## 4 Discussion
When fighting phytopathogens, it is important to consider and ideally avoid negative off-target effects on the above-mentioned beneficial microorganisms. Corresponding tests are therefore now frequently included in the first evaluation of antimicrobial activities. So far, no studies were investigating the effect of pure \(\beta\)-caryophyllene and \(\beta\)-caryophyllene oxide against a selection of phytopathogenic fungi aiming to determine and compare the potential antifungal properties of the two compounds and species-specific differences in sensitivity. In our current study, we show the potential of sustainable production of \(\beta\)-caryophyllene in the heterologous host _Rlodobacter capsulatus_ and the species-specific promoting or inhibitory effects for selected plant growth-promoting bacteria for both of the tested sesquiterpenoids at appropriate compound concentrations. Furthermore, we tested the bioactivities of both \(\beta\)-caryophyllene and \(\beta\)-caryophyllene oxide against several plant pathogenic fungi and showed that both substances were active against certain fungi. Interestingly, the oxidized form tended to be even more effective, and additionally has a more beneficial activity profile concerning the PGPB. These results are supported by previous reports which are introducing \(\beta\)-caryophyllene as a bioactive compound in its purified form [79] and as a component of several essential oils [109; 110]. The purified \(\beta\)-caryophyllene showed a MIC of 130 \(\mu\)g mL\({}^{-1}\) for _F. oxysporum_[79], which is below the maximal concentration tested in this study. However, our plate-based approach is not completely comparable with the method used for MIC determination in liquid medium. According to our results, the inhibitory effect of the hyphal growth was different depending on the tested fungus. Such a difference is dependent on the fungal species and frequently described by previous studies showing that the novel fungicide 3-[5-(4-chlorophenyl)]-2,3-dimethyl-3-isoxazolidinyl] pyridine (SYP-Z048) affected several pathogenic fungi in different ways [114]. Overall, our current results demonstrate that both \(\beta\)-caryophyllene and \(\beta\)-caryophyllene oxide exhibit bioactivity against plant pathogenic fungi and therefore could be suitable as potential fungicides in agriculture as, in contrast to many broad-spectrum pesticides, they do not harm many species of plant growth-promoting bacteria. However, despite the sesquiterpenes being natural compounds, which are often associated with non-harmful ecotoxicological profiles, effects against _Pseudomonas_ species were corroborated and will need to be taken into account. So far, there is only limited information about the individual activities of terpen against plant pathogens and the underlying molecular mechanisms. To be able to explain the differences we observed in the activity of the two terpenes against the different organisms and to get more data on the activity spectrum, our investigations need to be extended by including more target and non-target organisms. In addition, the respective modes of action on the molecular level have to be determined. Besides additional plant pathogens, this not only includes analyzing further plant growth-promoting bacteria, but it must be tested if plant growth-promoting fungi react sensitively to the terpen, as indicated by a previous promising study [80]. In particular, fungi of the genus _Trichoderma_, which are said to have many advantageous properties for plants, could be investigated more closely [115; 116]. To be able to provide appropriate quantities of an active antifungal substance, the heterologous production of promising sesquiterpenes in a suitable microbial host bears various benefits, such as the possibility to solely produce the desired compound without complex downstream processing and in high amounts. Therefore, we established the biosynthesis of the plant sesquiterpene \(\beta\)-caryophyllene in the heterologous production host \(R\).