Heavy Metal-Resistant PGPR Strains of Pseudomonas sp. Stimulating the Growth of Alfalfa under Cadmium Stress

Abstract

Three bacteria strains of Pseudomonas sp. resistant to heavy metals were isolated from chemically contaminated soil. An analysis of the nucleotide sequences of the 16S rRNA and rpoD genes identified the Pseudomonas sp. 17 НМ strain as Pseudomonas capeferrum, and the strains of Pseudomonas sp. 65 НМ and 67 НМ were most closely related to the type strains of Pseudomonas silesiensis and Pseudomonas umsongensis, respectively. It was shown that the strains of Pseudomonas sp. 17 НМ, 65 НМ, and 67 НМ were characterized by different levels of resistance to heavy metals, with the maximum tolerable concentration (MTC) of zinc being 1 mМ for all strains, that of cadmium, 1, 1.5, and 1 mМ; of lead, 5, 5, and 4 mМ; and of nickel, 7, 9, and 7 mМ, respectively. All Pseudomonas strains can form biofilms and have the properties of PGPR bacteria. Treatment of alfalfa seeds (Medicago sativa L.) with the strains of Pseudomonas sp. 17 НМ, 65 НМ, and 67 НМ under cadmium stress increased the dry weight of alfalfa seedlings up to 40% and the content of chlorophyll a and b in the leaves by 25–33% relative to the control.