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Along with the development of the economy, the number of ecosystems characterized by the high expansion of anthropogenic disorders increases. One of the side effects of human activity is synantrophization of flora in natural ecosystems. This type of vegetation can be divided into two broad types: weed and ruderal (Lososova et al. 2006, Silc 2010). Weed plants can be found on arable land, while the ruderal vegetation is found in waste deposits, along transportation routes, settlements and semi-natural landscape (Silc 2010).
The term ruderal comes from Latin word nidus and defines disturbance-adapted species (Laclmmd 2003). Thanks to the human activity, ruderal environments became nutrient-rich and thus are characterized by the presence of plants with high productivity and a variety of competitive relationships associated with continuous or occasional interference and rapid successive changes (Dietz et al. 1998). Ruderal plants encompass species from environments that are directly or indirectly influenced by human aird may grow in artificial habitats, which are disturbed such as surroundings of houses, gardens, roadsides, railways sides, etc. (Johnson and Klemens 2005, Szafer
2013). The communities of synanthropic plants are composed of not only native species but also alien ones. Native plants may persist in their old habitat, which has been damaged by humans (Pteridium aquilanum, a cereal which grows on fields, which were previously covered by forests). Another mechanism involves settling after arriving from a near to far neighborhood, like Urtica dioica (originally a forest plant, which is now growing in ruderal environments). Due to the environment in which these plants grow, highly affected by human activity, very often they are exposed to different xenobiotics like herbicides, pesticides, hydrocarbons, and heavy metals. Nevertheless, they are capable of surviving in such an inconvenient environment, thanks to microorganisms that are inhabiting the interior of their tissues (Figure 1).
Endophytes from ruderal plants against environment pollution
Ruderal plants, growing in a difficult and/or extreme environment, exhibit strong adaptation skills, related to the expression of genes responsible for defense mechanisms (Andreolli et al. 2013, Anyasi and Atagana 2019). Therefore, these plants may serve not only as a source of unique biologically active compounds but also new microorganisms with great metabolic potential (Afzal et al. 2014). This approach can be applied in the remediation of a polluted environment. Recently, several studies have shown that endophytic microorganisms may demonstrate relatively high degradation activity towards organic contaminations (Table 1) (Stepniewska and Kuzniar 2013, Sun et al. 2014, Syranidou et al. 2018).
Research performed by Moore et al. (2006) indicates that endophytic bacteiia like Pseudomonas sp., Bacillus sp., Arthrobacter sp. isolated from root, stem and leaves of poplar tree, which was growing on a site contaminated with BTEX compounds, have metabolic conditions to degrade this kind of pollution (Moore et al. 2006, Zhang et al. 2014).
Soleimani et al. (2010) reported that planting into a soil contaminated with hydrocarbons of two species of grass—Festuca arundinacea Schreb. and Festuca pratensis Huds. inoculated
Endophytic Microorganisms from Synanthropic Plants—A New Promising TooI for Bioremediation 321
Figure 1. Environmental application of endophytes isolated from synantrophic plants.
with endophytic fungi Neotyphodium coenophialum and Neotyphodium uncinatum, respectively, caused increased biomass and achieved higher levels of dehydrogenases activity in the soil than uninoculated grass samples. They also repoxted that polyaromatic and total hydrocarbons reduction in soil was higher than in a control sample by about 30-40% (Soleimani et al. 2010).
An interesting study was performed by Germaine et al. (2006). Authors inoculated plants with endophytic bacteria, Pseudomonas sp., and used it for herbicides degradation. Noteworthy, they did not observe the herbicide accumulation in the plant tissue in an inoculated plant, unlike for the plant without endophytes (Germaine et al. 2006, Anyasi and Atagana 2019). Although there is some research on degradation ability of endophytic fungi towards different xenobiotics, the use of endophytic bacteria as degradation agents has not been demonstrated for in situ remediation (Anyasi and Atagana 2019).
Urtica dioica L.
Urtica dioica L. (stinging nettle) is a connnon plant growing in the countryside in northern Europe and the majority of Asia. The positive effect of Urtica dioica has been known for a long time worldwide. It was used for healing different diseases and disorders like eczema, rheumatism, hemonhoids, hyperthyroidism, bronchitis, and cancer; thus, it is worth mentioning that it causes no adverse effect (Kavalali et al. 2003, Gaballu et al. 2015). Other authors claimed that a stinging nettle is a good tool for phytoremediation due to its hyperaccumulating ability (Hartley 2004, Viktorova et al. 2016).
