have numerous severe side effects on the environment, including the soil [65]. The soil is inhabited by an enormous diversity of organisms that are important players in maintaining a functional ecosystem and that comprise microorganisms with beneficial properties for plant development and health. For that reason, effective and sustainable alternatives are needed. Firstly, plants, as a part of a complex ecosystem, can produce enormous amounts of secondary metabolites for their survival and maintenance. Phenolics and terpen are examples of metabolites that are produced by plants and act as antimicrobial agents and feeding deterrents [66; 67; 68; 69; 70; 71; 72]. The presence of a wide range of terpen encouraged their use as nature-inspired plant protection agents in agriculture or their use for drug development. One of the commonly stress-associated terpenes is the sesquiterpene \(\beta\)-caryophyllene [73; 74; 75]. As mentioned in the previous section, various studies showed that \(\beta\)-caryophyllene exhibits diverse biological activities against many organisms.