**1. Introduction**

Each plant coexists with microorganisms residing within tissues and producing their metabolites, which are defined as endophytes if their occurrence does not cause apparent injuries [1,2]. Wilson [3] defined "endophytes" (from Greek endon—within; and phyton—plant) as microorganisms, commonly fungi and bacteria, spending their life cycle inter- and/or intra-cell space of the tissues of host plants, which do not show any symptoms of disease. Endophytes were isolated from plants belonging to all taxa investigated to date, occurring in all the world's ecosystems. In recent years, there has been an increased interest in explaining the endophytes/host plant cross-talk because the effects of these relationships could be beneficial to humans [1,4–6]. Host plants abide endophytes due to symbiotic relationships, profitable for microbes due to the availability of habitat and nutrients in the plant, while plants acquire a wide spectrum of microbial metabolites, including vitamins, hormones, and antibiotics [7,8]. Endophyte−host relationships can be so close, that microbes can even biosynthesize the same chemical compounds as the host, as myrtucommulones from *Myrtus communis*, camptothecin from *Camptotheca acuminata*, paclitaxel from *Taxus brevifolia*, or deoxypodophyllotoxin from *Juniperus communis* for better adaptation to the microenvironment of plant tissues [7,9–13]. It is an unresolved hypothesis that the production of secondary metabolites in plants is not achieved only by endophytes but arises from concomitant plant and fungal biosynthesis [13]. Endophytes occupy a unique ecological niche, their relationship with a host plant a balance between mutualistic, parasitic, or commensal

symbiosis, which is largely controlled via chemicals. That is the reason why endophytes produce highly specific metabolites [14]. Indeed, these microorganisms are being increasingly investigated as they play an important role in natural product discovery, especially when the source plant is used for medicinal purposes. In the latter respect, the healing action can be the result not only of the host plant metabolome but also the microorganism-derived active compounds and their interactions [4]. Moreover, organic extracts obtained from isolated endophytes show a wide spectrum of biological action and may be applied as antidiabetic, antimicrobial, antiviral, larvicidal, antimalarial, cytotoxic, and plant growth promoters [15,16]. The problem is that some endophyte genes responsible for secondary metabolite biosynthesis were found to be significantly expressed in planta but silent in vitro cultures. Plant and coexisting microbial signal molecules are required to induce particular pathways of endophyte metabolism leading to a balance of sexual to asexual reproduction and biochemical profile modification as well [17–20]. Moreover, the secondary metabolites are energy-consuming compounds, so endophytes can increase/decrease their production depending on specific needs, like competition with the other microorganisms or host plant communication and protection [9,21–23]. However, some fungal endophytes were shown to produce the desired compounds without a host plant association. Sustainable synthesis of tanshinone IIA and taxol by the axenic culture of endophytic fungi have been reported by Ma et al. [24] and Zhao et al. [25]. Karuppusamy [26] presented the possible origin of secondary metabolites in plant-endophyte systems, namely (i) parallel coevolution of plants and their microbiota possessing pathways to produce bioactive compounds; (ii) horizontal gene transfer between plants and microbes during their coevolution; (iii) plants or endophytic fungi synthesize and transfer metabolites to each other. Recent studies provided strong indications that endophytic fungi dispose host-independent machinery for secondary metabolite production [27–29]. Metabolites of fungal endophytes which were isolated from medicinal plants possess diverse and unique structural groups. That is the reason why they are good sources of novel secondary metabolic products contributing to the therapeutic activity [30–32]. Among medicinal plants, the members of Asteraceae family have been reported to be a source of natural remedies in all traditional medicine systems since their secondary metabolites exhibit strong antioxidant, antibacterial or anti-inflammatory activities [33].

The production of bioactive secondary metabolites by endophytic fungi colonizing medicinal plants has been largely ignored. The main idea of this review is that the Asteraceae evolutionary success is the effect of interaction between the host plant and fungal endophytic microbiota. We focused on determining the possible contribution of fungal biosynthesis to the secondary metabolome of Asteraceae, as a leading family of medicinal plants, to present the additional explanation for the distribution of bioactive compounds, including alkaloids, cardiac glycosides, and anthraquinones in the plant kingdom. We reviewed the available literature to assess therapeutic activity that had been reported previously from medicinal plants of the Asteraceae family that may likewise originate from endophytic fungi that coexist with these plants. We tried to estimate if the plants' taxonomic affinity affects the endophytic microbiome biodiversity and metabolic pathways.

