Streptomycetes from the Rhizosphere of Three Medicinal Plants as Plant Growth Stimulants and Biocontrol Agents

The number and functional structure of streptomycetes complexes isolated from the rhizosphere of three species of рlantae medicinales—Melissa officinalis L., Urtica dioica L., and Capsella bursa-pastoris L.—from the Vyatka–Kama Cis-Urals (Kirov oblast) was investigated. The total number of representatives of the genus Streptomyces varied in the rhizosphere from (3.0 ± 1.6) × 10⁵ to (1.4 ± 0.5) × 10⁶ CFU/g of substrate; depending on the type of plant, the proportion in the prokaryotic complex of different species varied from 4% (C. bursa-pastoris) to 31% (M. officinalis). Using selective treatment, 36 strains of bacteria were isolated, whose cultural and morphological features are characteristic of representatives of the genus Streptomyces. It was found that the majority (82%) of isolates from the rhizosphere of M. officinalis are capable of synthesizing water-soluble metabolites of antifungal action with moderate and high activity. In the rhizosphere of C. bursa-pastoris, 42% of the isolates are able to effectively carry out enzymatic hydrolysis of cellulose. The strains isolated from the roots of U. dioica stood out among others with the most pronounced phytostimulating effect. Based on the results of the evaluation of the antagonistic, cellulolytic, and phytoregulatory properties of streptomycetes, nine promising strains capable of synthesizing indole compounds (indolyl-3-acetic acid) in an amount of up to 24.0 ± 0.9 µg/mL and a radial growth rate of up to 65.7 ± 8.8 µm/h were selected, which can have a positive effect on the growth and development of agricultural plants. The results obtained will be useable for determining strategies for the search, identification, and use of this group of bacteria as biological control agents and plant growth stimulators.