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Endophytes in phytoremediation

Due to the increasing contamination of the environment with xenobiotics (hydrocarbons, nitrogen compounds, pesticides, herbicides), researchers are more likely to use phytoremediation technology. Unfortunately, it has many disadvantages, e.g., very long cleaning time. To overcome these drawbacks, a promising option is to inoculate plants with suitable endophytes capable of degrading xenobiotics like Burkholderia, Pseudomonas, Herbaspirillum, and Methanobacterium (Marmisto et al. 2001, Barac et al. 2004, Van Aken et al. 2004). Many other endophytic microorganisms have ability to degrade pollutants that are present in the environment where these plants grow. The most known and described endophytes were isolated from poplar tissues, wheat and yellow lupine (Mamiisto et al. 2001, Barac et al. 2004, Van Aken et al. 2004, Germaine et al. 2004).

Some of the endophytes can greatly increase resistance for the presence of heavy metals and thus may exhibit hyperaccumulating properties. These bacteiia belong to the genus Bacillus, Pseudomonas, and Achromobacter. It was reported that Bacillus thuringensis GDB-1 strain significantly enhanced plant growth and improved metal absorption by Pinus sylvestris (Babu et al. 2013). Another researcher proved that Microbacterium and Arthrobacter inhabiting interior tissues of Noccaea caerulenscens, despite strong plant growth promoting abilities, also notably enhanced phytoextraction, translocation, and removal of heavy metals like Fe, Co, Ni and Cu from soil (Visioli et al. 2015).

Expanded enzymatic apparatus of endophytic microorganisms, related to the colonization of plant tissues, resulted in rising interest in using those enzymes as potential biocatalysts in chemical transformation of not only natural compounds but also drugs. The transformation of benzoxazinones,

2-benzoxazolinone (BOA) and 2-hydroxy-l,4-benzoxazin-3-one (HBOA) into different compounds was described for endophytic fungus isolated from Aphelandra tetragona roots and shoots (Gunjal et al. 2018). Also, endophytic fungi Phomopsis spp. from Viguiera arenaria were reported to catalyze transformation of a tetrahydrofuran lignan, (-)-grandisin to a new compound-3,4- dimethyl-2-(4-hydroxy-3,5-diinethoxypheiiyl)-5-methoxy-tetrahydrofuran. What is interesting, this compound exhibits trypanocidal activity against Trypanosoma cruzi, which causes the Chagas disease. However, the studies on using endophytes for biotransformation processes are still scarce (Gunjal et al. 2019).

Despite remarkable abilities for promoting plant growth and increasing plants’ resistance for environmental stresses related to the presence of pollution in the soil, endophytes inside plants’ tissues may be not sufficient as a sole tool for cleaning the environment, especially in the presence of high concentrations of contaminants (Mitter et al. 2019).

Endophytes as a source of biosurfactants

Hydrophobic compounds present in the soil very often are bounded with soil matrix, thus their availability to microorganisms that are performing biodegradation process is limited. To increase the bioavailability of hydrophobic compounds, surface active agents may be applied. However, the chemical surfactants exhibit many hazardous features like toxicity and low biological compatibility and low biodegradability. However, it is a great alternative, namely, biological surface-active agents (biosurfactants) that demonstrate all advantages of synthetic surfactants and also do not have their disadvantages (Marchut-Mikolajczyk et al. 2018, Pinto et al. 2018, Sunkar et al. 2019).

Biosurfactants are surface active agents produced by microorganisms and they are very valuable during the biodegradation process; thus, they increase the solubility of hydrophobic compounds and heavy metals in soil matrix, which increase the bioavailability and mobility significantly (Marchut- Mikolajczyk et al. 2018, Pinto et al. 2018, Sunkar et al. 2019). Innovative sources of biosurfactants are endophytic bacteria.

Sunkar et al. (2019) isolated biosurfactant from endophytic Bacillus cereus strain, which is described as a glycolipid that contains rhamnose, fructose and glucose, and 9-hexadecanoic acid and 1-eicosanol. This compound has a good emulsifying activity, which predisposes its application for biodegradation enhancement. Also, this glycolipid has antibacterial activity against Staphylococcus sp. and Serratia sp., which is worth mentioning; nevertheless, it is not very effective against pathogenic bacteria (Sunkar et al. 2019).

 
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