Isolation and Characterization of Plant Growth-Promoting Bacteria from the Rhizosphere of Chamaecytisus ruthenicus (Russian Broom) Growing in Chalky Soil

Plant growth-promoting (PGPR) bacteria are beneficial soil bacteria that enhance plant growth against biotic and abiotic stress. Numerous studies have been carried out over the past three decades on the isolation and characterization of rhizospheric bacteria. However, no studies have been conducted on the bacteria present in the rhizosphere of the wild legume plant Chamacytisus ruthenicus when growing in chalky soil. The purpose of this study was to evaluate the abundance of culturable bacteria, assess the morphology of the bacterial cells, profile the chalky soil bacterial community, and characterize its ability to stimulate plant growth. Three soil samples were collected in January at a temperature of 2–4 °C. The first sample was taken from the topsoil, the second sample from the soil 15 cm beneath the surface, and the third sample from the rhizosphere. The results of the study revealed that the abundances of bacteria in the first, second, and third soil samples were 4.25 × 108 cfu/g, 3.58 × 108 cfu/g, and 10.1 × 107 cfu/g, respectively. Furthermore, a total of 23 rhizospheric bacteria were isolated based on differences in their morphological characteristics. The 16S rRNA soil profiling result showed that bacteria belonging to Proteobacteria, Actinobacteria, Gemmatimonadetes, Acidobacteria, and Bacteroidetes were the most dominant groups in the community. Six bacterial isolates (Z10, Z11, Z12, Z15, Z26, and Z44) were chosen to examine the inhibition effects on phytopathogenic microbes and their ability to promote plant growth. The bacterial isolates Z11 and Z15 showed good inhibition against all tested phytopathogenic fungi, while bacterial isolates Z10, Z12, Z15, Z26, and Z44 showed stimulatory effects on the length and fresh weight of the shoots and roots of wheat, maize, and oat seeds. In conclusion, the present study is the first report of chalky soil-associated bacteria found in the rhizosphere of the wild legume plant in the Belgorod region of Russia.