Woody plants growing in cities, especially large ones, are convenient model objects for studying the mechanisms of plant adaptation to technogenic stress. In the underground sphere, such mechanisms are partially coupled with the interaction between plants and fungi such as mycorrhizal, pathogenic and saprotrophic interactions. In nature, the formation of mycorrhiza is the rule, and its absence is an exception. The most common type of mycorrhiza is arbuscular (AM). Many works, mainly looking at young agricultural and ornamental crops, are devoted to the physiological and biochemical interactions of plants with AM fungi (AMF) associated with the absorption, transportation and accumulation of nutrients. The involvement of AM in the metabolism and supply of plants with N, P, Cu and Zn is considered the most critical [1–4].
There are indications of changes in the pigmentary apparatus and mechanisms for increasing plant resistance (to drought, salinity and acidity of soils, temperature stress, the content of heavy metals in soils, exposure of pathogenic microorganisms and pest damage) upon symbiotic interaction with microscopic fungi [5–9].The purpose of our research was to study the species composition of microscopic fungi in the root system of woody plants and rhizosphere soils in urban plantations.
The research was carried out in the large industrial center Izhevskof the Urals region (the population is more than 630,000 people: industry, the transport network and the social infrastructure are developed, while the pollution level is high). The objects of research are woody plants (Acer negundo L., Acer platanoides L., Betula pendula Roth.), which account for about 70% of the green area of the city. The species studied grow in various structural and functional types of plantations with different levels of soil contamination: park plantations (the park named after S.M. Kirov, which has a low level of heavy metal concentration in the soil); tree plantings along the highway (Udmurtskaya Street) and plantations in the sanitary protection zone (SPZ) of the industrial enterprise OJSC Izhstal, which is the main polluter of the city (high level of heavy metal concentration in the soil); the plantings in the yard territories of the residential area `Sever' and in the sanitary protective zone (SPZ) of the industrial enterprise Kerambloki (the average level of heavy metals concentration in the soil).
The record plants (5–10 trees of each species) were selected in each of the districts of the city. The selected plants were in a good living and middle-aged generative ontogenetic state . In September, during the formation of suction root hairs and root system inoculation with mycorrhiza-forming fungi, samples of the root system and rhizosphere soil of each record plant were collected to study the fungal microflora. Isolation of endophytes from the woody plants' roots was carried out . Samples of soil were stored at a temperature of +5–7ºC. The roots were surface sterilized and dried at room temperature to an airily dry state, or subjected to rapid freezing with liquid nitrogen. DNA extraction was performed using the Ultra Clean Soil DNA Isolation Kit. For the PCR analysis, the primers ITS1-ITS4, ITS F-ITS A, ITS F-ITS B and FLR3-FLR4 were used. Then, the PCR product was cloned by inserting E. coli DNA (Escherichia coli), followed by plasmid preparation and PCR analysis using the M13 primer. After confirming the presence of fungi DNA in the plasmid, the samples were sent for sequencing (the `Sangersequencing' method). To compare the results with known sequences, a list of annotated sequences present in the EMBL  and NSBI  databases were used. The extraction of DNA of roots, soil samples and isolated fungi was carried out at the Leibniz Institute of Vegetable and Decorative Cultures (Germany) and in the genetic laboratory of the Forestry Department of the Technical University of Zvolen (Slovakia).
The sampling of soils and their agrochemical analysis were carried out at the sites of the selection of plant samples. The analysis of the content of gross forms of heavy metals in soils was carried out at the Analytical Control Center (Accreditation Certificate No. POCCRU.0001.514685).
The soils in the park zone are referred to as natural because the transformation of the soil profile is no more than 50 cm and typical features are preserved. The sandy loam sod-podzol soils predominate there. The content of organic matter in these soils is 4.23%, the reaction of the soil solution is weakly acidic and close to neutral (pH 5.83, pH 6.70). On the whole, the soils are characterized by an average density; the field humidity is 17.08%. In the plantations growing in the district `Sever' and in the sanitary protection zone of the enterprise Kerambloki, the soil profile has been transformed into urban soil. The soil solution was characterized by a slightly alkaline reaction, close to neutral. The soils had a normal density and humidity and a fairly high content of humus. In plantations of the SPZ of OJSC Izhstal, anthropogenic soils with a prevalence of hemozems (anthropogenically transformed soils) were recorded. The content of organic matter was high and accounted for 2.17%. The reaction of the soil solution was neutral and alkaline (pH 6.95; pH 8.30). The soils were of average density and the field moisture was 13.61%. A complex of anthropogenic soils with a prevalence of stratozems (a mound on top of a natural profile) is revealed in plantings along the highway. The soil is characterized by pH 6.97 and pH 8.03, organic matter content is 2.29% and the average density and field humidity is 15.92%.
The results of soil analysis for the content of heavy metals are presented in Table 1. The identified species composition of microscopic fungi in plant roots and soils is presented in Table 2.
The results showed that the largest species diversity of microscopic fungi can be found in the soils and woody plants in the plantations of the sanitary protection zone of the enterprise Izhstal and in the plantings along the highway of UdmurtskayaStreet (Betulapendula, Acer negundo, Acer platanoides). No AMF was detected in the conventionally clean soils and roots of plants growing in the park zone.
We would like to note the presence of Fusarium equiseti fungi in the roots of plants. This endophyte is a modern object for studying the formation of plant resistance mechanisms based on interaction with fungi. For some plants, it provides protection against phytopathogenic fungi and viral infection while also increasing resistance to salts [14–17].