*4.5. Cytotoxic Secondary Metabolites*

The major difficulty in the treatment of cancer is the increase in drug resistance of commonly used chemotherapeutic agents, so the crucial task is to find out the novel compounds with high efficacy and low toxicity. The research and registration of new antitumor drugs are mostly based on the compounds extracted from medicinal plants including those of endophyte origin [127]. Martinez−Klimova et al. [5] found the endophytes that produce antibiotic metabolites belonging to phylum Ascomycota, which were isolated from the Asteraceae, Fabaceae, Lamiaceae, and Araceae families. The therapeutic activity of fungal endophytes was related to the production of compounds inhibiting the drug transporters of tumor cells. Moreover, the use of secondary metabolites produced by endophytes could mediate drug resistance reversal in cancer cells. A few reports are pointing out the use of endophytes isolated from Asteraceae species as a source of antitumor compounds targeted in the most common lines of cancer cells (Figure 6). Nectriapyrone, produced by the endophytic fungus *Glomerella cingulata*, a teleomorph stage of *C. gloeosporioides*, isolated from *V. arenaria* and *T. diversifolia* showed relevant cytotoxic activity towards tumor cells [105]. In the case of *Chaetomium globosum*, a fungal endophyte associated with *Viguiera robusta*, chaetoglobosins showed inhibition of Jurkat (leukemia) and B16F10 (melanoma) tumor cells with 89.55% and 57.1% inhibition at 0.1 mg mL<sup>−</sup>1, respectively [114]. Gallo et al. [99] isolated a fungus *P. immersa* from roots and leaves of *S. sonchifolius*. *P. immersa* extracts displayed strong cytotoxicity due to newly described secondary metabolites, i.e., 2,3-epoxy-1,2,3,4-tetrahydronaphthalene-c-1,c-4,8-triol, which showed highest activity against the human tumor cell lines MDA-MB435 (melanoma), HCT-8 (colon), SF295 (glioblastoma), and HL-60 (promyelocytic leukemia), with he half maximal inhibitory concentration (IC50) values of 3.3, 14.7, 5, and 1.6 mm, respectively. Moreover, sitostenone and tyrosol, other *P. immersa* secondary metabolites, showed anticancer effects when applied with isocoumarin [99]. The fungal endophytes of Asteraceae, especially the members of genera *Fusarium*, *Plectosphaerella*, *Stemphylium*, *Septoria*, *Alternaria*, *Didymella*, *Phoma*, *Chaetosphaeronema*, *Sarocladium*, *Nemania*, *Epicoccum*, and *Cladosporium* can produce the anticancer enzyme L-asparaginase used in the treatment of acute lymphoblastic leukemia. The isolates of fungi *Fusarium proliferatum* and *Plectosphaerella tracheiphilus*, obtained from an Asteraceae host *C. segetalis*, exhibited a maximum enzyme activity with 0.492 and 0.481 unit mL<sup>−</sup>1, respectively [56]. The milk thistle (*Silybum marianum*) is known as a source of silymarin, a mixture of flavonolignans used in cancer chemoprevention and hepatoprotection. El-Elimat et al. [103] showed that a fungal endophyte, *Aspergillus iizukae* (current name *Fennellia flavipes*), isolated from leaves of *S. marianum* can synthesize similar compounds as a host plant, namely silybin A, silybin

B, and isosilybin, the constituent compounds of silymarin. Endophytic fungi that can produce the same compounds of their associated host plants could be a sustainable and alternative source for secondary metabolites.

**Figure 6.** The molecular structure of chosen specific compounds with cytotoxic activity synthesized by fungal endophytes associated with Asteraceae species [99,103,105,110,114].

#### **5. Review Methodology**

The leading scientific databases dedicated to multidisciplinary as well as agricultural, biological, biomedical, and pharmacological sciences were screened. Relevant literature dated to the period 2000–2020 was collected, analyzed, and selected considering (i) the reports on endophyte isolation from the species of Asteraceae family, (ii) the reports on therapeutic utilization of the host plant or/and an endophyte, (iii) the reports on in vitro and in vivo bioactivity of chemical compounds produced by a host plant or/and an endophyte. Plant names were verified according to the Global Biodiversity Information Facility [128] and The Plant List [34], endophyte taxa were verified according to MycoBank database [129]. For clarity, the validated endophyte names used in the referenced literature were implemented in the text. In the tables and figures, the current taxa classification and nomenclature were used. Chemical structures were elaborated on the basis of referred publications, for new isolated compounds the number of C atoms was presented.

#### **6. Conclusions**

A growing spectrum of literature indicates that endophyte fungi colonizing different species of Asteraceae are responsible to some degree for their therapeutic potential reported in ethnobotanical and modern literature. Endophyte fungi are elements of a complex web of interactions of the plant host/endophyte/phytopathogen, and hence all elements of this system are expected to produce bioactive compounds that can improve their ability to survive in such a dynamic environment. Endophytes were involved in the superior adaptability and competitiveness reported for Asteraceae hosts and their evolutionary success. Plant/endophyte interactions regulated the energy costly process of production of secondary metabolites possessing therapeutic properties. In the case of the Asteraceae species analyzed, the host tissue's environment was more crucial than plant taxonomy for shaping the diversity and metabolite profile of fungal endophytes. Most endophyte fungi isolated from Asteraceae plants were wide-spreading. Despite that, they produced very specific secondary metabolites in planta and in vitro. The interactions between the endophyte and its host controlled by specific chemical compounds are dynamic and difficult to analyze but crucial for the composition of the medicinal plant extracts and their standardization.

**Author Contributions:** Conceptualization, A.S., and G.C.; writing—original draft preparation, A.S., and G.C.; writing—review and editing, M.T.A., A.K.; visualization, A.S.; supervision, A.S., and G.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was partially supported by the Ministry of Science and Higher Education of the Republic of Poland.

**Conflicts of Interest:** The authors declare no conflict of interest.