#### **2. Asteraceae Ecology and Biochemistry**

The family Asteraceae (Compositae) is the largest and most cosmopolitan group of angiosperms covering 32,913 accepted species, grouped in 1911 genera and 13 subfamilies [34]. Asteraceae comprise more than 40 economically important crops, including food crops (*Lactuca sativa*, *Cichorium* spp., *Cynara scolymus*, *Smallanthus sonchifolius*, and *Helianthus tuberosus*), oil crops (*Helianthus annuus, Carthamus tinctorius*), medicinal and aromatic plants (*Matricaria chamomilla*, *Chamaemelum nobile*, *Calendula* spp., *Echinacea* spp., and *Artemisia* spp.), ornamentals (*Chrysanthemum* spp., *Gerbera* spp., *Dendranthema* spp., *Argyranthemum* spp., *Dahlia* spp., *Tagetes* spp., and *Zinnia* spp.), and nectar producers (*Centaurea* spp., *H. annuus*, and *Solidago* spp.) [35]. Species of this family represent a great variation regarding the habit: annual, perennial, herbs, shrubs, vines, trees, epiphytes; with the inflorescence composed of one to more than a thousand florets; and chromosome numbers range from *n* = 2 to *n* = 114 [36]. The Asteraceae store energy in the form of inulin [37], they can produce acetylenes, alcohols, alkaloids, organic acids, pentacyclic triterpenes, sesquiterpene lactones, and tannins [38–40]. They are globally distributed although most are native to temperate climatic zones, the Mediterranean zone, or higher-elevation, cooler regions of the tropics [41]. The unique success of Asteraceae in worldwide distribution has been attributed to many factors, including diversity of secondary metabolites that improve overall fitness, a highly specialized inflorescence that maximizes fertilization, and a morphology promoting outcrossing [42]. Many species of the Asteraceae family have been used as medicinal plants, although the secondary metabolites responsible for the pharmacological efficiency were not always defined. The chemical diversity of bioactive compounds and pathways of their biosynthesis is dependent on a broad spectrum of biotic and abiotic factors and their interactions. Sometimes the benefits of plant-derived pharmacological products are controversial despite standard chemical composition with the use of commonly accepted pharmacopeia's methods [43]. Numerous papers have described the pharmacological activity and chemical constituents isolated from plants of the Asteraceae, covering polyphenols, sesquiterpenes, organic and fatty acids which have been associated with the successful treatment of cardiovascular diseases, cancer, microbial and viral infections, inflammation, and other diseases [43]. Most of the Asteraceae taxa, like *Artemisia*, are well known for their resistance to herbivores, bacterial and fungal pathogens [44]. Secondary metabolites are chemicals of a very diversified structure, not fundamental in the plant metabolism, but crucial for protection against pathogens and herbivores [45]. With the use of principal component analysis, Alvarenga et al. [46] showed the relationships between chemical composition and botanical classification of Asteraceae family, based on a huge group of 4000 species and 11 main chemical classes of secondary metabolites. *Barnadesieae* tribe revealed an anomalous position owing to the poor diversity of its secondary metabolites, particularly flavonoids. *Liabeae* and *Vernonieae* tribes were localized closely because of similar lactone composition, while *Asteridae* was separated because of monoterpenes, diterpenes, sesquiterpenes content. Moreover, the correlation matrix of Asteraceae secondary metabolites showed that benzofuranes and acetophenones, as well as diterpenes and phenylpropanoids, were highly correlated with each other [46]. The role of fungal endophytes in Asteraceae's evolutionary success has been recently recognized by the scientific community, although there is still a need for complex investigations in this area. The multifarious metabolome of Asteraceae is a dynamic patchwork of chemicals synthesized solely by the plant, by the microbial inhibiting the host species, or by both elements of this ecological system.

#### **3. Fungal Endophytes Associated with Asteraceae—Biodiversity, and Ecology**

The high diversity of endophytes indicates their multiple and variable relations with the host plants and ecological functions. The widest research program to find endophytes in medicinal Asteraceae has been performed in countries which are localized in the most important biodiversity hotspots, like Brazil, China, the Mediterranean region, Iran, or Thailand [47]. In Brazil, like the other South American countries, medicinal plants have been used as a traditional, cheap, and easily available alternative to drugs. Only a few tropical herbs were investigated with respect to endophytic fungal communities with bioactivity [48–50]. Another region of Asteraceae collection as host plants for fungal endophytes is the Panxi plateau in China [51] with xerothermic climate, diversified soil, and landscape conditions contributing to the high biodiversity in the area, concerning also medicinal plants having a long history of application by local communities [52]. The global screening reflected in the present review showed minimal knowledge on Asteraceae in this respect (Figure 1).

**Figure 1.** The Asteraceae hosts and endophyte fungi isolated from them in chosen countries (based on the references cited in this review).

Despite the high diversity and abundance of the Asteraceae worldwide, fungal endophytes associated with the plants of this family represented common or cosmopolitan species [53]. In light of the present review, about 23% of fungi taxa isolated from Asteraceae were associated with one host (Figure 2). They were mentioned in the footnote of Figure 2 as "The others". The most abundant fungi genera, *Colletotrichum*, *Alternaria*, *Penicillium*, etc., were ubiquitous and isolated from most plant species and environments [10].

