## 3 Discussion
thaliana_ plants cultivated on a nootkatone-containing medium and compared it with the profile of the plants cultivated on a medium with DMSO as the control. Our results revealed no detectable differences between the treatments. The measured volatile profiles overlap with the results from previous studies, where terpene profiles of six-week-old plants were analyzed [21; 44]. In addition, the sesquiterpenes \(\beta\)-cedrene, \(\beta\)-barbateate, 4-epi-\(\alpha\)-acoradiene and \(\alpha\)-curcumene were found in our analysis. With these results, we concluded that nootkatone is not acting through activating the emission of additional plant sesquiterpenes. Whether the emitted quantity of the individual sesquiterpenes differs between the control and the nootkatone-treated plant could not be determined, as the conducted analysis is purely qualitative. Moreover, we examined whether defense mechanisms involving the SA, JA and ET pathways are triggered by nootkatone by examining the transcript levels of several marker genes. We observed that the expression of several SA and JA marker genes were increased in the presence of nootkatone. This explains the reduced parasitism of _H. schachtii_ in our infection assay, as these results are in agreement with previous studies reporting that elevated concentrations of SA and JA negatively affected the parasitism of _H. schachtii_[20; 45]. Furthermore, it was shown that an increased ET level attracts nematodes towards the plant [20]. Our results revealed that ET biosynthesis was not affected by the presence of nootkatone, which could explain why nootkatone did not change the nematodes' chemotactic behavior. The presented results clearly demonstrate for the first time that nootkatone reduces _H. schachtii_ parasitism on its host plant _Arabidopsis_ and that this is caused by an indirect mode of action involving the plant. One way to explain this effect is the activation of the plant defense through elevating the expression of genes involved in the biosynthesis of the SA and JA genes upon nootkatone exposure. The molecular basis of how nootkatone interacts with the plant and initiates the observed effects still requires elucidation. Analogous to the observations by [PERSON] et al. (2019) for insects, it would moreover be interesting to determine whether and how the derivatization of nootkatone changes its activity and efficacy against nematodes. This will be an important factor to consider in terms of nootkatone formulation for field applications. One approach to increase the molecule's solubility in water, as well as its photo- and thermal stability, but ensuring its permanent release to the environment, is encapsulation in cyclodext [46]. In view of a possible application of nootkatone in nematode management strategies for plant protection, this is one among many open questions to be clarified. Knowledge about the suitable application strategy, the necessary exposure time and time point, as well as the physical and biochemical characteristics of nootkatone in the field in relation to the compound efficacy, will guide the way to product design and practical use. Taken together, our results present nootkatone as a promising natural compound and potentially environmentally safe option to manage nematodes in agricultural cropping systems. This is supported by the fact that nootkatone is naturally produced by, e.g., citrus fruits (it is a major component of the grapefruit aroma), and recognized as nontoxic to humans, that the compound is approved by the Food and Drug Administration (FDA) as a food additive and that nootkatone is already used as a citrus flavor and fragrance ingredient of commercial products. Moreover, it is considered as an environmentally friendly pesticide against mosquitos and ticks and does not persist in the environment [47]. Beside the possibility of extracting nootkatone from plants, it can alternatively be obtained by a recombinant microbial production.