Due to that fact, it was obvious that internal tissues of stinging nettle inhabit endophytes with special abilities.
Naoufal et al. (2018) isolated 54 endophytic bacterial strains from Urtica dioica L. from which authors chose gram-positive Bacilli characteiized by different moiphologies and abilities for further research, whose goal was to find the best phytopathogen inhibitors. Authors performed biochemical tests and on the basis of obtained results, they chose three isolates that exhibited the largest antagonistic activity against typical phytopathogens (Fusarium oxysporuni, Colletotrichum gloeosporioides, Rhizoctonia solani, Phytophthoraparasitica). The average size ofphytopathogens’ growth inhibition was 73.21% against the control sample which was uninoculated with endophytic
Table 1. Endophytic microorganisms with the ability for xenobiotics degradation (Feng et al. 2017).
bacteria. What is more, according to the authors, these endophytic Bacilli species may be valuable sources of different secondary' metabolites and enzymes (Naoufal et al. 2018).
Victorova et al. (2016) assessed the impact of endophytic bacteria on stinging nettle’s health and tolerance to toxic compounds during phytoremediation of soil contaminated with chlorinated biphenyls and heavy metals. Bacteria that were isolated by the authors mainly belong to the Bacillus and Arthrobacter genus. Authors noted that isolated endophytic B. shackletonii and Streptomyces badius represented all the beneficial abilities of endophytes-produced phytohormones, have ACC- deaminase and nitrogenase activity, perform solubilization of phosphate, and greatly increase the resistance of the host to heavy metals present in the environment. That is why authors claim that these microorganisms may constitute an effective tool for phytoremediation enhancement (Victorova et al. 2016).
Lolium perenne L.
Lolium perenne (perennial ryegrass) is a grass used in agriculture as a pasture all over the world, and because of an increasing demand of the public for meat and milk production, its role is significant. This grass usually grows in a moderate climate, so it is frequent to form an endosymbiosis with fungi like Neotyphodium and Epichloe, which provide an increased environmental stress tolerance and protection against pests (Saikkonen et al. 2006, Clay and Schardl 2002, Young et al. 2013). However, the reports about the usage of ryegrass and its endophytic fungi are scarce. Nevertheless, endophytic bacteria from that plant host are used more frequently. Jabeem et al. (2016) used Mezorhizobium sp. HN3 bacterial strain isolated from ryegrass for chlorpyrifos degradation (Jabeem et al. 2015).
Chelidonium majus L.
Chelidonium majus L. is a ruderal plant that naturally occurs in Europe, Asia, and South America. Although the plant is toxic, it produces many bioactive substances, e.g., alkaloids with high antimicrobial, antiviral and even anticancer properties (Goryluk et al. 2016, Marchut-Mikolajczyk et al. 2018). Therefore, it was essential to isolate endophytes from the plant and to examine the possibility of sharing the ability to produce the same biological active compounds. Goryluk et al. (2009) isolated 34 endophytic bacteria from internal stem tissues of Chelidonium majus L. and investigated their antifungal properties against six fungal species Alternaria alternata, Paecilo-myces variotti, Aureobasidium pullulans, Byssochla-mysfulva, Chaetomium sp. and Exophiala mesophila. The authors reported that eleven isolates exhibited the inhibition of growth of all fungi except B. fulva. Only one bacterial strain exhibited antifungal properties against all the tested fungi. The strain was classified on the basis of the API-20E, -50CHB tests and the analysis of 16S rDNA sequence as B. amyloliquefaciens (Goryluk et al. 2009). Studies on endophytic microorganisms from Chelidonium majus L. were extended by Marchut-Mikolajczyk et al. (2018). The authors isolated 11 endophytic bacteria from the tissues of Chelidonium majus L. plant growing in motorway neighborhood. They examined the ability of isolated bacteria for hydrocarbons degradation (diesel oil, waste engine oil) and biosurfactant production. All tested strains showed degradation activity. However, the strain marked as 2Ahad the highest degradation activity against both diesel and waste engine oil. The strain also exhibited the highest biosurfactant production. Based on the analysis of 16S rDNA sequence, the strain was classified as Bacillus pumihts 2A. Authors claim that produced biosurfactants not only enhance hydrophobic compounds degradation but also may act as plant-growth-promoting agents in a contaminated environment (Marchut-Mikolajczyk et al. 2018). Although the presented data is new and interesting, the application of the strain and obtained biosurfactant will be possible after acquired knowledge of the mechanism of an observed phenomenon, which will be important in evaluating the possibility of application of biosurfactants to promote plant growth, especially in contaminated areas.