**Figure 2.** The frequency of isolation of endophytes (%) from Asteraceae host plants. The others: *Acremonium*, *Ampelomyces*, *Bipolaris*, *Botryosphaeria*, *Botrytis*, *Calonectria*, *Cercospora*, *Coniochaeta*, *Cylindrocarpon*, *Epicoccum*, *Exserohilum*, *Memnoniella*, *Paecilomyces*, *Periconia*, *Podospora*, *Pezicula*, *Pyrenophora*, *Scopulariopsis*, *Seiridium*, *Trichoderma*, *Xylaria* (based on references cited in this review).

To date, most of the research was focused on the overall spectrum of endophytes of the particular host plant or the particular endophyte taxon isolated from a wide range of host plants. To validate, Rodríguez−Rodríguez et al. [49] compared microorganism diversity and abundance in *Aster grisebachii* (synonym of *Neja marginata*), *Erigeron bellidiastroides*, *Erigeron cuneifolius*, *Pectis juniperina*, and *Sachsia*

*polycephala* (Asteraceae), native to Cuba, collected in an area with a low-in-nutrients, acid, sandy soil with alternating dry, and rainy seasons. The colonization rate was higher than 50% in both the dry and rainy period for all species which is typical for changing and stressful ecosystems, with strong competition for soil resources. *Pestalotiopsis* spp. were isolated as dominant from the different medicinal plants originated to tropical and subtropical climatic zones [54]. *Preussia* spp. isolated from leaves of medicinal plants *Baccharis trimera* (Asteraceae) and *Stryphnodendron adstringens* (Fabaceae) are native to Brazilian savannah [55]. A study performed by Hatamzadeh et al. [56], on native Asteraceae medicinal plants of Iran, allowed to isolate 241 endophyte species from *Cota segetalis* (syn. *Anthemis altissima*), 163 from *Achillea millefolium*, 121 from *Anthemis triumfettii* (synonym of *Cota triumfettii* subsp. *triumfettii*), 132 from *Cichorium intybus*, 90 from *Achillea filipendulina*, and 59 from *M. chamomilla*. A few endophytic fungi such as *Acremonium sclerotigenum*, *Alternaria burnsii*, *Bjerkandera adusta*, *Colletotrichum tanaceti*, *Epicoccum nigrum*, *Fusarium acuminatum*, *Paraphoma chrysanthemicola*, *Plectosphaerella cucumerina*, and *Stemphylium amaranthi* were isolated from all host species [56], most of them colonizing the stem of the plant. Although Cheng et al. [57] concluded that the structure of the endophytic communities differed within plant tissues and habitats, similarities in the taxa of the endophytic fungi were rarely observed at the phylum order or even the host plant family level. Endophyte communities were characterized by ecological variation, different host preference, tissue specificity, spatial heterogeneity, and seasonal changes in terms of composition and quantity of fungal endophytic strains which can affect medicinal plant biochemical composition [58]. Investigations of endophytes coexisting with *Ageratina altissima* showed that the fungal microbiome was driven by host individual and geographic location. Moreover, the endophyte community of a single host collected in the urban zone was less abundant compared to the forest probably due to human disturbance and spatial isolation [59]. The expansion of the invasive species *Ageratina adenophora* was studied concerning the distribution of endophytes in tissues in surrounding environments [60–62]. The enrichment of *A. adenophora* endophytes was root tissue-specific, moreover, fungi rarely grew systemically within the plant. The roots were the habitat of *Fusarium*, the stems of *Allophoma*, the mature leaves of *Colletotrichum,* and *Diaporthe.* Additionally, some fungi might migrate tissue-to-tissue via the vascular system of the shoot, and this was the way airborne fungi infected roots, and soilborne fungi, shoots, and leaves. Leaf endophytes showed more fluctuations in the number of taxa than those in roots and stems, because of the stronger pressure of environmental factors [62]. Presented studies indicated that fungal endophyte communities varied based on host genotype or even specimen, plant tissue, growth stage, and growth conditions. The research referenced in this review were focused on the taxonomical analysis of endophytes collected in a particular area from different Asteraceae taxa, or one species, or from different tissues of that species. Another main field of investigation were secondary metabolites produced by endophytes in situ or in vitro. Table 1 summarizes the biological action of Asteraceae plant extracts and endophytes isolated from them. The evident similarities indicate that the therapeutic activity of Asteraceae plants used traditionally as herbal remedies can also be referred to associated fungal endophytes. Almost all internal symbiotic fungi showed in vitro similar activity to those of their host plant extract. However, the present review of the literature published during the last twenty years showed insufficient experimental evidence to describe the endophyte/host plant interactions on the metabolome level, so the biosynthetic pathway might be differently regulated in the fungus and the host plant.


 referred to host and/or endophyte taxon.

## *Agriculture* **2020**, *10*, 286


